Module 11.15_b1_rev 00.pdf

Module 11 – AIRCRAFT AERODYNAMICS AND STRUCTURES Sub Module 11.15 – OXYGEN

CATEGORY B1 - MECHANICAL

MODULE 11 SUB MODULE 11.15 OXYGEN (ATA 35)

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Module 11 – AIRCRAFT AERODYNAMICS AND STRUCTURES Sub Module 11.15 – OXYGEN

CATEGORY B1 - MECHANICAL Table of Contents

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Table of Contents

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Introduction ................................................................................... 2

Portable Oxygen Systems........................................................... 40

The Atmosphere ........................................................................... 2

Flight-Crew Portable Devices...................................................... 40

Human Respiration and Circulation .............................................. 4

Protective Breathing Equipment.................................................. 42

Hypoxia......................................................................................... 6

Oxygen System Maintenance Practices...................................... 48

Oxygen Systems........................................................................... 7

Safety Precautions When Handling Oxygen Systems................. 49

Characteristics of Oxygen............................................................. 7

Servicing Gaseous Oxygen Systems .......................................... 51

Sources of Supplemental Oxygen ................................................ 8

Purging A Gaseous Oxygen System........................................... 53

Gaseous Oxygen .......................................................................... 8

Oxygen System Tests ................................................................. 55

Liquid Oxygen............................................................................... 8

Leak Testing Gaseous Oxygen Systems .................................... 55

Chemical, or Solid, Oxygen ........................................................ 10

Pressure Tests ............................................................................ 55

Mechanically Separated Oxygen ................................................ 10

Flow Testing................................................................................ 55

Oxygen Systems and Components ............................................ 12

Components Maintenance .......................................................... 56

Gaseous Oxygen Systems ......................................................... 12

Chemical Oxygen Generators ..................................................... 57

Storage Cylinders ....................................................................... 12

Masks.......................................................................................... 58

Regulators .................................................................................. 17

Pipes and Fittings........................................................................ 59

Masks ......................................................................................... 25

Prevention of Oxygen Fires or Explosions .................................. 61

Lines and Fittings........................................................................ 32

Example Systems ....................................................................... 64

Typical Installed Gaseous Oxygen Systems............................... 34 Chemical Oxygen Systems......................................................... 38 Rev. 00 Oct 2006

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“The training notes and diagrams are compiled by SriLankan Technical Training and although comprehensive in detail, they are intended for use only with a Course of instruction. When compiled, they are as up to date as possible, and amendments to the training notes and diagrams will NOT be issued”.

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the air decreases. This change in air density has a tremendous effect on the operations of high altitude aircraft as well as physiological effects on humans. [Figure pg-3]

INTRODUCTION In order to understand the reasons for controlling the cabin atmosphere or environment, it is necessary to understand both the characteristics of the atmosphere and the physiological needs of the persons flying within that atmosphere.

Turbine engine-powered aircraft are efficient at high altitudes, but the human body is unable to exist in this cold and oxygen-deficient air, so some provision must be made to provide an artificial environment to sustain life.

Each type of aircraft will have specific requirements according to the altitudes and speeds at which the aircraft is flown.

Standard conditions have been established for all of the important parameters of the earth 's atmosphere. The pressure exerted by the blanket of air is considered to be 29.92 inches, or 1013.2 hectoPascals (millibars), which are the same as 14.69 pounds per square inch at sea level, and decreases with altitude as seen in figure. The standard temperature of the air at sea level is 15° Celsius, or 59° Fahrenheit. The temperature also decreases with altitude, as illustrated in figure. Above 36,000 feet, the temperature of the air stabilizes, remaining at-55° C (69.7° F).

THE ATMOSPHERE The atmosphere envelops the earth and extends upward for more than 20 miles, but because air has mass and is compressible, the gravity of the earth pulls on it and causes the air at the lower levels to be more dense than the air above it. This accounts for the fact that more than one- half of the mass of the air surrounding the earth is below about 18,000 feet. The atmosphere is a physical mixture of gases. Nitrogen makes up approximately 78% of the air, and oxygen makes up 21% of the total mixture. The remainder is composed of water vapor, carbon dioxide and inert gases. Oxygen is extremely important for both animal and plant life. It is so important for animals that if they are deprived of oxygen for even a few seconds, permanent damage to the brain or even death may result. Water vapor and carbon dioxide are also extremely important compounds. The other gases in the air, such as argon, neon, and krypton are relatively unimportant elements physiologically. The density of air refers to the number of air molecules within a given volume of the atmosphere. As air pressure decreases, the density of the air also decreases. Conversely, as temperature increases the density of

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CATEGORY B1 - MECHANICAL

There are two important considerations in providing sufficient oxygen for the body. There must be enough oxygen in the air to supply the body with the amount needed, and it must have sufficient pressure to enter the blood by passing through the membrane walls of the alveoli in the lungs.

HUMAN RESPIRATION AND CIRCULATION The human body is made up of living cells that must be continually supplied with food and oxygen and must have their waste carried away and removed from the body. Blood, circulated through the body by the heart, carries food and oxygen to the cells and carries away waste products.

Oxygen makes up approximately 21% of the mass of the air, and so 21% of the pressure of the air is caused by the oxygen. This percentage remains almost constant as the altitude changes, and is called the partial pressure of the oxygen. It is the partial pressure of the oxygen in the lungs that forces it through the alveoli walls and into the blood. At higher altitudes there is so little total pressure that there is not enough partial pressure of the oxygen to force it into the blood. This lack of oxygen in the blood is called hypoxia.

When people inhale, or take in air, the lungs expand and the atmospheric pressure forces air in to fill them. This air fills millions of tiny air sacs called alveoli, and the oxygen in the air diffuses through the extremely thin membrane walls of these sacs into blood vessels called arteries Nitrogen is not able to pass through these walls. The blood circulates through the body in the arteries and then into extremely thin capillaries to the cells, where the oxygen is used to convert the food in the blood into chemicals that are usable by the cells. The waste product, carbon dioxide, is then picked up by the blood and carried back into the lungs through blood vessels called veins. The carbon dioxide is able to diffuse through the membrane walls into the alveoli, where it is expelled during exhalation. [Figure pg-5]

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As an aircraft climbs from sea level to increasingly high altitudes, the crew and passengers move further and further from an ideal physiological condition. In order to compensate for an atmosphere that becomes thinner as altitude increases, two different approaches have been developed.

HYPOXIA Any time the body is deprived of the required amount of oxygen, it will develop hypoxia. As hypoxia becomes more severe, a person's time of useful consciousness decreases. Time of useful consciousness is defined as the time a person has to take corrective action before becoming so severely impaired that they cannot help themselves.

One of these is to provide pure oxygen to supplement the everdecreasing amount of oxygen available in the atmosphere.

One of the worst things about hypoxia is the subtle way it attacks. When the brain is deprived of the needed oxygen, the first thing people lose is their judgment. The effect is similar to intoxication; people are unable to recognize how badly their performance and judgment are impaired. Fortunately, hypoxia affects every individual the same way each time it is encountered. If a person can experience hypoxia symptoms in an altitude chamber under controlled conditions, they are more likely to recognize the symptoms during subsequent encounters.

The other is to pressurize the aircraft to create an atmosphere that is similar to that experienced naturally at lower altitudes. For aircraft that fly at extremely high altitude, a combination of pressurization and supplementary oxygen for emergencies is required.

Two of the more common first indications of hypoxia occur at about ten thousand feet altitude. These are an increased breathing rate and a headache. Some other signs of hypoxia are light- headedness, dizziness with a tingling in the fingers, vision impairment, and sleepiness. Coordination and judgment will also be impaired, but normally this is difficult to recognize. If exposed to this type of environment too long death could occur. When permanent physical damage results from lack of oxygen, the condition is defined as Anoxia. Because it is difficult to recognize hypoxia in its early stages, many pressurized aircraft have alarm systems to warn of a loss of pressurization.

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Oxygen will not burn, but it does support combustion so well that special care must be taken when handling. It should not be used anywhere there is any fire, hot material or petroleum products. If pure oxygen is allowed to come in contact with oil, grease or any other petroleum product, it will combine violently and generate enough heat to ignite the material.

OXYGEN SYSTEMS At higher altitudes (generally above 10,000 feet) the air is thin enough to require supplemental oxygen for humans to function normally. Modern aircraft with the capability to fly at high altitudes usually have oxygen systems installed for the use of crew and/or passengers. Studies have shown that the effects of hypoxia become apparent at approximately 5000 ft [1500 m] altitude in the form of reduced night vision. It is recommended, therefore, that a pilot flying above 5000 ft altitude at night use oxygen. As stated before, pilots flying above 10 000 ft [3048 m] altitude should use oxygen. Requirements for oxygen in aircraft are set forth in FAR Parts 23,25, and 91.

Commercial oxygen is used in great quantities for welding and cutting and for medical use in hospitals and ambulances. Aviator's breathing oxygen is similar to that used for commercial purposes, except that it is additionally processed to remove almost all of the water. Water in aviation oxygen could freeze in the valves and orifices and stop the flow of oxygen when an aircraft is flying in cold conditions found at high altitude. Because of the additional purity required, aircraft oxygen systems must never be serviced with any oxygen that does not meet the specifications for aviator's breathing oxygen. This is usually military specification MIL-0-27210. These specifications require the oxygen to have no more than two milliliters of water per liter of gas.

CHARACTERISTICS OF OXYGEN Oxygen is colorless, odorless and tasteless, and it is extremely active chemically. It will combine with almost all other elements and with many compounds. When any fuel burns, it unites with oxygen to produce heat, and in the human body, the tissues are continually being oxidized which causes the heat produced by the body. This is the reason an ample supply of oxygen must be available at all times to support life. Oxygen is produced commercially by liquefying air, and then allowing nitrogen to boil off, leaving relatively pure oxygen. Gaseous oxygen may also be produced by the electrolysis of water. When electrical current is passed through water (H2O), it will break down into its two elements, hydrogen and oxygen.

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LIQUID OXYGEN

SOURCES OF SUPPLEMENTAL OXYGEN

Most military aircraft now carry their oxygen in a liquid state. Liquid oxygen is a pale blue, transparent liquid that will remain in its liquid state as long as it is stored at a temperature of below -181°F. This is done in aircraft installations by keeping it in a Dewar flask that resembles a double-wall sphere having a vacuum between the walls. The vacuum prevents heat transferring into the inner container.

Oxygen systems, classified according to source of oxygen supply, may be described as stored-gas, chemical or solid-state, and liquid oxygen (LOX) systems. Aircraft oxygen systems employ several different sources of breathing oxygen. Among the more common ones are gaseous oxygen stored in steel cylinders, liquid oxygen stored in specially constructed containers called Dewars, and oxygen generated by certain chemicals that give off oxygen when heated.

Liquid oxygen installations are extremely economical of space and weight and there is no high pressure involved in the system. They do have the disadvantage, however, of the dangers involved in handling the liquid at its extremely low temperature, and even when the oxygen system is not used, it requires periodic replenishing because of losses from the venting system. [Figure b]

A Dewar, sometimes called a Dewar flask, is a special type of thermos bottle designed to hold extremely cold liquids. Recently, a system using microscopic filters to separate oxygen from other gases in the air has been developed for medical uses, and is being investigated for use in aircraft. GASEOUS OXYGEN Most of the aircraft in the general aviation fleet use gaseous oxygen stored in steel cylinders under a pressure of between 1,800 and 2,400 psi. The main reason for using gaseous oxygen is its ease of handling and the fact that it is available at most of the airports used by these aircraft. It does have all the disadvantages of dealing with high-pressure gases, and there is a weight penalty because of the heavy storage cylinders. [Figure a]

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Fig. a

Fig. b

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CATEGORY B1 - MECHANICAL CHEMICAL, OR SOLID, OXYGEN

MECHANICALLY SEPARATED OXYGEN

A convenient method of carrying oxygen for emergency uses and for aircraft that require it only occasionally is the solid oxygen candle. Many large transport aircraft use solid oxygen generators as a supplemental source of oxygen to be used in the event of cabin depressurization.

A new procedure for producing oxygen is its extraction from the air by a mechanical separation process. Air is drawn through a patented material called a molecular sieve. As it passes through, the nitrogen and other gases are trapped in the sieve and only the oxygen passes through. Part of the oxygen is breathed, and the rest is used to purge the nitrogen from the sieve and prepare it for another cycle of filtering.

Essentially, a solid oxygen generator consists of a shaped block of a chemical such as sodium chlorate encased in a protective steel case. When ignited, large quantities of gaseous oxygen are released as a combustion by-product. They are ignited either electrically or by a mechanical igniter (percussion device). Once they start burning, they cannot be extinguished and will continue to burn until they are exhausted.

This method of producing oxygen is currently being used in some medical facilities and military aircraft. It appears to have the possibility of replacing all other types of oxygen because of the economy of weight and space, and the fact that the aircraft is no longer dependent upon ground facilities for oxygen supply replenishment.

Solid oxygen candles have an almost unlimited shelf life and do not require any special storage conditions. There are specific procedures required for shipping these generators and they may not be shipped as cargo aboard passenger carrying aircraft. They can be shipped aboard cargo only aircraft and must be properly packaged, made safe from inadvertent activation, and identified properly for shipment. They are safe to use and store because no high pressure is involved and the oxygen presents no fire hazard. They are relatively inexpensive and lightweight. On the negative side, they cannot be tested without actually being used, and there is enough heat generated when they are used that they must be installed so that the heat can be dissipated without any damage to the aircraft structure.

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Solid oxygen generator

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STORAGE CYLINDERS

OXYGEN SYSTEMS AND COMPONENTS

Oxygen cylinders, also called oxygen bottles, are the containers used to hold the aircraft gaseous oxygen supply. The cylinders may be designed to carry oxygen at a high or low pressure.

The aviation maintenance technician will encounter oxygen systems during the course of servicing and repairing aircraft. Actual servicing or repair of the oxygen system itself must be accomplished in accordance with the manufacturer's instructions, but a general knowledge of gaseous, liquid, and chemical oxygen systems and how they operate will enable the technician to better prepare the aircraft for flight.

Most military aircraft at one time used a low-pressure oxygen system in which the gaseous oxygen was stored under a pressure of approximately 450 psi in large yellow-painted low-pressure steel cylinders. These cylinders were so large for the amount of oxygen they carried that they never became popular in civilian aircraft, and even the military has stopped using these systems.

Oxygen systems may be portable or fixed. The fixed system is permanently installed in an airplane where a need for oxygen may exist at any time during flight at high altitudes. Commercial airplanes are always equipped with fixed systems augmented by a few portable units for crewmembers who must be mobile and for emergency situations where only one or two persons may require oxygen for unusual physical reasons.

Low-pressure cylinders are made either of stainless steel with stainless steel bands seam-welded to the body of the cylinder or of low-alloy steel. The low-pressure cylinders are designed to store oxygen at a maximum of 450 psi, although they are not normally filled above 425 psi

GASEOUS OXYGEN SYSTEMS

Today, almost all-gaseous oxygen is stored in green painted highpressure steel cylinders under a pressure of between 1,800 and 2,400 psi. All cylinders approved for installation in an aircraft must be approved by the Department of Transportation (DOT) and are usually either the ICC/DOT 3AA 1800 or the ICC/DOT 3HT 1850 type.

Gaseous oxygen systems consist of the tanks the oxygen is stored in, regulators to reduce the pressure from the high pressure in the tanks to the relatively low pressure required for breathing, plumbing to connect the system components, and masks to deliver the oxygen to the crewmember or passenger.

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Oxygen storage cylinders

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feature, a surge of pressure can be sent into the oxygen system and cause damage.

Aluminum bottles are also available, but are much less common. Newer, lightweight "composite" bottles that comply with DOT-E-8162 are becoming more common. These bottles are made of lighter, thinner metals combined with a wrapping of composite material. Because of the high pressure in the cylinders they must be very strong to withstand the operational stress without shattering.

Another type of valve is of the self-opening design. When the valve is attached to the oxygen system, a check valve is moved off of its seat, allowing the cylinder to charge the system. A third type of valve uses a cabin-operated push pull control to operate a control lever on the top of the valve. This eliminates the necessity of always having the oxygen system charged but allows the pilot to activate the system whenever needed. This valve also can incorporate a pressure regulator so that there is no damage from a pressure surge in the system when the valve is opened.

Cylinders must be hydrostatically tested to 5/3 of their working pressure, which means that the 3AA cylinders are tested with water pressure of 3,000 psi every five years and stamped with the date of the test. 3HT cylinders must be tested with a water pressure of 3,083 psi every three years, and these cylinders must be taken out of service after 24 years, or after they have been filled 4,380 times, whichever comes first. E-8162 cylinders are tested to the same standards as the 3HT cylinders, but must be taken out of service after 15 years or 10,000 filling cycles, whichever occurs first.

Many high-pressure oxygen systems use pressure-reducing valves between the supply cylinders and the flight deck or cabin equipment. These valves reduce the pressure down to 300-400 PSI. Most systems incorporate a pressure relief valve that prevents high-pressure oxygen from entering the system if the pressure-reducing valve should fail.

All oxygen cylinders must be stamped near the filler neck with the approval number, the date of manufacture, and the dates of all of the hydrostatic tests. It is extremely important before servicing any oxygen system that all cylinders are proper for the installation and that they have been inspected within the appropriate time period. Oxygen cylinders may be mounted permanently in the aircraft and connected to an installed oxygen plumbing system. For light aircraft where oxygen is needed only occasionally, they may be carried as a part of a portable oxygen system. The cylinders for either type of system must meet the same requirements, and should be painted green and identified with the words AVIATOR'S BREATHING OXYGEN written in white letters on the cylinder. There are several types of cylinder valves in use. The hand-wheel type has a wheel on the top of the valve and operates like a water faucet. The valve opens, as the wheel is turned counter clockwise. If the cylinder does not incorporate a hand-wheel design with a slow-opening

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CATEGORY B1 - MECHANICAL On a hot day, the temperature inside a parked aircraft can cause the pressure in an oxygen cylinder to rise to dangerous levels. Permanently mounted gaseous oxygen systems, especially in large aircraft, normally have some type of thermal relief system to vent oxygen to the atmosphere if the cylinder pressure becomes too high. Venting systems may be temperature or pressure activated. To alert the crew that a thermal discharge has occurred, many systems use a "blow-out" disk as a thermal discharge indicator. A flush-type fitting containing a green plastic disk about 3/4 inches in diameter is mounted on the outside of the aircraft near the location of the oxygen bottles. If a thermal discharge occurs, the disk blows out of the fitting, and leaves the vent port visible. If the disk is found missing, there is no oxygen in the system and the aircraft must not be flown in conditions where supplemental oxygen might be required. A thermal discharge requires maintenance on the oxygen system. The discharge mechanism must be reset or replaced, the indicator disk replaced and the system serviced with oxygen to the correct pressure. Consult the maintenance manual for the particular aircraft to determine the proper procedures.

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CATEGORY B1 - MECHANICAL STUDENT NOTES

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CATEGORY B1 - MECHANICAL REGULATORS Regulators for the pressure and flow of oxygen are incorporated in stored-gas systems because the oxygen is stored in high-pressure cylinders under pressures of 1800 psig or more. The high pressure must be reduced to a value suitable for application directly to a mask or to a breathing regulator. This lower pressure is usually in the range of 40 to 75 psig, depending upon the system. One type of pressure regulator is illustrated in Figure. This pressure regulator is similar in design to many other gas- or airpressure regulators in that it utilizes a diaphragm balanced against a spring to control the flow of gas. This regulator consists of a housing, diaphragm, regulator spring, link actuator assembly, relief valve, and an inlet valve. With no inlet pressure on the regulator, spring tension on the diaphragm through the link actuator assembly forces the inlet valve to the open position. When oxygen is flowing, regulated pressure in the lower diaphragm chamber acts against the diaphragm, causing it to move upward, compressing the regulator spring. The link actuator assembly then mechanically causes the regulator valve to move toward the closed position, thus reducing the flow of oxygen. When the pressure in the lower chamber of the diaphragm is equal to the regulator spring force, the diaphragm ceases to move and positions the inlet valve to maintain the proper oxygen flow.

Pressure regulator

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Manual Continuous Flow Regulators typically consist of two gauges and an adjustment knob. One typical regulator has a gauge on the right that shows the pressure of the oxygen in the system and indicates indirectly the amount of oxygen available. The other gauge is a flow indicator and is adjusted by the knob in the lower center of the regulator. The user adjusts the knob so that the flow indicator needIe matches the altitude being flown. The regulator meters the correct amount of oxygen for the selected altitude. If the flight altitude changes, the pilot must remember to readjust the flow rate. [Figure a]

Oxygen systems are also classified according to the type of regulator, which controls the flow of oxygen. The mask employed must be compatible with the type of regulator. There are two basic types of regulators in use, and each type has variations. -

Continuous flow regulators

-

Demand/diluter demand regulators

CONTINUOUS FLOW REGULATOR This type of regulator allows oxygen to flow from the storage cylinder regardless of whether the user is inhaling or exhaling.

Automatic Continuous Flow Regulators have a barometric control valve that automatically adjusts the oxygen flow to correspond with the altitude. The flight crew need only open the valve on the front of the regulator, and the correct amount of oxygen will be metered into the system for the altitude being flown. [Figure b]

The majority of oxygen systems for both private and commercial aircraft are of the continuous-flow type. The regulator on the oxygen supply provides a continuous flow of oxygen to the mask. The mask valving provides for mixing of ambient air with the oxygen during the breathing process.

Oxygen is usually supplied to the flight crew of an aircraft by an efficient system that uses one of several demand-type regulators. Demand regulators allow a flow of oxygen only when the user is inhaling. This type of regulator is much more efficient than the continuous flow type. [Figure c]

Continuous flow systems do not use oxygen economically, but their simplicity and low cost make them desirable when the demands are low. The emergency oxygen systems that drop masks to the passengers of large jet transport aircraft in the event of cabin depressurization are of the continuous flow type. Continuous Flow Regulators are of either the manual or automatic type. Both of these are inefficient in that they do not meter the oxygen flow according to the individual's needs.

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Fig. a

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Fig. c

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Oxygen entering the regulator passes through the pressure reducer and the automatic-opening valve and then enters the altitude-compensating aneroid chamber. As the aneroid expands and contracts owing to the changes in cabin pressure altitude, the outlet flow and pressure vary. When the cylinder is fully charged to approximately 1850 psig, the maximum outlet flow at the regulator is 430 L/min, and the maximum pressure is approximately 80 psig at 35 000 ft [10668 m]. First-aid oxygen can be made available during normal operation of the pressurized airplane if necessary. This is accomplished by a crewmember placing the regulator manual control in the ON position.

The pressure regulator for the continuous-flow passenger oxygen system installed in an airliner is shown in Figure. This regulator is attached to the high-pressure oxygen storage cylinder and shutoff valve. It is an altitude-compensating type, which varies supply-line pressure in accordance with cabin altitude. The regulator is actuated automatically by sensing a rise in cabin-pressure altitude, or manually by controls located on the body of the regulator. A relief valve in the regulator will open to prevent outlet pressure from exceeding approximately 150 psig. At 10500- to 12000-ft [3200 to 3656-m] cabin pressure altitude, the automatic opening aneroid expands, thus causing the valve to open and supply pressure to the pressure surge unit. Increased pressure through the surge unit actuates the door-release check valve, unlatches the mask container doors, and pressurizes the dispensing manifolds. On descent, at a cabin pressure altitude of 6000 to 10 000 ft [1829 to 3048 m], the automatic-opening aneroid contracts, causing the valve to close and shutting off the oxygen supply. It is then necessary to place the manual control in the ON position to supply supplemental oxygen to passengers as necessary. If the automatic-opening valve fails to function properly, the system is pressurized by placing the regulator manual control in the ON position and turning the, manual oxygen-door release knob to the full-rotated position.

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Passenger oxygen regulator

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outside air passage and opens a supplemental oxygen valve inside the regulator so pure oxygen can flow to the mask.

DEMAND AND DILUTER DEMAND REGULATORS These used with demand masks supply oxygen to the user during inhalation. When the individual using the equipment inhales, he or she causes a reduction of pressure in a chamber in the regulator. This reduction in pressure activates the oxygen valve and supplies oxygen to the mask. A flow indicator shows when oxygen flow is taking place. The diluter demand regulator automatically adjusts the percentage of oxygen and air supplied to the mask in accordance with altitude. The demand masks cover most of the user's face and create an airtight seal. This is why a low pressure is created when the user inhales.

An additional safety feature is incorporated that bypasses the regulator. When the emergency lever is placed in the EMERGENCY position, the demand valve is held open and oxygen flows continuously from the supply system to the mask as long as the supply lever is in the ON position. .

Diluter Demand Regulators are used by the flight crews on most commercial jet aircraft. When the supply lever is turned on, oxygen can flow from the supply into the regulator. There is a pressure reducer at the inlet of the regulator that decreases the pressure to a value that is usable by the regulator. The demand valve shuts off all flow of oxygen to the mask until the wearer inhales and decreases the pressure inside the regulator. This decreased pressure moves the demand diaphragm and opens the demand valve so oxygen can flow through the regulator to the mask. [Figure a] A diluter demand regulator dilutes the oxygen supplied to the mask with air from the cabin. This air enters the regulator through the inlet air valve and passes around the air-metering valve. At low altitude, the air inlet passage is open and the passage to the oxygen demand valve is restricted so the user gets mostly air from the cabin. As the aircraft goes up in altitude, the barometric control bellows expands and opens the oxygen passage while closing off the air passage. At an altitude of around 34,000 feet, the air passage is completely closed off, and every time the user inhales, pure oxygen is metered to the mask. If there is ever smoke in the cabin, or if for any reason the user wants pure oxygen, the oxygen selector on the face of the regulator can be moved from the NORMAL position to the 100% position. This closes the

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Diluter demand regulator

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CATEGORY B1 - MECHANICAL When a person breathes normally, the lungs expand and atmospheric pressure forces air into them. But at altitudes above 40,000 feet not enough oxygen can get into the lungs even with the regulator on 100%. Operation of unpressurized aircraft at and above 40,000 feet requires the use of pressure demand regulators. These regulators have provisions to supply 100% oxygen to the mask at higher than ambient pressure, thus forcing oxygen into the user's lungs Pressure Demand Regulators operate in much the same way as diluter demand regulators except at extremely high altitudes, where the oxygen is forced into the mask under a positive pressure. Breathing at this high altitude requires a different technique from that required in breathing normally. The oxygen flows into the lungs without effort on the part of the user, but muscular effort is needed to force the used air out of the lungs. This is exactly the opposite of normal breathing. Pressure-demand regulators contain an aneroid mechanism, which automatically increases the flow of oxygen into the mask under positive pressure. This enables the user to absorb more oxygen under the conditions at very high altitudes. This type of equipment is normally used at altitudes above 40000 ft [13632 m]. The additional pressure is needed to enable the user to absorb oxygen at a greater rate than it would be absorbed at ambient pressure. A pressure demand mask must be worn with a pressure demand regulator. By action of special pressure-compensating valves, the mask provides for a build up of oxygen pressure from the regulator and creates the required input of oxygen into the lungs

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Pressure demand regulator

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CATEGORY B1 - MECHANICAL MASKS Masks are used to deliver the oxygen to the user. Oxygen masks vary considerably in size, shape, and design; however, each is designed for either a demand system or a continuous-flow (constant-flow) system. An oxygen mask for a demand system must fit the face, closely, enclosing both the mouth and nose, and must form an airtight seal with the face. Inhalation by the user will then cause a low pressure in the demand regulator, which results in opening of an oxygen valve and a flow of oxygen to the mask. When the user exhales, the flow of oxygen is cut off. An oxygen mask for a constant-flow system is designed so that some ambient air is mixed with the oxygen. The complete mask usually includes an oronasal face piece, a reservoir bag, valves, a supply hose, and a coupling fitting. Some models include a flow indicator in the supply hose. CONTINUOUS FLOW MASKS When the oxygen is turned on to a constant-flow mask, it fills the reservoir through a valve. When inhaling, the user draws oxygen directly from the reservoir bag. When the oxygen in the reservoir bag is depleted, the user breathes cabin air. When the user exhales, the reservoir bag refills with oxygen. The oxygen from the supply line flows continuously into the mask, sometimes filling the reservoir bag and at other times being breathed by the user. Exhaled oxygen and air are discharged from the mask into the cabin. Typical examples of continuous-flow oxygen masks are shown in Figure. Masks of these types are usually provided with space for the installation of a microphone.

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immediately when even a small amount of oxygen is flowing. The indicator is colored a bright green so the passenger can see at once whether oxygen is flowing into the mask. Oxygen masks on airliners are stowed in overhead compartments or in a compartment at the top of the seat back. If the cabin should depressurize, the compartments open automatically and present oxygen masks to the passengers. If the automatic system fails to work, a backup electrical or mechanical system can be activated by a member of the crew to open the oxygen compartments. These are either of the continuous flow or demand type.

Passenger oxygen masks for airliners are of the constant-flow type and the face piece is oronasal in design; that is, it is designed to cover both the nose and the mouth. They are referred to as phase-dilution masks because of the characteristics of their operation. When oxygen is turned on to the passenger mask, it enters the bottom of the reservoir bag and causes it to inflate. When inhaling, the user draws oxygen from the reservoir bag until it is deflated. At that time, the user begins to breathe cabin air plus a small amount of oxygen, which is flowing through the reservoir. Thus, there are two phases of oxygen consumption during inhalation. The first and largest part of the inhale action draws almost pure oxygen into the lungs. When the reservoir bag has deflated, the user continues to inhale but is breathing cabin air, primarily. The first part of the inhalation provides a very rich oxygen mixture, which goes deep into the lungs. The last part of the inhalation, in which cabin air is being breathed, affects only the upper part of the lungs, the bronchial tubes, and the windpipe (trachea). Since these parts of the respiratory system do not contribute to the absorption of oxygen by the blood, the low oxygen content of the cabin air breathed during the last part of inhalation is of little consequence. When the user of the mask exhales, the air is discharged through an exit valve in the mask to the cabin atmosphere. At this time, the reservoir bag refills with oxygen; however, the bag does not always fill completely, particularly if the user is breathing rapidly. If the flow rate of oxygen is only 1 L/min, which is normal for a cabin altitude of 15 000 ft [4573 m], the reservoir bag does not have time to fill between each inhalation by the passenger. This has caused concern among some passengers because they have thought they were not receiving an adequate flow of oxygen. A passenger oxygen mask is shown in Figure. This particular mask has a built-in flow indicator at the bottom of the reservoir bag. A small section of the bag has been partially sealed 'off so it will inflate

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Passenger oxygen mask

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Another type of continuous flow mask is the rebreather type and vary from a simple bag-type disposable mask used with some of the portable systems to the rubber bag-type mask used for some of the flight crew systems. Oxygen enters a rebreather mask at the bottom of the bag, and the mask fits the face of the user very loosely so air can escape around it. If the rebreather bag is full of oxygen when the user inhales, the lungs fill with oxygen. Oxygen continues to flow into the bag and fills it from the bottom at the same time the user exhales used air into the bag at the top. When the bag fills, the air that was in the lungs longest will spill out of the bag into the outside air, and when the user inhales, the first air to enter the lungs is that which was first exhaled and still has some oxygen in it. This air is mixed with pure oxygen, and so the wearer always breathes oxygen rich air with this type of mask. More elaborate rebreather-type masks have a close-fitting cup over the nose and mouth with a built-in check valve that allows the air to escape, but prevents the user from breathing air from the cabin.

Passenger mask with rebreather bag

The oxygen masks that automatically drop from the overhead compartment of a jet transport aircraft in the event of cabin depressurization are of the rebreather type. The plastic cup that fits over the mouth and nose has a check valve in it, and the plastic bag attached to the cup is the rebreather bag.

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CATEGORY B1 - MECHANICAL DEMAND MASKS Pressurized aircraft are normally equipped with diluter demand oxygen systems for use by the flight deck crew. The masks used by the crew are of an oronasal design and contain microphones and a strap harness arrangement that will hold the mask securely in position. With demand-type masks the regulator is set up to meter the proper amount of oxygen to the user, so outside air would upset the required ratio of air to oxygen. Demand-type masks must fit tightly to the face so no outside air can enter. A full-face mask is available for use in case the cockpit should ever be filled with smoke. These masks cover the eyes as well as the mouth and nose, and the positive pressure inside the mask prevents any smoke entering For some aircraft, which operate at very high altitudes, quick-donning masks are used. These masks can be put on in 5 s or less. Figure shows the pneumatic harness type of diluter demand masks. These masks are used primarily by aircrew members because they use the oxygen more efficiently and have higher altitude capabilities.

Demand type oxygen mask

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Flight crew quick donning mask

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CATEGORY B1 - MECHANICAL LINES AND FITTINGS Most of the rigid plumbing lines that carry high-pressure oxygen are made of stainless steel, with the end fittings silver soldered to the tubing. Lines that carry low-pressure oxygen are made of aluminum alloy and are terminated with the same type fittings used for any other fluid-carrying line in the aircraft. The fittings may be of either the flared or flareless type. It is essential in any form of aircraft maintenance that only approved components be used. This is especially true of oxygen system components. Only valves carrying the correct part number should be used to replace any valve in an oxygen system. Many of the valves used in oxygen systems are of the slow-opening type to prevent a rapid in-rush of oxygen that could cause excessive heat and become a fire hazard. Other valves have restrictors in them to limit the flow rate through a fully open valve.

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The pressure regulator reduces the pressure in the cylinder to a pressure that is usable by the masks. This regulator may be either a manual or an automatic type. There must be provision, one way or another, to vary the amount of pressure supplied to the masks as the altitude changes.

TYPICAL INSTALLED GASEOUS OXYGEN SYSTEMS If an aircraft has an installed oxygen system, it will be one of three types: the continuous flow type, the diluter demand type or the pressure demand type. Most single engine aircraft utilize a continuous flow oxygen system. The external filler valve is installed in a convenient location and is usually covered with an inspection door. It has an orifice that limits the filling rate and is protected with a cap to prevent contamination when the charging line is not connected.

The mask couplings are fitted with restricting orifices to meter the amount of oxygen needed at each mask. In the figure the pilot's coupling has an orifice considerably larger than that provided for the passengers. The reason is that the pilot and other flight crewmembers require more oxygen since they are more active, and their alertness is of more vital importance than that of the passengers.

The DOT approved storage cylinder is installed in the aircraft in a location that is most appropriate for weight and balance considerations. The shutoff valve on the cylinder is of the slow opening type and requires several turns of the knob to open or close it. This prevents rapid changes in the flow rate that could place excessive strain on the system or could generate too much heat. Some installations use a pressure-reducing valve on the cylinder. When a reducer is used, the pressure gauge must be mounted on the cylinder side of the reducer to determine the amount of oxygen in the cylinder. [ See Figure]

Some installations incorporate a therapeutic mask adapter. This is used for any passenger that has a health problem that would require additional oxygen. The flow rate through a therapeutic adapter is approximately two to three times that through a normal passenger mask adapter. Each tube to the mask has a flow indicator built into it. This is simply a colored indicator that is visible when no oxygen is flowing. When oxygen flows, it pushes the indicator out of sight.

The pressure gauge is used as an indication of the amount of oxygen in the cylinder. This is not, of course, a direct indication of quantity, but within the limitations seen when discussing system servicing, it can be used to indicate the amount of oxygen on board.

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Typical installation of a gaseous oxygen system

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CATEGORY B1 - MECHANICAL Pressurized aircraft do not normally have oxygen available for passengers all of the time, but regulations require that under certain flight conditions, the pilot operating the controls wear and use an oxygen mask. Because of this requirement, most executive aircraft that operate at high altitude are equipped with diluter demand or pressure demand oxygen regulators for the flight crew and a continuous flow system for the occupants of the cabin. Aircraft operating at altitudes above 40,000 feet will usually have pressure demand systems for the crew and passengers. [Figure pg-37] The masks for the flight crew normally feature a quick-donning system. The mask is connected to a harness system that fits over the head. This system is designed so that the mask can be put on with one hand and be firmly in place, delivering oxygen, within a few seconds. The oxygen filler valve is usually located under an access panel on the outside of the fuselage and near the oxygen cylinder. The filler valve consists of the valve incorporating a filter and valve cap. A check valve is installed in the high-pressure line at the regulator to prevent the escape of oxygen from the cylinder at the filler line port. A typical service panel is shown in the adjacent Figure.

Oxygen service panel

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An oxygen system which does not require oxygen for

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The emergency oxygen systems for pressurized aircraft have the oxygen generators mounted in either the overhead rack, in seat backs (Fig. b), lavatory ceilings, or in bulkhead panels. The masks are located with these generators and are enclosed, hidden from view by a door that may be opened electrically by one of the flight crewmembers or automatically by an aneroid valve in the event of cabin depressurization. Provision for manual opening of these doors are provided to cover against malfunction.

CHEMICAL OXYGEN SYSTEMS Another source of oxygen is the chemical system. This system uses chemical oxygen generators also called "oxygen candles" to produce breathing oxygen. The size and simplicity of the units, and minimal maintenance requirements make them ideal for many applications. The chemical oxygen generator requires approximately one-third the space for equivalent amounts of oxygen as a bottled system. The canisters are inert below 400°F. even under severe impact. Oxygen candles contain sodium chlorate mixed with appropriate binders and a fuel formed into a block. When the candle is activated, it releases oxygen. The shape and composition of the candle determines the oxygen flow rate. As the sodium chlorate decomposes. It produces oxygen by a chemical action. [Figure a]

When the door opens, the mask drops out where it is easily accessible to the user. Attached to the mask is a lanyard that, when pulled, releases the lock pin from the flow initiation mechanism, so the striker can hit the igniter and start the candle burning or alternately the percussion device is automatically electrically triggered. Once a chemical oxygen candle is ignited, it cannot be shut off. It must burn until it is exhausted, and the enclosure must not be closed until the cycle has completed. [Figure c]

An igniter actuated either electrically or by a spring, starts the candle bumming. The core of the candle is insulated to retain the heat needed for the chemical action and to prevent the housing from getting too hot. Filters are located at the outlet to prevent any contaminants entering the system. The long shelf life of unused chemical oxygen generators makes them an ideal source of oxygen for occasional flights where oxygen is needed, and for the emergency oxygen supply for pressurized aircraft where oxygen is required only as a standby incase cabin pressurization is lost..

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Fig. b

Fig. c

Fig. a

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oxygen to the crewmember(s) if there is a fire, or an emission of smoke and/or noxious gas. It permits the crewmember(s) to move freely to extinguish a fire. It can also supply emergency oxygen if the fixed emergency-oxygen system does not work. (Fig. a)

PORTABLE OXYGEN SYSTEMS Most commercial aircraft in addition to the mentioned oxygen systems will have a portable oxygen system. The portable oxygen system is used to supply oxygen to the cabin attendants, the Passengers and the crew in an emergency.

Cabin-Attendants Portable Devices

The simplest type of portable oxygen system includes a Department of Transportation (DOT)-approved oxygen cylinder of either 11 ft3 [311.5 L] capacity or 22 ft3 [623 L] capacity, a regulator assembly, a pressure gauge, an ON-OFF valve, hose couplings, flow indicator, and one or two oronasal masks. This system is charged to 1800 psi and is suitable for altitudes up to 28 000 ft [8536 m]. Portable oxygen systems are available with automatic flow-control regulators, which adjust oxygen flow in accordance with altitude.

The cabin-attendants portable oxygen system has these devices:

The flight-crew portable devices,

-

The cabin-attendants portable devices.

-

In the attendants seat areas,

-

Protective breathing equipment.

The primary use of the PBE is to supply oxygen to the cabin attendants if there is a fire, or an emission of smoke and/or noxious gas. It permits them to move freely to extinguish a fire.

The cabin attendant’s portable breathing-equipment is installed in several places in the cabin area easily accessible to the crew. The possible positions are: In the galley areas,

-

The high-pressure oxygen cylinders with continuous-flow type masks supply first aid oxygen for the passengers. If necessary they can also supply oxygen to the cabin attendants so that they are able to move about in the cabin for passenger assistance and other task during an emergency.

The flight crew portable breathing-equipment is usually installed behind the pilots seat.

-

High-pressure oxygen cylinders with continuous-flow type masks,

They are installed at different locations in the cabin and are immediately available for the cabin attendants to use. (Fig. b)

A portable oxygen system will have these subsystems: -

-

In the overhead stowage’s, in the cabin Stowage’s/doghouses.

FLIGHT-CREW PORTABLE DEVICES The flight crew portable device is a protective breathing equipment or a portable cylinder with a full-face mask. Its primary use is to supply

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Fig. a

Fig. b

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The yellow indicator inside the box shows the serviceability of the breathing hood, in addition the tamper seal indicates that the container has not been opened. Alternately a serviceability indicator which changes color with absorption of moisture is also in use. If the yellow indicator/ serviceable color is not apparent or if the tamper seal is broken the protective breathing equipment must be replaced.

PROTECTIVE BREATHING EQUIPMENT The flight crew emergency breathing hood system provides protection to the eyes and respiratory system for crewmembers. It is used when fighting a fire, against the emission of smoke and noxious gas. The breathing hood generally ensures a total autonomy of approximately 20 minutes.

Normally these equipment has a shelf or operational life of 10 years so it is imperative that one must check for the date of manufacture when inspecting this equipment.

Usually the complete hood is vacuum-packed in a transport /storage box. The storage box is provided with a good-condition indicator and tamper seal used for the pre-flight inspection. Generally a smoke hood will have -

A pictogram, which describes the utilization procedure (Fig. b)

-

An identification plate, located at the back of the container, which gives the date of its manufacture.

To gain access to the aluminized bag it is necessary to break the lead seal and open the box cover. The breathing hood shown (Fig. a) works with a closed breathing circuit. The expired air will be regenerated, enriched with oxygen, and inhaled again. The oxygen is generated in exothermic chemical reaction between the potassium peroxide (KO2), the humidity, and the carbon dioxide (CO2) in the air expired. This reaction releases heat.

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Fig. b Fig. a

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A typical high-pressure oxygen cylinder has a head, which will have the following components installed on it:

HIGH-PRESSURE OXYGEN CYLINDERS WITH CONTINUOUS-FLOW TYPE MASKS Portable Oxygen Cylinder The high-pressure oxygen cylinders with continuous-flow type masks supply first aid oxygen for the passengers. If necessary they can also supply oxygen to the cabin attendants. The oxygen source is a highpressure cylinder with a capacity of around 310 l at a pressure of 1800 psi. Brackets are installed with quick-release clamps to keep the cylinders in position.

-

A direct-reading pressure gauge to show the pressure in the cylinder,

-

A high-pressure relief valve with a rupture disk. (3000) psi

-

A low-pressure relief valve, which is installed in the lowpressure chamber of the pressure regulator. (90) psi

-

A rotary 'ON/OFF' valve to control the flow of oxygen into the high-pressure chamber of the pressure regulator,

-

A fill valve, which is directly connected to the high-pressure chamber of the pressure regulator,

-

A pressure regulator to give a low-pressure (35-80psi)

-

A constant flow outlet for the continuous-flow type oxygen mask,

-

An uncalibrated flow outlet for the full face/quick-donning type oxygen mask.

Continuous-Flow Type Oxygen Masks The continuous-flow type masks are used to give oxygen for first-aid treatment. They can also supply oxygen to the cabin attendants. The flexible supply hose is connected to the constant flow outlet of the portable oxygen cylinder.

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Portable oxygen cylinder

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The continuous-flow type oxygen mask is connected to the highpressure oxygen cylinder at the constant flow outlet. When the 'ON/OFF' valve is turned on the oxygen flows through the supply hose into the reservoir bag. The green flow indicator in the reservoir bag inflates. When the user breathes in, the oxygen flows from the reservoir bag through the inhalation valve into the face piece.

CONTINUOUS-FLOW TYPE OXYGEN MASK The continuous-flow oxygen masks will have: -

A face piece,

-

A diluter valve,

-

An exhale valve,

-

An inhale valve,

-

A reservoir bag with a flow indictor,

-

A flexible supply hose.

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When the oxygen in the reservoir bag is used, the diluter valve opens and ambient air is let into the face piece. When the user breathes out, the inhalation valve and the diluter valve close. The exhaled air goes through the exhalation valve to the atmosphere

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Continuous flow oxygen mask

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CATEGORY B1 - MECHANICAL OXYGEN SYSTEM MAINTENANCE PRACTICES To ensure that oxygen systems serve their purpose of supplying hygienically clean oxygen under emergency conditions in an efficient and safe manner, strict observance of servicing instructions and the necessary safety precautions is essential during the installation and maintenance of components. Failure to observe such precautions could result in fire and explosions and consequent serious injury to personnel and severe damage to an aircraft. The emphasis is, at all times, on cleanliness and on the standards of the work to be carried out at the appropriate stages of installation and maintenance. The information given in the following paragraphs is intended to serve as a guide to practices and precautions applicable to systems in general. Details relevant to specific types of aircraft systems are contained in the approved Maintenance Manuals and the schedules drawn up by an aircraft operator and reference must always be made to these documents. Servicing Personnel must fully understand the operation of an aircraft system, the relevant ground charging equipment and its connection to charging points and must have a full knowledge of any appropriate engineering and maintenance regulations in force. Personnel should also be alert to emergency situations, which could arise during oxygen system servicing.

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One of the most serious hazards with oxygen is the penetration of the gas into clothing, which can take place when a person has been exposed to an oxygen-rich atmosphere. In this state an infinitesimal particle of hot ash from a pipe or cigarette, can ignite the clothing, which will immediately burst into a fierce flame.

SAFETY PRECAUTIONS WHEN HANDLING OXYGEN SYSTEMS Before carrying out any work on an oxygen system, the following precautions against fire should be taken: -

Provide adequate equipment.

and

properly

manned

fire-fighting

-

Display 'No Smoking' and other appropriate warning placards outside the aircraft.

-

If artificial lighting is required, use explosion-proof lamps and hand torches

-

Testing of aircraft radio or electrical systems should be avoided.

-

Ensure that the aircraft is properly earthed.

-

Ensure that charging or servicing units, appropriate to oxygen systems are used and that they and all other necessary tools are serviceable and free of dirt, oil, grease or any other contaminants.

-

Where work on an oxygen system is to be performed in a confined space within the aircraft, adequate ventilation must be provided to prevent a high concentration of oxygen.

-

Pipe and component connections should be wiped clean and dry if contamination is present.

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Clothing which has been saturated by oxygen should be kept away from naked lights or any other source of heat until a period of a quarter of an hour has elapsed, or until thorough ventilation with air has been effected. A clean area, with bench surfaces and tools free of dirt and grease, should be used whenever it is necessary to carry out work on oxygen system components.

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CATEGORY B1 - MECHANICAL The following general procedures and precautions should be followed when handling, testing and cleaning any part of an oxygen system: -

Clean, white, lint-free cotton gloves should be worn by servicing personnel.

-

Before installing a component it must have been cleaned in accordance with the cleaning instructions laid down in relevant manuals. In order to avoid contamination, protective/blanking caps should not be removed until immediately before the installation of the component. When the caps are subsequently removed, the fittings of the component should be checked to ensure they are clean and free of contaminants, e.g. flaked particles from protective caps.

-

-

Shut-off valves should always be opened slowly to minimize the possibility of heat being generated by sudden compression of high-pressure oxygen within the confined spaces of valves or regulators. Particular attention must also be paid to any torque values specified for valve operation.

-

Certain components are stored in polythene bags, which should not be opened until immediately prior to installation. If a bag containing a component has been torn or unsealed during storage, the component should be re-cleaned.

-

All open pipe ends or component apertures should be kept capped or plugged at all times, except during installation or removal of components. Only protection caps or plugs designed for the purpose should be used.

-

On replacement of a component requiring electrical bonding or power supply connections. e.g. an electrical pressure transducer, the circuit should be tested.

-

For leak testing, only those solutions specified in the relevant manuals must be used. Care must be taken to prevent a solution from entering any connection, valve or component. All tested parts must be wiped clean and dried immediately.

-

For the testing of components, clean dry filtered air or nitrogen may be used instead of oxygen. On completion of the tests, components should be purged with breathing oxygen

Before uncoupling a connection the oxygen supply must be turned off. Connections should be unscrewed slowly to allow any residual pressure in the line or component to escape.

NOTE: If a cylinder valve is not completely closed or is leaking and there is a time lag after bleeding a line. Sufficient oxygen pressure could build up in the line to become potentially dangerous.

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Before filling any aircraft oxygen system, all of the cylinders being refilled must be checked to ensure that they are of the approved type, and have been hydrostatically tested within the required time interval

SERVICING GASEOUS OXYGEN SYSTEMS Some generic procedures are listed here as an orientation to the oxygen system servicing.

No oxygen system should be allowed to become completely empty. When there is no pressure inside the cylinder, air can enter, and most air contains water vapor. When the water vapor mixes with the oxygen the mixture expands as it is released through the small orifices in the system. This expansion lowers the temperature and the water is likely to freeze and shut off the flow of oxygen to the masks. Water in a cylinder can also cause it to rust on the inside and weaken it so it could fail with catastrophic results. A system is considered to be empty when the pressure gets down to 50 to 100 psi.

FILLING AN OXYGEN SYSTEM Fixed base operators who do a considerable amount of oxygen servicing will usually have an oxygen-servicing cart. Such carts usually consist of six large cylinders, each holding approximately 250 cubic feet of aviator's breathing oxygen. A seventh cylinder, facing the opposite direction and filled with compressed nitrogen, is normally carried to charge hydraulic accumulators and landing gear struts. Fittings on the nitrogen cylinders are different from those on the oxygen cylinders to minimize the possibility of using nitrogen to fill the oxygen system, or of servicing the other systems with oxygen.

If the system is ever allowed to get completely empty, the valve should be removed and the cylinder cleaned and inspected.

Each oxygen cylinder has its own individual shutoff valve, and all of the cylinders are connected into a common service manifold that has a pressure gauge. A flexible line with the appropriate fittings connects the charging manifold to the aircraft filler valve.

When an aircraft's oxygen system is being filled from a large supply cart, the cylinder having the lowest pressure should be used first. (The pressure in each tank should have been recorded on the container with chalk or in a record kept with the cart.) The valve on the cylinder should be opened slightly to allow some oxygen to purge all of the moisture, dirt and air from the line; then the line should be connected to the aircraft filler valve and the valve on the cylinder opened slowly. Most filler valves have restrictors that prevent an excessively high flow rate into the cylinder.

Various manufacturers of oxygen equipment use different types of connections between the supply and the aircraft, and a well-equipped service cart should have the proper adapters. These adapters must be kept clean and protected from damage. Leakage during the filling operation is not only costly, but is hazardous as well.

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An oxygen service cart

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DRAINING THE OXYGEN SYSTEM

When the pressure in the aircraft system and that in the cylinder with the lowest pressure stabilizes and there is no more flow, this new pressure should be recorded and the cylinder valve closed The valve on the cylinder having the next lowest pressure should be opened slowly and oxygen allowed to flow into the system until it again stabilizes. Continue this procedure until the aircraft system has been brought up to the required pressure. [Figure a]

Draining of the oxygen system should normally be done after the highpressure bottle has been removed or isolated from the system. Either outdoors or in a well-ventilated hangar, the system's pressure should be bled off by opening the appropriate fitting. Normally a system will require purging after the system has been drained. All the safety precautions mentioned later in this chapter should be followed during any oxygen draining procedure.

The ambient temperature determines the pressure that should be put into the oxygen system, and a chart should be used to determine the pressure needed. For example, if the ambient temperature is 90° F and a stabilized pressure in the system of 1,800 psi is desired, the oxygen should be allowed to flow until a pressure of 2,000 psi is indicated on the system pressure gauge. When the oxygen in the system drops to the standard temperature of 70°F, the pressure will stabilize at 1,800 psi. If the ambient temperature is low, the filling of the system must be stopped at a lower pressure, because the oxygen will expand and the pressure will rise when it warms up to its normal temperature. [Figure b]

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PURGING A GASEOUS OXYGEN SYSTEM If the oxygen system has been opened for servicing, it should be purged of any air that may be in the lines. To purge a continuous flow system, oxygen masks are plugged into each of the outlets and the oxygen supply valve turned on. Oxygen should be allowed to flow through the system for about ten minutes. Diluter demand and pressure demand systems may be purged by placing the regulators in the EMERGENCY position and allowing the oxygen to flow for about ten minutes. After the system has been thoroughly purged, the cylinders should be filled to the required pressure.

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Fig. a

Fig. b

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NOTE: Where very accurate leak-rate measurement is necessary, special leak-rate testing instruments are available and should be used as appropriate.

OXYGEN SYSTEM TESTS LEAK TESTING GASEOUS OXYGEN SYSTEMS Whenever a system component e.g. cylinder, pipe or regulator, etc., has been removed, re-installed, or the system has in any way been disconnected, tests for Leakage should be carried out. The system pressure should be at its normal maximum value.

PRESSURE TESTS Pressure testing of oxygen cylinders is required at stated periods (e.g. every four years) normally indicated in the relevant manuals and schedules. The date of pressure test is usually stamped on the neck ring of a cylinder or painted on the top hemispherical portion.

Leaks should be located using an Ieak detecting solution free from any combustible substances unless, of course, particular leaks arc large enough to be heard or felt. Solutions recommended for this purpose are those conforming to specifications MIL-L-25567 'B' and MIL-L-25567 'C' Type 1.

NOTE: The dates of any previous pressure tests should not be overstamped or obliterated. FLOW TESTING

The solution should be applied with a soft brush and the suspected connections checked for signs of frothing or bubble formation. After testing, all traces of solution must be removed by a thorough rinsing with clean water and drying with a soft lint-free cloth.

Where the testing of flow rates is required at various points in a system (e.g. at mask socket connections) special oxygen flow meters should be used in accordance with the manufacturer's instructions. These flow meters generally consist of a float inside a glass cylinder graduated for the appropriate flow ranges in liters per minute

If a leak is found, the pressure is released from the system, and the fittings checked for proper torque. Flareless fittings can leak from both under and over tightening. If the fitting is properly torqued and still leaks, remove the fitting and examine all of the sealing surfaces for indications of damage. It may be necessary to replace the fitting and reflare the tube or install a new flareless fitting. Where it may be necessary to check a Ieak-rate (e.g. through a valve) a leak-rate tester should be used. A simple tester consists of a flexible tube into which has been inserted a length of 1/4 inch bore glass tube. To check a leak-rate, the free end of the flexible tubing is fitted over the outlet to be tested whilst the glass tube is immersed one inch below the surface of water in a glass jar. The leak-rate can then be calculated from the number of bubbles passing through the water. Eight bubbles are considered equal to 1 c.c. Therefore eight bubbles per minute would show a leak-rate of 60 c.c. Per hour.

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If the acceptability of a cylinder is in question after making these checks it must be withdrawn for more detailed inspection and overhaul.

COMPONENTS MAINTENANCE The following paragraphs detail some of the procedures and precautions generally applicable to the installation and maintenance of the principal components comprising oxygen systems. Reference should always be made to the approved Maintenance Manual relevant to a specific aircraft and system for full details.

Checks on threads of connections should be carried out to ensure they are clean and free from damage. Thread lubricants should not be used. Protective caps should remain on the connections until a cylinder is ready for installation and should be replaced immediately a cylinder is removed.

CYLINDERS

During installation of cylinders a check must be made that they are properly aligned with their respective pipelines before finally tightening cylinder clamps and pipe connecting unions.

The handling and transportation of cylinders requires that extreme care be exercised at all times. They must not be allowed to fall over, or be knocked or jarred against hard or sharp objects, or against each other. On no account must they be rolled from a truck or trolley directly onto” the ground.

After installation, cylinder valves should be slowly opened to pressurize the high-pressure lines and a leak test carried out at the cylinder connections and any other connections, which may have been opened. On satisfactory completion of a leak test, cylinder pressures should be checked and recharging to normal system pressure carried out where necessary and valves should be wire locked in the open position.

Rapid opening of valves to allow a sudden release of oxygen under pressure from the outlet connections should be avoided. This applies particularly to cylinders, which do not incorporate a pressure-reducing valve. Apart from the fire risk, the reaction from the pressure discharge can cause an insecurely held cylinder to become a dangerous uncontrollable object. Cylinders must be checked to ensure that the date of the last pressure test has not expired and that the storage pressure is not below the minimum specified in the relevant manual. Where specified, it is necessary to carry out tests to ensure that there is no leakage of oxygen from the seats and spindle glands of cylinder valves. Control valves and, where appropriate, pressure regulators and gauges, are fitted by the cylinder manufacturers and no attempt should be made to remove them during service. The exterior of cylinders should be checked for signs of corrosion and damage such as dents, cuts, gouging; or marking by metal stamps other than that prescribed by the manufacturer on defined areas of the body.

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CATEGORY B1 - MECHANICAL CHEMICAL OXYGEN GENERATORS Unexpended generators should be handled with extreme care to prevent inadvertent removal of the firing pin. A blanking cap is normally fitted over the pin and this should remain in position until the generator is finally installed and the lanyard is tied to the disconnect ring of the mask. If a generator should become activated it should be immediately placed on a non-combustible surface. A minimum clearance of 5/8 inch must exist between a generator and its heat shield to allow proper cooling when the generator is activated. Oil or grease must not be used to lubricate the hinges or latch mechanism of a service panel door. When closing the door of a service panel it should be checked that the hoses between the generator and masks will fold without kinking or twisting.

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Masks should be cleaned and disinfected before installation and also whenever the oxygen system has been used and at the periods specified in approved Maintenance Schedules.

MASKS Disposable masks such as those used with many of the portable systems should be replaced with new masks after each use. But the permanent masks used by crewmembers are normally retained by each individual crewmember. These masks are fitted to the face to minimize leakage and are usually treated as personal flight gear. They should be occasionally cleaned by washing them with a cloth wet with a lukewarm detergent solution and then allowing them to dry at room temperature. The face portion of the mask may be disinfected with a mild antiseptic. The quick-donning masks for use by airliner flight crews are part of the aircraft and not crew personal equipment. Most airlines require each crewmember to don and test the mask as part of the required pre-flight inspection. Alcohol swabs in small sealed packets are provided to sterilize the mask before the crewmember dons the mask. The procedure for the installation of masks depends largely on whether they are of the plug-in type or automatic dropout type and full details should, therefore, be obtained from relevant manuals. In general, the following points should be observed during installation and maintenance: Masks should be properly stored without kinking or twisting of the hoses. The masks and hoses should be checked for leaks, holes or rips, and replaced rather than repaired. When storing the mask in the airplane, it should be protected from dust and dampness, and especially from any type of grease or oil. Plug-in couplings should be checked for proper insertion and removal. Stowage compartments should be inspected for cleanliness and general condition. Reservoir bags, where used in service panels, must be correctly positioned and folded to ensure efficient dropout.

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Clearances between pipes and aircraft structure should be in agreement with those specified in the relevant aircraft manuals and installation drawings, otherwise damage may be caused by vibration. Particular care is necessary to ensure clearance between pipes and moving parts such as aircraft control rods and levers.

PIPES AND FITTINGS It is extremely important when installing any oxygen line in an aircraft that no petroleum product is used as a thread lubricant, and that the lines are thoroughly cleaned of any trace of oil that was used in the flaring or presetting operation. Trichlorethylene or some similar solvent may be used to clean the tubing and fittings. After they are thoroughly clean, they should be dried either with heat or by blowing them with dry air or dry nitrogen.

Pipes should, in the first instance, be positioned and only partially coupled (i.e. turning union nuts through one or two threads) to each other or components as the case may be. The alignment of the tubes should then be adjusted for optimum clearance and the tubes partially secured to the structure by the appropriate attachment method. Commencing at either end of the pipe run, the union nuts should be backed off and a check made on the seating of the fittings. If satisfactory, union nuts should be re-fitted and tightened and the pipe attachments finally secured.

Tapered pipe threads must never be lubricated with a thread lubricant that contains any form of petroleum. Oxygen-compatible thread lubricant that meets specification MIL-G-27617 may be used, or the male threads may be wrapped with Teflon tape and the fittings screwed together. Before any tubing or fitting is replaced in an oxygen system, the part must be thoroughly cleaned and inspected. The part should be checked for evidence of corrosion or damage, and degreased with a vapor degreaser or ultrasonic cleaner. The new line should be flushed with stabilized Trichlorethylene, acetone, or some similar solvent, and dried thoroughly with dry air or nitrogen. If neither dry air nor nitrogen is available, the part may be dried by baking it at a temperature of about 250° F until it is completely dry.

Torque values specified for a particular oxygen system should be strictly observed when tightening the fittings. A fitting should never be over tightened to effect a seal or to establish a proper electrical bond; loosen the fitting and retorque it several times, if necessary, until the seal or bond has been established. When tightening or disconnecting a pipe coupling, a second spanner should be used as a back- up to prevent rotation of the fitting to which the pipe union is attached.

When the parts are dry, close them with properly fitting protective caps or plugs, but never use tape in any form to seal the lines or fittings, as small particles of the tape are likely to remain when it is removed.

If a section of the pipeline system is left open or disconnected during installation or removal, clean blanking caps must be fitted to open lines, fittings or parts to prevent contamination of the system. In connection with the application of blanking caps, the following points should be particularly noted: -

Pipes and fittings should be inspected for damage, cleanliness and signs of corrosion. If a pipe is damaged or deformed it should be removed and a new pipe fitted. The security of pipe attachments such as 'P' clips, support brackets, etc. and the conditions of electrical bonding connections should be checked.

Plastic caps should only be used on plain sections of pipes, e.g. flareless pipes. Plastic caps should not be re-used.

Only pipes and fittings designated for use with oxygen and cleaned by an approved method must be installed.

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Where caps are to be fitted to threaded unions or fittings they should be of the metal type.

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CATEGORY B1 - MECHANICAL Plugs, which can be jammed into pipes, should not be used.

On completing the installation of pipes a leak test must be carried out on all relevant connections and fittings. If a connection leaks, a check should be made that the specified torque values were used in tightening. If the leak persists, using the specified torque value, the connection should be re-opened and inspected to find the cause. Defective pipes or fittings should be replaced by serviceable items.

Metal caps may be re-used after cleaning in accordance with an approved method. Blanking caps should be scaled in polythene bags and should not be opened until ready for use. After opening, the bags should be re-sealed immediately to prevent contamination of unused caps.

THREAD LUBRICATION

Flexible hoses should not be twisted, kinked or collapsed during installation. In some aircraft, flexible hose assemblies are used in both the high and low-pressure systems and these can be connected to standard pipe connectors. Care must, therefore, be taken to ensure that the hoses are not interchanged.

With the exception of Teflon sealing tape to specification MIL-T-27730, it is recommended that the application of any other lubricants or anti-seize compounds to the threads of pipe or component connections be avoided. The tape, which contains a lubricating compound, should be applied to all except the first two threads of male fittings and not more than three wraps of tape should be used. The tape should be wrapped in a direction opposite to the running thread; any excess should be trimmed off. NOTE: All traces of previous tape should be removed from threads and extreme care must be taken to prevent debris from entering the oxygen system.

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be used exclusively with oxygen equipment. Any dirt, grease or oil that may be on the tools or on any of the hoses, adapters, cleaning rags, or even on clothing is a possible source of fire.

PREVENTION OF OXYGEN FIRES OR EXPLOSIONS Safety precautions for oxygen servicing are similar to those required for fueling or defueling an aircraft.

Many materials such as oils, grease, fuel, paint, flammable solvents and metal swarf (e.g. from a damaged thread or a pipe coupling) are liable to ignite or explode spontaneously when exposed to oxygen under pressure. Similarly, extraneous matter such as dust, lint from a cleaning rag or natural oil from the hands getting into the system or into a component could cause ignition or explosion. It is essential therefore to keep these materials and other extraneous matter away from exposed parts of oxygen systems to prevent contamination. Clean areas should be used for dismantling and assembly of all oxygen system components.

An oxygen fire or explosion depends on a combination of oxygen, a combustible material and heat. The danger of ignition is in direct ratio to the concentration of oxygen, the combustible nature of the material exposed to the oxygen and the temperature of either the oxygen or the material, or both. Oxygen itself does not burn but it supports and vigorously intensifies a fire with any combustible material. The term 'combustible material' is used in its widest sense, denoting not only flammable materials but also such materials as steel, normally considered to be non-combustible, but which is in fact combustible at high temperatures in the presence of oxygen under pressure.

Any oxygen system leak can lead to a build-up of near-pure oxygen in unventilated zones, particularly in aircraft that remain idle. A concentration of oxygen in such a zone, e.g. behind upholstery, or thermal/acoustic tagging, or in control panels, could result in a fire or explosion by contact with grease, oil or electrical hot spots. Any indication of pressure loss or leaks must, therefore, be treated as hazardous and must be traced and eliminated before further flight.

The airplane and service cart should be electrically grounded and all vehicles should be kept a safe distance away. There should be no smoking, open flame or items, which may cause sparks within 50 feet or more depending upon the ventilation of the area during servicing operations. Since the clothing of a person involved in servicing an oxygen system is likely to be permeated with oxygen, smoking should be avoided for ten to fifteen minutes after completing the oxygen servicing.

Heat can be generated in an oxygen system by sudden compression or by resonance of oxygen under relatively low pressure impinging into a dead-end cavity. It can also be caused by the vibration of a seal, '0' ring, or other non-metallic material which is exposed to oxygen under pressure. A small high-pressure leak could cause ignition of the material through which it is leaking due to heat generated by friction.

The most important consideration when servicing any type of oxygen system is the necessity for absolute cleanliness. The oxygen should be stored in a well ventilated part of the hangar away from any grease or oil, and all high pressure cylinders no mounted on a service cart should be stored upright out of contact with the ground and away from ice snow or direct rays of the sun.

Protective caps must always be in place to prevent possible damage to the shutoff valve. The storage area for oxygen should be at least 50 feet away from any combustible material or separated from such material by a fire resistant partition. When setting up an oxygen storage area, you

Because of the extreme incompatibility of oxygen and any form of petroleum products, it is a good idea to dedicate all necessary tools to

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Near the panel-mounted regulators are mask hose quick disconnects (QDs) and microphone cable jacks. These hose QDs will also accept the hose with a full-face smoke mask supplied with the flight station portable oxygen cylinder. To determine the oxygen level in the cylinder, it is weighed.

EXAMPLE SYSTEMS LOCKHEED L-1011 OXYGEN SYSTEM FLIGHT CREW OXYGEN SYSTEM The crew system, shown in Figure, will last a 5-man crew approximately 4 hours using diluted oxygen. An overpressure relief fitting will release all cylinder contents overboard if cylinder pressure exceeds a certain limit. It this happens. The overboard discharge indicator, a green plastic disc mounted at the skin line on the right hand side of the flight station, will blowout. If the oxygen cylinder's temperature is not excessive, an overpressure of the oxygen cylinder would normally be very rare. A slow-opening on-off valve releases cylinder pressure to the pressure reducer. The pressure reducer decreases cylinder pressure (1850 PSI at 70°F.) to 50-90 PSI. It also contains a relief valve that will safely relieve momentary overpressure. On a panel near the cylinder is a quick disconnect fitting for maintenance checks. Corrosion-resistant steel Tubing distributes the reduced pressure oxygen to five panel-mounted diluter demand regulators, which further reduce oxygen pressure to breathing-pressure level. These regulators indicate distribution line pressure, and contain a flow indicator and toggle control valves for oxygen pressure and dilution control. Aneroids within the regulators lessen oxygen dilution with increasing altitude and provide undiluted oxygen at positive pressure above 28,000 feet flight station altitude. `

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With the emergency-normal-test mask selector in the normal position, flow to the mask is on demand (inhalation). In the emergency position, flow to the mask is continuous, and at positive pressure. The emergency position mechanically overrides the normal position. In the test mask position, flow to the mask is continuous, and at a pressure higher than in emergency position. The selector is spring-Loaded out of the test mask position to the normal position.

The flight crew oxygen mask, hose, and diluter- demand regulator are shown in Figure. A panel-mounted oxygen regulator and a mic/oxy panel are located near each of five flight station crew seats. On the front of the oxygen regulator are controls for oxygen dilution and pressure. On the back of the oxygen regulator are oxygen inlet and outlet ports, and an electrical connector for the 5 VAC panel light. On the bottom of the oxygen regulator is an ambient air inlet. The end of the mask hose that connects to the mic/oxy panel has a warning type connector that will not allow oxygen to flow through it if the connector is not fully seated in the hose coupling. The regulated oxygen pressure gauge shows pressure entering the oxygen regulator. Oxygen will not flow from the oxygen regulator to the masks unless the on-off selector is in the ON position shown on the right side of the oxygen regulator panel in Figure. With the 100% oxygennormal oxygen selector in the 100% position, undiluted oxygen will be supplied to the mask regardless of flight station altitude. With this selector in the normal oxygen position, oxygen to the mask will be diluted with flight station ambient air in proportion to flight station air pressure. Dilution decreases with increasing altitude. At a flight station altitude of 28,000 feet or higher, oxygen to the masks will not be diluted at all. Normal flight station altitude (pressurized) is 8,000 feet.

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CATEGORY B1 - MECHANICAL The flight crew is required to have a portable breathing oxygen cylinder equipped with a full face smoke mask that operates in much the same manner as the crew oxygen system. It also operates with an undiluted on-demand regulator as shown in Figure. The mask hose can be connected to any crew station quick disconnect. Likewise, any crew mask can be connected to the portable cylinder. A pressure regulator at the top of the cylinder has a slow opening on-off valve, a charging valve, a pressure gauge, a relief valve, an overpressure safety plug, and a capped constant flow outlet, which is not used.

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The 15-paired outputs are completed in about 7 seconds. Then are redundantly repeated. The fifteenth output of the first cycle illuminates the OXYGEN FLOW light on the controller. Aircraft wiring connects the 15 output pulse pairs to the modules in each sequence group, initiating each generator in the group and energizing the solenoid for unlatching the mask rack door. The passenger oxygen controller incorporates a test panel, which contains provisions for testing the system.

PASSENGER OXYGEN SYSTEM Emergency oxygen for passengers is supplied from chemical oxygen generators through conventional face-cup masks. The generators, which are initiated by an electrical pulse, burn the two chemicals and produce oxygen for 15 to 18 minutes. Generators and associated rackmounted mask(s) are secured in modules, one about each passenger seat group, attendant seat, and in each lavatory ceiling. Oxygen masks, with associated supply tubes, are nested in a clear plastic rack and secured in the module by a hinged door. The door is magnetically latched, and for mask deployment, is electro magnetically unlatched by a solenoid. Oxygen generators and the masks associated with them are divided into fifteen groups; each group being identified numerically and composed of sectionally situated modules. System control components shown in Figure include a passenger oxygen controller, located at the Flight engineer's panel, and a sequence timer, located in the center cabin ceiling. The controller, through aneroid switching, automatically activates the system if cabin altitude rises to 13.000 feet. A manual switch on the controller provides for manual activation of the system if the aneroid switching circuits fail. The sequence timer, when signaled automatically or manually, sequentially develops fifteen paired, timed, and synchronized output pulses, each pair consisting of a generator initiation pulse and a mask drop pulse.

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CATEGORY B1 - MECHANICAL CHEMICAL OXYGEN GENERATOR Oxygen generators are available in three sizes to supply oxygen to one, two, or three masks, as shown in Figure. They are approximately 7 inches long and 3 inches in diameter. Hermetically sealed cases have a shelf life of 10 years. Date of manufacture is indicated on the nameplate. The case exterior contains black and white comparison stripes. The white stripe is heat-sensitive and will darken when the generator is used. Oxygen from all masks constitutes about 1 percent of the cabin atmosphere, so there is no fire hazard due to oxygen from unused masks. Oxygen flow is continuous until the generator is expended.

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CATEGORY B1 - MECHANICAL PASSENGER SERVICE MODULES An example passenger service module (PSM), shown in Figure is ceiling mounted above each set of two passenger seats on the left and right sides of the cabin centerline. Oxygen masks are rack mounted in a mask compartment in the passenger service module (PSM) and are retained by a latched door. A chemical oxygen generator, mounted adjacent to the mask compartment, is manifolded by clear plastic tubes to each oxygen mask. When the passenger oxygen system is activated, the door solenoid is energized; the door unlatches and springs open; the masks drop and are suspended by their supply tubes. Simultaneously, the chemical oxygen generator is actuated and supplies oxygen to each mask through its supply tube.

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In some continuous flow systems using compressed oxygen cylinders, a valve is kept closed by an actuating pin. The valve is opened when the mask is pulled toward the passenger's face; a lanyard attached to the mask hose pulls the actuating pin out of the valve, allowing the valve to open and oxygen to flow to the mask. When oxygen is no longer required, the flow control units are reset by the flight crew, stopping the flow of oxygen to the distribution manifold. Residual pressure in the manifold is depleted through the masks allowing the automatic vent valve to open. Test fittings allow connection of an external pressure source and gauge for system testing.

BOEING 747-400 OXYGEN SYSTEMS The flight crew and passenger oxygen systems use oxygen cylinders to supply oxygen if needed. Several passenger oxygen systems have used chemical oxygen generators to supply the passengers in an emergency. The 747 use nine high-pressure oxygen cylinders to store the passenger emergency oxygen. Each cylinder is equipped with a pressure gauge, shutoff valve and-frangible disc as shown in Figure. The frangible disc provides over-pressure relief through an overboard discharge port and manifold common to crew system. A coupling with an integral thermal compensator connects the cylinder to a pressure reducer. A medium pressure manifold carries oxygen from the pressure reducers to the flow control units. In the OFF (reset) condition, the flow control units prevent oxygen flow into the low-pressure distribution manifold by closing the flow control units. Any small amount of oxygen leakage through the flow control units is discharged by the automatic vent valve to prevent unscheduled mask deployment. When the flow control units are actuated, the resulting pressure surge unlatches the oxygen module doors, causing the masks to deploy. The surge also closes the automatic vent valve. Within several seconds of actuation, the pressure surge decays. While oxygen continues to flow to the masks, pressure in the manifold is sufficient to hold the automatic vent valve closed.

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CATEGORY B1 - MECHANICAL A guarded three-position switch (see adjacent Figure) controls operation of the passenger oxygen system through: The flow control units. With the switch in NORM, the; system will activate one of the flow control units automatically at 14,000 feet cabin altitude. Momentarily placing the switch to ON activates the two other flow control units. When oxygen is no longer required, the system is turned off by placing the switch momentarily to the RESET position. The switch is guarded to prevent unintentional system activation. Placing the switch to ON deploys all the passenger oxygen masks throughout the main and upper deck passenger cabins.

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