Chap 2 29 operating tests at the thrust/weight ratio corresponding to the one-engine-inoperative condition. As revised, § 25.107(d) specifies that V MU must be selected for the range of thrust/weight ratios to be certificated, rather than for the all-engines-operating and one-engine-inoperative conditions as was previously required.
In determining the all-engines-operating thrust/weight ratio that corresponds to the one-engine-inoperative condition, consideration should be given to trim and control drag differences between the two configurations in addition to the effect of the number of engines operating. The minimum thrust/weight ratio to be certificated is established by correcting, to the V MU speed, the thrust that results in the airplane achieving its limiting engine-out climb gradient in the appropriate configuration and at the normally scheduled speed.
(v) To conduct the VMU tests, rotate the airplane as necessary to achieve the V MU attitude.
It is acceptable to use some additional nose-up trim over the normal trim setting during V MU demonstrations. If additional nose-up trim is required, the additional considerations of paragraph (vii), below, apply. VMU is the speed at which the weight of the airplane is completely supported by aerodynamic lift and thrust forces. Some judgment may be necessary on airplanes that have tilting main landing gear bogies. Determining the liftoff point from gear loads and wheel speeds has been found acceptable in past programs. After liftoff, the airplane should be flown out of ground effect. During liftoff and the subsequent climbout, the airplane should be fully controllable.
(vi) VMU testing is a maximum performance flight test maneuver, and liftoff may occur very near the angle of attack for maximum lift coefficient. Also, even though pitch attitude may be held fairly constant during the maneuver, environmental conditions and transiting through ground effect may result in consequential changes in angle of attack. It is permissible to lift off at a speed that is below the normal stall warning speed, provided no more than light buffet is encountered. An artificial stall warning system (e.g., a stick shaker) may be disabled during V MU testing, although doing so will require extreme caution and depend upon a thorough knowledge of the airplane’s stall characteristics. If the airplane is equipped with a stick pusher, for flight test safety reasons it should normally be active and set to the minimum angle of attack side of its rigging tolerance band. However, depending on the airplane’s stall characteristics and the stick pusher design, disabling the pusher or delaying activation of the system until a safe altitude is reached may be the safer course. Again, this decision should be made only with a thorough knowledge of the airplane’s stall characteristics combined with a complete understanding of the stick pusher design.
(vii) VMU Testing for Airplanes Having Limited Pitch Control Authority.
(A) For some airplanes with lim ited pitch control authority, it may not be possible, at forward c.g. and normal trim, to rotate the airplane to a liftoff attitude where the airplane could otherwise perform a clean flyaway at a minimum speed had the required attitude been achieved. This may occur only over a portion of the takeoff weight range in some configurations. When limited pitch control authority is clearly shown to be the case, V MU test conditions may be modified to allow testing aft of the forward c.g.
limit and/or with use of more airplane nose-up trim than normal. The V MU data determined with this procedure should be corrected to those values representative of the appropriate forward limit; the variation of VMU with c.g. may be assumed to be like the variation of free air stalling speed with c.g. Although the development of scheduled takeoff speeds may proceed from these corrected V MU data, additional tests are required (see paragraph (B) below) to check that the relaxed V MU criteria have not neglected problems that might arise from operational variations in rotating airplanes with limited pitch control authority.
(B) In the following assurance test, the airplane must demonstrate safe flyaway characteristics.
(1) Minimum speed liftoff should be demonstrated at the c ritical 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. These tests should be conducted at minimum thrust/weight
Chap 2 30 for both the simulated one-engine-inoperative test (symmetrical reduced thrust) and the all-enginesoperating case.
(2) 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 overrotation, but the liftoff attitude should be maintained as the airplane flies off the ground and out of ground effect.
(3) Resulting liftoff speeds do not affect AFM speed schedules if the test proves successful and the resulting liftoff speed is at least 5 knots below the normally scheduled liftoff speed. Adjustments should be made to the scheduled V R, forward c.g. limit, etc., if necessary, to achieve this result.
(4) This minimum 5-knot reduction below 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 V MU margins of the minimum liftoff speeds demonstrated in this test, relative to those specified in § 25.107(e)(1)(iv), result from the reduced probability of a pitch control authority-limited airplane getting into a high drag condition due to overrotation.
(viii) VMU Testing for Geometry Limited Airplanes.
(A) For airplanes that are geometry limited, the 11 0 percent of VMU required by §
25.107(e)(1)(iv) may be reduced to an operationally acceptable value of 108 percent on the basis that equivalent airworthiness is provided for the geometry-limited airplane. For acceptance of the 108 percent of VMU liftoff speed, safeguards protecting the geometry-limited airplane against both overrotation on the ground and in the air must be provided. Also, the airplane should be geometry limited to the extent that a maximum gross weight takeoff with the tail dragging will result in a clean liftoff and fly-away in the allengines-operating condition. During such a takeoff for the all-engines-operating condition, the resulting distance to the 35 ft. height must not be greater than 105 percent of the normal takeoff distance un der similar weight, altitude, and temperature conditions before the 15 percent margin is added. Lastly, the V MU demonstrated must be sound and repeatable. Compliance with these criteria should be formally addressed in a finding of equivalent safety.
(B) The criteria for demonstrating the capability for a clean liftoff and fly-away are as follows:
(1) The airplane’s pitch attitude from a speed of 96 percent of the actual liftoff speed must be within 5 percent (in degrees) of the tail-dragging attitude to the point of liftoff.
(2) During the above speed range (96 to 100 percent of the actual liftoff speed), the aft under-surface of the airplane must have achieved actual runway contact. It has been found acceptable in tests for contact to exist approximately 50 percent of the time that the airplane is in this speed range.
(3) Beyond the point of liftoff to a height of 35 ft., the airplane’s pitch attitude must not decrease below that at the point of liftoff, or the speed must not increase more than 10 percent.
(4) The airplane shall be at the critical thrust/weight condition, as defined in §
25.107(d), but with all engines operating.
(ix) VMU for a Stretched Version of a Tested Airplane.
(A) It has been considered that the VMU speed represents the aerodynamic potential minimum liftoff speed related to a safe liftoff and fly-away, even though the body contact attitude may
Chap 2 31 prevent achieving the lift coefficient (C L) for this speed. Such is the case when V MU is determined with body contact, whether or not geometry-limited “credit” is sought. It is therefore concluded that a V MU schedule obtained on one model of an airplane type may be applied to a geometry-limited stretched version of that tested airplane. The 108 percent speed factor of paragraph 10b(5)(viii)(A), above, is only applicable to the stretched version if it was applicable to the tested airplane.
(B) Since the concern for tail strikes is increased with the stretched airplane, the following shall be accomplished, in addition to normal takeoff tests, when applying a shorter body airplane’s VMU schedule to the stretched derivative:
(1) Scheduled rotation speeds (V R) for the stretched airplane should result in at least the required liftoff speed margins above V MU (i.e., 1.05 and 1.10 or 1.08, as applicable) of the shorter body airplane, corrected for the reduced runway pitch attitude capability and revised c.g. range of the stretched airplane.
(2) At both the forward and aft c.g. limits, and over the th rust-to-weight range for each takeoff flap, the following abuse takeoff tests must be accomplished. The tests described in paragraphs ( i) and (ii), below, should be accomplished with not more than occasional, minor (i.e., nondamaging) tail strikes.
(i) All-engines-operating, early rotation abuse tests specified in paragraph 10b(6)(iii)(B), including both the rapid rotations and over-rotations as separate test conditions.
(ii) One-engine-inoperative, early rotation abuse tests specified in paragraph 10b(6)(ii).
(iii) All-engines-operating, moderate rotation rate (i.e., more rapid than normal) takeoff abuse tests, using the scheduled V R, and normal pitch attitude after liftoff. No tail strikes may occur for this condition.
(6) Section 25.107(e) - Rotation Speed (V R).
(i) The rotation speed, (V R) 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):
(A) VR may not be less than V 1; however, it can be equal to V 1 in some cases.
(B) VR may not be less than 105 percent of the air minimum control speed (V MCA).
(C) VR must be a speed that will allow the airplane to reach V 2 at or before reaching a height of 35 ft. above the takeoff surface.
(D) 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 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.
(ii) Early rotation, one-engine-inoperativ e abuse test.
(A) 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 dealing with a rotation speed abuse test only specifies an early rotation (V R-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 somewhat below
Chap 2 32 the normal scheduled V 2 speed. However, the amount of permissible V 2 speed reduction must be limited to a reasonable amount as described below.
(B) These test criteria are applicable to all unapproved, new, basic model airplanes.
They are also applicable to previously approved airplanes when subsequent abuse testing is warranted.
However, for those airplanes where the criteria herein are more stringent than that previously applied, consideration will be given to permitting some latitude in the test criteria.
(C) In conducting the flight tests required by § 25.107(e)(3), the test pilot shall use a normal/natural rotation technique as associated with the use of scheduled takeoff speeds for the airplane being tested. Intentional tail or tailskid contact is not considered acceptable. Further, the airspeed attained at the 35-ft. height during this test must not be less than the scheduled V 2 value minus 5 knots. These speed limits should not be considered or used as target V 2 test speeds, but rather are intended to provide an acceptable range of speed departure below the scheduled V 2 value.
(D) In this abuse test, the simulated engine failure should be accomplished sufficiently in advance of the V R test speed to allow for engine spin-down, unless this would be below the VMCG, in which case V MCG should govern. The normal one-engine-inoperative takeoff distance may be analytically adjusted to compensate for the effect of the early thrust reduction. Further, in those tests where the airspeed achieved at the 35-ft. height is slightly less than the V 2-5 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.
(iii) All-engines-operating abuse tests.
(A) 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:
(1) An abuse on rotation speed, and
(2) Out-of-trim conditions, but with rotation at the scheduled V R speed.
NOTE: The expression “marked increase” in the takeoff distance is defined as any amount in excess of 1 percent of the scheduled takeoff distance. Thus, the abuse 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.
(B) For the early rotation abuse 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 test that when the airplane is overrotated at a speed below the scheduled V R, no “marked increase” in the scheduled AFM field length will result. For this demonstration, the airplane should be rotated at a speed 7 percent or 10 knots, whichever is less, below the scheduled V R. Tests should be conducted at a rapid rotation rate or should include an overrotation of 2 degrees above normal attitude after liftoff. Tail strikes during this demonstration are acceptable if they are minor and do not result in unsafe conditions.
(C) 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 V R speed. (See paragraph 21(c)(7)(ii) for additional guidance regarding the evaluation of flight characteristics for out-of-trim conditions.) 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
Chap 2 33 in lieu of actual testing if the analysis shows that the out-of-trim condition would not present unsafe flight characteristics or “marked increase” in the scheduled AFM field lengths.
(iv) Stall Warning During Takeoff Speed Abuse Tests. The presumption is that if a n operational pilot was to make an error in takeoff speeds that resulted in an encounter with stall warning, the likely response would be to aggressively recover to a safe flight condition rather than making a conscious effort to duplicate the AFM takeoff performance data. Therefore, the activation of any stall warning devices, or the occurrence of airframe buffeting during takeoff speed abuse testing, is unacceptable.
(7) Section 25.107(f) - Liftoff Speed (V LOF). The liftoff speed (V LOF) 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. V LOF differs from VMU in that VMU is the minimum possible VLOF speed for a given configuration, and depending upon landing gear design, V MU 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 V MU 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 flyoff is in progress by virtue of lift being greater than weight.
*11. ACCELERATE-STOP DISTANCE - § 25.109 .
a. Explanation. This section describes test demonstrations and data expansion methods necessary to determine accelerate-stop distances for publication in the FAA-approved Airplane Flight Manual (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 accelerate-stop requirements by Amendment 25-92 are listed below. (For other material related to the use of acceleratestop distances, see Parts 121 and 135 of the FAR.) *
Chg. 1 Chap 2 80- 1
(1) 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.
(2) 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.
(3) Section 25.107(a)(2) was revised to remove the reference to “takeoff decision speed” from the definition of V1.
(4) Section 25.109 was revised to add a requirement to determine acce lerate-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 thrust, as introduced by Amendment 25-42, was replaced with a distance increment equivalent to two seconds at V 1. 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 V 1.
(5) 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.
(6) Section 25.109(i) was added to require a maximum brake energy accelerate-stop test to be conducted with not more than 10 percent of the allowable brake wear range remaining on each individual wheelbrake assembly.
(7) 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.
(8) 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 Federal Aviation Regulations (FAR) are § 25.109, and the following:
§ 25.101(f) Airplane configuration and procedures.
§ 25.101(h) Pilot retarding means time delay allowances.
§ 25.101(i) Worn brake stop ping performance.
§ 25.105 Takeoff configuration and environmental and runway conditions.
§§ 25.107(a)(l) & (2) Defines V1 and VEF speeds.
§ 25.735 Brakes.
§ 25.1301 Function and installation.
§ 25.1309 Equipment, systems, and inst allation.
§ 25.1533 Additional operating limitations - maximum takeoff weights and minimum takeoff distances.
Chg. 1 Chap 2 80- 2 § 25.1583(h) FAA-approved Airplane Flight Manual - operating limitations.
§ 25.1587 FAA-approved Airplane Flight Manual - performan ce information.
c. Procedures . The following paragraphs provide guidance for accomplishing accelerate-stop flight tests and expanding the resulting data for the determination of AFM performance information.
(1) Accelerate-stop testing . The following guidance is applicable 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).
(i) In order to establish a distance that would be representative of the distance needed in the e vent of a rejected takeoff, where the first action to stop the airplane is taken at or below V 1, 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 advisory circular.)
(ii) The guidance outlined in paragraph 11c(3) describes how to apply the appropriate time delays, as required by § 25.101(h)(3), for the flightcrew to accomplish the rejected takeoff operating procedures.
(iii) 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 amount of wear allowed before the brake must be removed from the airplane for overhaul. The allowable wear should be defined in terms of a lineardimension in the axial direction, which is typically determined by measuring the wear pin extension.
(A) The only accelerate-stop test that mu st 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 (see paragraph 11c(2)(iii)). 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.
(B) Either airplane-worn or mechanically-worn brakes (i.e., machined or dynamometer worn) may be used. If mechanically-worn brakes are used, it must 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.
(iv) Section 25.109(f)(1) denies credit for the use of reverse thrust as a decelerati ng means in determining the accelerate-stop distance for a dry runway. This provision applies to both turbine engine and propeller engine reverse thrust. 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)(v) for guidance related to obtaining accelerate-stop performance credit for reverse thrust on wet runways.)
(v) 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 that must be absorbed by the brakes. The limiting brake energy value in the AFM shall 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.)
Chg. 1 Chap 2 80- 3
(vi) 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 V 1 speed to fully investigate the energy range and weight envelope.
(vii) 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 must be conducted on hard surfaced, dry runways.
(viii) For approval of dispatch capability with anti-skid inoperative, nose wheel brakes or specified main wheel-brake(s) inoperative, autobraking systems, etc., a full set of tests, as in paragraph (vii) 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.
(ix) Either ground or airborne instrumentation sh ould include means to determine the horizontal distance time-history.
(x) 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 advisory circular.)
(xi) The accelerate -stop tests should be conducted in the following configurations:
(A) Heavy to light weight as required.
(B) Most critical c.g. position.
(C) Wing flaps in the takeoff pos ition(s).
(D) Tire pressure: before taxi and with cold tires, set to the highest value appropriate to the takeoff weight for which approval is being sought.
(E) Engine idle thrust: set at the recommended upper limit for use on the ground or t he effect of maximum ground idle 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 thrust.
(xii) Engine thrust should be appropriate to ea ch segment of the rejected takeoff and should include accounting for thrust decay rates (i.e., spindown) for failed or throttled back engines. At the speed that corresponds to the energy level defined for the test demonstration, the stopping sequence is initiated by employing the first acceptable braking means.
(A) 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. (Note: If, due to the certification basis of the airplane, all-engine-accelerate-stop distances are not being considered, the one-engineinoperative AFM distances should be based on the critical engine failing to maximum ground idle thrust rather than the windmilling condition.) The thrust from the operative engine(s) should be consistent with a throttle chop to maximum ground idle thrust. For determining the all-engines-operating dry runway accelerate-stop AFM distances, the stopping portion should be based on all engines producing maximum ground idle thrust (after engine spindown), as noted in paragraph 11c(1)(xi)(E). The accelerate-stop tests may be conducted with either concurrent or sequential throttle chops to idle 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 thrust and the idle thrust level achieved during the test. For the criteria relating to reverse thrust credit for wet runway accelerate-stop distances, see paragraph 11c(4)(v).
Chg. 1 Chap 2 80- 4 (B) 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 one-engine-inoperative accelerate-stop distances, the high drag groundidle 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 §§ 25.109(f) and (h). Wet runway controllability may either be demonstrated by using the guidance available in paragraph 11c(4)(v)(F) 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 § 25.109(f) and (h)). For the criteria relating to reverse thrust credit for wet runway accelerate-stop distances, see paragraph 11c(4)(v).
(xiii) 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)(ix)).
(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.
(i) The maximum brake energy accel erate-stop demonstration should be conducted at not less than the maximum takeoff weight and should be preceded by at least a 3-mile taxi with all engines operating at maximum ground idle 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.
(ii) Section 25.735(h) requires the rejected takeoff brake kinetic energy capacity rating of each main wheel-brake assembly to be determined at the fully worn limit of its allowable wear range. The calculation of maximum brake energy limited takeoff weights and speeds, for presentation in the AFM performance section, must therefore be based on each airplane main wheel-brake being in the fully worn condition.
(iii) 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.
(iv) 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 Technical Standard Order (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.
(v) 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 11(c)(2)(i), were not able to accurately account for the associated energy increments.
(3) Accelerate-Stop Time Delays . Section 25.101(h) of the FAR 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, V EF, and clarified the meaning of V 1 with relation to VEF. The preamble to Amendment 25-42 states that “V 1 is determined by adding to V EF (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
Chg. 1 Chap 2 80- 5 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 V 1 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. 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 to stop as the airplane passes through V 1.
Since the typical transport category airplane requires three pilot actions (i.e., brakes-throttles-spoilers) to achieve the final braking configuration, Amendment 25-42 defined a two-second time period, in § 25.109, to account for delays in activating the second and third deceleration devices. Amendment 25-92 (effective March 20, 1998) redefined, and reinterpreted the application of that two-second delay time as a distance increment equivalent to two seconds at V1. No credit may be taken for system transient effects (e.g., engine spin-down, brake pressure ramp-up, etc.) in determining this distance. The following paragraphs provide guidance related to the interpretation and application of delay times to show compliance with the accelerate-stop requirements of Amendment 25-92.
(i) Figure 11-1 presents a pictorial representation of the accelerate-stop time delays considered acceptable for compliance with § 25.101(h) as discussed above.
FIGURE 11-1. ACCELERATE-STOP TIME DELAYS Initiation of first pilot action Initiation of second pilot action Initiation of third pilot action Engine failure V1VEF tact 1 tact 1 Event Demonstrated Time Delays Flight Manual Expansion Time Delays Initiation of subsequent pilot actions tact 4 n tact+ 1 sec.
tact 2 tact 2 tact 3 tact 3 Distance equivalent to 2 sec. at V1
(ii) VEF is the calibrated airspeed selected by the applicant at which the critical engine is assumed to fail. The relationship between V EF and V1 is defined in § 25.107.
(iii) ∆tact 1 = the demonstrated time interval between engine failure and initiation of the first pilot action to stop the airplane. This time interval is defined as beginning at the instant the critical engine is failed and ending when the pilot recognizes and reacts to the engine failure, as indicated by the pilot’s application of the first retarding means during accelerate-stop tests. A sufficient number of demonstrations should be conducted using both applicant and FAA test pilots to assure that the time increment is representative and repeatable. The pilot’s feet should be on the rudder pedals, not the brakes, during the tests. For AFM data expansion purposes, in order to provide a recognition time increment that can be executed consistently in service, this time increment must be equal to the demonstrated time or one second, whichever is greater. If the airplane incorporates an engine failure warning light, the recognition time includes the time increment necessary for the engine to spool down to the point of warning light activation, plus the time increment from light “on” to pilot action indicating recognition of the engine failure.
(iv) ∆tact 2 = the demonstrated time interval between initiation of the first and second pilot actions to stop the airplane.
(v) ∆tact 3 = the demonstrated time interval between initiation of the second and third pilot actions to stop the airplane.
(vi) ∆tact 4→n = the demonstrated time interval between initiation of the third and fourth (and any subsequent) pilot actions to stop the airplane. For AFM expansion, a one-second reaction time delay to account for in-service variations should be added to the demonstrated time interval between the third and fourth (and any subsequent) pilot actions. If a command is required for another crewmember to initiate an action to stop the
Chg. 1 Chap 2 80- 6 airplane, a two-second delay, in lieu of the one-second delay, should be applied for each action. For automatic deceleration devices that are approved for performance credit for AFM data expansion, established systems actuation times determined during certification testing may be used without the application of the additional time delays required by this paragraph.
(vii) The sequence for ini tiation of pilot actions may be selected by the applicant, but it must match the sequence established for operation in service, as prescribed by § 25.101(f). If, on occasion, the specified sequence is not achieved during testing, the test need not be repeated; however, sufficient testing must be conducted to establish acceptable values of ∆tact.
(viii) Sections 25.109(a)(1)(iv) and (a)(2)(iii) require the one-engine-inoperative and all-enginesoperating accelerate-stop distances, respectively, to include a distance increment equivalent to two seconds at V 1.
(Although the requirement for the distance increment equivalent to two seconds at V 1 is explicitly stated in the “dry runway” criteria of § 25.109, it is also applied to the “wet runway” accelerate-stop distances by reference in § 25.109(b).) This distance increment is represented pictorially on the right side of the “Flight Manual Expansion Time Delays” presentation in Figure 11-1, and in the speed versus distance plot of Figure 11-2, below. The twosecond time period is only provided as a method to calculate the required distance increment, and is not considered to be a part of the accelerate-stop braking transition sequence. Consequently, no credit for pilot actions, or engine and systems transient responses (e.g., engine spin-down) may be taken during this two-second time period.
Similarly, the two-second time period may not be reduced for airplanes incorporating automated systems that decrease the number of pilot actions required to obtain the full braking configuration (e.g., autospoiler systems).
Chg. 1 Chap 2 80- 7 FIGURE 11-2. ACCELERATE-STOP SPEED VS. DISTANCE SPEED
DISTANCE 3rd Action 2nd Action V1 (1st Action) VEF AFM model must accurately represent demonstrated energy state of the airplane after V1.
} Distance for 2 secs. at V1
(ix) Any residual acceleration that occurs after V1, while the airplane and its systems become stabilized in the braking configuration, must be accounted for in the expansion of accelerate-stop performance data for presentation in the AFM. The effects of system transients, such as engine spin-down, brake pressure ramp-up, spoiler actuation times, etc., should be accounted for in this time period. The area of interest is noted at the top of the graphical representation of the speed versus distance relationship in Figure 11-2.
(x) All-Engine Accelerate-Stop Distance . For the all-engines-operating accelerate-stop distance prescribed by § 25.109(a)(2), apply the demonstrated time intervals, and associated delays, of paragraphs 11c(3)(iv) through (vi) after the airplane has accelerated to V 1.
(xi) The procedures used to determine the accelerate-stop distance must be described in the performance section of the AFM.
(4) Wet Runway Accelerate-Stop Distance . The following guidance is provided for showing compliance with the requirements stated in §§ 25.109(b) through (d) for determining accelerate-stop distances applicable to wet runways. In general, the wet runway accelerate-stop distance is determined in a similar manner to the dry runway accelerate-stop distance. The only differences are in reflecting the reduced stopping force available from the wheel brakes on the wet surface and in provisions for performance credit for the use of reverse thrust as an additional decelerating means. The general method for determining the reduced stopping capability of the wheel brakes on a smooth wet runway is as follows: First, determine the maximum tire-to-ground wet runway braking coefficient of friction versus ground speed from the relationships provided in § 25.109(c)(1). Then, adjust this braking coefficient