18 thrust, if applicable, over the range of power settings, altitudes, temperatures, and airspeeds for which approval is sought. Although it is not possible to definitively list or foresee all of the types of instrumentation that might be considered adequate for determining thrust (or power) output, two examples used in past certification programs are: (1) engine pressure rakes, with engines calibrated in a ground test cell, and (2) fan speed, with engines calibrated in a ground test cell and the calibration data validated by the use of a flying test bed. In any case, the applicant should substantiate the adequacy of the instrumentation to be used for determining the thrust (or power) output.
(b) Lapse rate takeoff testing to characterize the behavior of power setting, rotor speeds, propeller effects (i.e., torque, RPM, and blade angle), or gas temperature as a function of time, thermal state, or airspeed, as appropriate. These tests should include the operation of an automatic takeoff thrust control system (ATTCS), if applicable, and should cover the range of power settings for which approval is sought.
1 Data for higher altitude power settings may be acquired via overboost
(i.e., operating at a higher than normal power setting for the conditions) with the consent of the engine and propeller manufacturer(s), when applicable. When considering the use of overboost on turbopropeller propulsion system installations to simulate higher altitude and ambient temperature range conditions, the capability to achieve an appropriate simulation should be evaluated based on the engine and propeller control system(s) and aircraft performance and structural considerations. Engine (gearbox) torque, rotor speed, or gas temperature limits, including protection devices to prohibit or limit exceedances, may prevent the required amount of overboost needed for performance at the maximum airport altitude sought for approval.
Overboost may be considered as increased torque, reduced propeller speed, or a combination of both, in order to achieve the appropriate blade angle for the higher altitude and ambient temperature range simulation. Consideration for extrapolations will depend on the applicant’s substantiation of the proper turbopropeller propulsion system simulated test conditions.
2 Lapse rate characteristics should be validated by takeoff demonstrations at the maximum airport altitude for which takeoff approval is being sought. Alternatively, if overboost (see paragraph 1 above) is used to simulate the thrust setting parameters of the maximum airport altitude for which takeoff approval is sought, the takeoff demonstrations of lapse rate characteristics can be performed at an airport altitude up to 3,000 feet lower than the maximum airport altitude.
(c) Thrust calculation substantiation. Installed thrust should be calculated via a mathematical model of the propulsion system, or other appropriate means, adjusted as necessary to match the measured inflight performance characteristics of the installed propulsion system.
The propulsion system mathematical model should define the relationship of thrust to the power setting parameter over the range of power setting, airspeed, altitude, and temperature for which approval is sought. For turbojet airplanes, the propulsion system mathematical model should be substantiated by ground tests in which thrust is directly measured via a calibrated load cell or equivalent means. For turbopropeller airplanes, the engine power measurement should be substantiated by a calibrated dynamometer or equivalent means, the engine jet thrust should be
19 established by an acceptable engine model, and the propeller thrust and power characteristics should be substantiated by wind tunnel testing or equivalent means.
(d) Effects of ambient temperature. The flight tests of paragraph 9b(l)(a) above will typically provide data over a broad range of ambient temperatures. Additional data may be obtained from other flight or ground tests of the same type or series of engine. The objective is to confirm that the propulsion system model accurately reflects the effect of temperature over the range of ambient temperatures for which approval is being sought (operating envelope).
Because thrust (or power) data can usually be normalized versus temperature using either dimensionless variables (e.g., theta exponents or a thermodynamic cycle model), it is usually unnecessary to obtain data over the entire ambient temperature range. There is no need to conduct additional testing if:
1 The data show that the behavior of thrust and limiting parameters versus ambient temperature can be predicted accurately, and
2 Analysis based upon the test data shows that the propulsion system will operate at rated thrust without exceeding propulsion system limits.
(2) Extrapolation of propulsion system performance data to 3,000 feet above the highest airport altitude tested (but no higher than the maximum takeoff airport altitude to be approved) is acceptable, provided the supporting data, including flight test and propulsion system operations data (e.g., engine and propeller control, limits exceedance, and surge protection devices scheduling), substantiates the proposed extrapolation procedures.
Considerations for extrapolation depend upon an applicant’s determination, understanding, and substantiation of the critical operating modes of the propulsion system. This understanding includes a determination and quantification of the effects that propulsion system installation and variations in ambient conditions have on these modes.
10. Takeoff and Takeoff Speeds - §§ 25.105 and 25.107.
a. Explanation. The primary objective of the takeoff tests required by § 25.107 is to determine the takeoff speed schedule for all takeoff configurations at all weight, altitude, and temperature conditions within the operational limits selected by the applicant. The provisions of § 25.105 are self evident and are not repeated or amplified in this discussion. Guidance material for unpaved runway evaluation is contained in Chapter 8 (paragraph 232) of this AC.
b. Procedures. Although the following speed definitions are given in terms of calibrated airspeed, the AFM presentations should be given in terms of indicated airspeed. Attention should be paid to all potential sources of airspeed error, but special consideration should be given to airplanes with electronic instruments in the cockpit that apply electronic filtering to the airspeed data. This filtering, which causes a time delay in the airspeed indication, can be a source of significant systematic error in the presentation of airspeed to the flightcrew. During a normal takeoff acceleration, the airplane will be at a higher speed than is indicated by the cockpit instrument, which can result in longer distances than are presented in the AFM, particularly in the event of a rejected takeoff near the indicated V1 speed. The effects of any time delays caused
20 by electronic filtering, pneumatic system lag, or other sources should be adequately addressed in the AFM speed and distance presentations. Further explanation of airspeed lag, particularly pertaining to airplanes with electronic instruments in the cockpit, and procedures for calibrating the airspeed indicating system (§ 25.1323(b)) are presented in paragraph 177 of this AC.
(1) Section 25.107(a)(1) - Engine Failure Speed (VEF). The engine failure speed (VEF) is defined as the calibrated airspeed at which the critical engine is assumed to fail and must be selected by the applicant. VEF cannot be less than the ground minimum control speed (VMCG).
(2) Section 25.107(a)(2) - Takeoff Decision Speed (V1). The takeoff decision speed (V1) may not be less than VEF plus the speed gained with the critical engine inoperative during the time interval between VEF and the instant at which the pilot takes action after recognizing the engine failure. This is indicated by pilot application of the first deceleration device such as brakes, throttles, spoilers, etc. during accelerate-stop tests, or by the first control input during VMCG testing. The applicant may choose the sequence of events. Refer to paragraph 11 of this AC, addressing § 25.109, for a more complete description of rejected takeoff (RTO) transition procedures and associated time delays. If it becomes evident in expansion of takeoff data for the AFM that excessive variation in V1 exists, resulting from the many performance variables involved (variations of +1.5 knots or +100 ft. have been found acceptable), then measures should be taken to ensure that scheduled performance variations are not excessive. Examples of such measures are small field length factors, or increments, and multiple web charts (acceleratego/stop, V1/VR) for a particular configuration.
(3) Section 25.107(b) - Minimum Takeoff Safety Speed (V 2MIN).
(a) V 2MIN, in terms of calibrated airspeed, cannot be less than:
1 1.1 times the V MCA defined in § 25.149.
2 1.13 times V SR for two-engine and three-engine turbopropeller and reciprocating engine-powered airplanes and for all turbojet airplanes that do not have provisions for obtaining significant reduction in the one-engine inoperative power-on stalling speed (i.e., boundary layer control, blown flaps, etc.). The value of VS to be used in determining V2MIN is the stall speed in the applicable takeoff configuration, landing gear retracted, except for those airplanes with a fixed landing gear or for gear-down dispatch.
(b) V 2MIN may be reduced to 1.08 times VSR for turbopropeller and reciprocating engine-powered airplanes with more than three engines, and turbojet powered airplanes with adequate provisions for obtaining significant power-on stall speed reduction through the use of such things as boundary layer control, blown flaps, etc.
(c) For propeller-driven airplanes, the difference between the two margins, based upon the number of engines installed on the airplane, is because the application of power ordinarily reduces the stalling speed appreciably. In the case of the two-engine propeller-driven airplane, at least half of this reduction is eliminated by the failure of an engine. The difference in the required factors therefore provides approximately the same margin over the actual stalling speed under the power-on conditions that are obtained after the loss of an engine, no matter what the number of engines (in excess of one) may be. Unlike the propeller-driven airplane, the turbojet/turbofan powered airplane does not show any appreciable difference between the poweron and power-off stalling speed. This is due to the absence of the propeller, which ordinarily induces a slipstream with the application of power causing the wing to retain its lift to a speed lower than the power-off stalling speed. The applicant’s selection of the two speeds specified will influence the nature of the testing required in establishing the takeoff flight path.
(4) Section 25.107(c) - Takeoff Safety Speed (V 2). V2 is the calibrated airspeed that is attained at or before the airplane reaches a height of 35 ft. above the takeoff surface after an engine failure at VEF using an established rotation speed (VR). During the takeoff speeds demonstration, V2 should be continued to an altitude sufficient to assure stable conditions beyond the 35 ft height. V2 cannot be less than V2MIN. In addition, V2 cannot be less than the liftoff speed, VLOF, which is defined in § 25.107(f). In accordance with § 25.107(c), V2 in terms of calibrated airspeed may not be less than VR plus the speed increment attained before reaching a height of 35 feet above the takeoff surface and a speed that provides the maneuvering capability specified in § 25.143(h). Section 25.111(c)(2) stipulates that the airplane must reach V2 before it is 35 feet above the takeoff surface and continue at a speed not less than V2 until it is 400 feet above the takeoff surface. These requirements were first expressed in SR-422A, paragraphs 4T.114(b)(4) and (c)(3) and 4T.116(e). The intent of these requirements is discussed in the preamble to SR-422A, which states, in part, “For these reasons, this regulation permits the airplane to lift off the ground at a speed lower than the V2 speed.” The concern that the regulation change was addressing was the overshoot of V2 after liftoff when it was required that the airplane attain V2 on, or near, the ground. It was therefore the intent of the regulation to allow an acceleration to V2 after liftoff but not to allow a decrease in the field length required to attain a height of 35 feet above the takeoff surface by attaining a speed greater than V2, under low drag ground conditions, and using the excess kinetic energy to attain the 35 foot height.
(a) In the case of turbojet powered airplanes, when the bulk of the one-engineinoperative data have been determined with idle cuts, V2, and its relationship to VR, should be substantiated by a limited number of fuel cuts at VEF. For derivative programs not involving a new or modified engine type (i.e., a modification that would affect thrust decay characteristics), fuel cuts are unnecessary if thrust decay characteristics have been adequately substantiated.
(b) For propeller-driven airplanes, the use of fuel cuts can be more important in order to ensure that the takeoff speeds and distances are obtained with the critical engine’s propeller attaining the position it would during a sudden engine failure. The number of tests that should be conducted using fuel cuts, if any, depends on the correlation obtained with the idle cut data and substantiation that the data analysis methodology adequately models the effects of a sudden engine failure.
(5) Section 25.107(d) - Minimum Unstick Speed (V MU).
(a) An applicant should comply with § 25.107(d) by conducting minimum unstick speed (VMU) determination tests with all engines operating and with one engine inoperative. If a stick pusher is installed, it should normally be active and set to the minimum angle-of-attack side of its rigging tolerance band. If desired, artificial stall warning systems may be disabled for this demonstration. (See paragraph (f) below for further discussion regarding the setting of stick pusher and stall warning systems during VMU testing.) During this demonstration, the takeoff should be continued until the airplane is out of ground effect. The airplane pitch attitude should not be decreased after liftoff.
(b) In lieu of conducting actual one-engine-inoperative VMU tests, the applicant may conduct all-engines-operating VMU tests that simulate and account for all pertinent factors that would be associated with an actual one-engine-inoperative VMU test. To account fully for pertinent factors, it may be necessary to adjust the resulting VMU test values analytically. The factors to be accounted for should include at least the following:
1 Thrust/weight ratio for the one-engine-inoperative range.
2 Controllability (may be related to one-engine-inoperative free air tests, such as VS, VMCA, etc.).
3 Increased drag due to lateral/directional control systems.
4 Reduced lift due to devices such as wing spoilers used for lateral control.
5 Adverse effects of any other systems or devices on control, drag, or lift.
(c) The number of VMU tests required may be minimized by testing only the critical all-engines-operating and one-engine-inoperative thrust/weight ratios, provided that the VMU speeds determined at these critical conditions are used for the range of thrust/weights appropriate to the all-engines-operating and one-engine-inoperative configurations. The critical thrust/weight is established by correcting, to the VMU speed, the thrust that results in the airplane achieving its limiting one-engine-inoperative climb gradient at the normally scheduled speed and in the appropriate configuration.
(d) Amendment 25-42, effective March 1, 1978, revised §§ 25.107(d) and
25.107(e)(1)(iv) in order to permit the one-engine-inoperative VMU to be determined by allengines-operating tests at the thrust/weight ratio corresponding to the one-engine-inoperative
23 condition. As revised, § 25.107(d) specifies that VMU must be selected for the range of thrust/weight ratios to be certificated, rather than for the all-engines-operating and one-engineinoperative 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 VMU 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.
(e) To conduct the VMU tests, rotate the airplane as necessary to achieve the VMU attitude. It is acceptable to use some additional nose-up trim over the normal trim setting during VMU demonstrations. If additional nose-up trim is required, the additional considerations of paragraph (g), 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.
(f) V MU 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 VMU 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.
(g) V MU Testing for Airplanes Having Limited Pitch Control Authority.
1 For some airplanes with limited 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, VMU 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 VMU data determined with this procedure should be
Change 1 24 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 VMU data, additional tests are required (see paragraph 2 below) to check that the relaxed VMU criteria have not neglected problems that might arise from operational variations in rotating airplanes with limited pitch control authority.
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. These tests should be conducted at minimum thrust/weight for both the simulated one-engine-inoperative test (symmetrical reduced thrust) and the all-engines-operating case.
(bb) 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.
(cc) 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 VR, forward c.g. limit, etc., if necessary, to achieve this result.
(dd) 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 VMU 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.
(h) VMU Testing for Geometry Limited Airplanes.
1 For airplanes that are geometry limited (i.e., the minimum possible V MU 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
Change 1 25 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.
(i) VMU for a Stretched Version of a Tested Airplane.
1 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 prevent achieving the lift coefficient (CL) for this speed. Such is the case when VMU is determined with body contact, whether or not geometry limited “credit” is sought.
It is therefore concluded that a VMU schedule obtained on one model of an airplane type may be applied to a geometry limited stretched version of that tested airplane. The 108 and 104 percent speed factors of § 25.107(e)(1)(iv)(B), as discussed in paragraph 10b(5)(h)1, above, are only applicable to the stretched version if they were applicable to the tested airplane.
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 applying a shorter body airplane’s VMU schedule to the stretched derivative:
(aa) Scheduled rotation speeds (V R) for the stretched airplane should result in at least the required liftoff speed margins above VMU (i.e., 1.05 and 1.10, or 1.04 and
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.
(bb) At both the forward and aft c.g. limits, and over the thrust-to-weight range for each takeoff flap, the following abuse 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.
Change 1 26
(i) All-engines-operating, early rotation abuse tests specified in paragraph 10b(6)(c)2, 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)(b).
(iii) All-engines-operating, moderate rotation rate (i.e., more rapid than normal) takeoff abuse tests, using the scheduled VR, and normal pitch attitude after liftoff.
Tail strikes should not occur for this condition.
(6) 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 V R may not be less than V1; however, it can be equal to V1 in some cases.
2 V R may not be less than 105 percent of the air minimum control speed (VMCA).
3 V R 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 V R 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 abuse 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 dealing with a rotation speed abuse test only specifies an early rotation (VR-5 knots), it is interpreted that pilot technique is to remain the same as normally used for a oneengine-inoperative condition. With these considerations in mind, it is apparent that the airspeed achieved at the 35 ft. point can be 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 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.
3 In conducting the flight tests required by § 25.107(e)(3), the test pilot should use a normal/natural rotation technique as associated with the use of scheduled takeoff speeds for the airplane being tested. Intentional tail or tail skid contact is not considered acceptable. 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 abuse 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 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 abuse 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) An abuse on 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 defined as any amount in excess of one 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.
2 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 over-rotated at a speed below the scheduled VR, 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 VR. Tests should be conducted at a rapid rotation rate or should include an
28 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.
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. (See paragraph 21b(7)(b) for additional guidance regarding the evaluation of flight characteristics for out-of-trim conditions.) The amount of mis-trim 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 “marked increase” in the scheduled AFM field lengths.
(d) Stall Warning During Takeoff Speed Abuse 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 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 (VLOF). 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.
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 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)
(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 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 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.
(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 wheel-brake 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.
NOTE: Section 25.735, including § 25.735(h) has been more recently revised by amendment 25-107.
(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 part 25 regulations 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 stopping performance § 25.105 Takeoff configuration and environmental and runway