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
30 conditions §§ 25.107(a)(l) & (2) Defines V 1 and VEF speeds § 25.735 Brakes § 25.1301 Function and installation § 25.1309 Equipment, systems, and installation § 25.1533 Additional operating limitations - maximum takeoff weights and minimum takeoff distances § 25.1583(h) Airplane Flight Manual - operating limitations § 25.1587 Airplane Flight Manual - performance information
c. Procedures. The following paragraphs provide guidance for accomplishing acceleratestop flight tests and expanding the resulting data for the determination of AFM performance information.
(1) Accelerate-stop testing. The following guidance 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).
(a) In order to establish a distance that would be representative of the distance needed in the event of a rejected takeoff, where the first action to stop the airplane is taken at or below V1, a sufficient number of test runs should be conducted for each airplane configuration specified by the applicant. (For intermediate configurations, see paragraph 3 of this AC.)
(b) The guidance outlined in paragraph 11c(3) describes how to include allowances for any time delays, as required by § 25.101(h)(3), for the flightcrew to accomplish the rejected takeoff operating procedures.
(c) Section 25.101(i) states that the accelerate-stop distances must be determined with all the airplane wheel-brake assemblies at the fully worn limit of their allowable wear range. The fully worn limit is defined as the maximum amount of wear allowed before the brake is to be removed from the airplane for overhaul. The allowable wear should be defined in terms of a linear dimension in the axial direction, which is typically determined by measuring the wear pin extension.
1 The only accelerate-stop test that must be conducted at a specific brake wear state is the maximum brake kinetic energy demonstration, which must use brakes that have no more than 10 percent of the allowable brake wear range remaining, as required by § 25.109(i).
(See paragraph 11c(2)(c) of this AC). The remainder of the accelerate-stop tests may be conducted with the brakes in any wear state as long as a suitable combination of airplane and dynamometer tests is used to determine the accelerate-stop distances corresponding to fully worn brakes. For example, dynamometer testing may be used to determine whether there is a
31 reduction in brake performance from the wear state used in the airplane tests to a fully worn brake. The airplane test data could then be adjusted analytically for this difference without additional airplane testing.
2 Either airplane-worn or mechanically-worn brakes (i.e., machined or dynamometer worn) may be used. If mechanically-worn brakes are used, it should be shown that they can be expected to provide similar results to airplane-worn brakes. This comparison can be based on service experience on the test brake or an appropriate equivalent brake, or on dynamometer wear test data when service data are unavailable.
(d) Section 25.109(f)(1) denies credit for the use of reverse thrust as a decelerating means in determining the accelerate-stop distance for a dry runway. This provision applies to both turbine engine and propeller engine reverse thrust. Credit for the additional deceleration available from reverse thrust is permitted for wet runway accelerate-stop distances, provided the thrust reverser system is shown to be safe, reliable, capable of giving repeatable results, and does not require exceptional skill to control the airplane. (See paragraph 11c(4)(e) for guidance related to obtaining accelerate-stop performance credit for reverse thrust on wet runways.)
(e) The accelerate-stop test runs should be conducted at weight/speed combinations that will provide an even distribution of test conditions over the range of weights, speeds, and brake energies for which takeoff data will be provided in the AFM. The effects of different airport elevations can be simulated at one airport elevation, provided the braking speeds employed are relevant for the range of airplane energies to be absorbed by the brakes. The limiting brake energy value in the AFM should not exceed the maximum demonstrated in these tests or the maximum for which the brake has been approved. (See paragraph 11c(2) for further guidance related to tests and analyses for the demonstration of the maximum brake energy absorption capability.)
(f) The V 1 speeds used in the accelerate-stop tests need not correspond precisely to the AFM values for the test conditions since it may be necessary to increase or decrease the AFM V1 speed to investigate fully the energy range and weight envelope.
(g) A total of at least six accelerate-stop flight tests should be conducted. Unless sufficient data are available for the specific airplane type showing how braking performance varies with weight, kinetic energy, lift, drag, ground speed, torque limit, etc., at least two tests should be conducted for each configuration when the same braking coefficient of friction is being claimed for multiple aerodynamic configurations. These tests should be conducted on hard surfaced, dry runways.
(h) For approval of dispatch capability with anti-skid inoperative, nose wheel brakes or specified main wheel-brake(s) inoperative, automatic braking systems, etc., a full set of tests, as described in paragraph (g) above, should normally be conducted. A lesser number of tests may be accepted for “equal or better” demonstrations, to establish small increments or if adequate conservatism is used during testing.
(i) Either ground or airborne instrumentation should include means to determine the horizontal distance time-history.
(j) The wind speed and direction relative to the test runway should be determined and corrected to a height corresponding to the approximate height of the mean aerodynamic chord. (See paragraph 3 of this AC.)
(k) The accelerate-stop tests should be conducted in the following configurations:
1 Heavy to light weight as required.
2 Most critical c.g. position.
3 Wing flaps in the takeoff position(s).
4 Tire pressure: before taxi and with cold tires, set to the highest value appropriate to the takeoff weight for which approval is being sought.
5 Engine idle thrust: set at the recommended upper limit for use on the ground or the 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.
(l) Engine thrust should be appropriate to each segment of the rejected takeoff and should include accounting for thrust decay rates (i.e., spin down) 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.
1 Turbojet powered airplanes . For AFM calculation purposes, the critical engine failure accelerate-stop data may be based on the failed engine spinning down to a windmilling condition.
NOTE: If, due to the certification basis of the airplane, all-engine-acceleratestop distances are not being considered, the one-engine-inoperative AFM distances should be based on the critical engine failing to maximum ground idle thrust rather than the windmilling condition.
2 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 spin down), as noted in paragraph 11c(1)(k)5. The accelerate-stop tests may be conducted with either concurrent or sequential throttle chops to idle 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
33 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)(e).
3 Turbopropeller-powered airplanes . For the one-engine-inoperative accelerate-stop distances, the critical engine’s propeller should be in the position it would normally assume when an engine fails and the power levers are closed. For dry runway oneengine-inoperative accelerate-stop distances, the high drag ground-idle position of the operating engines’ propellers (defined by a pitch setting that results in not less than zero total thrust, i.e., propeller plus jet thrust, at zero airspeed) may be used provided adequate directional control is available on a wet runway and the related operational procedures comply with § 25.101(f) and
(h). Wet runway controllability may either be demonstrated by using the guidance available in paragraph 11c(4)(e)6 at the appropriate power level, or adequate control can be assumed to be available at ground idle power if reverse thrust credit is approved for determining the wet runway accelerate-stop distances. For the all-engines-operating accelerate-stop distances on a dry runway, the high drag ground-idle propeller position may be used for all engines (subject to § 25.101(f) and (h)). For the criteria relating to reverse thrust credit for wet runway acceleratestop distances, see paragraph 11c(4)(e).
(m) System transient effects (e.g., engine spin-down, brake pressure ramp-up, etc.)
should be determined and properly accounted for in the calculation of AFM accelerate-stop distances (see paragraph 11c(3)(i)).
(2) Maximum Brake Energy Testing. The following paragraphs describe regulatory requirements and acceptable test methods for conducting an accelerate-stop test run to demonstrate the maximum energy absorption capability of the wheel-brakes.
(a) The maximum brake energy accelerate-stop demonstration should be conducted at not less than the maximum takeoff weight 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.
(b) Section 25.735(f)(2) requires the maximum kinetic energy accelerate-stop absorption capability of each wheel, tire, and brake assembly to be determined. It also requires dynamometer testing to show that the wheel, brake, and tire assembly is capable of absorbing not less than this level of kinetic energy throughout the defined wear range of the brake. The calculation of maximum brake energy limited takeoff weights and speeds, for presentation in the AFM performance section, therefore should be based on the most critical wear range of the brake.
(c) Section 25.109(i) requires a flight test demonstration of the maximum brake kinetic energy accelerate-stop distance to be conducted with not more than 10 percent of the allowable brake wear range remaining on each of the airplane wheel-brakes. The 10 percent allowance on the brake wear state is intended to ease test logistics and increase test safety, not to allow the accelerate-stop distance to be determined with less than fully worn brakes. If the
34 brakes are not in the fully worn state at the beginning of the test, the accelerate-stop distance should be corrected as necessary to represent the stopping capability of fully worn brakes.
(d) The maximum airplane brake energy allowed for dispatch should not exceed the value for which a satisfactory after-stop condition exists, or the value documented under the applicable TSO (or an acceptable equivalent), whichever value is less. A satisfactory after-stop condition is defined as one in which fires are confined to tires, wheels, and brakes, such that progressive engulfment of the rest of the airplane would not occur during the time of passenger and crew evacuation. The application of fire fighting means or artificial coolants should not be required for a period of 5 minutes following the stop.
(e) Landings are not an acceptable means for demonstrating the maximum rejected takeoff brake energy. Though permitted in the past, service experience has shown that methods used to predict brake and tire temperature increases that would have occurred during taxi and acceleration, as specified in paragraph 11c(2)(a), were not able to accurately account for the associated energy increments.
(3) Accelerate-Stop Time Delays. Section 25.101(h) requires allowance for time delays in the execution of procedures. Amendment 25-42 (effective March 1, 1978) amended the airworthiness standards to clarify and standardize the method of applying these time delays to the accelerate-stop transition period. Amendment 25-42 also added the critical engine failure speed, VEF, and clarified the meaning of V1 with relation to VEF. The preamble to amendment 25-42 states that “V1 is determined by adding to VEF (the speed at which the critical engine is assumed to fail) the speed gained with the critical engine inoperative during the time interval between the instant at which the critical engine is failed and the instant at which the test pilot recognizes and reacts to the engine failure, as indicated by the pilot’s application of the first retarding means during accelerate-stop tests.” Thus it can be seen that V1 is not only intended to be at the end of the decision process, but it also includes the time it takes for the pilot to perform the first action to stop the airplane. (See Appendix 5 of this AC for further discussion on the historical development of accelerate-stop time delays.) The purpose of the time delays is to allow sufficient time (and distance) for a pilot, in actual operations, to accomplish the procedures for stopping the airplane. The time delays are not intended to allow extra time for making a decision to stop as the airplane passes through V1. 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.
(a) 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 o f first pilot action Initiation of second pilot action Initiation of third pilot action Engine failure
V 1 V EF t act 1 t ac t 1 Ev ent Demonstrated Time Delays Flight Ma nual Expansion Time Delays Initiation of subsequent pilot actions tact 4 n tact + 1 sec.
t act 2 t act 2 tact 3 tact 3 Dist ance equivalent to 2 sec . a t V 1
(b) V EF is the calibrated airspeed selected by the applicant at which the critical engine is assumed to fail. The relationship between VEF and V1 is defined in § 25.107.
(c) 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 should 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.
(d) tact 2 = the demonstrated time interval between initiation of the first and second pilot actions to stop the airplane.
(e) tact 3 = the demonstrated time interval between initiation of the second and third pilot actions to stop the airplane.
(f) tact 4n = 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 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.
(g) The sequence for initiation 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 should be conducted to establish acceptable values of tact.
(h) Sections 25.109(a)(1)(iv) and (a)(2)(iii) require the one-engine-inoperative and all-engines-operating accelerate-stop distances, respectively, to include a distance increment equivalent to two seconds at V1. (Although the requirement for the distance increment equivalent to two seconds at V1 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, on the following page. The two-second 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., automatic spoiler systems).
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
(i) Section 25.109 (a)(1)(ii) requires that any residual acceleration causing the airplane to exceed V1, while the airplane and its systems become stabilized in the braking configuration, must be included in the accelerate-stop distance. 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.
(j) 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)(d) through (f) after the airplane has accelerated to V1.
(k) Describe the procedures used to determine the accelerate-stop distance 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
38 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-toground wet runway braking coefficient of friction versus ground speed from the relationships provided in § 25.109(c)(1). Then, adjust this braking coefficient to take into account the efficiency of the anti-skid system. (See paragraph 11c(4)(b) of this AC for a definition of antiskid efficiency.) Next, determine the resulting braking force and adjust this force for the effect of the distribution of the normal load between braked and unbraked wheels at the most adverse center-of-gravity position approved for takeoff, as prescribed by § 25.109(b)(2)(ii). In accordance with § 25.109(b)(2)(i), apply further adjustments, if necessary, to ensure that the resulting stopping force attributed to the wheel brakes on a wet runway never exceeds (i.e., during the entire stop) the wheel brakes stopping force used to determine the dry runway accelerate-stop distance (under § 25.109(a)). Neither the dry runway brake torque limit nor the dry runway friction (i.e., anti-skid) limit should be exceeded. Alternative methods of determining the wet runway wheel brakes stopping force may be acceptable as long as that force does not exceed the force determined using the method just described.
(a) Maximum Tire-to-Ground Braking Coefficient of Friction . The values specified in § 25.109(c)(1) were derived from data contained in Engineering Sciences Data Unit (ESDU) 71026, “Frictional and Retarding Forces on Aircraft Types - part II: Estimation of Braking Force,” (August 1981). The data in ESDU 71026 is a compilation from many different sources, including the National Aeronautics and Space Administration, the British Ministry of Aviation, and others. ESDU 71026 contains curves of wet runway braking coefficients versus speed for smooth and treaded tires at varying inflation pressures. These data are presented for runways of various surface roughness, including grooved and porous friction course runways.
Included in the data presentation are bands about each of the curves, which represent variations in: water depths from damp to flooded, runway surface texture within the defined texture levels, tire characteristics, and experimental methods. In defining the standard curves of wet runway braking coefficient versus speed that are prescribed by the equations in § 25.109(c)(1), the effects of the following variables were considered: tire pressure, tire tread depth, runway surface texture, and the depth of the water on the runway.
1 Tire Pressure : Lower tire pressures tend to improve the airplane’s stopping capability on a wet runway. The effect of tire pressure is taken into account by providing separate curves (equations) in § 25.109(c)(1) for several tire pressures. As stated in the rule, the tire pressure used to determine the maximum tire-to-ground braking coefficient of friction must be the maximum tire pressure approved for operation. Linear interpolation may be used for tire pressures other than those listed.
2 Tire Tread Depth : The degree to which water can be channeled out from under the tires significantly affects wet runway stopping capability. The standard curves of braking coefficient versus speed prescribed in § 25.109(c)(1) are based on a tire tread depth of 2 mm. This tread depth is consistent with tire removal and retread practices reported by airplane