49 determine the accelerate-stop distances (either by using a variable efficiency or by using a conservative single value).
( i i ) Wheel Slip Method:
(a) 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.
(b) 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.
(c) For each wheel and ground speed data point, the instantaneous anti-skid efficiency value should be determined from the relationship shown in figure 11-7.
Figure 11-7. Anti-Skid Efficiency – Wheel Slip Relationship
Wheel Slip Ratio0 free rolling 1 locked wheel
1.0 0 Optimal Slip
0.5 Anti-Skid Efficiency
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
(d) 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:
where s = stopping distance
(e) 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).
(f) 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 11-8 for an example and note how both the actual wheel slip and the optimal wheel slip can vary during the stop.
instantaneous brake forceds s anti-skid efficiency =
51 Figure 11-8. Substantiation of the Optimal Slip Value
Brake Force Time Slip Optimal Ratio Wheel
4 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 11c(4)(b)(2).
(c) 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 center-of-gravity 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 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 center-of-gravity position approved for takeoff, considering any redistribution in loads that occur due to the dynamics of the stop. The most adverse center-ofgravity position is the position that results in the least load on the braked wheels.
(d) Grooved and Porous Friction Course (PFC) Runways. 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.
1 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
52 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.
2 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.
3 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 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 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, “Measurement, Construction, and Maintenance of Skid-Resistant Airport Pavement Surfaces,” provides guidance relative to acceptable design, construction, and maintenance practices for grooved and PFC runway surfaces.
(e) 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
53 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:
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.
2 It should be demonstrated that using reverse thrust during a rejected takeoff complies with the engine operating characteristics requirements of § 25.939. The engine should not exhibit any of the adverse engine operating characteristics described in AC 25.939-1, “Evaluating Turbine Engine Operating Characteristics,” dated March 19, 1986 (or later revision). 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.
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 11-1).
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.
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.
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
54 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.
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).
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-engines-operating 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.
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).
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).
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 acceleratestop 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.
12 For turbopropeller powered airplanes, the criteria of paragraphs 1 through 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.
(f) AFM wet runway accelerate-stop distances . Section 25.1583(h) requires the operating limitations established under § 25.1533, including wet runway accelerate-stop distances, to be presented in the AFM. At the option of the applicant, grooved and PFC wet runway accelerate-stop distances may also 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. The page(s) in the AFM containing the wet runway accelerate-stop distances for grooved and PFC runways should contain a note equivalent to the following: “These accelerate-stop distances apply only to runways that are grooved or treated with a porous friction course (PFC) overlay that the operator has determined have been designed, constructed, and maintained in a manner acceptable to the FAA Administrator.” Information should also be included describing the method and assumptions used in generating both the smooth and grooved/PFC wet runway information and generally describing the effect of operational variables on wet runway stopping performance (e.g., tire tread depth, runway surface texture, water depth, brakes-on speed).
12. Takeoff Path - § 25.111.
a. Section 25.111(a) .
(1) Explanation.
(a) The takeoff path requirements of § 25.111, and the reductions to that path required by § 25.115, are established so that the AFM performance can be used in making necessary decisions relative to takeoff weights when obstacles are present. Such considerations are required by § 121.189 when operations are conducted in accordance with 14 CFR part 121.
(b) The required performance is provided in AFMs by either pictorial paths at various thrust-to-weight (T/W) conditions, with corrections for wind, or by a series of charts for each segment, along with a procedure for connecting these segments into a continuous path.
(c) The height references in § 25.111 should be interpreted as geometric heights.
(2) Procedures.
(a) Section 25.111(a) requires that the actual takeoff path (from which the AFM net takeoff flight path is derived) extend to the higher of where the airplane is 1,500 ft. above the takeoff surface or to the altitude at which the transition to en route configuration is complete and a speed is reached where compliance with the final segment requirements of § 25.121(c) can be met. Section 25.115(b) allows termination of the AFM “net” flight path below 1,500 ft. in some cases.
(b) The AFM should contain information required to show compliance with the climb requirements of §§ 25.111 and 25.121(c). This should include information related to the transition from the takeoff configuration and speed to the final segment configuration and speed.
56 The effects of changes from takeoff thrust to maximum continuous thrust should also be included.
(c) Generally, the AFM shows low T/W takeoff paths ending at 1500 ft. above the takeoff surface, with acceleration segments between 400 and 1,500 ft., and high T/W takeoff paths extending considerably higher than 1,500 ft. above the takeoff surface. On some airplanes, the takeoff speed schedules, or certain flap configurations, do not require acceleration below 1,500 ft., even at limiting performance gradients. Although § 25.111(a) permits the takeoff path to be terminated as low as 1,500 feet above the takeoff surface, it is recommended that the flight path data, or associated AFM methodology, be presented so that the flight path can be determined to 3,000 ft. above the takeoff surface. This will permit obstacle clearance analysis for distant obstacles of considerable elevation that may be encountered in operations from mountain airports.
(d) The § 25.115(b) net takeoff flight path, required by § 25.1587(b) to be included in the AFM, need not extend to the altitude specified in § 25.111(a). It may be terminated at a height, generally called “NET HEIGHT,” that is directly related to the actual airplane height specified in § 25.111(a). The “NET HEIGHT” is calculated using the actual airplane takeoff climb performance, to the point where the altitude requirements of § 25.111(a) are met, reduced by the climb gradient decrements specified in § 25.115(b).
b. Section 25.111(a)(1) - Takeoff Path Thrust Conditions .
(1) Explanation . The takeoff path established from continuous demonstrated takeoffs must at all points represent the actual expected performance, or be conservative, per §§ 25.111(d)(2) and 25.111(d)(4), if the path is constructed by the segmental method.
(2) Procedures.
(a) To be assured that the predicted takeoff path is representative of actual performance, construct the path using the thrust required by § 25.101(c). This requires, in part, that the thrust be based on the particular ambient atmospheric conditions that are assumed to exist along the path. The standard lapse rate for ambient temperature is specified in part 1 of 14 CFR under “Standard Atmosphere,” and should be used for thrust determination associated with each pressure altitude during the climb.
(b) In accordance with § 25.111(c)(4), the thrust up to 400 ft. above the takeoff surface must represent the thrust available along the path resulting from the power lever setting established during the initial ground roll in accordance with AFM procedures. This resulting thrust may be less than that available from the rated inflight setting schedule.
(c) A sufficient number of takeoffs, to at least the altitude above the takeoff surface scheduled for V2 climb, should be made to establish the fixed power lever thrust lapse.
An analysis may be used to account for various engine bleeds (e.g. ice protection, air conditioning, etc.). In some airplanes, the thrust growth characteristics are such that less than full rated thrust is used for AFM takeoff power limitations and performance. This is to preclude
57 engine limitations from being exceeded during the takeoff climbs to 400 ft. above the takeoff surface.
(d) Engine thrust lapse with speed and altitude during the takeoff and climb, at fixed power lever settings, can be affected by takeoff pressure altitude.
(e) 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 thrust setting procedure, provided the AFM takeoff field length and the takeoff 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 on all engines (power setting selected by applicant).
2 Release brakes and advance power levers.
3 Set target power 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 ft. 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.
(f) 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.
c. Section 25.111(a)(2) - Engine Failure .
(1) Explanation .
(a) 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 thrust to drop immediately, with the associated performance going from all-engines-operating to one-engine-inoperative at the point of engine failure.
(b) This conservative rationale notwithstanding, there is a basis for assuming that the failed engine thrust will not decay immediately. Unlike reciprocating engines, the lockingup 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.
(c) With these considerations, it may be acceptable to use the transient thrust as the failed engine spools down at VEF. The thrust time-history used for data reduction and expansion should be substantiated by test results.
(d) 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 thrust in this case will be much more sudden than the turbojet engine spooldown described above.
(2) Procedures .
(a) For turbojet powered airplanes, if transient thrust credit is used during engine failure in determining the accelerate-go AFM 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.
(b) For propeller driven airplanes, the use of fuel cuts can be more important in order to ensure that the takeoff speeds and distances are obtained with the critical engine’s propeller attaining the position it would during a sudden engine failure. The number of tests that should be conducted using fuel cuts, if any, depends on the correlation obtained with the idle cut data and substantiation that the data analysis methodology adequately models the effects of a sudden engine failure.
d. Section 25.111(a)(3) - Airplane Acceleration.
(1) Explanation. None.
(2) Procedures. None.
e. Section 25.111(b) - Airplane Rotation and Gear Retraction.
(1) Explanation. The rotation speed, VR, is intended to be the speed at which the pilot initiates action to raise the nose gear 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 gear off the ground will not be initiated until the speed VR has been reached.
(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.
f. Section 25.111(c)(1) - Takeoff Path Slope .
(1) Explanation .
(a) The establishment of 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).
(b) The net takeoff flight path is the flight path used to determine the airplane obstacle clearance for turbine powered airplanes (§ 121.189(d)(2)). 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 applicant exchanges altitude reduction for increased distance to accelerate in level flight in determination of the level flight portion of the net takeoff path.
(2) Procedures.
(a) The level acceleration segment in the AFM net takeoff profile should begin at the same horizontal distance along the takeoff flight path that the climb segment, without the gradient reductions of § 25.115(b), reaches the AFM specified acceleration height.
(b) The AFM acceleration height should be presented in terms of pressure altitude increment above the takeoff surface. This information should allow the establishment of the pressure altitude “increment” (hp) for off-standard ambient temperatures so that the geometric height required for obstacle clearance is assured. For example:
Given :
o Takeoff surface pressure altitude (hp) = 2,000 ft.
o Airport std. temp. abs. (T S) = 11º C+273.2º = 284.2º K o Airport ambient temp. abs. (T AM) = -20º C+273.2º = 253.2º K o Geometric height required ( h) = 1,700 ft. above the takeoff surface
Find :
o Pressure altitude increment ( hp) above the takeoff surface hp = h(TS/TAM) = 1,700 ft. (284.2º K/253.2º K) hp = 1,908 ft.
g. Section 25.111(c)(2) - Takeoff Path Speed .
(1) Explanation.
(a) It is intended that the airplane be flown at a constant indicated airspeed to at least 400 ft. above the takeoff surface. This speed must meet the constraints on V2 of § 25.107(b) and (c).
(b) The specific wording of § 25.111(c)(2) should not be construed to imply that above 400 ft. the airspeed may be reduced below V2, but instead that acceleration may be commenced.
(2) Procedures.
(a) For those airplanes that take advantage of reduced stall speeds at low pressure altitude, the scheduling of V2 should not be factored against the stall speed obtained at the takeoff surface pressure altitude. Such a procedure would result in a reduced stall speed margin during the climb, which would be contrary to the intent of § 25.107(b).
(b) For those airplanes mentioned in paragraph (a), above, the V2 should be constrained, in addition to the requirements of § 25.107(b) and (c), by the stall speed 1,500 ft.
above the takeoff surface. Weight reduction along the takeoff path, due to fuel burn, may be considered in the calculation of the stall speed ratios, provided it is well established. However, many applicants have measured stall speeds at 10,000 to 15,000 ft., which provides stall margin conservatism at lower takeoff field pressure altitudes.
h. Section 25.111(c)(3) - Required Gradient.
(1) Explanation . None.
(2) Procedures . None.
i. Section 25.111(c)(4) - Configuration Changes .
(1) Explanation .
(a) The intent of this requirement is to permit only those crew actions that are conducted routinely to be used in establishing the one-engine-inoperative takeoff path. The power levers may only be adjusted early during the takeoff roll, as discussed in paragraph 12b(2), and then left fixed until at least 400 ft. above the takeoff surface.
(b) Simulation studies and accident investigations have shown that when heavy workload occurs in the cockpit, as with an engine failure during takeoff, the crew might not advance the operative engines to avoid the ground, even if the crew knows the operative engines have been set at reduced power. This same finding applies to manually feathering a propeller.
The landing gear may be retracted, however, as this is accomplished routinely once a positive rate of climb is observed. This also establishes the delay time to be used for data expansion purposes.