Instruments

Attitude Indicator Sometimes called the "artificial horizon," the attitude indicator is the only instrument that simultaneously displays both pitch and bank information.

How It Works The gyro mounted in the attitude indicator rotates in the horizontal plane and maintains its orientation relative to the real horizon as the airplane banks, climbs, and descends. Note, however, that the attitude indicator alone can't tell you whether the airplane is maintaining level flight, climbing, or descending. It simply shows the aircraft's attitude relative to the horizon. To determine your flight path, you must crosscheck the airspeed indicator, altimeter, heading indicator, and other instruments. The pointer at the top of the attitude indicator moves along a scale with marks at 10, 20, 30, 60, and 90 degrees of bank. The horizontal lines show the aircraft's pitch attitude in degrees above or below the horizon. The converging white lines in the bottom section of the indicator can also help you establish specific bank angles.

Limitations The gyros in the attitude indicators used in most small aircraft tumble if the pitch attitude exceeds +/-70 degrees or if the angle of bank exceeds 100 degrees. When the gyro tumbles, it gives unreliable indications until it realigns itself, a process that usually requires several minutes of straight and level flight. Aerobatic and large aircraft are often equipped with gyros that are reliable through 360 degrees of pitch and bank. Many modern attitude indicators have a blue "sky" and brown "earth," which is the origin of the phrase "keep the blue side up."

Heading Indicator The heading indicator, sometimes called the "directional gyro" or "DG," is one of the three gyro instruments. When aligned with the compass, it provides an accurate, stable indication of the aircraft's magnetic heading. The heading indicator is an important aid because the compass is subject to errors caused by acceleration, deceleration, and the curvature of the earth's magnetic field, especially at high latitud es. The compass often oscillates or leads or lags a turn and it is especially hard to read in turbulence or during maneuvers.

How It Works The gyro in the heading indicator rotates in the vertical plane. A card marked with headings maintains its orientation as the airplane turns. The apparent movement of the card gives the pilot an immediate, precise indication of the airplane's heading and the direction in which the airplane is turning. The card is marked off in five-degree increments, with numbers every 30 degrees and the cardinal directions indicated by N, S, E, W.

Turn Coordinator The turn coordinator is really two instruments. The gyro portion shows the aircraft's rate of turn—how fast it's changing direction. A ball in a tube calle d the "inclinometer" or "slip/skid indicator" shows the quality of the turn—whether the turn is "coordinated."

How It Works The gyro in the turn coordinator is usually mounted at a 30-degree angle. When the airplane turns, forces cause the gyro to precess. The rate of precession makes a miniature airplane on the face of the instrument bank left or right. The faster the turn, the greater the precession, and the steeper the bank of the miniature airplane. Standard Rate Turn When the wings of the miniature airplane align with the small lines next to the "L" and "R," the aircraft is making a standard rate turn. Balancing Act The black ball in the slip/skid indicator stays between the two vertical reference lines when the forces in a turn are balanced and the airplane is in coordinated flight. If the ball drops toward the inside of the turn, the airplane is slipping. If the ball moves toward the outside of the turn, the airplane is skidding. To correct a skid, reduce rudder pressure being held in the direction of the turn and/or increase the bank angle. To correct a slip, add rudder pressure in the direction of the turn and/or decrease the bank angle.

Gyroscopic Instruments Gyroscopes work like spinning tops. They have two properties— rigidity in space and precession— that make them useful in flight instruments. The attitude indicator and heading indicator are based on a gyro's rigidity in space. Because a gyro resists being tipped over, it can provide a stable reference to the real horizon or to a specific direction. The turn coordinator uses precession to display information about the direction and rate of turn.

Horizontal Situation Indicator An HSI, or horizontal situation indicator, is a fancy VOR indicator superimposed on a heading indicator. It still has an omni-bearing selector (OBS) or omni-bearing indicator (OBI), a course deviation indicator, and an ambiguity indicator. But an HSI gives you an overhead view of the airplane's position relative to aVOR radial or other electronic course.

Bird’s-Eye View The HSI shows you a top-down view. Your airplane is at the center of the instrument. Your magnetic heading is at the top. The omni-bearing selector is a yellow arrow with a central segment that moves back and forth and acts as a course deviation indicator. A white triangle near the center replaces the TO-FROM-OFF flag. It points toward the VOR station.

Radio Magnetic Indicator (RMI) A radio magnetic indicator (RMI) combines an ADF (and often a VOR indicator) with a heading indicator. This arrangement makes it much easier to fly bearings to or from NDBs and can help you easily keep track of your position relative to a VOR. With an RMI, relative bearings are a thing of the past— the ADF needle points to magnetic bearings. The head of the needle always indicates the magnetic bearing TO the station, and the tail of the needle always indicates the magnetic bearing of the airplane FROM the station. Most RMIs have two needles— one for the ADF and one for the VOR. No TO-FROMOFF flag is needed for the VOR indicator, because the needle always points TO the station. The VOR function is used exactly the same as the ADF function.

VOR Navigation Throughout most of the world the primary electronic navigation aid is the VOR. Aircraft fly routes called "airways" defined by a network of stations.

How It Works VOR transmitters and receivers operate in the 108.0–117.95 MHz range. The transmitter sends out two signals. The reference phase signal radiates in all directions. A second, variable-phase signal, rotates through 360 degrees, like the beam from a lighthouse. Both signals are in phase when the variable signal passes 360 degrees (referenced to magnetic north) and they are 180 degrees out of phase when the rotating signal passes 180 degrees.

Radials The two signals from a VOR transmitter generate 360 lines, like spokes in a wheel. Each line is called a "radial." VOR navigation equipment in an airplane can determine which of those 360 radials the airplane is on. The pilot can also select a radial to define a magnetic course toward or away from a VOR station. The VOR equipment displays the airplane's position to or from the station and left or right of the selected course.

Limitations Radio signals in the Very High Frequency (VHF) range are limited to line-of-sight, like FM radio and television broadcasts. This limitation means that hills or other obstacles between you and a VOR transmitter can block the navigation signal unless you climb to a higher altitude. A VOR signal's range is al so limited. Below about 18,000 ft (5,486 m), a typical VOR's range is 40 –130 nm, depending on terrain and other factors. Above 18,000 ft (5,486 m) range is about 130 nm. Omni-Bearing Indicator The VOR indicator (or "VOR head") on the instrument panel includes a dial called the omni-bearing selector (OBS) or omni-bearing indicator (OBI); a needle or course deviation indicator (CDI) that pivots at the top like a windshield wiper or moves smoothly from side to side; and an ambiguity indicator, or TO-FROM-OFF flag. You use the OBS to select a radial or to determine which radial you're on. The TO-FROM-OFF flag indicates your position relative to the station and the radial you've selected with the OBS. It also tells you if you're receiving a usable navigation signal— "OFF" means you aren't.

A: Rotating Course Card B: Omni Bearing Selector, or OBS knob C: CDI, or Course Deviation Indicator D: TO-FROM indicator

Non-Directional Beacons (NDB)Non-directional beacons (NDBs) are the oldest type of navigational aid still in use today. They operate in the 200–400 KHz frequency range. NDBs are inexpensive to install and operate, so instrument approaches based on NDBs are frequently found at relatively small airports with no other instrument landing equipment. As the name implies, its signal is transmitted in all directions. How It Works The airborne equipment is an automatic direction finder (ADF). The terms "NDB approach" and "ADF approach" are often used interchangeably. Just as the needle of a magnetic compass points toward the magnetic north pole, the needle of an ADF points toward the source of the radio signal to which its receiver is tuned. As the airplane turns, the needle continues to point toward the transmitting antenna. Many ADF indicators have a fixed-card dial with marks every five degrees and numbers at the 0, 90, 180, and 270 degree points; most have dots at 45-degree increments for easy reference. Relative Bearing (RB) When the airplane proceeds directly toward the station, the needle points straight ahead, or zero degrees relative to the nose of the airplane. A 90-degree left turn makes the needle point 90 degrees to the right, or 090 degrees relative to the nose. These angles measured relative to the nose of the airplane are called relative bearings. Magnetic Bearing (MB) By themselves, relative bearings are of little value for navigation because we navigate using magnetic courses, bearings, and tracks. To figure out which magnetic bearing you're on, you must add the relative bearings to the airplane's magnetic heading (MH). Use the formula MB = RB + MH. For example, if you are maintaining a heading of 090 degrees magnetic and the needle points toward the right wing at 090 degrees relative bearing, the magnetic bearing to the transmitting antenna is 180 degrees magnetic— so you are north of the station.

Activating and Tuning the ADF The automatic direction finder (ADF) receives signals from nondirectional radio beacons (NDBs). It's useful for en -route navigation and for flying NDB approaches. On these aircraft, the ADF indicator is a large instrument with a yellow or white needle that points to numbers on a compass card. In the When you click the Toggle ADF Needle switch at the lower right of primary electronic flight instrument display. After tuning a VOR, ILS, or NDB, always identify the station to confirm that you’ve selected the correct frequency and that the station is transmitting a usable signal. To do so, listen for its three-letter Morse code ID after you select the frequency.

Vertical Speed Indicator (VSI) The vertical speed indicator (sometimes called the VSI or rate-of-climb indicator) shows how fast an aircraft is climbing or descending. The VSI is usually calibrated in feet per minute. Pilots use the VSI primarily during instrument flight to help them establish the correct rate of descent during approaches and to maintain steady rates of climb or descent.

The VSI has a restrictor in the connection to the case which cases a difference between diaphragm and case pressure during climbs and descents

How It Works The VSI is connected to the static system. Air pressure inside the instrument case decreases as the airplane climbs and increases as the airplane descends. Inside the case, a sealed wafer, much like the one used in the altimeter, expands and contracts as the pressure changes. A needle connected to the wafer rotates as the wafer expands and contracts, indicating a rate of climb or descent. The wafer also has a small, calibrated leak to allow the pressure in the wafer to equalize with the pressure in the case when the airplane levels off. When the pressure inside the wafer equals the pressure in the case, the needle returns to zero, indicating level flight. Reading the VSI You shouldn't use the VSI as the primary indicator of whether you're maintaining level flight. If the airplane begins to climb or descend, the VSI initially indicates the change in the proper direction. But the indicator takes several seconds to catch up to the aircraft's actual rate of climb or descent. "Chasing" the needle on the VSI can make you feel like you're riding a roller coaste r. Rely instead on the airspeed indicator and altimeter— they give quick, accurate indications of deviations from level flight. Then crosscheck the VSI to verify that the airplane is climbing or descending at the rate you want.

Airspeed Indicator The airspeed indicator is a differential pressure gauge. It measures the difference between the air pressure in the pitot tube and the static, relatively undisturbed air surrounding the airplane. A needle displays this difference as airspeed. Aircraft manufactured in the U.S. after 1976 have airspeed indicators with markings based on indicated airspeed in knots. Older aircraft typically have markings that reflect calibrated airspeed in statute miles per hour.

How It Works The airspeed indicator is the only instrument connected to both the pitot tube and the static system. Air from the static system fills the case of the airspeed indicator, providing a "base" pressure against an expandable diaphragm. Ram air forced into the pitot tube as the airplane moves fills the diaphragm, which expands as ram air pressure (and speed) increase. A needle connected to the diaphragm rotates as the diaphragm expands. The needle's position on the instrument face indicates airspeed. The airspeed indicators for the Boeing 737-400 include an additional needle with red and white stripes called the "barber pole." A flight data computer takes information about the current altitude, air temperature, and pressure and continuously computes the maximum allowable airspeed as the aircraft climbs and descends. The barber pole shows this speed.

Instrument Landing System (ILS) The instrument landing system is the most precise approach system currently in use. A typical ILS includes a localizer to provide left-right alignment with the runway and a glide slope to define the proper descent path, usually about three degrees. An ILS also includes marker beacons that define specific points along the final approach path.

The ILS indicator has two needles: a vertical needle for localizer and a horizontal needle for glideslope Decision Height Each ILS approach chart has a decision height (DH) for that approach. This is the indicated altitude at which you must decide to either continue the approach to a landing or to initiate the missed approach procedure. If your airplane is stabilized on the glide slope at approach airspeed and is configured for landing, the transition from instrument meteorological conditions to visual meteorological conditions at or above decision height requires no action at all— just continues the descent and land. If you reach DH and do not make visual contact with the runway environment, add power, pitch up, retract the gear and flaps, and execute the missed approach procedure. Ignore any deviation below DH, but do not attempt to salvage the approach at the last minute if you make visual contact. Back Course Approaches Because the localizer antenna radiates bidirectional, all localizers transmit a "back course" signal away from the approach path and runway. In many cases, electronic screening is provided to block the back course signal so that it cannot be received by users departing the airport. However, at some airports the back course is usable and a back course instrument approach is authorized. A back course approach does not have a glide slope. When inbound on a back course approach your course will be the reciprocal of the front course and you must fly away from the needle to correct for wind drift. If you have a horizontal situation indicator (HSI), however, you always fly toward the needle. At some major airports, the same runway has two instrument landing systems, one for each direction. However, only one ILS can be activated at a given time.

The Localizer In the U.S., localizer transmitters operate on frequencies between 108.100–111.950 MHz. Glide slope transmitters operate in the UHF range, but they are paired with the localizer frequencies. The localizer antenna is installed at the far end of the runway. Its beam is adjusted so that it is 700 ft (213 m) wide at the runway threshold as measured by fullscale deflection of the course deviation indicator needle. The typical width of the localizer beam is five degrees, although the actual width varies with runway length. The signal is highly directional— it cannot be used if your airplane's position is more than 35 degrees away from the runway heading. At the outer marker, typically 4–7 nm (7.5–13 km) from the runway, a five-degree localizer signal is 2,000–3,500 ft (610–1,067 m) wide. The ILS becomes more sensitive as you approach the runway.

Flying the Localizer When using the localizer to fly an ILS approach you always fly toward the needle. You have no choice— the omni-bearing selector does not work when a localizer frequency is selected. Many pilots set the OBS to the published course for the approach as a memory aid, not as a part of the procedure. Remember, however, that if you're using the HSI in the Boeing 737-400, you must set the inbound course in the course selector on the autopilot. Tracking the Localizer Whether you are vectored to intercept the localizer by a controller or do it on your own, you will intercept a localizer at no more than a 45-degree angle several miles from the final approach fix. As the localizer needle approaches the center during the intercept, turn to the published inbound heading and note the rate of movement of the needle. To avoid over controlling, do not bank more than five degrees or change heading more than two degrees at a time after you intercept the localizer. You may find that making small heading changes with the rudder is more efficient than banking. When the needle is within one-half scale of being centered, concentrate on making very small heading changes into the wind to move it to the centered position. Remember that when you reach the threshold, full-scale deflection on either side of the indicator represents only 350 ft (106 m).

The Glide Slope The glide slope antenna is located on one side of the runway, about 1000 ft (305 m) down the runway from the threshold. Its beam is normally inclined three degrees, although local conditions may require shallower or steeper glide paths. The glide slope beam is approximately 1.4 degrees. Because one degree = 100 ft per nm from the antenna, the glide slope is 140 ft thick (70 ft above and 70 ft below the centerline) one mile from the antenna. Tracking the Glide Slopes you track the localizer while maintaining altitude the glide slope needle is at the top of the indicator. As it starts to move toward the center, make sure that your airspeed is constant at the correct approach speed. Just before the needle centers (about one dot), extend the landing gear. With a fixed power setting and the added drag of the gear, the airplane should begin to lose altitude at approach airspeed. There are many techniques for flying a smooth descent path. One common method is to adjust pitch to follow the glide slope and adjust the throttle to maintain approach speed. However you choose to fly the glide slope, remember to use small, smooth corrections. The beam is very narrow and tolerances are tight.

Marker Beacons Marker beacons are used to alert the pilot that an action (e.g., altitude check) is needed. This information is presented to the pilot by audio and visual cues. The ILS may contain three marker beacons: inner, middle and outer. The inner marker is used only for Category II operations. The marker beacons are located at specified intervals along the ILS approach and are identified by discrete audio and visual characteristics (see the table below). All marker beacons operate on a frequency of 75 MHz. Indications a pilot receives when passing over a marker beacon. MARKER 

CODE 

LIGHT 

SOUND 

OM 

___

BLUE 

400 Hz two dashes/second 

MM 

._._._ 

AMBER 

1300 Hz Alternate dot and dash 

IM 

....

WHITE 

3000 Hz only dots 

BC 

.. ..

WHITE 



Notice above that the sound gets "quicker" and the tone "higher" as the aircraft moves towards the airport— first dashes, then dots and dashes, finally just dots.

The OM, 4 to 7 NM from the runway threshold, normally indicates where an aircraft intercepts the glide path when at the published altitude. The MM, 3500 feet from the runway threshold, is the Decision Height point for a normal ILS approach. On glide path at the MM an aircraft will be approximately 200 feet above the runway. The IM. 1000 feet from the runway threshold, is the Decision Height point for a Category II approach. BC ... Most, but not all, airports with an ILS also offer guidance on the back course. The BC marker identifies the FAF for the back course. A Back-Course approach is nonprecision since there is no glide path associated with it. The majority of problems in locating marker beacons are the availability of real estate and access to utilities.