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4.2.4 Procedures: Section 25.107(a)(2)—V1.
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 means such as brakes, throttles, spoilers, etc. during accelerate-stop tests.
The applicant may choose the sequence of events. Refer to paragraph 4.3 of this AC, addressing § 25.109, for a more complete description of rejected takeoff (RTO) transition procedures and associated time delays.
4.2.5 Procedures: Section 25.107(b)—Minimum Takeoff Safety Speed (V2MIN).
4.2.5.1 V2MIN, in terms of calibrated airspeed, cannot be less than:
4.2.5.1.1 1.1 times the VMC defined in § 25.149.
4.2.5.1.2 1.13 times VSR for two-engine and three-engine turbopropeller and reciprocating engine-powered airplanes and for all turbojet airplanes that do not have provisions for obtaining a significant reduction in the one-engine-inoperative power-on stalling speed (i.e., boundary layer control, blown flaps, etc.). The value of VSR 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.
4.2.5.2 V2MIN 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.
4.2.5.3 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 power-on 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.
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4.2.6 Procedures: Section 25.107(c)—Takeoff Safety Speed (V2).
4.2.6.1 V2 is the calibrated airspeed that is attained at or before the airplane reaches a height of 35 feet above the takeoff surface after an engine failure at VEF using an established rotation speed (VR). From the liftoff point, the takeoff surface extends to the end of the takeoff distance continuing at the same slope as the runway. During the takeoff speeds demonstration, V2 should be continued to an altitude sufficient to assure stable conditions beyond the 35-foot 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). In addition, § 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 Special Civil Air Regulation No. SR-422, Turbine-Powered Transport Category Airplanes of Current Design (SR-422A), paragraphs 4T.114(b)(4) and (c)(3) and 4T.116(e). The concern that the regulation change was addressing was the overshoot of V2 after liftoff under the previous requirement that the airplane attain V2 on, or near, the ground. The intent of the current requirement is 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.
4.2.6.2 In the case of turbojet powered airplanes, when most of the one-engine-inoperative data have been collected using throttle chops, V2, and its relationship to VR, should be substantiated by at least a limited number of fuel cuts at VEF. For derivative programs not involving a modification that would affect thrust decay characteristics, demonstrations of fuel cuts may be unnecessary.
4.2.6.3 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 depends on the correlation obtained with the throttle chop data and substantiation that the data analysis methodology adequately models the effects of a sudden engine failure.
4.2.7 Procedures: Section 25.107(d)—Minimum Unstick Speed (VMU).
4.2.7.1 Section 25.107(d) states, “VMU speeds must be selected by the applicant.”
An applicant can either determine the lowest possible VMU speeds or select
4-7 a higher speed that supports the takeoff performance targets of the airplane. Regardless of how the applicant selects the VMU speeds, compliance must be shown with § 25.107(d), (e)(1)(iv), (e)(3), and (e)(4) to show that the selected VMU speeds allow the airplane to safely lift off the ground and continue the takeoff.
4.2.7.2 An applicant should comply with § 25.107(d) by conducting VMU tests with all engines operating and also with one engine inoperative. During these tests, the takeoff should be continued until the airplane is out of ground effect. The airplane pitch attitude should not be decreased after liftoff.
4.2.7.3 VMU testing to demonstrate the lowest VMU speed 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 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.
4.2.7.3.1 It is important for the flight test team to understand the control laws and any transitions between control laws during takeoff (e.g., based on weight on wheels) for an electronic flight control system that may present unique hazards that should be taken into account.
4.2.7.3.2 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, both in and out of ground effect.
4.2.7.3.3 If the airplane is equipped with a stick pusher, angle-of-attack limiter, or other system that may affect the conduct of the test, the angle of attack setting for activation of the system may be selected by the applicant and differ from the nominal setting. The system may alternatively be disabled or its activation delayed for test purposes until a safe altitude is reached.
However, for airplanes equipped with a stick pusher that is not designed to be inhibited during takeoff, the VMU test demonstrations will need to be assessed and will only remain valid if the stick pusher would not have activated with the angle-of-attack indication means set at the lowest angle within production tolerances.
4.2.7.4 In lieu of conducting one-engine-inoperative VMU tests, the applicant may conduct all-engines-operating VMU tests if all pertinent factors that would be associated with an actual one-engine-inoperative VMU test are simulated or otherwise taken into account. To take into account all pertinent factors,
4-8 it may be necessary to adjust the resulting VMU test values analytically.
The factors to be accounted for should include at least the following:
Thrust/weight ratio for the one-engine-inoperative range.
Controllability (may be related to one-engine-inoperative free air tests, such as minimum control speed in the air (VMCA)).
Increased drag due to use of lateral/directional control systems.
Reduced lift due to use of devices such as wing spoilers for lateral control.
Adverse effects of use of any other systems or devices on control, drag, or lift.
4.2.7.5 The number of VMU tests needed may be minimized by testing only the critical all-engines-operating and one-engine-inoperative thrust/weight ratios, provided 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.
4.2.7.6 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 all-engines-operating tests at the thrust/weight ratio corresponding to the one-engine-inoperative 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-enginesoperating 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, consider 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.
4.2.7.7 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 4.2.7.8 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.
4-9 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.
4.2.7.8 VMU Testing for Airplanes having Limited Pitch Control Authority.
4.2.7.8.1 For some airplanes with limited pitch control authority, it may not be possible, at forward CG 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.
Though generally associated with the inability of the pitch control surfaces to provide adequate pitching moment to rotate the airplane to the desired pitch attitude at low thrust/weight ratio conditions, the same phenomenon may occur at high thrust/weight ratio conditions for airplanes with high thrust lines (e.g., aft engines mounted high on the fuselage). 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 CG limit and/or with use of more airplane nose-up trim than normal. The VMU data determined with this procedure should be corrected to those values representative of the appropriate forward limit; the variation of VMU with CG may be assumed to be like the variation of free air stalling speed with CG.
Although the development of scheduled takeoff speeds may proceed from these corrected VMU data, additional tests are required (see paragraph 4.2.7.8.2 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.
4.2.7.8.2 In the following assurance test, the airplane should demonstrate safe flyaway characteristics: Minimum speed liftoff should be demonstrated at the critical forward CG limit with normal trim. For airplanes with a cutback forward CG at heavy weight, two weight/CG 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 CG. The full forward CG 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/CG combination with regard to limited pitch attitude capability for liftoff.
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.
4.2.7.8.3 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-engine-inoperative test (i.e., symmetrical reduced thrust) case and the all-engines-operating case.
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4.2.7.8.4 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.
4.2.7.8.5 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.
4.2.7.8.6 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.
4.2.7.8.7 This minimum 5 knot margin from the scheduled liftoff speed provides some leeway for operational variations such as mistrim, CG 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.
4.2.7.9 VMU Testing for Geometry Limited Airplanes.
4.2.7.9.1 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.
4.2.7.9.2 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.
4.2.7.9.3 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-engines-operating condition:
1. 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
4-11 contact to exist approximately 50 percent of the time that the airplane is in this speed range.
2. 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.
3. 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.
4.2.7.10 VMU for a Stretched Version of a Tested Airplane.
4.2.7.10.1 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 4.2.7.9.1 above, then the flight tests described in paragraph 4.2.7.9.3 above should be performed on the stretched derivative. Otherwise, the flight tests described in paragraph 4.2.7.10.2.2 below should be performed on the stretched derivative.
4.2.7.10.2 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:
1. The 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 CG 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 for the stretched airplane should result in at least the required liftoff speed margins above the corrected VMU required by § 25.107(e)(1)(iv) for the one-engine-inoperative and all-engines-operating takeoff conditions.
2. At both the forward and aft CG limits, and over the thrust-to-weight range for each takeoff flap, the following takeoff tests should be accomplished. The tests described in paragraphs a and b below should be accomplished with not more than occasional, minor (i.e., non-damaging) tail strikes.
a. All-engines-operating, early rotation tests specified in paragraph 4.2.8.3.2 below, including both the rapid rotations and over-rotations as separate test conditions.
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b. One-engine-inoperative, early rotation tests specified in paragraph 4.2.8.2 below.
c. 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.
4.2.8 Procedures: Section 25.107(e)—Rotation Speed (VR).
4.2.8.1 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):
4.2.8.1.1 VR may not be less than V1; however, it can be equal to V1 in some cases.
4.2.8.1.2 VR may not be less than 105 percent of VMCA.
4.2.8.1.3 VR must be a speed that will allow the airplane to reach V2 at or before reaching a height of 35 feet above the takeoff surface.
4.2.8.1.4 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.
4.2.8.2 Early Rotation, One-Engine-Inoperative Test.
4.2.8.2.1 In showing compliance with § 25.107(e)(3), some guidance relative to the airspeed attained at the 35-foot 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-foot 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.
4.2.8.2.2 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.
4.2.8.2.3 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
4-13 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-foot 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.
4.2.8.2.4 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-foot 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.
4.2.8.3 All-Engines-Operating Tests.
4.2.8.3.1 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:
A lower than scheduled rotation speed, and 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.
4.2.8.3.2 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 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° 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
4-14 ground effect. Tail strikes during this demonstration are acceptable if they are minor and do not result in unsafe conditions.
4.2.8.3.3 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.
4.2.8.3.4 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 CG position and mistrimmed for an aft CG 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 CG position and mistrimmed for a forward CG loading, it may not be possible to readily arrest a self-rotating 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:
1. The test pilot should determine that no unsafe characteristics exist with the airplane loaded to the forward CG 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.
2. The test pilot should determine that no unsafe characteristics exist with the airplane loaded to the aft CG 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
4-15 control input, or an excessive pitch control force required to maintain the airplane in the normal pitch attitude prior to the scheduled rotation speed or during rotation and initial climb.
3. For the tests described in paragraphs 1 and 2 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.
4.2.8.4 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.
4.2.8.5 Stick Forces during Takeoff Speed Tests.
According to § 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 1 and 2 (on page 4-14) 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).
4.2.9 Procedures: Section 25.107(f)—Liftoff Speed (VLOF).
4.2.9.1 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.
4.2.9.2 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).
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4.3 Accelerate-Stop Distance—§ 25.109.
4.3.1 Explanation.
4.3.1.1 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.
4.3.1.2 This section describes test demonstrations and data expansion methods necessary to determine accelerate-stop distances for publication in the FAA-approved 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 accelerate-stop distances, see 14 CFR parts 121 and 135.)
4.3.1.2.1 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.
4.3.1.2.2 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.
4.3.1.2.3 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 either continue or reject the takeoff and, if the latter, has initiated the first action to stop the airplane.
4.3.1.2.4 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 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.
4.3.1.2.5 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.