4-37 Figure 4-8. Instantaneous Brake Force and Peak Brake Force Time Instantaneous Brake Force Peak Brake Force Time Intervals Brake Force
2. Using data obtained from the wet runway stopping tests of paragraph 4.3.7.6 of this AC, instantaneous brake force can be calculated from the following relationship:
𝐹𝑏 = (𝑇𝑏+∝ 𝐼) 𝑅𝑡𝑖𝑟𝑒
Where:
𝐹𝑏 = 𝑏𝑟𝑎𝑘𝑒 𝑓𝑜𝑟𝑐𝑒 𝑇𝑏 = 𝑏𝑟𝑎𝑘𝑒 𝑡𝑜𝑟𝑞𝑢𝑒 ∝= 𝑤ℎ𝑒𝑒𝑙 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝐼 = 𝑤ℎ𝑒𝑒𝑙 𝑎𝑛𝑑 𝑡𝑖𝑟𝑒 𝑚𝑜𝑚𝑒𝑛𝑡 𝑜𝑓 𝑖𝑛𝑒𝑟𝑡𝑖𝑎 𝑅𝑡𝑖𝑟𝑒 = 𝑡𝑖𝑟𝑒 𝑟𝑎𝑑𝑖𝑢𝑠
3. For brake installations where measuring brake torque directly is impractical, torque may be determined from other parameters (e.g., brake pressure) if a suitable correlation is available. Wheel acceleration is obtained from the first derivative of wheel speed.
Instrumentation recording rates and data analysis techniques for wheel speed and torque data should be well matched to the anti-skid response characteristics to avoid introducing noise and other artifacts of the instrumentation system into the data.
4. Since the derivative of wheel speed is used in calculating brake force, smoothing of the wheel speed data is usually necessary to give good results. The smoothing algorithm should be carefully designed as it can affect the resulting efficiency calculation. Filtering or smoothing of the brake torque or brake force data should not normally be done. If conditioning is applied, it should be done in a conservative manner
(i.e., result in a lower efficiency value) and should not misrepresent actual airplane/system dynamics.
5. Both the instantaneous brake force and the peak brake force should be integrated over the stopping distance. The anti-skid efficiency value
4-38 for determining the wet runway accelerate-stop distance is the ratio of the instantaneous brake force integral to the peak brake force integral:
𝐴𝑛𝑡𝑖-– 𝑠𝑘𝑖𝑑 𝑒𝑓𝑓𝑖𝑒𝑛𝑐𝑒𝑛𝑐𝑦 = ∫ 𝑖𝑛𝑠𝑡𝑎𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠 𝑏𝑟𝑒𝑎𝑘 𝑓𝑜𝑟𝑐𝑒 𝑑𝑠∫ 𝑝𝑒𝑎𝑘 𝑏𝑟𝑎𝑘𝑒 𝑓𝑜𝑟𝑐𝑒 𝑑𝑠 Where:
𝑠 = 𝑠𝑡𝑜𝑝𝑝𝑖𝑛𝑔 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
6. The stopping distance is defined as the distance traveled during the specific wet runway stopping demonstration, beginning when the full braking configuration is obtained and ending at the lowest speed at which anti-skid cycling occurs (i.e., the brakes are not torque-limited), except that this speed need not be less than 10 knots. Any variation in the anti-skid efficiency with speed should also be investigated, which can be accomplished by determining the efficiency over segments of the total stopping distance. If significant variations are noted, this variation should be reflected in the braking force used to determine the accelerate-stop distances (either by using a variable efficiency or by using a conservative single value).
4.3.7.6.11 Wheel Slip Method.
1. At brake application, the tire begins to slip with respect to the runway surface (i.e., the wheel speed slows down with respect to the airplane’s ground speed). As the amount of tire slip increases, the brake force also increases until an optimal slip is reached. If the amount of slip continues to increase past the optimal slip, the braking force will decrease.
2. Using the wheel slip method, the anti-skid efficiency is determined by comparing the actual wheel slip measured during a wet runway stop to the optimal slip. Since the wheel slip varies significantly during the stop, sufficient wheel and ground speed data should be obtained to determine the variation of both the actual wheel slip and the optimal wheel slip over the length of the stop. A sampling rate of at least 16 samples per second for both wheel speed and ground speed has been found to yield acceptable fidelity.
3. For each wheel and ground speed data point, the instantaneous antiskid efficiency value should be determined from the relationship shown in figure 4-9 below.
4-39 Figure 4-9. Anti-Skid Efficiency–Wheel Slip Relationship W h e e l S lip R a tio0 fre e ro llin g 1 lo c k e d w h e e l 1 .0 0 O p tim a l S lip 0 .5 A n ti-S k id E ffic ie n c y
ratio slip optimal theis OPS and speed Ground speed Wheel1=ratio slip wheelthe= WSRwhere OPS1 WSR1+10.5=Efficiency OPS>WSR
1.0=Efficiency OPS=WSR OPS WSR5.0OPSWSR5.1=Efficiency OPS<for WSR 3
4. To determine the overall anti-skid efficiency value for use in calculating the wet runway accelerate-stop distance, the instantaneous anti-skid efficiencies should be integrated with respect to distance and divided by the total stopping distance:
𝐴𝑛𝑡𝑖-– 𝑠𝑘𝑖𝑑 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = ∫ 𝑖𝑛𝑠𝑡𝑎𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠 𝑏𝑟𝑒𝑎𝑘 𝑓𝑜𝑟𝑐𝑒 𝑑𝑠𝑠 Where:
𝑠 = 𝑠𝑡𝑜𝑝𝑝𝑖𝑛𝑔 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
5. The stopping distance is defined as the distance traveled during the specific wet runway stopping demonstration, beginning when the full braking configuration is obtained and ending at the lowest speed at which anti-skid cycling occurs (i.e., the brakes are not torque-limited), except that this speed need not be less than 10 knots. Any variation in the anti-skid efficiency with speed should also be investigated, which can be accomplished by determining the efficiency over segments of the total stopping distance. If significant variations are noted, this variation should be reflected in the braking force used to determine the
4-40 accelerate-stop distances (either by using a variable efficiency or by using a conservative single value).
6. The applicant should provide substantiation of the optimal wheel slip value(s) used to determine the anti-skid efficiency value. An acceptable method for determining the optimal slip value(s) is to compare time history plots of the brake force and wheel slip data obtained during the wet runway stopping tests. For brake installations where measuring brake force directly is impractical, brake force may be determined from other parameters (e.g., brake pressure) if a suitable correlation is available. For those skids where wheel slip continues to increase after a reduction in the brake force, the optimal slip is the slip value corresponding to the brake force peak. See figure 4-10 below for an example and note how both the actual wheel slip and the optimal wheel slip can vary during the stop.
Figure 4-10. Substantiation of the Optimal Slip Value B ra k e F o rc e T im e S lip O p tim a l R a tio W h e e l
4.3.7.7 For dispatch with an inoperative anti-skid system (if approved), the wet runway accelerate-stop distances should be based on an efficiency no higher than that allowed by § 25.109(c)(2) for an on-off type of anti-skid system. The safety of this type of operation should be demonstrated by flight tests conducted in accordance with paragraph 4.3.7.5 of this AC.
4.3.7.8 Distribution of the Normal Load between Braked and Unbraked Wheels.
In addition to taking into account the efficiency of the anti-skid system, § 25.109(b)(2)(ii) also requires adjusting the braking force for the effect of the distribution of the normal load between braked and unbraked wheels at the most adverse CG position approved for takeoff. The stopping force due to braking is equal to the braking coefficient multiplied by the normal load (i.e., weight) on each braked wheel. The portion of the airplane’s
4-41 weight being supported by the unbraked wheels (e.g., unbraked nose wheels) does not contribute to the stopping force generated by the brakes.
In accordance with § 25.21(a), this effect must be taken into account for the most adverse CG position approved for takeoff, considering any redistribution in loads that occur due to the dynamics of the stop. The most adverse CG position is the position that results in the least load on the braked wheels.
4.3.7.9 Grooved and Porous Friction Course (PFC) Runways.
4.3.7.9.1 Properly designed, constructed, and maintained grooved and PFC runways can offer significant improvements in wet runway braking capability. A conservative level of performance credit is provided by § 25.109(d) to reflect this performance improvement and to provide an incentive for installing and maintaining such surfaces.
4.3.7.9.2 In accordance with §§ 25.105(c) and 25.109(d), applicants may optionally determine the accelerate-stop distance applicable to wet grooved and PFC runways. These data would be included in the AFM in addition to the smooth runway accelerate-stop distance data. The braking coefficient for determining the accelerate-stop distance on grooved and PFC runways is defined in § 25.109(d) as either 70 percent of the braking coefficient used to determine the dry runway accelerate-stop distances, or a curve based on ESDU 71026 data and derived in a manner consistent with that used for smooth runways. In either case, the brake torque limitations determined on a dry runway may not be exceeded.
4.3.7.9.3 Using a simple factor applied to the dry runway braking coefficient is acceptable for grooved and PFC runways because the braking coefficient’s variation with speed is much lower on these types of runways. On smooth wet runways, the braking coefficient varies significantly with speed, which makes it inappropriate to apply a simple factor to the dry runway braking coefficient.
4.3.7.9.4 For applicants who choose to determine the grooved/PFC wet runway accelerate-stop distances in a manner consistent with that used for smooth runways, § 25.109(d)(2) provides the maximum tire-to-ground braking coefficient applicable to grooved and PFC runways. This maximum tire-to-ground braking coefficient must be adjusted for the anti-skid system efficiency, either by using the value specified in § 25.109(c)(2) appropriate to the type of anti-skid system installed, or by using a specific efficiency established by the applicant. As anti-skid system performance depends on the characteristics of the runway surface, a system that has been tuned for optimum performance on a smooth surface may not achieve the same level of efficiency on a grooved or porous friction course runway, and vice versa. Consequently, if the applicant elects to establish a specific efficiency for use with grooved or PFC surfaces, anti-skid
4-42 efficiency testing should be conducted on a wet runway with such a surface, in addition to testing on a smooth runway. Means other than flight testing may be acceptable, such as using the efficiency previously determined for smooth wet runways, if that efficiency is shown to be representative of, or conservative for, grooved and PFC runways. Per § 25.109(b)(2)(ii), the resulting braking force for grooved/PFC wet runways must be adjusted for the effect of the distribution of the normal load between braked and unbraked wheels. This adjustment will be similar to that used for determining the braking force for smooth wet runways, except that the braking dynamics should be appropriate to the braking force achieved on grooved and PFC wet runways. Due to the increased braking force on grooved and PFC wet runways, an increased download on the nose wheel and corresponding reduction in the download on the main gear is expected.
4.3.7.9.5 In accordance with §§ 25.1533(a)(3) and 25.1583(h), grooved and PFC wet runway accelerate-stop distances may be established as operating limitations and be presented in the AFM, 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. Airplane operators who wish to use the grooved or PFC runway accelerate-stop distances will need to determine that the design, construction, and maintenance aspects are acceptable for each runway for which such credit is sought. AC 150/5320-12C provides guidance relative to acceptable design, construction, and maintenance practices for grooved and PFC runway surfaces.
4.3.7.10 Reverse Thrust Performance Credit.
In accordance with § 25.109(f), reverse thrust may not be used to determine the accelerate-stop distances for a dry runway. For wet runway accelerate-stop distances, however, § 25.109(f) allows credit for the stopping force provided by reverse thrust, if the requirements of §
25.109(e) are met. In addition, the procedures associated with the use of reverse thrust, which § 25.101(f) requires the applicant to provide, must meet the requirements of § 25.101(h). The following criteria provide acceptable means of demonstrating compliance with these requirements:
4.3.7.10.1 Procedures for using reverse thrust during a rejected takeoff should be developed and demonstrated. These procedures should include all of the pilot actions necessary to obtain the recommended level of reverse thrust, maintain directional control and safe engine operating characteristics, and return the reverser(s), as applicable, to either the idle or the stowed position. These procedures need not be the same as those recommended for use during a landing stop, but should not result in additional hazards
(e.g., cause a flameout or any adverse engine operating characteristics), nor should they significantly increase flightcrew workload or training needs.
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4.3.7.10.2 It should be demonstrated that using reverse thrust during a rejected takeoff complies with the engine operating characteristics requirements of § 25.939. The reverse thrust procedures may specify a speed at which the reverse thrust is to be reduced to idle in order to maintain safe engine operating characteristics.
4.3.7.10.3 The time sequence for the actions necessary to obtain the recommended level of reverse thrust should be demonstrated by flight test. The time sequence used to determine the accelerate-stop distances should reflect the most critical case relative to the time needed to deploy the thrust reversers.
For example, on some airplanes the outboard thrust reversers are locked out if an outboard engine fails. This safety feature prevents the pilot from applying asymmetric reverse thrust on the outboard engines, but it may also delay the pilot’s selection of reverse thrust on the operable reversers.
In addition, if the selection of reverse thrust is the fourth or subsequent pilot action to stop the airplane (e.g., after manual brake application, thrust/power reduction, and spoiler deployment), a one-second delay should be added to the demonstrated time to select reverse thrust. (See figure 4-1 of this AC.)
4.3.7.10.4 The response times of the affected airplane systems to pilot inputs should be taken into account. For example, delays in system operation, such as thrust reverser interlocks that prevent the pilot from applying reverse thrust until the reverser is deployed, should be taken into account. The effects of transient response characteristics, such as reverse thrust engine spin-up, should also be included.
4.3.7.10.5 To enable a pilot of average skill to consistently obtain the recommended level of reverse thrust under typical in-service conditions, a lever position that incorporates tactile feedback (e.g., a detent or stop) should be provided. If tactile feedback is not provided, a conservative level of reverse thrust should be assumed.
4.3.7.10.6 The applicant should demonstrate that exceptional skill is not required to maintain directional control on a wet runway with a ten-knot crosswind from the most adverse direction. For demonstration purposes, a wet runway may be simulated by using a nose wheel free to caster on a dry runway. Symmetric braking should be used during the demonstration, and both all-engines-operating and critical-engine-inoperative reverse thrust should be considered. The brakes and thrust reversers may not be modulated to maintain directional control. The reverse thrust procedures may specify a speed at which the reverse thrust is reduced to idle in order to maintain directional controllability.
4.3.7.10.7 Compliance with the requirements of §§ 25.901(b)(2), 25.901(c),
25.1309(b), and 25.1309(c) will be accepted as providing compliance with the “safe and reliable” requirements of §§ 25.101(h)(2) and 25.109(e)(1).
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4.3.7.10.8 The number of thrust reversers used to determine the wet runway accelerate-stop distance data provided in the AFM should reflect the number of engines assumed to be operating during the rejected takeoff, along with any applicable system design features. The all-enginesoperating accelerate-stop distances should be based on all thrust reversers operating. The one-engine-inoperative accelerate-stop distances should be based on failure of the critical engine. For example, if the outboard thrust reversers are locked out when an outboard engine fails, the one-engine-inoperative accelerate stop distances can only include reverse thrust from the inboard engine thrust reversers.
4.3.7.10.9 For the engine failure case, it should be assumed that the thrust reverser does not deploy (i.e., no reverse thrust or drag credit for deployed thrust reverser buckets on the failed engine).
4.3.7.10.10 For approval of dispatch with one or more inoperative thrust reverser(s), the associated performance information should be provided either in the AFM or the master minimum equipment list (MMEL).
4.3.7.10.11 The effective stopping force provided by reverse thrust in each, or at the option of the applicant, the most critical takeoff configuration, should be demonstrated by flight test. (One method of determining the reverse thrust stopping force would be to compare unbraked runs with and without the use of thrust reversers.) Regardless of the method used to demonstrate the effective stopping force provided by reverse thrust, flight test demonstrations should be conducted using all of the stopping means on which the AFM wet runway accelerate-stop distances are based in order to substantiate the accelerate-stop distances and ensure that no adverse combination effects are overlooked. These demonstrations may be conducted on a dry runway.
4.3.7.10.12 For turbopropeller powered airplanes, the criteria of paragraphs 4.3.7.10.1 through 4.3.7.10.11 above remain generally applicable. Additionally, the propeller of the inoperative engine should be in the position it would normally assume when an engine fails and the power lever is closed.
Reverse thrust may be selected on the remaining engine(s). Unless this selection is achieved by a single action to retard the power lever(s) from the takeoff setting without encountering a stop or lockout, it should be regarded as an additional pilot action for the purposes of assessing delay times. If this action is the fourth or subsequent pilot action to stop the airplane, a one-second delay should be added to the demonstrated time to select reverse thrust.
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4.4 Takeoff Path—§ 25.111.
4.4.1 Section 25.111(a).
4.4.1.1 Explanation.
The takeoff path requirements of § 25.111, and the reductions to that path required by § 25.115, are established so that performance data can be provided in the AFM for use in assessing vertical clearance of obstacles under the takeoff flight path.
4.4.1.2 Procedures.
The height references in § 25.111 should be interpreted as geometric heights. Section 25.111(a) requires the actual takeoff path (from which the net takeoff flight path is derived) to extend to the higher of where the airplane is 1,500 feet above the takeoff surface or to the altitude at which the transition to en route configuration is complete and the final takeoff speed (VFTO) is reached.
4.4.2 Section 25.111(a)(1)—Takeoff Path Power/Thrust Conditions.
4.4.2.1 Explanation.
In accordance with § 25.111(d), the takeoff path must be established either from continuous demonstrated takeoffs or by synthesis from segments. If determined from segments, it must be shown by continuous demonstrated takeoff to be conservative, in accordance with § 25.111(d)(4).
4.4.2.2 Procedures.
4.4.2.2.1 To be assured that the predicted takeoff path is representative of actual performance, construct the path using the power or thrust required by §
25.101(c). This requires, in part, that the power or thrust be based on the particular ambient atmospheric conditions that are assumed to exist along the path. The standard lapse rate for ambient temperature as specified in the 1962 U.S. Standard Atmosphere part 1 should be used for power or thrust determination associated with each pressure altitude during the climb.
4.4.2.2.2 In accordance with § 25.111(c)(4), the power or thrust up to 400 feet above the takeoff surface must represent the power or thrust available along the path resulting from the power lever setting established during the initial ground roll in accordance with AFM procedures. This resulting power or thrust may be less than that available from the rated inflight setting schedule.
4.4.2.2.3 A sufficient number of takeoffs, to at least the altitude above the takeoff surface scheduled for V2 climb, should be made to establish the power or thrust lapse for a fixed power or thrust lever setting. An analysis may be
4-46 used to account for various engine bleeds (e.g., ice protection, air conditioning, etc.) and for electrical, pneumatic, and mechanical power extraction. In some airplanes, the power or thrust growth characteristics are such that less than full rated power or thrust is used for AFM takeoff power/thrust limitations and performance. This is to preclude engine limitations from being exceeded during the takeoff climbs to 400 feet above the takeoff surface.
4.4.2.2.4 Engine power or thrust lapse with speed and altitude during the takeoff and climb, at fixed power or thrust lever settings), can be affected by takeoff pressure altitude.
4.4.2.2.5 Most turbine engines are sensitive to crosswind or tailwind conditions, when setting takeoff power under static conditions, and may stall or surge.
To preclude this problem, it is acceptable to establish a rolling takeoff power or thrust setting procedure, provided the AFM takeoff field length and the takeoff power or thrust setting charts are based on this procedure.
Demonstrations and analyses have been accepted in the past showing a negligible difference in distance between static and rolling takeoffs. A typical test procedure is as follows:
1. After stopping on the runway, set an intermediate power or thrust on all engines (power setting selected by applicant).
2. Release brakes and advance power or thrust levers.
3. Set target power or thrust setting as rapidly as possible prior to reaching 60 to 80 knots.
4. No adverse engine operating characteristics should exist after completion of the power setting through the climb to 1,500 feet above the airport and attainment of the en route configuration. Tests should be conducted to determine if any engine operating problems exist for takeoffs conducted throughout the altitude range for which takeoff operations are to be scheduled in the AFM.
4.4.2.2.6 If the applicant wishes to use a different procedure, it should be evaluated and, if found acceptable, the procedure should be reflected in the AFM.
4.4.3 Section 25.111(a)(2)—Engine Failure.
4.4.3.1 Explanation.
4.4.3.1.1 Since the regulations cannot dictate what type of engine failures may actually occur, it could be assumed that the engine failure required by the regulation occurs catastrophically. Such a failure would cause the power or thrust to drop immediately, with the associated performance going from all-engines-operating to one-engine-inoperative at the point of engine failure.
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4.4.3.1.2 This conservative rationale notwithstanding, there is a basis for assuming that the failed engine power or thrust will not decay immediately. Unlike reciprocating engines, the locking-up of a jet engine fan without causing the engine to separate from the airplane is highly unlikely. Separation of the engine or fan, or fan disintegration, would remove weight and/or the ram drag included in the engine inoperative performance, providing compensation for the immediate thrust loss.
4.4.3.1.3 With these considerations, it may be acceptable to use the transient power or thrust as the failed engine spools down at VEF. The power or thrust time history used for data reduction and expansion should be substantiated by test results.
4.4.3.1.4 In the case of propeller-driven airplanes, consideration should also be given to the position of the failed engine’s propeller during the engine failure. These airplanes typically incorporate an automatic system to drive the propeller to a low drag position when an engine fails. The loss of power in this case will be much more sudden than the turbojet engine spooldown described above.
4.4.3.2 Procedures.
4.4.3.2.1 For turbojet powered airplanes, if transient thrust credit is used during engine failure in determining the one-engine-inoperative AFM takeoff performance, sufficient tests should be conducted using actual fuel cuts to establish the thrust decay as contrasted to idle engine cuts. 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.
4.4.3.2.2 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.
4.4.4 Section 25.111(a)(3)—Airplane Acceleration.
4.4.4.1 Explanation.
None.
4.4.4.2 Procedures.
None.
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4.4.5 Section 25.111(b)—Airplane Rotation and Gear Retraction.
4.4.5.1 Explanation.
The rotation speed, VR, is intended to be the speed at which the pilot initiates action to raise the nose wheel(s) off the ground during the acceleration to V2. Consequently, the takeoff path, determined in accordance with § 25.111(a) and (b), should assume that pilot action to raise the nose wheel(s) off the ground will not be initiated until the speed VR has been reached.
4.4.5.2 Procedures.
The time between liftoff and initiation of gear retraction during takeoff distance demonstrations should not be less than that necessary to establish an indicated positive rate of climb plus one second. For the purposes of flight manual expansion, the average demonstrated time delay between liftoff and initiation of gear retraction may be assumed; however, this value should not be less than 3 seconds.
4.4.6 Section 25.111(c)(1)—Takeoff Path Slope.
4.4.6.1 Explanation.
4.4.6.1.1 Establishing a horizontal segment as part of the takeoff flight path is considered to be acceptable, per § 25.115(c), for showing compliance with the positive slope required by § 25.111(c)(1).
4.4.6.1.2 The net takeoff flight path is the flight path used to determine compliance with the airplane obstacle clearance requirements of the applicable operating rules. Section 25.115(b) states the required climb gradient reduction to be applied throughout the flight path for the determination of the net flight path, including the level flight acceleration segment. Rather than decreasing the level flight path by the amount required by § 25.115(b), § 25.115(c) allows the airplane to maintain a level net flight path during acceleration, but with a reduction in acceleration equal to the gradient decrement required by § 25.115(b). By this method, the altitude reduction is exchanged for increased distance to accelerate in the level flight portion of the net takeoff path.
4.4.6.2 Procedures.
The level acceleration segment in the AFM net takeoff profile should begin at the horizontal distance along the takeoff flight path where the actual airplane height, without the gradient reductions of § 25.115(b), reaches the AFM specified acceleration height.