Module 10

Aircraft Performance

Module 10

Where are we? 1 : Introduction to aircraft performance, atmosphere 2 : Aerodynamics, air data measurements 3 : Weights / CG, engine performance, level flight 4 : Turning flight, flight envelope 5 : Climb and descent performance 6 : Cruise and endurance 7 : Payload-range, cost index 8 : Take-off performance 9 : Take-off performance 10 : Enroute and landing performance 11 : Wet and contaminated runways 12 : Impact of performance requirements on aircraft design

Enroute and landing performance

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Enroute performance  Introduction  OEI enroute climb gradient  OEI enroute climb ceiling  Driftdown  Terrain clearance requirements  Return, Continue or Divert  ETOPS

Enroute and landing performance

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Enroute - Introduction  This section addresses the main performance considerations in the event of a system failure during cruise • Mostly addressing an engine failure but other failures such as loss of pressurization may also have to be considered

 A number of performance calculations may have to be carried out during the flight preparation phase to ensure safe continuation of the flight after a failure during cruise • OEI enroute climb ceiling • Driftdown flight path • Decision points: return, continue or divert

Enroute and landing performance

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Enroute – OEI enroute climb gradient  In the event of an engine failure after take-off and before landing, the aircraft will be flown in the OEI enroute climb configuration • Clean (Flaps and slats retracted) • One engine at MCT • One engine inoperative • Airspeed is equal to Venr : - Venr must not be less than 1.18 Vsr - Venr must provide a minimum maneuvering margin of 40 degree bank prior to stall warning or buffet - Normally Venr is selected by the manufacturer so as to provide best climb gradient

 The enroute climb gradient is presented in the AFM  The gross enroute climb gradient represents actual climb performance  The net enroute climb gradient is obtained by subtracting 1.1 % from the gross gradient (for a two engine aircraft)

Enroute and landing performance

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Enroute – OEI enroute climb ceiling  The OEI enroute climb ceiling is the pressure altitude at which the airplane can maintain a net enroute climb gradient of 0 %  Critical case is with anti-ice on (lower thrust level) and ice accumulated on unprotected surfaces  It is highly desirable to have an OEI enroute climb ceiling of at least 15,000 ft so to ensure obstacle clearance after an engine failure during cruise in mountainous areas such as the alps for example • • • •

Higher OEI enroute climb ceiling will be required in the Himalayas Typically 21,000 – 25,000 ft Very difficult to achieve with a two-engine aircraft Four engine aircraft have an advantage in this case

 Typical OEI enroute climb ceiling chart is shown on the next slide

Enroute and landing performance

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Enroute – OEI enroute climb ceiling (Cont’d)

Enroute and landing performance

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Enroute – Driftdown  Following an engine failure during cruise, the so-called driftdown procedure must be followed to minimize loss of range and maximize the time to descend to the OEI climb ceiling

Enroute and landing performance

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Enroute – Terrain clearance requirements 

The enroute terrain clearance requirements in FAR 121.191 for one engine inoperative are:

A) The net flight path shall have a positive slope 1500 ft above airport intended for landing

Enroute and landing performance

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Enroute – Terrain clearance requirements (Cont’d)

Compliance with B) or C) below: B)

C)

Net flight path slope shall be positive at an altitude of at least 1,000 ft above all terrain within 5 statute miles (4.34 nm) on either side of the intended track, or

The net flight path clearing all terrain and obstacles by at least 2,000 ft within 5 statute miles (4.34 nm) on either side of the intended track

Enroute and landing performance

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Enroute – Return, Continue or Divert  A pilot may be faced with the decision to continue, turn back, or divert if an emergency such as an engine failure arises during the flight  Decision points A and B can be defined based on (1) fuel required to return or continue and / or (2) obstacles that must be cleared

Enroute and landing performance

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Enroute – Return, Continue or Divert (Cont’d)

Enroute and landing performance

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Enroute – ETOPS 

Basic aircraft operations have traditionally been predicated on capability to reach a suitable alternate airport within 60 minutes in the event of an emergency • •



This requirement does not allow flight over water for long periods of time and forces the use of a trajectory that is relatively close to the shore line Not optimum for planning of long range missions

With the increased reliability of aircraft systems, it is possible to obtain airworthiness and operational approval for more than 60 minutes for two engine aircraft •

Referred to as Extended Twin-engine Operations (ETOPS)

•

The 60-minute requirement can be extended up to 180 minutes with appropriate system reliability and redundancy (Ref. FAA Advisory Circular 120-42A on ETOPS)

•

Allows the use of direct (optimum) flight trajectories during long range missions over water

Enroute and landing performance

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Enroute – ETOPS (Cont’d)

60 minutes Light blue indicates acceptable flight zones

120 minutes

180 minutes

Enroute and landing performance

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Landing        

Definition of landing distance Landing speed Calculation of landing distance Main factors affecting performance Landing field length Performance-limited landing weight Landing WAT limits Brake energy considerations

Enroute and landing performance

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Landing – Definition of landing distance  The actual landing distance extends from the point where the aircraft is 50 ft above the runway to the point where the aircraft has reached a full stop on the runway

Enroute and landing performance

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Landing – Definition of landing distance (Cont’d)

 Segment A – Air distance • Transition from a stabilized approach at a speed of VREF on a 3 degree glideslope to the touchdown point • Thrust may be reduced to idle at or after the 50 ft point • Pilot applies longitudinal control to reduce the rate of descent before the touchdown point (referred to as the landing flare) • Touchdown is defined as the time when the wheels first touch the ground

Enroute and landing performance

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Landing – Definition of landing distance (Cont’d)

 Segment B – Delay distance from VTD to VFB • Transition from the touchdown point to the full braking configuration • No performance credit for reverse thrust on dry or wet runways • After touchdown, brakes are applied and GLDs are extended as rapidly as possibly (GLD extension may be automatic) • At VFB, the aircraft is in the full braking configuration with brakes fully applied and GLDs fully extended • Brakes may be applied before nosewheel touchdown but in a manner such that nosewheel does not hit the runway at a vertical speed exceeding than 8 ft/sec Enroute and landing performance

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Landing – Definition of landing distance (Cont’d)

 Segment C – Braking distance from VFB to full stop • From VFB to a full stop • Maximum anti-skid braking is used • Dry runway conditions are assumed • GLDs are fully extended

Enroute and landing performance

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Landing – Landing speed  The landing approach speed VREF is determined by the aircraft manufacturer based on the following considerations • VREF is not less than 1.23 Vsr and not less than VMCL (as per FAR/JAR 25.125) - VMCL is the minimum control speed in the landing configuration, i.e. the minimum speed at which the aircraft can be controlled in the landing configuration with one engine failed and the other engine at maximum go-around thrust

• VREF is the minimum speed that provides acceptable handling characteristics during a stabilized landing approach • A minimum maneuvering margin of 40 degree bank prior to stall warning is available at VREF -

 It is highly desirable to have a VREF speed not greater than 140 KIAS at MLW in order to ensure that the aircraft can operate in the class C approach category • Higher VREF would result in restrictions at some airports Enroute and landing performance

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Landing – Calculation of landing distance  Landing distance (or actual landing distance ALD) is the sum of segments A, B and C  Typically calculated for ISA conditions and zero runway slope (as per FAR/JAR 25.125)  Segment A – Air distance • Air distance = air time * average ground speed • Most aircraft manufacturers use the “parametric method” in order to calculate air distance • A certified air distance of approximately 1,000 ft is is typical for transport category airplanes • Typical speed at touchdown when an “aggressive” landing is carried out is approximately 0.98 VREF (speed decay = 2 % between 50 ft and touchdown)

Enroute and landing performance

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Landing – Calculation of landing distance (Cont’d)  Description of the parametric method • Methodology is defined in FAA Advisory Circular (AC) 25-7A • A minimum of 40 “normal” landings are carried out with glideslope at 50 ft varying from 2.5 to 3.5 degrees and with rate of descent at touchdown varying from 2 to 6 ft/sec • Parametric equations are determined from the the results of the 40 landings • The intent of the parametric method is to allow extrapolation to “aggressive performance landing “ conditions while reducing the risk for aircraft damage during landing performance tests • The certified air distance is estimated, from the parametric equations, for an “aggressive” condition where the aircraft approaches on a 3.5 degrees at 50 ft and touches down with a rate of descent of 8 ft/sec

Enroute and landing performance

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Landing – Calculation of landing distance (Cont’d)  Description of the parametric method (Cont’d) • The air time obtained from the parametric equations must checked against the minimum demonstrated air time - If calculated air time is less than 90 % of the minimum demonstrated air time, then an air time equal to 90 % of the minimum demonstrated air time must be used

• Extracts from the relevant sections of AC 25-7A follow (see http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCi rcular.nsf)

Enroute and landing performance

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Landing – Calculation of landing distance (Cont’d)

Enroute and landing performance

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Landing – Calculation of landing distance (Cont’d)

Enroute and landing performance

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Landing – Calculation of landing distance (Cont’d)

Enroute and landing performance

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Landing – Calculation of landing distance (cont’d)  Segment B – Delay distance • Distance from VTD to VFB is equal to the time delay from VTD to VFB multiplied by the average ground speed - ∆ s = ∆ t delay * average ground speed - average ground speed = VTDG – (0.5* adelay* ∆ t delay)

• Typical deceleration during the delay distance (adelay) is approx. 3 ft/sec2 • Time delay from VTD to VFB (∆ t delay) is typically equal to -

aircraft without slats and with automatic GLD : 1 second aircraft without slats and without automatic GLD : 2 seconds aircraft with slats and with automatic GLD : 2 seconds aircraft with slats and without automatic GLD : 3 seconds

Enroute and landing performance

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Landing – Calculation of landing distance (cont’d) 

Segment C – Braking distance •

Similarly to the ASD braking segment, the braking distance can be calculated with either -

a step by step integration process, or with a simplified methodology (average deceleration calculated at VRMS)

Enroute and landing performance

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Landing – Main factors affecting performance 

Landing approach speed •



•

The main factor that affects landing performance Landing distance is proportional to VREF2

•

VREF is essentially a function of VSR and CLMAX (slats are important)

Braking performance •



•

The second most important factor Good lift dumping is necessary (negative CL)

•

High anti-skid efficiency is very important

Time delay • •

Reduction in the time delay is beneficial Automatic GLD reduces the time delay

Enroute and landing performance

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Landing – Landing field length  The landing field length (LFL) defines the minimum runway length required for a landing as per FAR 121 and JAR OPS 1 requirements  The LFL is obtained by applying a factor to the actual landing distance (ALD)  For a dry runway • LFLDRY = ALD / 0.6 = 1.67 ALD • The aircraft must be able to stop within 60 % of the available runway length

 For a wet runway, the runway length required for a dry runway must be increased by 15 % •

LFLwet = LFLDRY * 1.15 = 1.92 ALD

Enroute and landing performance

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Landing – Landing field length

Enroute and landing performance

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Landing – Performance-limited landing weight  The maximum permissible landing weight is limited by the most limiting of • landing field length requirements • landing weight limited by climb requirements

Enroute and landing performance

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Landing – Landing WAT limits  Similarly to the take-off phase, to ensure safety, FAR/JAR 25 requirements define minimum climb gradients for the approach and landing phase (Ref. FAR 25.121)  The following table summarizes minimum climb gradient requirements for the approach and landing phase

Requirement FAR 25.121 (d) FAR 25.121 (e)

Name Flap/Slat Ldg gear Thrust Speed Approach climb Approach up GA, OEI Vac Landing climb Landing down GA,AEO Vlc

Min. grad. Req'd 2.1% 3.2%

 Approach climb speed Vac (speed used for go-around climb with OEI) must not be less than the greater of 1.13Vsr and 1.10 Vmcl  Landing climb speed (Vlc) must not be less than the greater of 1.13 Vsr or Vmcl

Enroute and landing performance

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Landing – Landing WAT limits (Cont’d)  Go-around is a critical phase of flight • The thrust used for calculation of landing climb gradient is the thrust available 8 seconds following a thrust increase from flight idle to goaround thrust [FAR 25.119 (a)] - An important requirement for engine design : in order to ensure safety during go-around, the engine must be able to accelerate quickly from idle to go-around thrust

• Drag in the landing configuration is very high (up to twice as much as in the take-off configuration) • It is important to be able to increase thrust and reduce drag (flap retraction and landing gear retraction) as rapidly as possible in the event of a go-around • The aircraft must also maintain an acceptable margin to stall during flap retraction if it does not accelerate during go-around -

Far 25.121(d) requires that VSR in the approach configuration must not exceed 1.1 VSR in the landing configuration

Enroute and landing performance

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Landing – Brake energy considerations  During a landing, brakes will absorb a certain amount of energy  Following a high energy landing, heat will be evacuated from the brake and will be partially transmitted to the wheel / tire assembly  The heat input to the tire will result in increased tire pressure and excessive heat transfer to the tire may cause tire explosion  Wheels are protected by fuseplugs that will melt and release tire pressure if a critical wheel temperature is reached • Fuseplug may only release some time after the stop (e.g. 5-40 minutes) due to the lag associated with the heat transfer process

 Means must be provided to the crews to ensure that fuseplugs will not release during a subsequent take-off that may take place shortly after the landing

Enroute and landing performance

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Landing – Brake energy considerations (cont’d)  During certification, a high energy landing must be carried to demonstrate the maximum level of energy that the brakes can absorb without causing subsequent fuseplug release (fuseplug integrity test)  The demonstrated energy level is then used to calculate “maximum quick turnaround landing weights” • The maximum quick turnaround landing weight chart provides operational conditions (weight, altitude, temperature, wind, runway slope) that would result in a brake energy level that is equal to the brake energy level obtained during the fuseplug integrity test • Calculations based on performance-type landing with no reversers

 When landing at weights equal to or greater than the maximum quick turnaround landing weight, a minimum waiting time and an inspection of the wheels is required before the subsequent take-off

Enroute and landing performance

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Landing – Brake energy considerations (cont’d)  Some airplanes are equipped with a Brake Temperature Monitoring System (BTMS)  The BTMS provides indications of brake temperatures to the crew  The BTMS displays temperatures based on color codes • Green : OK • White : be careful • Red : brake overheat

 When aircraft are equipped with BTMS, there is no need to rely on a quick turnaround landing weight chart • the aircraft may have to stay a minimum period of time on the ground (typically 15 minutes) before the next take-off to ensure that temperature has reached its maximum level (thermal lag associated with the location of BTMS temperature sensors )

Enroute and landing performance

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