Module 11

Aircraft Performance

Module 11

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

Wet and contaminated runways

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Agenda – Module 11

 Wet runways  Contaminated runways

Wet and contaminated runways

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Wet runways – Take-off performance  Take-off performance on wet runways is based on the same considerations as take-off performance on dry runways with the following exceptions  Screen height is reduced from 35 to 15 ft for TODOEI • Screen height remains at 35 ft for TODAEO

 TOR OEI = TOD OEI • No clearway credit for engine out distance on a wet runway

 Reverse thrust can be used for ASD calculation • Subject to demonstration that aircraft the can be controlled on the ground during a stop with one engine at maximum reverse thrust and one engine at forward idle thrust • Demonstration is carried out on a dry runway with nosewheel steering inoperative and with 50 % of normal braking (to simulate a wet runway) Wet and contaminated runways

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Wet runways – Take-off performance (cont’d)  Braking coefficient is reduced • Data for wet smooth runway must be provided in the AFM • Data for wet grooved runway or for runway covered with porous friction course material may be presented in the AFM at the option of the applicant (better braking performance) • Braking force FB is calculated as follows FB = η AS µ B WBR Where η AS = anti-skid system efficiency µ B = wheel-braking coefficient defined in 25.109 WBR = Vertical load on braked wheels • Anti-skid efficiency can be determined by flight test or a default value of 0.8 can be used Wet and contaminated runways

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Wet runways – Take-off performance (cont’d)  Braking coefficient is reduced (Cont’d) • Anti-skid efficiency will be lower if the brake system cannot maintain the optimum slip ratio during the stop

Wet and contaminated runways

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Wet runways – Take-off performance (cont’d)  Braking coefficient is reduced (Cont’d) • Variation of wheel braking coefficient as a function of tire pressure (psi) and ground speed (knots) for a smooth wet runway :

• Deceleration on a smooth wet runway is typically about 50 % of the deceleration on a dry runway

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Wet runways – Take-off performance (cont’d)  Braking coefficient is reduced (Cont’d) • Wheel braking coefficient as a function of tire pressure (psi) and ground speed (knots) for a wet runway that is grooved or covered with porous friction course material (FAR 25.109)

• Deceleration on a wet grooved runway is typically about 80-90 % of the deceleration on a dry runway

Wet and contaminated runways

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Wet runways – Take-off performance (cont’d)  Overall, the field length-limited take-off weight on a wet runway is normally slightly lower than on a dry runway • Reduction of approximately 0-2 % in weight limit is typical

 In some cases, distances may be shorter on wet runways because of the alleviations (screen height and reverse thrust) • In this case, the dry runway weight limit must be used when operating on a wet runway (i.e. it is not acceptable to have a higher field length-limited take-off weight when operating on a wet runway)

 For given operating conditions (weight, altitude, temperature, …), the dry runway V1 must be reduced when operating on a wet runway • Mainly due to the fact that ASD increases significantly on a wet runway (will be discussed in more detail later) • VR and V2 are unchanged when operating on a wet runway

Wet and contaminated runways

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Wet runways – Landing performance  Landing performance on wet runways has already been addressed in the last module  Covered by an operational factor rather than by an actual calculation based on reduced braking friction  LFLWET = 1.15 * LFLDRY

Wet and contaminated runways

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Contaminated runways – Take-off performance  Take-off performance is significantly degraded when operating on contaminated runways • The presence of precipitation such as standing water, slush or loose snow on the runway will increase drag during take-off roll • Braking coefficients will be even lower than on a wet smooth runway • Aquaplaning may take place above a certain speed • Calculation methods are detailed in JAR 25 AMJ25X1591 (main points are summarized in this presentation) • Field length-limited take-off weight can be reduced by as much as 25 % relative to dry runway conditions!

 Take-off performance on contaminated runways is calculated based on the same alleviations as take-off performance on wet runways but : • Additional precipitation drag must be considered • Reduced braking coefficients must be considered

Wet and contaminated runways

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Contaminated runways – Take-off performance (cont’d)  Additional precipitation drag is due to two effects • Landing gear displacement drag : drag caused by the fact that contamination is displaced by the tires • Impingement drag : drag due to airframe impingement of contamination spray from tires • Landing gear displacement drag and impingement drag are proportional to contamination Water Equivalent Depth (WED = depth * SG) and increase as function of VG2 until the aquaplaning speed is reached and then they reduce gradually as speed in increased

Wet and contaminated runways

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Contaminated runways – Take-off performance (cont’d)  Aquaplaning speed • An airplane will aquaplane at high speed due to the incompressibility of water on a surface contaminated by standing water, slush or wet snow • The aquaplaning speed (VP) in knots (ground speed) can be estimated as where P is tire pressure in lb/in2

Vp = 9 P

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Contaminated runways – Take-off performance (cont’d)  Current certification requirements define braking coefficients for different contaminated runway conditions

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Contaminated runways – Take-off performance (cont’d)  Runway friction measurement devices are used in many airports • Airport personnel measure friction • Friction measurements are reported to the crews • Canadian Runway Friction Index (CRFI) has been defined

Wet and contaminated runways

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Contaminated runways – Landing performance  JAR 25 AMJ25X1591 also provides details for calculation of landing distance on contaminated runways  Main points are summarized below • Air time of 7.0 seconds • Touchdown speed = 93 % of speed at 50 ft • Delay distance must consider additional time delay for selection of reverse thrust • Braking coefficient is as defined before for take-off on contaminated runways

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Contaminated runways – Other considerations 

Rutting of snow/slush may cause directional control difficulties



Reduced tire to ground cornering friction adversely affects aircraft controllability, particularly in crosswind and/or with reverse thrust

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Contaminated runways – Other considerations (Cont’d)



Potential for wing leading edge contamination due to slush/snow impingement and freezing, leading to loss of lift/angle of attack margin during takeoff

 Water/slush/snow ingestion into engine inlets leading to powerplant operating anomalies  Water/slush/snow impingement and subsequent freezing leading to system operating anomalies (e.g. flight control restrictions, freezing brakes)

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Wet and contaminated runways – impact on V1  Three cases are compared • Dry runway • Shallow contamination - Low mu - No precipitation drag - Examples : wet runway, icy runway, … • Deep contamination - Low mu - Precipitation drag - Examples: Slush, standing water, … Wet and contaminated runways

Wet and contaminated runways – impact on V1 (Cont’d) Dry runway DISTANCE

ASD (AEO) ASD (OEI) Minimum Distance (BFL)

TOD (AEO) TOD (OEI) Optimum V1/VR

Wet and contaminated runways

V1/VR

Wet and contaminated runways – impact on V1 (Cont’d) Shallow contamination – Low mu Contaminated Field Length (Dry V1/VR)

ASD (low Mu)

Contaminated BFL(Optimum V1/VR)

ASD (dry)

Dry BFL TOD (dry) TOD (15 ft)

Optimum V1/VR

Dry V1/VR

Wet and contaminated runways

V1/VR

Wet and contaminated runways – impact on V1 (Cont’d) Deep contamination – low mu and precipitation drag Contaminated Field Length (Dry V1/VR)

ASD (low Mu)

Contaminated BFL(Optimum V1/VR)

ASD (dry) TOD (15 ft and DCONTAM)

Dry BFL

TOD (dry)

Optimum V1/VR

Dry V1/VR

Wet and contaminated runways

V1/VR 22

Example of Takeoff Performance Shallow Contamination (Low Mu)  2 engine commuter jet • Maximum structural takeoff weight • Sea level, ISA - 15 (0o C) temperature • Zero wind, zero runway slope • No clearway, no stopway

 Runway Conditions • Dry • Shallow Contamination, CRFI = 0.1, 0.2, 0.3, 0.4, 0.5 • CRFI / Mu relationship established by JWRFP

 For contaminated runway, full reverse thrust to stop

Wet and contaminated runways

Example of Takeoff Performance (continued) Shallow Contamination (Low Mu)

Optimum V1/VR V1/VR

BFL (ft)

Dry V1/VR (0.98) Field Length (ft)

Dry Runway

0.98

6340

6340

CRFI = 0.5

0.94

CRFI = 0.4

0.93

CRFI = 0.3

0.91

CRFI = 0.2

0.88

CRFI = 0.1

0.84

6570 (+3.6%) 6710 (+5.8%) 6980 (+10.1%) 7350 (+15.9%) 7810 (+23.2%)

7020 (+10.7%) 7420 (+17.0%) 7970 (+25.7%) 8760 (+38.1%) 10050 (+58.5%)

Wet and contaminated runways

Example of Takeoff Performance (continued) Shallow Contamination (Low Mu)  Example of Takeoff Performance on a runway with shallow contamination shows: • The optimum V1/VR ratio reduces with decreasing CRFI and is much lower than the dry runway balanced V1/VR • BFL increases with decreasing CRFI • If dry runway balanced V1/VR is maintained, takeoff field length is higher than BFL and increases significantly with decreasing CRFI

Wet and contaminated runways

Example of Takeoff Performance Deep Contamination (DCONTAM and Low Mu)  2 engine commuter jet • • • •

Maximum structural takeoff weight Sea level, ISA - 15 (0o C) temperature Zero wind, zero runway slope No clearway, no stopway

 Runway Conditions • Dry • Snow (SG = 0.2), Depth = 0.5, 1.0, 1.5, 2.0, 2.5 inch • DCONTAM/EWD = 2000 lbf/inch (from JWRFP) • Mu = 0.15 (from JWRFP)

 For contaminated runway, full reverse thrust to stop

Wet and contaminated runways

Example of Takeoff Performance (continued) Deep Contamination (DCONTAM and Low Mu) Optimum V1/VR V1/VR

BFL (ft)

Dry V1/VR (0.98) Field Length (ft)

Dry Runway

0.98

6340

6340

Depth = 0.5 inch

0.91

Depth = 1.0 inch

0.91

Depth = 1.5 inch

0.92

Depth = 2.0 inch

0.93

Depth = 2.5 inch

0.94

7320 (+15.4%) 7410 (+16.8%) 7500 (+18.3%) 7600 (+19.8%) 7760 (+22.3%)

8400 (+32.5%) 8410 (+32.6%) 8410 (+32.6%) 8430 (+32.9%) 8440 (+33.1%)

Wet and contaminated runways

Example of Takeoff Performance (continued) Deep Contamination (DCONTAM and Low Mu)  Example of Takeoff Performance on a runway with deep contamination (snow) shows: • The optimum V1/VR ratio increases with increasing depth of snow (but is lower than the dry runway balanced V1/VR) • BFL increases with increasing depth of snow • If dry runway balanced V1/VR is maintained, takeoff field length is higher than BFL value but does not significantly increase with increasing depth of snow

Wet and contaminated runways

For more information about aircraft performance on contaminated runways  See article titled “Slush on the Runway and What it Does to Aircraft Performance” • By Gerard van Es, Senior Research Engineer, Flight Testing & Safety Department, National Aerospace Laboratory NLR, The Netherlands.

 See Powerpoint presentation titled “Aircraft Take-Off Performance on Contaminated Runways” • By J.C.T. Martin, Flight Test Engineer, Transport Canada Aircraft Certification 

www.tc.gc.ca/civilaviation/commerce/operationalstandards/crfi/CRFI.pdf

Wet and contaminated runways