2 In the following assurance test, the airplane should demonstrate safe flyaway characteristics.
(aa) Minimum speed liftoff should be demonstrated at the critical forward
c.g. limit with normal trim. For airplanes with a cutback forward c.g. at heavy weight, two weight/c.g. conditions should be considered. The heavy weight tests should be conducted at maximum structural or maximum sea level climb-limited weight with the associated forward c.g.
The full forward c.g. tests should be conducted at the highest associated weight. Alternatively, testing may be conducted at a single weight if an analysis is provided that identifies the critical weight/c.g. combination with regard to limited pitch attitude capability for liftoff.
(bb) These assurance tests should be conducted at the thrust/weight ratio that is most critical for attaining a pitch attitude that will provide a minimum liftoff speed.
(i) For airplanes that are limited by low thrust/weight conditions, tests should be conducted at the minimum thrust/weight ratio for both the simulated one-engineinoperative test (i.e., symmetrical reduced thrust) case and the all-engines-operating case.
(ii) For airplanes that are limited by high thrust/weight conditions, tests should be conducted at the highest thrust/weight ratio within the airplane’s operating envelope for both the simulated one-engine-inoperative case (i.e., symmetrical reduced thrust) and the all-engines-operating case.
(cc) One acceptable test technique is to hold full nose-up control column as the airplane accelerates. As pitch attitude is achieved to establish the minimum liftoff speed, pitch control may be adjusted to prevent over-rotation, but the liftoff attitude should be maintained as the airplane flies off the ground and out of ground effect.
(dd) The resulting liftoff speeds are acceptable if the test proves successful and the liftoff speed is at least 5 knots below the normally scheduled liftoff speed.
(ee) This minimum 5 knot margin from the scheduled liftoff speed provides some leeway for operational variations such as mis-trim, c.g. errors, etc., that could further limit the elevator authority. The reduced VMU margins arising from this test, relative to those specified in § 25.107(e)(1)(iv), are considered acceptable because of the reduced probability of a pitch control authority-limited airplane getting into a high drag condition due to over-rotation.
(i) VMU Testing for Geometry Limited Airplanes.
1 For airplanes that are geometry limited (i.e., the minimum possible VMU speeds are limited by tail contact with the runway), § 25.107(e)(1)(iv)(B) allows the VMU to VLOF speed margins to be reduced to 108 percent and 104 percent for the all-engines-operating and one-engine-inoperative conditions, respectively. The VMU demonstrated should be sound and repeatable.
2 An airplane that is deemed to be geometry limited at the conditions tested is expected to be geometry limited over its entire takeoff operating envelope. If this is not the case, the airplane is not considered geometry limited and the reduced VMU to VLOF speed margins do not apply.
3 One acceptable means for demonstrating compliance with §§ 25.107(d) and 25.107(e)(1)(iv) with respect to the capability for a safe liftoff and fly-away from the geometry limited condition is to show that at the lowest thrust-to-weight ratio for the all-enginesoperating condition:
(aa) In the speed range from 96 to 100 percent of the actual liftoff speed), the aft under-surface of the airplane should be in contact with the runway. Because of the dynamic nature of the test, it is recognized that contact will probably not be maintained during this entire speed range, so some judgment is necessary. It has been found acceptable for contact to exist approximately 50 percent of the time that the airplane is in this speed range.
(bb) Beyond the point of liftoff to a height of 35 feet, the airplane’s pitch attitude should not decrease below that at the point of liftoff, nor should the speed increase more than 10 percent.
(cc) The horizontal distance from the start of the takeoff to a height of 35 feet above the takeoff surface should not be greater than 105 percent of the distance determined in accordance with § 25.113(a)(2) without applying the 115 percent factor.
(j) VMU for a Stretched Version of a Tested Airplane.
1 VMU speeds obtained by flight testing one model of an airplane type may be used to generate VMU speeds for a geometry-limited stretched version of that airplane. If the short body airplane met the criteria for the 104/108 percent VMU/VLOF speed margin for geometry limited airplanes as permitted by § 25.107(e)(1)(iv)(B) and discussed in paragraph 10b(8)(i)1, the flight tests described in paragraph 10b(8)(i)3 should be performed on the stretched derivative.
Otherwise, the flight tests described in paragraph 10b(8)(j)2(bb) should be performed on the stretched derivative.
2 Since the concern for tail strikes is increased with the stretched airplane, the following should be accomplished, in addition to normal takeoff tests, when the VMU schedule of the stretched derivative is derived from that of the shorter body parent airplane:
(aa) The minimum unstick speed (VMU) of the stretched derivative airplane should be determined by correcting the VMU of the shorter body tested airplane for the reduced runway pitch attitude capability and revised c.g. range of the stretched airplane.
Alternatively, stretched airplane VMU speeds not determined in this manner should be substantiated by flight testing or a rational analysis. Scheduled rotation speeds (VR) for the stretched airplane should result in at least the required liftoff speed margins above the corrected
28 VMU required by § 25.107(e)(1)(iv) for the one-engine-inoperative and all-engines-operating takeoff conditions.
(bb) At both the forward and aft c.g. limits, and over the thrust-to-weight range for each takeoff flap, the following takeoff tests should be accomplished. The tests described in paragraphs (i) and (ii), below, should be accomplished with not more than occasional, minor (i.e., non-damaging) tail strikes.
(i) All-engines-operating, early rotation tests specified in paragraph 10b(9)(c)2, including both the rapid rotations and over-rotations as separate test conditions.
(ii) One-engine-inoperative, early rotation tests specified in paragraph 10b(9)(b).
(iii) All-engines-operating, moderate rotation rate (i.e., more rapid than normal) takeoff tests, using the scheduled VR and normal pitch attitude after liftoff. Tail strikes should not occur for this condition.
(9) Section 25.107(e) - Rotation Speed (VR).
(a) The rotation speed, (VR) in terms of calibrated airspeed, must be selected by the applicant. VR has a number of constraints that must be observed in order to comply with § 25.107(e):
1 VR may not be less than V1; however, it can be equal to V1 in some cases.
2 VR may not be less than 105 percent of the air minimum control speed (VMCA).
3 VR must be a speed that will allow the airplane to reach V2 at or before reaching a height of 35 ft. above the takeoff surface.
4 VR must be a speed that will result in liftoff at a speed not less than 110 percent of VMU (unless geometry limited) for the all-engines-operating condition and not less than 105 percent of the VMU (unless geometry limited) determined at the thrust/weight ratio corresponding to the one-engine-inoperative condition for each set of conditions such as weight, altitude, temperature, and configuration when the airplane is rotated at its maximum practicable rate.
(b) Early rotation, one-engine-inoperative test.
1 In showing compliance with § 25.107(e)(3), some guidance relative to the airspeed attained at the 35 ft. height during the associated flight test is necessary. As this requirement only specifies an early rotation (VR-5 knots), it is interpreted that pilot technique is to remain the same as normally used for a one-engine-inoperative condition. With these considerations in mind, it is apparent that the airspeed achieved at the 35 ft. point can be
29 somewhat below the normal scheduled V2 speed. However, the amount of permissible V2 speed reduction should be limited to a reasonable amount as described below.
2 These test criteria apply to all unapproved, new, basic model airplanes.
They also apply to previously approved airplanes when subsequent testing is warranted.
However, for those airplanes where these criteria are more stringent than those previously applied, consideration will be given to permitting some latitude in the test criteria.
3 In conducting the flight tests required by § 25.107(e)(3), the test pilot should use the normal/natural rotation technique associated with the use of scheduled takeoff speeds for the airplane being tested. Intentional tail or tail skid contact is not considered acceptable. Non-damaging contact due to inadvertent over-rotation is acceptable provided there is a prompt recovery to the normal one-engine-inoperative takeoff pitch attitude. Further, the airspeed attained at the 35 ft. height during this test should not be less than the scheduled V2 value minus 5 knots. These speed limits should not be considered or used as target V2 test speeds, but rather are intended to provide an acceptable range of speed departure below the scheduled V2 value.
4 In this test, the simulated engine failure should be accomplished sufficiently in advance of the VR test speed to allow for engine spin-down, unless this would be below the VMCG, in which case VMCG should govern. The normal one-engine-inoperative takeoff distance may be analytically adjusted to compensate for the effect of the early power or thrust reduction. Further, in those tests where the airspeed achieved at the 35-ft. height is slightly less than the V2-5 knots limiting value, it will be permissible, in lieu of conducting the tests again, to analytically adjust the test distance to account for the excessive speed decrement.
(c) All-engines-operating tests.
1 Section 25.107(e)(4) states that there must not be a “marked increase” in the scheduled takeoff distance when reasonably expected service variations such as early and excessive rotation and out-of-trim conditions are encountered. This has been interpreted as requiring takeoff tests with all engines operating with:
(aa) A lower than scheduled rotation speed, and
(bb) Out-of-trim conditions, but with rotation at the scheduled VR speed.
NOTE: The expression “marked increase” in the takeoff distance is considered to be any amount in excess of 1 percent of the scheduled takeoff distance. Thus, the tests should not result in field lengths more than 101 percent of the takeoff field lengths calculated in accordance with the applicable requirements of part 25 for presentation in the AFM.
2 For the early rotation condition with all engines operating, and at a weight as near as practicable to the maximum sea level standard day takeoff weight limit, it should be shown by tests that when the airplane is rotated at a speed below the scheduled VR, no “marked
30 increase” in the scheduled AFM field length will result. For these tests, the airplane should be rotated at a speed equal to the scheduled VR minus 7 percent or the scheduled VR minus 10 knots, whichever results in the higher rotation speed. Tests should be conducted at: (1) a rapid rotation rate to the normal takeoff attitude, and (2) an over-rotation of 2 degrees above normal attitude after liftoff at the normal rotation rate. For tests using over rotations, the resulting increased pitch attitude should be maintained until the airplane is out of ground effect. Tail strikes during this demonstration are acceptable if they are minor and do not result in unsafe conditions.
3 For reasonably expected out-of-trim conditions with all engines operating and as near as practicable to the maximum weight allowed under sea level standard day conditions, it should be shown that there will not be a “marked increase” in the scheduled AFM takeoff distance when rotation is initiated in a normal manner at the scheduled VR speed. The amount of mistrim should be the maximum mistrim that would not result in a takeoff configuration warning, including taking into account the takeoff configuration warning system rigging tolerance. It is permissible to accept an analysis in lieu of actual testing if the analysis shows that the out-of-trim condition would not present unsafe flight characteristics or a “marked increase” in the scheduled AFM field lengths.
4 Section 25.107(e)(4) also states that the reasonably expected variations in service from the established takeoff procedures for the operation of the airplane may not result in unsafe flight characteristics. For example, for an airplane loaded to obtain a forward c.g.
position and mistrimmed for an aft c.g. loading, it may not be possible to rotate at the normal operating speeds due to excessive control force or lack of primary pitch control authority. This may result in an excessive delay in accomplishing the rotation. Such a condition would be considered an unsafe flight characteristic. Similarly, for an airplane loaded to obtain an aft c.g.
position and mistrimmed for a forward c.g. loading, it may not be possible to readily arrest a selfrotating tendency. This rotation, if abrupt enough and rapid enough, could lead to stall.
Qualitative assessments should be made by the test pilot in the following takeoff tests with all engines operating:
(aa) The test pilot should determine that no unsafe characteristics exist with the airplane loaded to the forward c.g. limit and the stabilizer mistrimmed in the airplane nose-down direction. The amount of mistrim should be the maximum mistrim that would not result in a configuration warning (including taking into account takeoff warning system tolerances). Rotation should be initiated at the scheduled rotation speed for the airplane weight and ambient conditions. Unsafe characteristics include an excessive pitch control force to obtain normal airplane response or an excessive time to achieve perceptible rotation.
(bb) The test pilot should determine that no unsafe characteristics exist with the airplane loaded to the aft c.g. limit and the stabilizer mistrimmed in the airplane nose-up direction. The amount of mistrim should be the maximum mistrim that would not result in a configuration warning (including taking into account takeoff warning system tolerances). The airplane should be rotated at the scheduled rotation speed for the airplane weight and ambient conditions. Unsafe characteristics include: an abrupt self rotating tendency that cannot be checked with normal control input, or an excessive pitch control force required to maintain the
31 airplane in the normal pitch attitude prior to the scheduled rotation speed or during rotation and initial climb.
(cc) For the tests described in paragraphs (aa) and (bb) above, the flight characteristics should be assessed at the most critical combinations of airplane weight, wing flap position and engine power or thrust for the out of trim position being considered.
(d) Stall Warning During Takeoff Speed Tests. The presumption is that if an operational pilot was to make an error in takeoff speeds that resulted in an encounter with stall warning, the likely response would be to recover aggressively to a safe flight condition rather than trying to duplicate the AFM takeoff flight path. Therefore, the activation of any stall warning devices, or the occurrence of airframe buffeting during takeoff speed testing, is unacceptable.
(e) Stick Forces During Takeoff Speed Tests. Per § 25.143(a)(1) and (b), stick forces to initiate rotation and continue the takeoff during takeoff flight testing must comply with the control force limits of § 25.143(d). This includes the mistrim takeoff tests described in paragraphs 10b(9)(c)4(aa) and (bb) to show compliance with § 25.107 (e)(4), which are considered to represent probable operating conditions under § 25.143(b). Stick forces should be those that result from using the takeoff procedures established by the manufacturer for use in operational service in accordance with § 25.101(f) and must comply with § 25.101(h).
(10) Section 25.107(f) - Liftoff Speed (VLOF).
(a) The liftoff speed (VLOF) is defined as the calibrated airspeed at which the airplane first becomes airborne (i.e., no contact with the runway). This allows comparison of liftoff speed with tire limit speed. VLOF differs from VMU in that VMU is the minimum possible VLOF speed for a given configuration, and depending upon landing gear design, VMU liftoff is shown to be the point where all of the airplane weight is being supported by airplane lift and thrust forces and not any portion by the landing gear. For example, after the VMU speed is reached, a truck tilt actuator may force a front or rear wheel set to be in contact with the runway, even though the liftoff is in progress by virtue of lift being greater than weight.
(b) The maximum ground speed at liftoff, considering the entire takeoff operating envelope and taking into account 50 percent of the headwind and 150 percent of the tailwind, in accordance with § 25.105(d)(1), must not exceed the tire speed rating established under § 25.733(a) or (c).
11. Accelerate-Stop Distance - § 25.109.
a. Explanation.
(1) The accelerate-stop distance is the horizontal distance from a reference point on the airplane at initial brake release to that same reference point after the airplane is brought to a stop.
(2) This section describes test demonstrations and data expansion methods necessary to determine accelerate-stop distances for publication in the FAA AFM, as required by § 25.1583(h) (by reference to § 25.1533). Amendment 25-92 revised some aspects of the part 25 accelerate-stop criteria and added new requirements related to the stopping capability of the airplane as affected by brake wear and wet runways. The changes imparted to the acceleratestop requirements by Amendment 25-92 are listed below. (For other material related to the use of accelerate-stop distances, see parts 121 and 135 of the Code of Federal Regulations)
(a) Section 25.101(i) was added to require accelerate-stop distances to be determined with all the airplane wheel brake assemblies at the fully worn limit of their allowable wear range.
(b) Section 25.105(c)(1) was revised to require takeoff data to be determined for wet, in addition to dry, hard surfaced runways. At the applicant’s option, takeoff data may also be determined for wet runways that have grooved or porous friction course surfaces.
(c) Section 25.107(a)(2) was revised to remove the reference to “takeoff decision speed” from the definition of V1. V1 is the speed by which the pilot has already made the decision to reject the takeoff and has initiated the first action to stop the airplane.
(d) Section 25.109 was revised to add a requirement to determine accelerate-stop distances for wet runways. Additionally, the requirement for the AFM expansion to include two seconds of continued acceleration beyond V1, with the operating engines at takeoff power or thrust, as introduced by Amendment 25-42, was replaced with a distance increment equivalent to two seconds at V1. Also, the text of § 25.109(a) was modified to clarify that the accelerate-stop distances must take into account the highest speed reached during the rejected takeoff maneuver, including, as applicable, speeds higher than V1.
(e) Section 25.109(f) was added to permit credit for the use of reverse thrust in determining wet runway accelerate-stop distances (subject to the requirements of § 25.109(e)) and to explicitly deny reverse thrust credit for determining dry runway accelerate-stop distances.
(f) Section 25.109(i) was added to require a maximum brake energy acceleratestop test to be conducted with not more than 10 percent of the allowable brake wear range remaining on each individual wheel-brake assembly.
(g) Section 25.735(h) was added to require the maximum rejected takeoff brake energy absorption capacity rating used during qualification testing to the applicable Technical Standard Order to be based on the fully worn limit of the brake’s allowable wear range.
NOTE: Section 25.735, has been more recently revised by Amendment 25-107, which moved the rejected takeoff kinetic energy rating requirements into § 25.735(f).
(h) Section 25.1533(a)(3) was revised to add runway surface condition (dry or wet) as a variable that must be accounted for in establishing minimum takeoff distances. Section
25.1533(a)(3) was also revised to allow wet runway takeoff distances on grooved and porous friction course (PFC) runways to be established as additional operating limitations, but approval to use these distances is limited to runways that have been designed, constructed, and maintained in a manner acceptable to the FAA Administrator.
b. The applicable part 25 regulations are § 25.109, and the following:
§ 25.101(f) Airplane configuration and procedures § 25.101(h) Pilot action time delay allowances § 25.101(i) Worn brake stopping performance § 25.105 Takeoff configuration and environmental and runway conditions § 25.107(a)(l) & (2) V1 and VEF speed definitions § 25.735 Brakes and braking systems § 25.1301 Function and installation § 25.1309 Equipment, systems, and installation § 25.1533 Additional operating limitations - maximum takeoff weights and minimum takeoff distances § 25.1583(h) AFM - operating limitations § 25.1587 AFM - performance information
c. Procedures. The following paragraphs provide guidance for accomplishing acceleratestop flight tests and expanding the resulting data for the determination of AFM performance information.
(1) Accelerate-stop testing. The following guidance applies to turbine-powered airplanes with and without propellers. Guidance regarding flight testing applies only to dry runway accelerate-stop distances. Guidance for expanding the flight test data to determine AFM distances applies to both dry and wet runways, unless otherwise noted. Further guidance for determining wet runway accelerate-stop distances is provided in paragraph 11c(4).
(a) In order to establish a distance that would be representative of the distance needed in the event of a rejected takeoff, where the first action to stop the airplane is taken at V1, a sufficient number of test runs should be conducted for each airplane configuration specified by the applicant. (For intermediate configurations, see paragraph 3 of this AC.)
(b) The guidance outlined in paragraph 11c(3) describes how to include allowances for any time delays, as required by § 25.101(h)(3), for the flightcrew to accomplish the rejected takeoff operating procedures.
(c) Section 25.101(i) states that the accelerate-stop distances must be determined with all the airplane wheel-brake assemblies at the fully worn limit of their allowable wear range.
The fully worn limit is defined as the maximum amount of wear allowed before the brake is to be removed from the airplane for overhaul. The allowable wear should be defined in terms of a linear dimension in the axial direction, which is typically determined by measuring the wear pin extension.
1 The only accelerate-stop test that must be conducted at a specific brake wear state is the maximum brake kinetic energy demonstration, which must use brakes that have no more than 10 percent of the allowable brake wear range remaining, as required by § 25.109(i).
(See paragraph 11c(2)(c) of this AC). The remainder of the accelerate-stop tests may be conducted with the brakes in any wear state as long as a suitable combination of airplane and dynamometer tests is used to determine the accelerate-stop distances corresponding to fully worn brakes. For example, dynamometer testing may be used to determine whether there is a reduction in brake performance from the wear state used in the airplane tests to a fully worn brake. The airplane test data could then be adjusted analytically for this difference without additional airplane testing.
2 Either airplane-worn or mechanically-worn brakes (i.e., machined or dynamometer worn) may be used. If mechanically-worn brakes are used, it should be shown that they can be expected to provide similar results to airplane-worn brakes. This comparison can be based on service experience on the test brake or an appropriate equivalent brake, or on dynamometer wear test data when service data are unavailable.
(d) Section 25.109(f)(1) denies credit for the use of reverse thrust as a decelerating means in determining the accelerate-stop distance for a dry runway. This provision applies to both turbine engine and propeller engine reverse thrust (but not to any drag resulting from the ground idle power setting for a propeller engine). Credit for the additional deceleration available from reverse thrust is permitted for wet runway accelerate-stop distances, provided the thrust reverser system is shown to be safe, reliable, capable of giving repeatable results, and does not require exceptional skill to control the airplane. (See paragraph 11c(4)(e) for guidance related to obtaining accelerate-stop performance credit for reverse thrust on wet runways.)
(e) The accelerate-stop test runs should be conducted at weight/speed combinations that will provide an even distribution of test conditions over the range of weights, speeds, and brake energies for which takeoff data will be provided in the AFM. The effects of different airport elevations can be simulated at one airport elevation, provided the braking speeds employed are relevant for the range of airplane energies to be absorbed by the brakes. The limiting brake energy value in the AFM should not exceed the maximum demonstrated in these tests or the maximum for which the brake has been approved. (See paragraph 11c(2) for further guidance related to tests and analyses for the demonstration of the maximum brake energy absorption capability.)
(f) The V1 speeds used in the accelerate-stop tests need not correspond precisely to the AFM values for the test conditions since it may be necessary to increase or decrease the AFM V1 speed to investigate fully the energy range and weight envelope.
(g) A total of at least six accelerate-stop flight tests should be conducted. Unless sufficient data are available for the specific airplane type showing how braking performance varies with weight, kinetic energy, lift, drag, ground speed, torque limit, etc., at least two tests should be conducted for each configuration when the same braking coefficient of friction is being claimed for multiple aerodynamic configurations. These tests should be conducted on smooth, hard surfaced, dry runways.
(h) For approval of dispatch capability with anti-skid inoperative, nose wheel brakes or specified main wheel-brake(s) inoperative, automatic braking systems, etc., a full set of tests, as described in paragraph (g) above, should normally be conducted. A lesser number of tests may be accepted for “equal or better” demonstrations, to establish small increments or if adequate conservatism is used during testing.
(i) Either ground or airborne instrumentation should include means to determine the horizontal distance time-history.
(j) The wind speed and direction relative to the test runway should be determined and corrected to a height corresponding to the approximate height of the mean aerodynamic chord. (See paragraph 3 of this AC.)
(k) The accelerate-stop tests should be conducted in the following configurations:
1 Heavy to light weight as required.
2 Most critical c.g. position.
3 Wing flaps in the takeoff position(s).
4 Tire pressure: before taxi and with cold tires, set to the highest value appropriate to the takeoff weight for which approval is being sought.
5 Engine idle power or thrust: set at the recommended upper limit for use on the ground or the effect of maximum ground idle power or thrust may be accounted for in data analyses. For maximum brake energy and fuse plug no-melt tests, data analysis may not be used in place of maximum ground idle power or thrust.
(l) Engine power or thrust should be appropriate to each segment of the rejected takeoff and should include accounting for power or thrust decay rates (i.e., spin down) for failed or throttled back engines.
1 Turbojet powered airplanes. For AFM calculation purposes, the critical engine failure accelerate-stop data may be based on the failed engine spinning down to a windmilling condition.
36 NOTE: If, due to the certification basis of the airplane, all-engine-accelerate-stop distances are not being considered, the one-engine-inoperative AFM distances should be based on the critical engine failing to maximum ground idle power or thrust rather than the windmilling condition.
2 The power or thrust from the operative engine(s) should be consistent with a throttle chop to maximum ground idle power or thrust. For determining the all-enginesoperating dry runway accelerate-stop AFM distances, the stopping portion should be based on all engines producing maximum ground idle power or thrust (after engine spin down), as noted in paragraph 11c(1)(k)5. The accelerate-stop tests may be conducted with either concurrent or sequential throttle chops to idle power or thrust as long as the data are adjusted to take into account pilot reaction time, and any control, system, or braking differences (e.g., electrical or hydraulic/mechanical transients associated with an engine failing to a windmilling condition resulting in reduced braking effectiveness). Test data should also be analytically corrected for any differences between maximum ground idle power or thrust and the idle power or thrust level achieved during the test. For the criteria relating to reverse thrust credit for wet runway accelerate-stop distances, see paragraph 11c(4)(e).
3 Turbopropeller-powered airplanes. For the one-engine-inoperative accelerate-stop distances, the critical engine’s propeller should be in the position it would normally assume when an engine fails and the power levers are closed. For dry runway oneengine-inoperative accelerate-stop distances, the high drag ground-idle position of the operating engines’ propellers (defined by a pitch setting that results in not less than zero total thrust, i.e., propeller plus jet thrust, at zero airspeed) may be used provided adequate directional control is available on a wet runway and the related operational procedures comply with § 25.101(f) and
(h). Wet runway controllability may either be demonstrated by using the guidance available in paragraph 11c(4)(e)6 at the appropriate power level, or adequate control can be assumed to be available at ground idle power if reverse thrust credit is approved for determining the wet runway accelerate-stop distances. For the all-engines-operating accelerate-stop distances on a dry runway, the high drag ground-idle propeller position may be used for all engines (subject to § 25.101(f) and (h)). For the criteria relating to reverse thrust credit for wet runway acceleratestop distances, see paragraph 11c(4)(e).
(m) System transient effects (e.g., engine spin-down, brake pressure ramp-up, etc.)
should be determined and properly accounted for in the calculation of AFM accelerate-stop distances (see paragraph 11c(3)(i)).
(2) Maximum Brake Energy Testing. The following paragraphs describe regulatory requirements and acceptable test methods for conducting an accelerate-stop test run to demonstrate the maximum energy absorption capability of the wheel brakes.
(a) The maximum brake energy accelerate-stop demonstration should be conducted at not less than the maximum takeoff weight. It should be preceded by at least a 3- mile taxi with all engines operating at maximum ground idle power or thrust, including three full stops using normal braking. Following the maximum brake energy stop, it will not be necessary to demonstrate the airplane’s ability to taxi.
(b) Section 25.735(f)(2) requires the maximum kinetic energy accelerate-stop absorption capability of each wheel, tire, and brake assembly to be determined. It also requires dynamometer testing to show that the wheel, brake, and tire assembly is capable of absorbing not less than this level of kinetic energy throughout the defined wear range of the brake. The calculation of maximum brake energy limited takeoff weights and speeds, for presentation in the AFM performance section, therefore should be based on the most critical wear range of the brake.
(c) Section 25.109(i) requires a flight test demonstration of the maximum brake kinetic energy accelerate-stop distance to be conducted with not more than 10 percent of the allowable brake wear range remaining on each of the airplane wheel-brakes. The 10 percent allowance on the brake wear state is intended to ease test logistics and increase test safety, not to allow the accelerate-stop distance to be determined with less than fully worn brakes. If the brakes are not in the fully worn state at the beginning of the test, the accelerate-stop distance should be corrected as necessary to represent the stopping capability of fully worn brakes.
(d) The maximum airplane brake energy allowed for dispatch should not exceed the value for which a satisfactory after-stop condition exists, or the value documented under the applicable TSO (or an acceptable equivalent), whichever value is less. A satisfactory after-stop condition is defined as one in which fires are confined to tires, wheels, and brakes, such that progressive engulfment of the rest of the airplane would not occur during the time of passenger and crew evacuation. The application of fire fighting means or artificial coolants should not be required for a period of 5 minutes following the stop.
(e) Landings are not an acceptable means for demonstrating the maximum rejected takeoff brake energy. Though permitted in the past, service experience has shown that methods used to predict brake and tire temperature increases that would have occurred during taxi and acceleration, as specified in paragraph 11c(2)(a), were not able to accurately account for the associated energy increments.
(3) Accelerate-Stop Time Delays. Section 25.101(h) requires allowance for time delays in the execution of procedures. Amendment 25-42 (effective March 1, 1978) amended the airworthiness standards to clarify and standardize the method of applying these time delays to the accelerate-stop transition period. Amendment 25-42 also added the critical engine failure speed, VEF, and clarified the meaning of V1 with relation to VEF. The preamble to Amendment 25-42 states that “V1 is determined by adding to VEF (the speed at which the critical engine is assumed to fail) 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 test pilot recognizes and reacts to the engine failure, as indicated by the pilot’s application of the first retarding means during accelerate-stop tests.” Thus it can be seen that V1 is not only intended to be at the end of the decision process, but it also includes the time it takes for the pilot to perform the first action to stop the airplane. (See Appendix 3 for further discussion on the historical development of accelerate-stop time delays.) The purpose of the time delays is to allow sufficient time (and distance) for a pilot, in actual operations, to accomplish the procedures for stopping the airplane. The time delays are not intended to allow extra time for making a decision