Aircraft Performance Takeoff Speeds PDF

Getting to Grips with Aircraft Performance TAKEOFF C. TAKEOFF 1. INTRODUCTION The possibility of engine failure during takeoff should always be considered, and the crew must be provided with the appropriate means of deciding on t...

Getting to Grips with Aircraft Performance TAKEOFF C. TAKEOFF 1. INTRODUCTION The possibility of engine failure during takeoff should always be considered, and the crew must be provided with the appropriate means of deciding on the safest procedure in the event of such a failure. Ground acceleration Rotation Airborne Acceleration Brake Start of Lift off release Rotation Figure C1: Takeoff Profile During the takeoff phase, the pilot must achieve the sufficient speed and angle of attack conditions to balance the aircraft’s lift and weight forces. At the end of the ground acceleration phase, the pilot pulls the stick to start the rotation. During this phase, acceleration is maintained and the angle of attack is increased in order to achieve a higher lift. The ground reactions progressively decrease until lift off. As mentioned above, the performance determination must take into account the possibility of an engine failure during the ground acceleration phase. For FAR/JAR certified aircraft, failure of the most critical engine must be considered. JAR 1.1 FAR 1.1 “JAR/FAR 1.1 : 'Critical Engine' means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft”, i.e. an outer engine on a four engine aircraft. 43 TAKEOFF Getting to Grips with Aircraft Performance 2. TAKEOFF SPEEDS 2.1. Operational Takeoff Speeds 2.1.1. Engine Failure Speed: VEF JAR 25.107 Subpart B FAR 25.107 Subpart B “JAR/FAR 25.107 (a)(1) VEF is the calibrated airspeed at which the critical engine is assumed to fail. VEF must be selected by the applicant, but may not be less than VMCG.” 2.1.2. Decision Speed: V1 JAR 25.107 Subpart B FAR 25.107 Subpart B V1 is the maximum speed at which the crew can decide to reject the takeoff, and is ensured to stop the aircraft within the limits of the runway. “JAR/FAR 25.107 (a)(2) V1, in terms of calibrated airspeed, is selected by the applicant; however, V1 may not be less than VEF plus the speed gained with the critical engine inoperative during the time interval between the instant at which the critical engine is failed, and the instant at which the pilot recognises and reacts to the engine failure, as indicated by the pilot's initiation of the first action (e.g. applying brakes, reducing thrust, deploying speed brakes) to stop the aeroplane during accelerate-stop tests.” V1 can be selected by the applicant, assuming that an engine failure has occurred at VEF. The time which is considered between the critical engine failure at VEF, and the pilot recognition at V1, is 1 second. Thus: VMCG ≤ VEF ≤ V1 44 Getting to Grips with Aircraft Performance TAKEOFF Stop the Continue the takeoff takeoff V1 The failure is recognized, the pilot being ready for the first braking action. V EF V1 ∆ T= Recognition time = 1s Engine Failure Figure C2: Decision Speed This speed is entered by the crew in the Multipurpose Control and Display Unit (MCDU) during flight preparation, and it is represented by a “1” on the speed scale of the Primary Flight Display (PFD) during takeoff acceleration (See Figure C3). V2 V1 Figure C3: Information provided by the PFD 45 TAKEOFF Getting to Grips with Aircraft Performance 2.1.3. Rotation Speed: VR JAR 25.107 Subpart B FAR 25.107 Subpart B VR is the speed at which the pilot initiates the rotation, at the appropriate rate of about 3° per second. “JAR/FAR 25.107 (e) VR, in terms of calibrated air speed, […] may not be less than: V1, 105% of VMCA The speed that allows reaching V2 before reaching a height of 35 ft above the take-off surface, or A speed that, if the aeroplane is rotated at its maximum practicable rate, will result in a [satisfactory] VLOF” VR is entered in the MCDU by the crew during the flight preparation. VR ≥ 1.05 VMCA 2.1.4. Lift-off Speed: VLOF JAR 25.107 Subpart B FAR 25.107 Subpart B FAR AC 25-7A “JAR/FAR 25.107 (f) VLOF is the calibrated airspeed at which the aeroplane first becomes airborne.” Therefore, it is the speed at which the lift overcomes the weight. “JAR/FAR 25.107 (e) […] VLOF [must] not [be] less than 110% of VMU in the all-engines-operating condition and not less than 105% of VMU determined at the thrust-to-weight ratio corresponding to the one-engine-inoperative condition.” The regulations consider the particular case of aircraft which are geometrically-limited, or limited by the elevator efficiency at high angle of attack. An aircraft is said to be geometrically-limited, when, at its maximum angle of attack (the tail of the aircraft hits the ground while the main landing gear is still on ground), the maximum lift coefficient is not reached. In these conditions, the margins can be reduced, as follows: “JAR 25.107 (only valid for JAR) (e) […] in the particular case that lift-off is limited by the geometry of the aeroplane, or by elevator power, the above margins may be reduced to 108% in the all-engines- operating case and 104% in the one-engine-inoperative condition.” 46 Getting to Grips with Aircraft Performance TAKEOFF “AC 25-7A (only valid for FAR) For airplanes that are geometry limited, the 110 percent of VMU required by § 25.107(e) may be reduced to an operationally acceptable value of 108 percent on the basis that equivalent airworthiness is provided for the geometry-limited airplane.” Airbus aircraft, as most commercial airplanes, are generally geometrically- limited. For those aircraft, certification rules differ between JAR and FAR, as summarized in Table C1: JAR FAR Geometric VLOF ≥ 1.04 VMU (N-1) VLOF ≥ 1.05 VMU (N-1) Limitation VLOF ≥ 1.08 VMU (N) VLOF ≥ 1.08 VMU (N) Aerodynamic VLOF ≥ 1.05 VMU (N-1) Limitation VLOF ≥ 1.10 VMU (N) Table C1: VLOF Limitation 2.1.5. Takeoff Climb Speed: V2 JAR 25.107 Subpart B FAR 25.107 Subpart B V2 is the minimum climb speed that must be reached at a height of 35 feet above the runway surface, in case of an engine failure. “JAR/FAR 25.107 (b) V2min, in terms of calibrated airspeed, may not be less than: 1.13 VSR1 (JAR) or 1.2 VS (FAR) for turbo-jet powered aeroplanes […] 1.10 times VMCA (c) V2, in terms of calibrated airspeed, must be selected by the applicant to provide at least the gradient of climb required by JAR 25.121(b) but may not be less than: V2min; and VR plus the speed increment attained before reaching a height of 35 ft above the take-off surface.” This speed must be entered by the crew during flight preparation, and is represented by a magenta triangle on the speed scale (see Figure C3). V2 ≥ 1.1 VMCA V2 ≥ 1.13 Vs1g (Airbus Fly-By-Wire aircraft)2 V2 ≥ 1.2 Vs (Other Airbus types) 1 VSR is the 1-g stall speed VS1g (refer to the “Aircraft limitations” chapter). 2 Airbus FBW aircraft are FAA approved, under special condition, with the 1-g reference stall speed. 47 TAKEOFF Getting to Grips with Aircraft Performance 2.2. Takeoff Speed Limits 2.2.1. Maximum Brake Energy Speed: VMBE When the takeoff is aborted, brakes must absorb and dissipate the heat corresponding to the aircraft’s kinetic energy at the decision point (1/2.TOW.V12). JAR 25.109 Subpart B FAR 25.109 Subpart B “JAR/FAR 25.109 (h) A flight test demonstration of the maximum brake kinetic energy accelerate-stop distance must be conducted with no more than 10% of the allowable brake wear range remaining on each of the aeroplane wheel brakes.” Brakes have a maximum absorption capacity, known as maximum brake energy. For certification purposes, this absorption capacity must be demonstrated with worn brakes (post-amendment 42 only). As a result, the speed at which a full stop can be achieved for a given takeoff weight is limited to a maximum value (VMBE). Thus, for a given takeoff weight: V1 ≤ VMBE 2.2.2. Maximum Tire Speed: VTIRE The tire manufacturer specifies the maximum ground speed that can be reached, in order to limit the centrifugal forces and the heat elevation that may damage the tire structure. Thus: VLOF ≤ VTIRE For almost all Airbus aircraft models, VTIRE is equal to 195 knots (Ground Speed). 2.3. Speed Summary The following Figure illustrates the relationships and the regulatory margins between the certified speeds (VS1G, VMCG, VMCA, VMU, VMBE, VTIRE), and the takeoff operating speeds (V1, VR, VLOF, V2). 48 Getting to Grips with Aircraft Performance TAKEOFF 1.13 V S1g V2 1.1 VMCA 1.05 VMCA VR VMCG VMBE V1 VEF 35 ft 1.08 V MU (N) VLOF VTIRE 1.04 or 1.05 V MU (N-1) Figure C4: Takeoff Speed Summary and Limitations related to V1,VR,VLOF and V2 3. RUNWAY LIMITATIONS 3.1. Takeoff Distances 3.1.1. Regulatory Background The different Airbus types have been certified at different times and comply with different certification rules. A major change occurred when the FAA published an amendment to FAR Part 25, known as “Amendment 25-42”. This amendment, which became effective on March 1, 1978, revised the takeoff performance standards and made them more restrictive. To summarize, Amendment 25-42 required the accelerate-stop distance to include two seconds of continued acceleration beyond V1 speed, before the pilot takes any action to stop the airplane. It also introduced the notion of Accelerate-Stop Distance all engines. This revision resulted in longer accelerate-stop distances for airplanes whose application for a type certificate was made after amendment 25-42 became effective. The A320 was the first airplane to be certified under this rule, as no retroactivity was required. It was also the last one. Although the airplane types were originally certified at different times, thus allowing the use of different amendments, both groups of airplanes were continuing in production and competing for sales and for use over some common routes. Airplanes whose designs were type-certified to the standards introduced by Amendment 25-42 were penalized in terms of payload, even though the airplane’s takeoff performance 49 TAKEOFF Getting to Grips with Aircraft Performance might be better from a safety perspective, than the design of a competing airplane that was not required to meet the latest standards. This disparity in airworthiness standards has created an unfair international trade situation, affecting the competitiveness of a later design of the A320. At the June 1990 annual meeting, the FAA and JAA agreed to jointly review the current takeoff performance standards to reduce the above-discussed inequities, without adversely affecting safety. In March 1992, the JAA Notice for Proposed Amendment (NPA) 25B,D,G-244: “Accelerate-Stop Distances and Related Performance Matters” was issued, followed by the FAA Notice of Proposed Rule Making (NPRM) 93-8 on July 1993. The rule changes proposed in the NPA and in the NPRM were essentially the same, and are better known as Post-Amendment 42. To summarize, NPA 244 and NPRM 93-08 (post-amendment 42) proposed the following rule changes: 1 – Replace the two seconds of continued acceleration beyond V1, with a distance margin equal to two seconds at V1 speed 2 – Require that the runway surface condition (dry or wet) be taken into account, when determining the runway length that must be available for takeoff. 3 – Require that the capability of the brakes to absorb energy and stop the airplane during landings and rejected takeoffs be based on brakes that are worn to their overhaul limit. After industry feedback, NPA 244 was incorporated into JAR 25 on October 2000 (change 15), whereas NPRM 93-08 was incorporated into FAR 25 on February 1998 (amendment 25-92). The definitions provided in the following sections refer to the latest airworthiness standards (i.e. post amendment 42). As a reminder, the certification status of the Airbus models is the following: Pre-amendment 42 : A300, A300-600, A310 Amendment 25-42 : A3201 Post-amendment 42 : A318, A319, A3201, A321, A330, A340 3.1.2. Takeoff Distance (TOD) JAR 25.113 Subpart B FAR 25.113 Subpart B For given operational conditions (temperature, pressure altitude, weight, etc.): a) The takeoff distance on a dry runway is the greater of the following values: TODN-1 dry = Distance covered from the brake release to a point at which the aircraft is at 35 feet above the takeoff surface, assuming the failure of the critical engine at VEF and recognized at V1, 1.15 TODN dry = 115% of the distance covered from brake release to a point at which the aircraft is at 35 feet above the takeoff surface, assuming all engines operating. 1 Some A320s are certified with amendment 25-42, others with post-amendment 42. 50 Getting to Grips with Aircraft Performance TAKEOFF TODdry = max of {TODN-1 dry, 1.15 TODN dry} b) The takeoff distance on a wet runway is the greater of the following values: TODdry = Takeoff distance on a dry runway (see above), TODN-1 wet = Distance covered from brake release to a point at which the aircraft is at 15 feet above the takeoff surface, ensuring the V2 speed to be achieved before the airplane is 35 feet above the takeoff surface, assuming failure of the critical engine at VEF and recognized at V1. TODwet = max of {TODdry, TODN-1 wet} V2 WITH CRITICAL ENGINE FAILURE TOGAN TOGA N-1 35 ft V=0 VEF V1 VR VLOF 15 ft TOD N-1 (wet) TOD N-1 (dry) V2 WITHOUT ENGINE FAILURE TOGAN V=0 VR VLOF 35 ft TOD N (dry) 1.15 TOD N (dry) Figure C5: Takeoff Distance (TOD) 51 TAKEOFF Getting to Grips with Aircraft Performance 3.1.3. Takeoff Run (TOR) JAR 25.113 Subpart B FAR 25.113 Subpart B 3.1.3.1. Runway with Clearway a) The takeoff run on a dry runway is the greater of the following values (Figure C6): TORN-1 dry = Distance covered from brake release to a point equidistant between the point at which VLOF is reached and the point at which the aircraft is 35 feet above the takeoff surface, assuming failure of the critical engine at VEF and recognized at V1, 1.15 TORN dry = 115 % of the distance covered from brake release to a point equidistant between the point at which VLOF is reached and the point at which the aircraft is 35 feet above the takeoff surface, assuming all engines operating. TORdry = max of {TORN-1 dry, 1.15 TORN dry} b) The takeoff run on a wet runway is the greater of the following values: TORN-1 wet = Distance covered from the brake release to a point at which the aircraft is at 15ft above the takeoff surface, ensuring the V2 speed to be achieved before the airplane is 35 feet above the takeoff surface, assuming the failure of the critical engine at VEF and recognized at V1. It is equal to TODN-1 wet. 1.15 TORN wet = 115 % of the distance covered from brake release to a point equidistant between the point at which VLOF is reached and the point at which the aircraft is 35 feet above the takeoff surface, assuming all engines operating. TORwet = max of {TORN-1 wet, 1.15 TORN wet} V2 WITH CRITICAL ENGINE FAILURE TOGA N TOGA N-1 V=0 VEF V1 VR VLOF 35 ft 15 ft TOR N-1 (dry) TOR N-1 (wet) 52 Getting to Grips with Aircraft Performance TAKEOFF WITHOUT ENGINE FAILURE V2 TOGA N V=0 VR VLOF 35 ft TOR N (dry or wet) 1.15 TOR N (dry or wet) Figure C6: Takeoff Run (TOR) with a Clearway 3.1.3.2. Runway without Clearway The takeoff run is equal to the takeoff distance, whatever the takeoff surface (dry or wet). 3.1.3.3. Clearway Influence on a Wet Runway With a wet runway, the takeoff run with one engine-out is always equal to the takeoff distance with one engine-out (i.e. from brake release to 15 feet). Therefore, a clearway does not give any performance benefit on a wet runway, as the TOR is always more limiting (TORA less than TODA). 3.1.4. Accelerate-Stop Distance (ASD) JAR 25.109 Subpart B FAR 25.109 Subpart B a) The accelerate-stop distance on a dry runway is the greater of the following values: ASDN-1 dry = Sum of the distances necessary to: - Accelerate the airplane with all engines operating to VEF, - Accelerate from VEF to V11 assuming the critical engine fails at VEF and the pilot takes the first action to reject the takeoff at V1 - Come to a full stop23 - Plus a distance equivalent to 2 seconds at constant4 V1 speed 1 Delay between VEF and V1 = 1 second 2 ASD must be established with the “wheel brakes at the fully worn limit of their allowable wear range” [JAR/FAR 25.101] 3 ASD shall not be determined with reverse thrust on a dry runway 4 Pre-amendment 42 : no additional distance Amendment 25-42 : 2 seconds of continuing acceleration after V1 53 TAKEOFF Getting to Grips with Aircraft Performance ASDN dry = Sum of the distances necessary to: - Accelerate the airplane with all engines operating to V1, assuming the pilot takes the first action to reject the takeoff at V1 - With all engines still operating come to a full stop - Plus a distance equivalent to 2 seconds at constant V1 speed ASDdry = max of {ASDN-1 dry, ASDN dry} b) The accelerate-stop distance on a wet runway is the greater of the following values: ASDdry ASDN-1 wet = same definition as ASDN-1 dry except the runway is wet1 ASDN wet = same definition as ASDN dry except the runway is wet ASDwet = max of {ASDdry, ASDN-1 wet, ASDN wet} WITH CRITICAL ENGINE FAILURE TOGAN TOGAN-1 Thrust Reduction + Brakes Application V=0 VEF V 1 V1 V=0 1s 2s ASD N-1 (dry or wet) WITHOUT ENGINE FAILURE Thrust Reduction TOGA N + Brakes Application V=0 V1 V1 V=0 2s ASD N (dry or wet) Figure C7: Accelerate Stop Distance (ASD) 1 ASD determination on a wet runway may include the use of the reverse thrust provided it is safe and reliable [JAR/FAR 25-109 (e)(f)] 54 Getting to Grips with Aircraft Performance TAKEOFF 3.1.5. Influence of V1 on Accelerate-Go/Stop Distances For a given takeoff weight, any increase in V1 leads to a reduction in both TODN-1 and TORN-1. The reason is that the all engine acceleration phase is longer with a higher V1 speed, and, consequently, in case of an engine failure occurring at VEF, the same V2 speed can be achieved at 35 feet at a shorter distance. On the other hand, TODN and TORN are independent of V1 as there is no engine failure, and thus no consequence on the acceleration phase and the necessary distance to reach 35 feet. On the contrary, for a given takeoff weight, any increase in V1 leads to an increase in both the ASDN-1 and ASDN. Indeed, with a higher V1 speed, the acceleration segment from brake release to V1 is longer, the deceleration segment from V1 to the complete stop is longer, and the 2 second segment at constant V1 speed is longer. As a result, the following graph providing the takeoff/rejected takeoff distances as a function of V1 can be plotted. This graph clearly shows that a minimum distance is achieved at a particular V1 speed. This speed is called “balanced V1”, and the corresponding distance is called “balanced field”. ASD TOD N TOR N TOD N-1 TOR N-1 Figure C8: Influence of V1 on Accelerate-go/stop Distances for a Given Weight 55 TAKEOFF Getting to Grips with Aircraft Performance 3.2. Available Takeoff Lengths 3.2.1. Takeoff Run Available (TORA) JAR-OPS 1.480 Subpart F “JAR-OPS 1.480 (a)(9) TakeOff Run Available (TORA): The length of runway which is declared available by the appropriate authority and suitable for the ground run of an aeroplane taking off.” TORA is either equal to the runway length, or to the distance from the runway entry point (intersecting taxiway) to the end of the runway (Figure C9). RWY = TORA Figure C9: Definition of TORA JAR-OPS 1.490 Subpart G FAR 121.189 (c)(3) Subpart I “JAR-OPS 1.490 (b)(3) The Takeoff run must not exceed the takeoff run available.” TOR ≤ TORA 3.2.2. Takeoff Distance Available (TODA) JAR 1.1 General definitions FAR 1.1 General definitions The runway may be extended by an area called the clearway. The clearway is an area beyond the runway, which should have the following characteristics: It must: Be centrally located about the extended centerline of the runway, and under the control of the airport authorities. Be expressed in terms of a clearway plane, extending from the end of the runway with an upward slope not exceeding 1.25%. Have a minimum width not less than 152 m (500 feet) wide. Have no protruding objects or terrain. Threshold lights may protrude above the plane, if their height above the end of the runway is 0.66 m (26 in) or less, and if they are located on each side of the runway. 56 Getting to Grips with Aircraft Performance TAKEOFF JAR-OPS 1.480 Subpart F “JAR-OPS 1.480 (a)(7) Takeoff Distance Available (TODA): The length of the takeoff run available plus the length of the clearway available.” As shown in Figure C10, the Takeoff Distance Available (TODA) corresponds to the Takeoff Run Available (TORA) plus the clearway (CWY), if any. 66 cm Obstacle (26 in max) 152 m (500 ft min) CWY TORA 1.25% max 1/2 TORA (max) TODA Figure C10: TODA Definition JAR-OPS 1.490 Subpart G FAR 121.189 (c)(2) Subpart I “JAR-OPS 1.490 (b)(2) The Takeoff distance must not exceed the takeoff distance available, with a clearway distance not exceeding half of the takeoff run available.” TOD ≤ TODA 3.2.3. Accelerate-Stop Distance Available (ASDA) JAR 1.1 General Definitions FAR 1.1 General Definitions The runway may be extended by an area called the stopway. The stopway is an area beyond the runway, which should have the following characteristics. It must be : At least as wide as the runway, and centered upon the extended centerline of the runway. Able to support the airplane during an abortive takeoff, without causing structural damage to the airplane. Designated by the airport authorities for use in decelerating the airplane during an abortive takeoff. 57 TAKEOFF Getting to Grips with Aircraft Performance JAR-OPS 1.480 Subpart F “JAR-OPS 1.480 (a)(1) Accelerate-Stop Distance Available (ASDA): The length of the takeoff run available plus the length of the stopway, if such stopway is declared available by the appropriate Authority and is capable of bearing the mass of the aeroplane under the prevailing operating conditions.” RWY = TORA SWY ASDA Figure C11: ASDA Definition JAR-OPS 1.490 Subpart G FAR 121.189 (c)(1) Subpart I “JAR-OPS 1.490 (b)(1) The accelerate-stop distance must not exceed the accelerate-stop distance available.” ASD ≤ ASDA 3.2.4. Loss of Runway Length due to Alignment Airplanes typically enter the takeoff runway from an intersecting taxiway. The airplane must be turned so that it is pointed down the runway in the direction for takeoff. FAA regulations do not explicitly require airplane operators to take into account the runway distance used to align the airplane on the runway for takeoff. On the contrary, JAA regulations require such a distance to be considered: JAR-OPS 1.490 Subpart G IEM OPS 1.490 “JAR-OPS 1.490 (c)(6) […] an operator must take account of the loss, if any, of runway length due to alignment of the aeroplane prior to takeoff.” Lineup corrections should be made when computing takeoff performance, anytime runway access does not permit positioning the airplane at the threshold. The takeoff distance / takeoff run (TOD / TOR) adjustment is made, based on the initial distance from the beginning of the runway to the main gear, since the screen height is measured from the main gear, as indicated by distance "A" in Figure 58 Getting to Grips with Aircraft Performance TAKEOFF C12. The accelerate-stop distance (ASD) adjustment is based on the initial distance from the beginning of the runway to the nose gear, as indicated by distance "B" in Figure C12. Figure C12: Lineup Corrections Runways with displaced takeoff thresholds, or ample turning aprons, should not need further adjustment. Accountability is usually required for a 90° taxiway entry to the runway and a 180° turnaround on the runway. The following tables (C2 and C3) contain the minimum lineup distance adjustments for both the accelerate-go (TOD/TOR) and accelerate-stop (ASD) cases that result from a 90° turn onto the runway and a 180° turn maneuver on the runway. For further details, refer to the Airbus Performance Program Manual (PPM). 3.2.4.1. 90 Degree Runway Entry 90 Degree Runway Entry Aircraft Model Maximum Minimum Line up Effective Distance Correction Steering Angle TODA (m) ASDA (m) A300 all models 58.3° 21.5 40.2 A310 all models 56° 20.4 35.9 A320 all models 75° 10.9 23.6 A319 all models 70° 11.5 22.6 A321 all models 75° 12.0 28.9 A330-200 (Mod 47500) 62° 22.5 44.7 A330-200 (Mod 46810) 55.9° 25.8 48.0 A330-300 (Mod 47500) 65° 22.9 48.3 A330-300 (Mod 46863) 60.5° 25.1 50.5 A340-200 (Mod 47500) 62° 23.3 46.5 A340-200 (Mod 46863) 59.6° 24.6 47.8 A340-300 (Mod 47500) 62° 24.4 50 A340-300 (Mod 46863) 60.6° 25.2 50.8 A340-500 65° 23.6 51.6 A340-600 67° 24.6 57.8 Table C2: 90° Lineup Distances 59 TAKEOFF Getting to Grips with Aircraft Performance Figure C13: 90° taxiway 3.2.4.2. 180 Degree Turnaround 180 Degree Turnaround Required Minimum Nominal Line up Aircraft Model Minimum Line up Distance on a 60 m Runway Distance correction * runway width ** width TODA (m) ASDA (m) (m) TODA (m) ASDA (m) A300 all models 26.5 45.2 66.1 38.0 56.7 A310 all models 23.3 38.8 61.6 29.0 44.5 A320 all models 16.5 29.1 28.7 16.5 29.1 A319 all models 15.1 26.2 31.1 15.1 26.2 A321 all models 20.9 37.8 33.1 20.9 37.8 A330-200 (Mod 47500) 30.1 52.3 68.2 43.3 65.5 A330-200 (Mod 46810) 31.9 54.1 81.6 55.0 77.1 A330-300 (Mod 47500) 33.2 58.5 70.0 47.9 73.3 A330-300 (Mod 46683) 34.2 59.6 78.8 55.4 80.8 A340-200 (Mod 47500) 31.5 54.8 71.4 47.4 70.6 A340-200 (Mod 46683) 32.2 55.4 76.6 51.8 75.1 A340-300 (Mod 47500) 34.1 59.7 76.0 53.3 78.9 A340-300 (Mod 46683) 34.4 60.0 79.2 55.9 81.5 A340-500 35.9 63.9 72.8 52.8 80.8 A340-600 41.1 74.3 76.6 60.7 93.9 Table C3: 180° Lineup Distances * Lineup distance required to turn 180 degrees at maximum effective steering angle and end aligned with the centerline of the pavement. The indicated minimum runway width is required (Figure C14, left hand side). ** Lineup distance required to turn 180 degrees and realign the airplane on the runway centerline on a 60 m wide runway (Figure C14, right hand side). 60 Getting to Grips with Aircraft Performance TAKEOFF Figure C14: 180° Turnaround 3.2.5. Influence of V1 on the Runway-Limited Takeoff Weight Considering the runway requirements (TOR≤TORA, TOD≤TODA, and ASD≤ASDA), a Maximum Takeoff Weight (MTOW) can be obtained for each runway limitation. As an example, when for a given takeoff weight the TOD is equal to the TODA, this takeoff weight is maximum regarding the Takeoff Distance limitation. As previously seen, for a given takeoff weight, any increase of V1 leads to shortening the TODN-1 and TORN-1, and increasing the ASD, but has no influence on TODN and TORN. Therefore, for a given runway (i.e. given TORA, TODA and ASDA), any increase in V1 leads to an increase in the MTOW TOD(N-1) and MTOW TOR(N-1), and to a reduction in MTOW ASD, but has no influence on MTOW TOD(N) and MTOW TOR(N). The following graph (Figure C15) provides the runway-limited accelerate- go/stop takeoff weights as a function of V1. This graph clearly shows that a maximum takeoff weight is achieved in a particular range of V1. TOR N-1 TOD N-1 TOR N TOD N ASD Figure C15: Runway-Limited Takeoff Weight 61 TAKEOFF Getting to Grips with Aircraft Performance 4. CLIMB AND OBSTACLE LIMITATIONS 4.1. Takeoff Flight Path 4.1.1. Definitions JAR 25.111 Subpart B FAR 25.111 Subpart B JAR 25.115 Subpart B FAR 25.115 Subpart B “JAR/FAR 25.111 (a) The takeoff path extends from a standing start to a point at which the aeroplane is at a height: Of 1500 ft above the takeoff surface, or At which the transition from the takeoff to the en-route configuration1 is completed and the final takeoff speed2 is reached, whichever point is higher”. “JAR/FAR 25.115 (a) The takeoff flight path begins 35 ft above the takeoff surface at the end of the takeoff distance.” The takeoff path and takeoff flight path regulatory definitions assume that the aircraft is accelerated on the ground to VEF, at which point the critical engine is made inoperative and remains inoperative for the rest of the takeoff. Moreover, the V2 speed must be reached before the aircraft is 35 feet above the takeoff surface, and the aircraft must continue at a speed not less than V2, until it is 400 feet above the takeoff surface. 4.1.2. Takeoff Segments and Climb Requirements JAR 25.121 Subpart B FAR 25.121 Subpart B The takeoff flight path can be divided into several segments. Each segment is characteristic of a distinct change in configuration, thrust, and speed. Moreover, the configuration, weight, and thrust of the aircraft must correspond to the most critical condition prevailing in the segment. Finally, the flight path must be based on the aircraft’s performance without ground effect. As a general rule, the aircraft is considered to be out of the ground effect, when it reaches a height equal to its wing span. 1 En route configuration: Clean configuration, Maximum Continuous Thrust (MCT) setting. 2 Final takeoff speed: Speed greater than 1.25 Vs, chosen equal to Green Dot speed (best climb gradient speed) 62 Getting to Grips with Aircraft Performance TAKEOFF T.O distance Takeoff Flight Path Climb First Second Third Final 1,500 ft seg. segment segment segment 400 ft mini Gross flight 10 min max path gear retracted 35 ft BR VEF V1 VR VLOF V2 acceleration green dot N N-1 engines TOGA MCT slats / flaps take-off configuration slats / flaps retraction clean configuration Figure C16: Takeoff Path and Definition of Various Segments After an engine failure at VEF, whatever the operational conditions, the aircraft must fulfill minimum climb gradients, as required by JAR/FAR 25.121. The following Table (C4) summarizes the different requirements and aircraft status during the four takeoff segments : Minimum required climb gradient one- engine inoperative, flaps / slats configuration, engine rating, speed reference, landing gear configuration… 63 TAKEOFF Getting to Grips with Aircraft Performance FIRST SEGMENT SECOND THIRD SEGMENT FINAL SEGMENT SEGMENT Minimum climb Twin 0.0% 2.4% - 1.2% gradient (N-1) Quad 0.5% 3.0% - 1.7% engines Start when VLOF reached Gear fully Acceleration En route retracted height reached configuration (min 400 feet) Achieved Slats / Flaps Takeoff Takeoff Slats / Flaps Clean Configuration retraction Engine rating TOGA/FLEX TOGA/FLEX TOGA/FLEX MCT Speed reference VLOF V2 Acceleration Green Dot from V2 to Green Dot Landing gear Retraction Retracted Retracted Retracted Weight reference Weight at the Weight when Weight at the Weight at the start of the the gear is fully start of the end of the gear retraction retracted acceleration acceleration segment segment Ground effect Without Without Without Without Table C4: Takeoff Segment Characteristics 4.1.3. Minimum and Maximum Acceleration Heights 4.1.3.1. Minimum Acceleration Height JAR 25.111 Subpart B FAR 25.111 Subpart B 64 Getting to Grips with Aircraft Performance TAKEOFF “JAR/FAR 25.111 (c)(2) The aeroplane must reach V2 before it is 35 ft above the takeoff surface and must continue at a speed not less than V2 until it is 400 ft above the takeoff surface” “JAR/FAR 25.111 (c)(3) At each point along the takeoff flight path, starting at the point at which the aeroplane reaches 400 ft above the takeoff surface, the available gradient of climb may not be less than: 1.2% for a two-engined airplane 1.7% for a four-engined airplane” So, below 400 feet, the speed must be maintained constant to a minimum of V2. Above 400 feet, the aircraft must fulfill a minimum climb gradient, which can be transformed into an acceleration capability in level flight. Therefore, the regulatory minimum acceleration height is fixed to 400 feet above the takeoff surface. Nevertheless, during the acceleration segment, obstacle clearance must be ensured at any moment. Therefore, the operational minimum acceleration height is equal to or greater than 400 feet (Figure C16). 4.1.3.2. Maximum Acceleration Height The Maximum Takeoff Thrust (TOGA) is certified for use for a maximum of 10 minutes, in case of an engine failure at takeoff, and for a maximum of 5 minutes with all engines operating. The Maximum Continuous Thrust (MCT), which is not time-limited, can only be selected once the enroute configuration is achieved (i.e. when the aircraft is in clean configuration at green dot speed). As a result, the enroute configuration (end of the third segment) must be achieved within a maximum of 10 minutes after takeoff, thus enabling the determination of a maximum acceleration height (Figure C16). 4.1.4. Takeoff Turn Procedure Some airports are located in an environment of penalizing obstacles, which may necessitate turning to follow a specific departure procedure. Turning departures are subject to specific conditions. The turn conditions differ between JAR and FAR regulations. Thus, the following paragraphs deal separately with both requirements. JAR-OPS 1.495 Subpart G “JAR-OPS 1.495 (c)(1) Track changes shall not be allowed up to the point at which the net take-off flight path has achieved a height equal to one half the wingspan but not less than 50 ft above the elevation of the end of the take-off run available.” 65 TAKEOFF Getting to Grips with Aircraft Performance Minimum height above end of AIRCRAFT TYPE WINGSPAN TORA to start a track change = Max {Half of Wingspan , 50 ft} A300-B2/B4/600 44.84 m (147 ft 1 in) Half of wingspan = 74 ft A310-200/300 43.90 m (144 ft 1 in) Half of wingspan = 73 ft A318/A319/A320/A321 34.10 m (111 ft 10 in) Half of wingspan = 56 ft A330-200/300 60.30 m (197 ft 10 in) Half of wingspan = 99 ft A340-200/300 60.30 m (197 ft 10 in) Half of wingspan = 99 ft A340-500/600 63.50 m (208 ft 2 in) Half of wingspan = 105 ft Table C5: Minimum Height to Initiate a Track Change “JAR-OPS 1.495 (c)(1) Thereafter, up to a height of 400 ft it is assumed that the aeroplane is banked by no more than 15°. Above 400 ft height bank angles greater than 15°, but not more than 25° may be scheduled.” (see table C6) “JAR-OPS 1.495 (c)(3) An operator must use special procedures, subject to the approval of the Authority, to apply increased bank angles of not more than 20º between 200 ft and 400 ft, or not more than 30º above 400 ft” Maximum Bank angle during a turn (JAR) Standard Specific procedure approval Below 200 ft 15° 15° Between 200 ft 15° 20° and 400 ft Above 400 ft 25° 30° Table C6: Maximum Bank Angle During a Turn FAR 121.189 Subpart I “FAR 121.189 (f) For the purpose of this section, it is assumed that the airplane is not banked before reaching a height of 50 ft, […] and thereafter that the maximum bank is not more than 15 degrees1.” 1 The FAA rule is similar to the ICAO annex 6 recommendations. 66 Getting to Grips with Aircraft Performance TAKEOFF 4.2. Obstacle Clearance 4.2.1. Gross and Net Takeoff Flight Paths Most of the time, runways have surrounding obstacles which must be taken into account prior to takeoff, to ascertain that the aircraft is able to clear them. A vertical margin has to be considered between the aircraft and each obstacle in the takeoff flight path. This margin, based on a climb gradient reduction, leads to the definitions of the Gross Takeoff Flight Path and the Net takeoff flight Path. JAR 25.115 Subpart B FAR 25.115 Subpart B GROSS Flight Path = Takeoff flight path actually flown by the aircraft, i.e.: “JAR/FAR 25.115 (a) […] from 35 ft above the takeoff surface at the end of the takeoff distance [to the end of the takeoff path]” NET Flight Path = Gross takeoff flight path minus a mandatory reduction. “JAR/FAR 25.115 (b) The net takeoff flight path data must be determined so that they represent the actual [Gross] takeoff flight path reduced at each point by a gradient equal to: 0.8% for two-engine aeroplanes 1.0% for four-engine aeroplanes” Net Gradient = Gross Gradient - Gradient Penalty Gradient Penalty Two-engine aircraft 0.8% Four-engine aircraft 1.0% Table C7: Values of Gradient Penalties The gradient penalty between the net and the gross flight path must be taken into account during the first, second, and final takeoff segments (Figure C17). 67 TAKEOFF Getting to Grips with Aircraft Performance T.O distance Takeoff Flight Path Climb First Second Third Final 1,500 ft seg. segment segment segment gradient Gross flight path reduction 35 ft Net flight path 35 ft 35 ft BR VEF V1 VR VLOF V2 acceleration green dot N N-1 engines Figure C17: Gross and Net Takeoff Flight Paths 4.2.2. Obstacle Clearance during a Straight Takeoff JAR-OPS 1.495 Subpart G FAR 121.189 (d)(2) Subpart I “JAR–OPS 1.495 (a) An operator shall ensure that the net take-off flight path clears all obstacles by a vertical distance of at least 35 ft.” As an example, the minimum required climb gradient during the second segment must be 2.4% for a two-engine aircraft. But, as per regulation, the net flight path must clear any obstacle by at least 35 feet (Figure C17). This may sometimes require the second segment gradient to be greater than 2.4% and, consequently, the Maximum Takeoff Weight may have to be reduced accordingly. This is a case of obstacle limitation. 4.2.3. Obstacle Clearance during a Turn Once again, the obstacle clearance margins during a turn differ between JAR and FAR regulations. The FAR regulation doesn’t consider any additional vertical margin during a turn, as the bank angle is limited to 15º. The following rule is then purely JAR-OPS: JAR-OPS 1.495 Subpart G “JAR-OPS 1.495 (c)(2) Any part of the net take-off flight path in which the aeroplane is banked by more than 15° must clear all obstacles […] by a vertical distance of at least 50 ft.” 68 Getting to Grips with Aircraft Performance TAKEOFF Obstacle clearance margin Bank angle ≤ 15° 35 ft Bank angle > 15° 50 ft Table C8: Minimum Vertical Clearance Between the Net Flight Path and the Obstacles 4.2.4. Loss of Gradient during a Turn During a turn, an aircraft is not only subjected to its weight (W), but also to a horizontal acceleration force (Fa). The resulting force is called “apparent weight” (W a), and its magnitude is equal to the load factor times the weight (nz.W). Fa Φ Wa = nz.m.g W=mg Figure C18: Load Factor in Turn Considering the above Figure C18, the load factor (nz) can be expressed versus the bank angle (Φ) as follows: 1 nz = cosφ So, as soon as the aircraft is banked, the load factor becomes greater than one. This induces a loss of climb gradient, as the climb angle can be expressed as follows (refer to the “Climb” chapter) : Thrust 1 γ% = − n z.Weight L/D AMC-OPS 1.495 “AMC OPS 1.495 (c)(4) The Aeroplane Flight Manual generally provides a climb gradient decrement for a 15° bank turn. For bank angles of less than 15°, a proportionate amount should be 69 TAKEOFF Getting to Grips with Aircraft Performance applied, unless the manufacturer or Aeroplane Flight Manual has provided other data.” The loss of gradient versus the bank angle is provided in the Airbus Flight Manual (AFM), as well as in the Airbus Performance Program Manual (PPM) as shown in Figure C19. Figure C19: Loss of Gradient versus Bank Angle (A320 family example) On airbus fly-by-wire aircraft, the autopilot limits the bank angle at takeoff with one engine inoperative to 15°. Some Engine Out Standard Instrument Departures (EOSID) require a turn to be performed with a bank angle of 20° or more. When a turn with more than a 15° bank angle must be carried out, the aircraft must be manually flown. 4.2.5. Takeoff Flight Path with Obstacles Once the obstacles are taken into account, the maximum takeoff weight at brake release must be calculated so that the net flight path clears the most penalizing obstacle with a vertical margin of 35 feet (or 50 feet when the bank angle is greater than 15°). 70 Getting to Grips with Aircraft Performance TAKEOFF t.o. dist. Take Off Flight Path climb gross f.p. 35 ft min net F.P. B obst. envelope 35 ft A Segments: 1 2 3 final Figure C20: Takeoff Flight Path with Obstacles Obstacle A (Figure C20), imposes a minimum Net second segment gradient and, therefore, a minimum Gross second segment gradient. This results in a takeoff weight limitation. Obstacle B helps determine the minimum acceleration height. This height must be between 400 feet and the maximum acceleration height (10 minutes at TOGA). The minimum acceleration height ensures a vertical clearance of 35 feet (or 50 feet) between the net flight path and the obstacle. The net acceleration segment is longer than the gross one, as the end of both segments is assumed to be reached after the same flight time. 4.2.6. Takeoff Funnel The takeoff funnel represents an area surrounding the takeoff flight path, within which all obstacles must be cleared, assuming they are all projected on the intended track. The contours of this area, also called departure sector, differ between the JAR and the FAR regulations, and will be dealt with separately in the following section. JAR-OPS 1.495 Subpart G AMC-OPS 1.495 “JAR-OPS 1.495 (a) An operator shall ensure that the net take-off flight path clears all obstacles […] by a horizontal distance of at least 90 m plus 0.125 x D, where D is the horizontal distance the aeroplane has traveled from the end of the take-off distance available or the end of the take-off distance if a turn is scheduled before the end of the take-off distance available. For aeroplanes with a wingspan of less than 60 m a horizontal obstacle clearance of half the aeroplane wingspan plus 60 m plus 0.125 x D may be used.” 71 TAKEOFF Getting to Grips with Aircraft Performance The semi-width at the start of the departure sector is a function of the aircraft’s wingspan. The following Table (C9) provides the values for each aircraft type: AIRCRAFT TYPE WINGSPAN Semi-width at the start of the departure sector (1/2 E0) A300-B2/B4/600 44.84 m (147 ft 1 in) 83 m (271 ft) A310-200/300 43.90 m (144 ft 1 in) 82 m (269 ft) A318/A319/A320/A321 34.10 m (111 ft 10 in) 78 m (253 ft) A330-200/300 60.30 m (197 ft 10 in) 90 m (296 ft) A340-200/300 60.30 m (197 ft 10 in) 90 m (296 ft) A340-500/600 63.50 m (208 ft 2 in) 90 m (296 ft) Table C9: JAR-OPS Semi-Width at the Start of the Departure Sector “JAR-OPS 1.495 (d) For those cases where the intended flight path does not require track changes of more than 15°, an operator need not consider those obstacles which have a lateral distance greater than: 300 m, if the pilot is able to maintain the required navigational accuracy through the obstacle accountability area, or 600 m, for flights under all other conditions.” “JAR-OPS 1.495 (e) For those cases where the intended flight path does require track changes of more than 15°, an operator need not consider those obstacles which have a lateral distance greater than: 600 m, if the pilot is able to maintain the required navigational accuracy through the obstacle accountability area, or 900 m for flights under all other conditions.” The Required Navigational Accuracy is defined in AMC-OPS 1.495. It can either be obtained via navigation aids, or by using external references in case of Visual Course guidance (VMC day flights). The following Figures C21 and C22 represent the JAR-OPS departure sectors: 72 Getting to Grips with Aircraft Performance TAKEOFF Start of the Start of the takeoff departure 12.5% (7.1º) flight path sector 1/2E 2 3 1/2E0 1 cwy D 1 TOD 2 3 TORA TODA Figure C21: JAR-OPS Departure Sector (Track change ≤ 15º) Start of : Start of. takeoff flight path turn. departure sector* 12.5% (7.1º) 1/2E 1/2E0 cwy 2 1 D TOD 3 TORA 1 TODA 2 3 * The start of the departure sector is: - The end of TOD when the turn starts before the end of TODA, or - The end of TODA when the turn starts after the end of TODA Figure C22: JAR-OPS Departure Sector (Track change > 15º) Note that the ICAO recommendations for the departure sector (Annex 6) are the same as the JAR-OPS definitions. FAR 121.189 Subpart I “FAR 121.189 (d)(2) No person operating a turbine engine powered transport category airplane may take off that airplane at a weight greater than that listed in the Airplane Flight Manual […] that allows a net takeoff flight path that clears all obstacles […] by at least 200 feet horizontally within the airport boundaries and by at least 300 feet horizontally after passing the boundaries.” 73 TAKEOFF Getting to Grips with Aircraft Performance Airport boundary Start of the Start of the takeoff departure flight path sector 1/2E = 200 ft cwy TOD TORA TODA Figure C23: FAR Departure Sector 5. OUTSIDE ELEMENTS Determination of the performance limited Takeoff Weight must be done considering the external conditions of the day. These conditions affect the MTOW, which can vary considerably from one day to the other. JAR-OPS 1.490 Subpart G FAR 121.189 (e) Subpart I JAR 25.105 Subpart B FAR 25.105 Subpart B JAR 25.237 Subpart B FAR 25.237 Subpart B “JAR-OPS 1.490 (c) an operator must take account of the following [when determining the maximum takeoff mass]: Not more than 50% of the reported head-wind component or not less than 150% of the reported tailwind component; The pressure altitude at the aerodrome; The ambient temperature at the aerodrome; The runway slope in the direction of take-off; The runway surface condition and the type of runway surface“ 5.1. Wind The wind component along the runway axis is an important influencing factor for takeoff. It affects the takeoff ground speed and, therefore, the takeoff distances, which are reduced in case of headwind and increased in case of tailwind. 74 Getting to Grips with Aircraft Performance TAKEOFF True Air Speed Ground Speed Headwind Figure C24: Headwind Effect on Ground Speed The MTOW calculated prior to takeoff, must be determined considering 50% of the actual headwind component, or 150% of the actual tailwind component. This condition forms part of the Airbus performance software, so that an operator just has to consider the actual wind component for the MTOW determination. JAR 25.237 Subpart B FAR 25.237 Subpart B “JAR/FAR 25.237 (a) A 90° cross component of wind velocity, demonstrated to be safe for take-off and landing, must be established for dry runways and must be at least 20 knots or 0.2 VS01, whichever is greater, except that it need not exceed 25 knots.” The crosswind component does not affect takeoff performance. Nevertheless, it is necessary to demonstrate the safety of takeoff and landing procedures up to 25 knots of crosswind. The maximum demonstrated value must be published in the Aircraft Flight Manual. 5.2. Pressure Altitude Pressure altitude influences airframe and engine performance. When the pressure altitude increases, the corresponding static pressure Ps and air density ρ decrease. 5.2.1. Effect on Aerodynamics The force balance in level flight can be illustrated as follows: 1 Weight = m g = Lift = ρ S TAS 2 C L 2 1 VS0 is the reference stall speed in clean configuration. 75 TAKEOFF Getting to Grips with Aircraft Performance As a conclusion, when the pressure altitude increases for a given weight, the true air speed (TAS) must be increased to compensate for the air density reduction. Therefore, the takeoff distance is increased. 5.2.2. Effect on Engines When the pressure altitude increases, the available takeoff thrust is reduced. Therefore, takeoff distances are longer and takeoff climb gradients are reduced. 5.2.3. Summary When the pressure altitude Ò ⇒  Takeoff distances Ò  Takeoff climb gradients Ô ⇒  MTOW Ô 5.3. Temperature 5.3.1. Effect on Aerodynamics When the Outside Air Temperature (OAT) increases, the air density ρ decreases. As mentioned above, the true air speed (TAS) must be increased to compensate for the air density reduction. As a result, the takeoff distance is increased. 5.3.2. Effect on Engines The Takeoff thrust (TOGA) remains constant, equal to the Flat Rated Thrust, until the OAT reaches the Flat Rating Temperature (Tref). Above this temperature, the takeoff thrust starts decreasing (Figure C25). TOGA Flat Rated Thrust EGT limit T REF OAT Figure C25: Engine Thrust versus Outside Air Temperature Consequently, when the Outside Air Temperature increases, the takeoff distances are longer and takeoff climb gradients are reduced. 76 Getting to Grips with Aircraft Performance TAKEOFF 5.3.3. Summary When the Outside Temperature Ò ⇒  Takeoff distances Ò  Takeoff climb gradients Ô ⇒  MTOW Ô 5.4. Runway Slope A slope is generally expressed in percentages, preceded by a plus sign when it is upward, or a minus sign when it is downward. Airbus aircraft are all basically certified for takeoff on runways whose slopes are between -2% and +2%. Nevertheless, these values can be extended to higher limits for operations on particular runways, but it remains marginal as it requires additional certification tests. From a performance point of view, an upward slope degrades the aircraft’s acceleration capability and, consequently, increases takeoff distance. On the other hand, the stopping distance is shortened in case of a rejected takeoff. This is why, depending on the takeoff limitation, an upward slope can sometimes improve MTOW and sometimes lower it. Upward slope ⇒  Takeoff distances Ò  Accelerate stop distance Ô Downward slope ⇒  Takeoff distances Ô  Accelerate stop distance Ò 5.5. Runway Conditions (Dry, Damp, Wet, Contaminated) JAR-OPS 1.480 Subpart F The previously-discussed performance aspects only concerned dry and wet runways. But contaminants also affect takeoff performance, and have to be considered for takeoff weight calculation. The following section aims at defining the different runway states that can be encountered at takeoff. 77 TAKEOFF Getting to Grips with Aircraft Performance 5.5.1. Definitions “JAR-OPS 1.480 (4) Dry runway: A dry runway is one which is neither wet nor contaminated, and includes those paved runways which have been specially prepared with grooves or porous pavement and maintained to retain ‘effectively dry’ braking action even when moisture is present” “JAR-OPS 1.480 (3) Damp runway: A runway is considered damp when the surface is not dry, but when the moisture on it does not give it a shiny appearance.” The FAA does not make any reference to damp runways, which are considered as wet, whereas JAR-OPS 1.475 states that a damp runway is equivalent to a dry one in terms of takeoff performance. Recently, JAR 25 and JAR- OPS Study Groups came to the conclusion that a damp runway should be considered closer to a wet one than to a dry one in terms of friction coefficient (µ)1. As of today, a JAA Notice for Proposed Amendment (NPA) is under discussion, so that in the future, a damp runway may have to be considered as wet. “JAR-OPS 1.480 (10) Wet runway: A runway is considered wet when the runway surface is covered with water or equivalent, [with a depth less than or equal to 3 mm], or when there is a sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water.” In other words, a runway is considered to be wet, as soon as it has a shiny appearance, but without risk of hydroplaning due to standing water on one part of its surface. The water depth is assumed to be less than 3 mm. For “grooved” or “porous friction course”2 wet runways, specific friction coefficients wet (between µdry and µwet) can be used, if provided in the Aircraft Flight Manual. The resulting ASD improvement can sometimes result in higher takeoff weights than on smooth wet runways. Nevertheless, Airbus AFMs don’t provide any specific data for these runway types. “JAR-OPS 1.480 (2) Contaminated runway: A runway is considered to be contaminated when more than 25% of the runway surface area within the required length and width being used is covered by the following:” Standing water: Caused by heavy rainfall and/or insufficient runway drainage with a depth of more than 3 mm (0.125 in). Slush: Water saturated with snow, which spatters when stepping firmly on it. It is encountered at temperature around 5q C, and its density is approximately 0.85 kg/liter ( 7.1 lb / US GAL). 1 µ = friction coefficient = ratio of maximum available tire friction force and vertical load acting on a tire. 2 Runways specially prepared and treated with a porous friction course (PFC) overlay 78 Getting to Grips with Aircraft Performance TAKEOFF Wet snow: If compacted by hand, snow will stick together and tend to form a snowball. Its density is approximately 0.4 kg/liter ( 3.35 lb / US GAL). Dry snow: Snow can be blown if loose, or if compacted by hand, will fall apart again upon release. Its density is approximately 0.2 kg/liter ( 1.7 lb / US GAL). Compacted snow: Snow has been compressed (a typical friction coefficient is 0.2). Ice : The friction coefficient is 0.05 or below. 5.5.2. Effect on Performance There is a clear distinction of the effect of contaminants on aircraft performance. Contaminants can be divided into hard and fluid contaminants. Hard contaminants are : Compacted snow and ice. They reduce friction forces. Fluid contaminants are : Water, slush, and loose snow. They reduce friction forces, and cause precipitation drag and aquaplaning. 5.5.2.1. Reduction of Friction Forces The friction forces on a dry runway vary with aircraft speed. Flight tests help to establish the direct relation between the aircraft’s friction coefficient (µ) and the ground speed (Figure C26). µ Speed Figure C26: µdry versus aircraft’s speed Until recently, regulations stated that, for a wet runway and for a runway covered with standing water or slush, the aircraft’s friction coefficient could be deduced from the one obtained on a dry runway, as follows: µwet = µdry/2 (limited to 0.4) µconta = µdry/4 This concerns A300, A300-600, A310, A320 (except A320-233), A321-100 (JAA certification only), A330-300 (JAA certification only) and A340 basic versions. 79 TAKEOFF Getting to Grips with Aircraft Performance As of today, a new method, known as ESDU, has been developed and introduced by post-amendment 42 in JAR/FAR 25.109. The proposed calculation method of the µwet accounts for the tire pressure, the tire wear state, the type of runway and the anti-skid efficiency demonstrated through flight tests. The µconta (water and slush) results from an amendment based on a flight test campaign. The ESDU model concerns all aircraft types which are not mentioned above. For snow-covered or icy runways, the following values are considered, whatever the aircraft type: µsnow = 0.2 µicy = 0.05 5.5.2.2. Effective µ and Reported µ Airport authorities publish contaminated runway information in a document called “SNOWTAM”, which contains: The type of contaminant The mean depth for each third of total runway length The reported µ or braking action. The reported µ is measured by such friction-measuring vehicles, as: Skidometer, Saab Friction Tester (SFT), MU-Meter, James Brake Decelerometer (JDB), Tapley meter, Diagonal Braked Vehicle (DBV). ICAO Airport Services Manual Part 2 provides information on these measuring vehicles. The main problem is that the resulting friction forces of an aircraft (interaction tire/runway) depend on its weight, tire wear, tire pressure, anti-skid system efficiency and… ground speed. The only way to obtain the aircraft’s effective µ would be to use the aircraft itself in the same takeoff conditions, which is of course not realistic in daily operations. Another solution is to use one of the above-mentioned vehicles, but these vehicles operate at much lower speeds and weights than an aircraft. Then comes the problem of correlating the figures obtained from these measuring vehicles (reported µ), and the actual braking performance of an aircraft (effective µ). To date, scientists have been unsuccessful in providing the industry with reliable and universal values. But tests and studies are still in progress. This is why Airbus publishes contaminated runway information as a function of the type of contaminant and depth of contaminant, and not as a function of the aircraft’s effective µ. Regulation states that: IEM OPS 1.485 Subpart F “IEM OPS 1.485 (b) If the performance data has been determined on the basis of measured runway friction coefficient, the operator should use a procedure correlating the measured runway friction coefficient and the effective braking coefficient of friction of the aeroplane type over the required speed range for the existing runway conditions.” 80 Getting to Grips with Aircraft Performance TAKEOFF 5.5.2.3. Precipitation Drag Precipitation drag is composed of: Displacement drag: Produced by the displacement of the contaminant fluid from the path of the tire. Spray impingement drag: Produced by the spray thrown up by the wheels (mainly those of the nose gear) onto the fuselage. The effect of these additional drags is to : Improve the deceleration rate: Positive effect, in case of a rejected takeoff. Worsen the acceleration rate: Negative effect for takeoff. So, the negative effect on the acceleration rate leads to limit the depth of a fluid contaminant to a maximum value. On the other hand, with a hard contaminant covering the runway surface, only the friction coefficient (effective µ) is affected, and the depth of contaminant therefore has no influence on takeoff performance. 5.5.2.4. Aquaplaning Phenomenon The presence of water on the runway creates an intervening water film between the tire and the runway, leading to a reduction of the dry area (Figure C27). This phenomenon becomes more critical at higher speeds, where the water cannot be squeezed out from between the tire and the runway. Aquaplaning (or hydroplaning) is a situation where the tires of the aircraft are, to a large extent, separated from the runway surface by a thin fluid film. Under these conditions, tire traction drops to almost negligible values along with aircraft wheels’ braking; wheel steering for directional control is, therefore, virtually ineffective. no reaction tire/runway rotation no braking capacity Water Dry Runway Contaminated Runway Figure C27: Hydroplaning Phenomenon Aquaplaning speed depends on tire pressure, and on the specific gravity of the contaminant (i.e. how dense the contaminant is). 81 TAKEOFF Getting to Grips with Aircraft Performance VAQUAPLANING (kt) = 34 (PT/σ)0.5 With PT = tire pressure (kg/cm2) σ = specific gravity of the contaminant. In other words, the aquaplaning speed is a threshold at which friction forces are severely diminished. Performance calculations on contaminated runways take into account the penalizing effect of hydroplaning. 5.5.3. Aircraft Manufacturer Data The aircraft manufacturer has to provide relevant data for operations on runways contaminated by one of the above contaminants, as quoted below: JAR 25X1591 “JAR 25X1591 (a)(c) Supplementary performance information for runways contaminated with standing water, slush, loose snow, compacted snow or ice must be furnished by the manufacturer in an approved document, in the form of guidance material, to assist operators in developing suitable guidance, recommendations or instructions for use by their flight crews when operating on contaminated runway surface conditions.” “JAR 25X1591 (d) The information [on contaminated runways] may be established by calculation or by testing.” As far as performance determination is concerned, Airbus provides guidance material for the following runway contaminants and maximum depths (Table C10): Contaminant Wet runway or Contaminated runway equivalent Water (fluid) < 3 mm (0.12 in) 3 to 12.7 mm (0.5 in) Slush (fluid) < 2 mm (0.08 in) 2 to 12.7 mm (0.5 in) wet snow (fluid) < 4 mm (0.16 in) 4 to 25.4 mm (1 in) dry snow (fluid) < 15 mm (0.59 in) 15 to 50.8 mm (2 in) Compacted snow (hard) / No depth limit Ice (hard) / No depth limit Table C10: Wet and Contaminated Runways Note that takeoff is not recommended, when conditions are worse than the above-listed. 82 Getting to Grips with Aircraft Performance TAKEOFF 5.5.4. Takeoff Performance on Wet and Contaminated Runways 5.5.4.1. Acceleration Stop Distance JAR 25X1591 The ASD definition on a contaminated runway is the same as on a wet runway. Reversers’ effect may be taken into account in the ASD calculation, as soon as the surface is not dry. The distances can either be established by calculation or testing. 5.5.4.2. Takeoff Distance and Takeoff Run JAR 25X1591 IEM-OPS 1.495 (b) The TOD and TOR definitions on a contaminated runway are similar to the ones on a wet runway. They can either be established by calculation or testing. 5.5.4.3. Takeoff Flight Path JAR-OPS 1.495 Subpart G FAR 121.189 IEM-OPS 1.495 Subpart G JAR 25.115 FAR 25.115 “JAR-OPS 1.495 (a) The net flight path must clear all relevant obstacles by a vertical distance of 35 ft.” “JAR 25.115 (a) The takeoff flight path begins 35 ft above the takeoff surface at the end of the takeoff distance.” On a wet or contaminated runway, the screen height (height at the end of the TOD) is 15 feet. The net takeoff flight path starts at 35 feet at the end of the TOD. So, the gross flight path starts at 15 feet while the net flight path starts at 35 feet at the end of the TOD (see Figure C28). “IEM-OPS 1.495 When taking off on a wet or a contaminated runway and an engine failure occurs at V1, this implies that the aeroplane can initially be as much as 20 ft below the net takeoff flight path, and therefore may clear close-in obstacles by only 15 ft”. 83 TAKEOFF Getting to Grips with Aircraft Performance Start of the takeoff flight path net f.p. 20 ft gross f.p. net f.p. - 35 ft 15 ft Runway Terrain Figure C28: Gross and Net Takeoff Flight Paths on Wet and Contaminated Runways While the net flight path clears the obstacles by 35 feet all along the takeoff flight path, the gross flight path can initially be at less than 35 feet above close-in obstacles. 5.5.4.4. Takeoff Weight The TOD and ASD requirements differ between wet and contaminated runways on one side, and dry runways on the other side. Indeed, on wet and contaminated runways, the screen height is measured at 15 feet rather than 35 feet on dry runways. Moreover, the use of reverse thrust is allowed for ASD determination on wet and contaminated runways, whereas it is forbidden to take it into account for the ASD determination on dry runways. Therefore, it is possible to obtain shorter TODs and ASDs on wet and contaminated runways than on dry runways for the same takeoff conditions. Thus, it is possible to obtain higher takeoff weights on surfaces covered with water, slush, or snow than on dry runways. This is why the regulation indicates that: JAR-OPS 1.490 Subpart G “JAR-OPS 1.490 (b)(5) On a wet or contaminated runway, the takeoff mass must not exceed that permitted for a takeoff on a dry runway under the same conditions”. 6. MAXIMUM TAKEOFF WEIGHT DETERMINATION 6.1. Speed Optimization Process Airbus recommends that the MTOW on a given runway and given conditions be computed by optimizing both the V1/VR ratio and the V2/VS ratio. The performance software provided by Airbus automatically carries out this optimized computation, whose aim is to achieve the highest possible MTOW. This optimization process is described in Appendix 2 of this manual. 84 Getting to Grips with Aircraft Performance TAKEOFF 6.2. Regulatory Takeoff Weight Chart (RTOW Chart) To determine the regulatory takeoff weight for repetitive takeoff planning, it is mandatory to provide pilots with data, which enable quick calculations of the Maximum Allowed Takeoff Weight and its associated speeds. This can be done via ground or onboard computerized systems, such as the LPC (Less Paper Cockpit: see Appendix 3), or through paper documents. These paper documents are referred to as “Regulatory TakeOff Weight” charts (RTOW). The charts must be generated for each runway heading, and can be produced for different takeoff conditions at the convenience of the applicant (temperature, wind, QNH, flap setting, runway status, inoperative items). They provide the: Maximum Takeoff Weight (MTOW) Takeoff speeds (V1,VR,V2) Limitation code Minimum and maximum acceleration heights. Figure C29 shows an example of an A319 RTOW chart. Example: MTOW and speeds determination DATA Takeoff from Paris-Orly, Runway 08 Slat/Flap configuration: 1+F OAT = 24ºC Wind = Calm QNH = 1013 hPa Air conditioning: Off Runway state: Dry RESULT MTOW = 73.6 tons V1 = 149 Kt, VR = 149 Kt, V2 = 153 Kt MTOW limited by: second segment and obstacle(2/4) Note: In case of deviation from the chart reference conditions (QNH, air conditioning…), corrections have to be applied to the MTOW and the speeds. 85 TAKEOFF Getting to Grips with Aircraft Performance Figure C29: A319 RTOW Chart Example 86 Getting to Grips with Aircraft Performance TAKEOFF 7. FLEXIBLE AND DERATED TAKEOFF The aircraft actual takeoff weight is often lower than the maximum regulatory takeoff weight. Therefore, in certain cases, it is possible to takeoff at a thrust less than the Maximum Takeoff Thrust. It is advantageous to adjust the thrust to the actual weight, as it increases engine life and reliability, while reducing maintenance and operating costs. These takeoff operations generally fall into two categories: Those using the reduced thrust concept, known as flexible takeoffs in the Airbus world, and those using a specific derated thrust level named derated takeoffs. 7.1. Flexible Takeoff A takeoff at reduced thrust is called a flexible takeoff, and the corresponding thrust is called flexible thrust. AMJ 25-13 AC 25-13 7.1.1. Definition “AMJ 25-13 / AC 25-13 (4)(c) Reduced takeoff thrust, for an aeroplane, is a takeoff thrust less than the takeoff (or derated takeoff) thrust. The aeroplane takeoff performance and thrust setting are established by approved simple methods, such as adjustments, or by corrections to the takeoff thrust setting and performance.” In this case, “the thrust for takeoff is not considered as a takeoff operating limit.” As shown in Figure C30, the actual takeoff weight is less than the maximum permissible takeoff weight obtained from a RTOW chart. Therefore, it is possible to determine the temperature at which the needed thrust would be the maximum takeoff thrust for this temperature. This temperature is called “flexible temperature (TFlex)” or “assumed temperature”. Moreover: “AMJ 25-13 / AC 25-13 (5)(a) The reduced takeoff thrust setting (2) Is based on an approved takeoff thrust rating for which complete aeroplane performance data is provided (3) Enables compliance with the aeroplane controllability requirements in the event that takeoff thrust is applied at any point in the takeoff path (4) Is at least 75% of the maximum takeoff thrust for the existing ambient conditions” 87 TAKEOFF Getting to Grips with Aircraft Performance Weight Thrust flat rated thrust available MTOW thrust EGT limit needed 25% actual TOW reduction max thrust OAT OAT Tref Flex temp T Flex max Figure C30: Flexible Temperature Principle Consequently, the flexible temperature is the input parameter through which the engine monitoring computer adapts the thrust to the actual takeoff weight. This method is derived from the approved maximum takeoff thrust rating, and thus uses the same certified minimum control speeds. In addition, thrust reduction cannot exceed 25% of the maximum takeoff thrust, thus leading to a maximum flexible temperature, as shown in Figure C30. To comply with the above requirements, flexible takeoff is only possible when the flexible temperature fulfils the following three conditions: TFlex > TREF TFlex > OAT TFlex ≤ TFlex Max Regulations require operators to conduct periodic takeoff demonstrations, using the maximum takeoff thrust setting, in order to check takeoff parameters (N1, N2, EPR, EGT). The time interval between takeoff demonstrations may be extended, provided an approved engine condition-monitoring program is used. 7.1.2. Flexible Takeoff and Runway State “AMJ 25-13 / AC 25-13 (f) The AFM states that [reduced thrust takeoffs] are not authorised on contaminated runways and are not authorised on wet runways unless suitable performance accountability is made for the increased stopping distance on the wet surface". Airbus operational documentation (RTOW, FCOM) provides performance information for flexible takeoffs on wet runways. As a result, a flexible takeoff is allowed on a wet runway, while it is forbidden on a contaminated one. 88 Getting to Grips with Aircraft Performance TAKEOFF 7.1.3. Flexible Temperature Determination The following example illustrates how to determine a flexible temperature, with the use of a RTOW chart (Figure C29). Example: Flexible Temperature and Speeds Determination DATA Takeoff from Paris-Orly, Runway 08 Slat/Flap configuration: 1+F Actual TOW = 66 tons OAT = 24ºC Wind = +20 Kt headwind QNH = 1013 hPa Air conditioning: Off Runway state: Dry RESULT Flex Temp = 68ºC V1 = 145 Kt, VR = 145 Kt, V2 = 150 Kt Note: In case of deviation from the chart reference conditions (QNH, air conditioning…), corrections have to be applied to the flexible temperature. 7.1.4. Flexible Takeoff Procedure To carry out a flexible takeoff, which is always at the discretion of the pilot, a flexible temperature has to be determined from an RTOW chart computed with no derate or an equivalent computerized system. This temperature value must then be entered in the MCDU (Multipurpose Control and Display Unit) during the takeoff preparation phase (Figure C31). At the brake release point, the thrust throttles must be pushed to the FLX position (Figure C32) as per the Standard Operating Procedure (SOP). TOGA thrust remains available at any moment during the takeoff phase. But, in the event of an engine failure after V1, its selection is not required. Figure C31: MCDU Takeoff Performance Page Figure C32: Thrust Throttle Positions 89 TAKEOFF Getting to Grips with Aircraft Performance 7.2. Derated Takeoff 7.2.1. Definition AMJ 25-13 AC 25-13 “AMJ 25-13 / AC 25-13 (4)(b) Derated takeoff thrust, for an aeroplane, is a takeoff thrust less than the maximum takeoff thrust, for which exists in the AFM a set of separate and independent takeoff limitations and performance data that complies with all requirements of Part 25.” In this case, “the thrust for takeoff is considered as a normal takeoff operating limit.” For a derated takeoff, the limitations, procedures and performance data must be included in the Aircraft Flight Manual (AFM). For each derate level, a specific RTOW chart can be established for a given runway, taking into account such new limitations as the minimum control speeds. 7.2.2. Minimum Control Speeds with Derated Thrust A given derate level corresponds to the basic ma