J - ADVANCE RATIO
CP - POWER COEFFICIENT CT =.10 CT=.08 CT=.06 CT=.04 CT=.02 CT=0 B=10 O B=15 O B=2 0O B=2 5O B=30 O
FIGURE 17-2 PROPELLER COEFFICIENTS
V-TAS IN KNOTS P R O P E L L E R R P M
FIGURE 17-3 - ZERO THRUST
b. Procedures.
(1) Instrumentation.
(i) Test Systems. As previously mentioned, the production airspeed system is normally not sufficiently predictable or repeatable at high angles of attack to accurately measure the performance stall speeds of an airplane. However, a production airspeed system should be installed during stall speed tests to define the airspeed indicator markings required by § 23.1545.
The performance stall speed test system utilized in a type certification program should be calibrated to a minimum speed at least as low as the predicted minimum stall speed anticipated on the test airplane. Test systems that have been utilized to accurately define the performance stall speeds include, but are not limited to the following:
(A) Boom Systems. Swivel-head, boom-mounted, pitot-static systems with sufficient free-swivel angle to cover the stall angle-of-attack range of the airplane have been found to be acceptable. Some angle-of-attack compensated fixed pitot heads have also been found to be acceptable over a wind tunnel defined angle-of-attack range. In all wing-mounted boom systems, the boom mounted static source should be at least one chord length ahead of the wing leading edge. On nose-boom mounted systems, it has been generally accepted that the static source should be at least one and one-half fuselage diameters ahead of the nose. All boom systems should be installed in a manner which assures that the boom and boom pitot-static head are structurally sound (both static and dynamic) within the proposed operating range.
(B) Pitot-Static Bombs. Pitot-static bombs that are stable through the stall maneuvers have been found to provide acceptable data.
(C) Trailing Cones. A trailing cone static source dynamically balanced with a swivel head pitot source, or dynamically balanced with a fixed pitot source of proven accuracy in the stall angle-of-attack range has been acceptable. The stability of the cone should be verified during stall tests and throughout its intended operating range.
The length of the cone may need to be adjusted on individual airplane installations to assure cone stability.
(ii) Lag Equalization. All of the systems described in paragraph (i) could involve the use of long lengths of pressure tubing, and the associated pressure lags then occur whenever either speed or altitude, or both, are changed. Probably the most important consideration in these installations (on most small general aviation airplanes) is that the test pitot-static systems should be dynamically balanced. This is easily accomplished experimentally by putting both the total head and static orifices in a common chamber and varying the pressure in the chamber at a rate corresponding to a 2,000 to 3,000 feet-per-minute rate of descent. Various volumes are inserted in the total head line until the airspeed indicator has no tendency to move in either direction from zero during the simulated rate of descent. This method results in approximately the same volume in both systems, and, for the same size tubing, the Reynolds Number of the flow through both lines will be the same. A dynamically balanced airspeed system has equal lag in both the total and static sides. Use of a balanced system simplifies the interpretation of recorded stall time histories.
(iii) Lag Correction. When a balanced test airspeed system is used, it is often unnecessary to determine the actual amount of lag present. When such a determination is necessary, a method of accounting for lag errors is described in NASA Reference Publication 1046, "Measurement of Aircraft Speed and Altitude," by W. Gracey, May 1980.
(2) Test.
(i) Stall Speed. The actual test should be commenced with the airplane in the configuration desired and trimmed at approximately 1.5 VS1 or the minimum speed trim, whichever is greater. The airplane should be slowed to about 10 knots above the stall, at which time the speed should be reduced at a rate of one knot per second or less, until the stall occurs or the control reaches the stop. Where exact determination of stalling speed is required, entry rate should be varied to bracket one knot per second, and data should be recorded to allow the preparation of time histories similar to those shown in figure 17-1.
The indicated airspeed at the stall should be noted, using the production airspeed system.
Both the indicated airspeeds and the calibrated stall speeds may then be plotted versus entry rate to determine the one knot per second values.
(ii) Bomb. When using a bomb, caution should be used in recovering from the stall so that the bomb is not whipped off the end of the hose.
(iii) Weight and C.G. The stalling speed should be determined at all weight and c.g.
positions defining the corners of the loading envelope to determine the critical condition.
Data should be recorded so that the weight and c.g. at the time of the test can be accurately determined. This can often be done by recording the time of takeoff, time of test, time of landing, and total fuel used during the flight.
(iv) Power and Configuration. The stall should be repeated enough times for each configuration to ensure a consistent speed. If a correction is to be made for zero thrust, then the stall speed and power at several power settings may be recorded for later extrapolation to zero thrust.
(v) Control Stops. The elevator up stop should be set to the minimum allowable deflection. Flap travels should be set to minimum allowable settings.
(3) Data Reduction. The correction involves:
(i) Correction for airspeed error. IAS to CAS (correct for instrument as well as position error) when CAS is required.
(ii) Correction for weight. Multiply the test calibrated stall speed times the square root of the standard weight divided by the test weight.
VS = VST W W S T
Where: VS = Stall speed (CAS) VST = Test stall speed (CAS) 19
WS = Standard weight (lbs.)
WT = Test weight (lbs.)
(CAUTION — Do not use for minimum steady flight speed)
(iii) The correction for weight shown above applies only where the c.g. is not also changing with weight. Where c.g. is changing with weight, such as between forward regardless and forward gross, stall speed should account for this. A straight line variation between the measured stall speeds for the two weight and c.g. conditions has been found to be an acceptable method.
18. SECTION 23.51 TAKEOFF SPEEDS.
a. Explanation. The primary objective of this section is to determine the normal takeoff speeds for non-weight, altitude, and temperature limited airplanes. For WAT limited airplanes, the objective is to determine the takeoff speed schedules for all takeoff configurations at weight, altitude, and temperature conditions within the operational limits selected by the applicant.
b. For normal, utility, and acrobatic category airplanes, the rotation speed, (VR) in terms of inground effect calibrated airspeed, must be selected by the applicant. VR is constrained by § 23.51(a), as follows:
(1) For multiengine landplanes, VR must not be less than the greater of 1.05 VMC or 1.10 VS1;
(2) For single-engine landplanes, VR must not be less than VS1; and
(3) For seaplanes and amphibians taking off from water, VR may be any speed that is shown to be safe under all reasonably expected conditions, including turbulence and complete failure of the critical engine.
c. For normal, utility, and acrobatic category airplanes, the speed at 50 feet.
(1) Multiengine 50-foot Speed. For multiengine airplanes, § 23.51(b)(1) requires the speed at the 50-foot point to be the higher of the following:
(i) A speed that is shown to be safe for continued flight (or land back, if applicable) under all reasonably expected conditions, including turbulence and complete engine failure; or
(ii) 1.1 VMC; or
(iii) 1.2 VS1.
(2) Single Engine 50-foot Speed. For single-engine airplanes, § 23.51(b)(2) requires the speed at the 50-foot point to be the higher of the following:
(i) A speed that is shown to be safe under all reasonably expected conditions, including turbulence and complete engine failure; or
(ii) 1.2 VS1.
(3) Takeoff Speed Investigations - General. Investigation of the acceptability of the takeoff speed and of the associated takeoff procedure should include a demonstration that controllability and maneuverability in the takeoff configuration are adequate to safely proceed with the takeoff in turbulent crosswind conditions and maximum approved lateral imbalance.
(4) Single-engine Airplane Takeoff Speeds. The takeoff speed investigation should include a demonstration that controllability and maneuverability following engine failure at any time between lift-off and the 50-foot point are adequate for safe landing.
(5) Multiengine Airplane Takeoff Speeds. For multiengine airplanes, the investigation should include a demonstration that the controllability and maneuverability following critical engine failure at any time between lift-off and the 50-foot point are adequate for either safe landing or for safe continuation of the takeoff. There will be some combinations of weight, altitude, and temperature where positive climb at the 50-foot height with one engine inoperative is not possible. Because of this, a satisfactory re-land maneuver should be demonstrated. Rotation speed should be scheduled so that the speed at 50 feet is in accordance with § 23.51(b)(1).
(6) Multiple Takeoff Weights. For those multiengine airplanes for which takeoff distance data are to be approved for a range of weights, and for which the takeoff distance is based upon takeoff speeds that decrease as the weight decreases, the investigations of paragraph (3) of this section also should include consideration of the minimum control speed, VMC. The
1.2 VS design limit imposed on VMC by § 23.149 is intended to provide a controllability margin below the takeoff speed that is sufficient for adequate control of the airplane in the event of engine failure during takeoff. Hence, to maintain the intended level of safety for the lower takeoff speeds associated with the lighter takeoff weights, investigation of the acceptability of such speeds for compliance with § 23.51(b)(1) should include demonstration of acceptable characteristics following engine failure at any time between liftoff and the 50-foot point during takeoff in accordance with the established takeoff procedures.
(7) Complete Engine Failure. The term "complete engine failure" has been consistently interpreted to require that, for multiengine airplanes that meet the powerplant isolation requirements of § 23.903(c) in the takeoff configuration, only one engine needs be made inoperative in the specified investigations.
d. Commuter Category Airplanes.
(1) Takeoff Speeds. The following speed definitions are given in terms of calibrated airspeed. The AFM presentations are required, by § 23.1581(d), in indicated airspeed (IAS).
(i) Section 23.51(c)(1) - Engine Failure Speed (VEF). The engine failure speed VEF is defined as the calibrated airspeed at which the critical engine is assumed to fail and must be selected by the applicant. VEF cannot be less than 1.05 VMC, as determined in § 23.149. Ground controllability should also be determined to be adequate at VEF to ensure meeting the requirements of § 23.51(c)(1), that is, speed adequate to safely continue the takeoff. During the demonstration, the airplane's ground run should not deviate more than 30 feet from the pre-engine-cut projected ground track. VMCg determined under 14 CFR Part 25, § 25.149(d) is acceptable in lieu of 1.05 VMC. At the applicant's option, in crosswind conditions, the runs may be made on reciprocal headings or an analytical correction may be applied to determine the zero crosswind deviation. If 21
nose wheel steering is an integral part of the rudder system and is required to be operative, then nose wheel steering may be active. Otherwise, control of the airplane should be accomplished by use of the rudder only. If the applicant elects to use VMCg, then the nosewheel steering must be disconnected as required in § 25.149(d). All other controls, such as ailerons and spoilers, should only be used to correct any alterations in the airplane attitude and to maintain a wings level condition. Use of those controls to supplement the rudder effectiveness should be avoided.
(ii) Section 23.51(c)(l) - Takeoff Decision Speed (V1). The takeoff decision speed V1 may not be less than VEF plus the speed gained with the critical engine inoperative during the time interval between VEF and the instant at which the pilot recognizes the engine failure. This is indicated by pilot application of the first decelerating device, such as brakes, throttles, spoilers, and so forth, during accelerate-stop tests. The applicant may choose the sequence of events. V1 should include any airspeed system errors determined during accelerate-takeoff ground runs. Refer to the requirements of § 23.1323(c).
(iii) Section 23.51(c)(2)- Rotation Speed (VR).
(A) The rotation speed, VR in terms of in-ground effect calibrated airspeed, must be selected by the applicant. VR is constrained by § 23.51(c)(2) as follows:
(1) V1; or
(2) 1.05 VMC determined under § 23.149(b); or
(3) 1.10 VS1; or
(4) The speed that allows attaining the initial climbout speed, V2, before reaching a height of 35 feet above the takeoff surface in accordance with § 23.57(c)(2).
(B) Early rotation, one-engine inoperative abuse test.
(1) In showing compliance with § 23.51(c)(5), some guidance relative to the airspeed attained at a height of 35 feet during the associated flight test is necessary. As this requirement dealing with a rotation speed abuse test only specifies an early rotation (VR-5 knots), it is assumed that pilot technique is to remain the same as normally used for an engine-out condition. With these considerations in mind, it is apparent that the airspeed achieved at a height of 35 feet can be somewhat below the normal scheduled V2 speed. However, the amount of permissible V2 speed reduction should be limited to a reasonable amount, as described in paragraphs (2) and (3) below.
(2) In conducting the flight tests required by § 23.51(c)(5), 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 a height of 35 feet during this test is required to be not less than the scheduled V2 value minus 5 knots. These speed limits should not be considered or utilized as target V2 test 22
speeds but rather are intended to provide an acceptable range of speed departure below the scheduled V2 value.
(3) In this abuse test, the engine cut should be accomplished prior to the VR test speed (that is, scheduled VR-5 knots) to allow for engine spin down. The normal one-engine-inoperative takeoff distance may be analytically adjusted to compensate for the effect of the early engine cut. Further, in those tests where the airspeed achieved at a height of 35 feet is slightly less than the V2-5 knots limiting value, it is permissible, in lieu of re-conducting the tests, to analytically adjust the test distance to account for the excessive speed decrement.
(C) All-engines-operating abuse tests.
(1) Section 23.51(c)(6) requires that there 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 is considered as requiring takeoff tests with all engines operating with:
(aa) An abuse at 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 5 percent of the takeoff distance as determined in accordance with § 23.59. Thus, the abuse tests should not result in a takeoff distance of more than 105 percent of the scheduled takeoff distance.
(2) For the early rotation abuse condition with all engines operating and at a weight as near as practicable to the maximum sea level takeoff weight, 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 takeoff distance will result. For this demonstration, the airplane should be rotated at a speed of 10 knots or 7 percent, whichever is less, below the scheduled VR. Tests should be conducted at a rapid rotation rate or should include an over-rotation of 2 degrees above normal attitude after liftoff. Rapid rotation should be taken to mean significantly above the normal pitch rate of rotation. It should be noted that 4 or 5 degrees per second have previously proved satisfactory. Tail strikes, should they occur during this demonstration, are acceptable only if a fault analysis (structural, electrical, hydraulic, and so forth) has been accomplished and indicates no possible degradation in the control of aircraft, engines, or essential systems necessary for continued safe flight after a reasonable worst case tail strike.
(3) For out-of-trim conditions with all engines operating and at a weight as near as practicable to the maximum sea level takeoff weight, it should be shown that with the airplane mis-trimmed, as would reasonably be expected in service, there should not be a "marked increase" in the takeoff distance when rotation is initiated in a normal manner at the scheduled VR speed. The amount of mis-trim used should be with the longitudinal control trimmed to its most adverse position within the allowable takeoff trim band as shown on the cockpit indicator.
(iv) Lift-off Speed (VLOF). VLOF is the calibrated airspeed at which the airplane first becomes airborne.
(v) Section 23.51(c)(4) - Takeoff Safety Speed (V2). V2 is the calibrated airspeed that is attained at or before 35 feet above the takeoff surface after an engine failure at VEF using an established rotation speed VR. During the takeoff speed demonstration, V2 should be continued to an altitude sufficient to assure stable conditions beyond 35 feet. Section
23.51(c)(4) requires V2 not be less than 1.1 VMC or 1.2 VS1. Attainment of V2 by 35 feet should be substantiated by use of procedures consistent with those that will be experienced in service with an actual engine failure (that is, if auto feather is required), then auto feather should be activated as an integral part of testing.
19. SECTION 23.53 TAKEOFF PERFORMANCE.
a. Explanation.
(1) Normal, Utility, and Acrobatic Category Airplanes.
(i) Objective of Takeoff Requirement. The primary objective of the takeoff requirement is to establish, for information of the operator, a takeoff distance within which the airplane may be expected to achieve a speed and height sufficient to ensure capability of performing all maneuvers that may become necessary for safe completion of the takeoff and for safe landing if necessitated by power failure. An airspeed margin above stall in conjunction with a height of 50 feet is presumed to assure the desired maneuvering capability.
(ii) AFM Takeoff Distance. Section 23.1587(c)(1) requires the takeoff distance determined under this section be furnished in the AFM. The data should be furnished at the most critical c.g. (usually forward). Section 23.1587 further requires the effect of altitude from sea level to 10,000 feet and the following:
(A) Temperature from standard to 30 °C above standard; or
(B) For airplanes greater than 6,000 lbs. (2730 kg) and turbine-powered airplanes, the AFM should furnish temperature from standard to 30 °C above standard, or the maximum ambient atmospheric temperature at which compliance with the cooling provisions of § 23.1041 to § 23.1047 is shown, if lower. Propulsive thrust available should be accounted for in accordance with § 23.45 and Appendix 1 of this AC. For turbine-powered airplanes, distances should be presented up to the maximum takeoff temperature limit. A data expansion method appropriate to the airplane's features should be used.
(iii) AFM Takeoff Technique. For multiengine airplanes, § 23.1585(d)(1) requires the AFM to furnish the procedures for the § 23.53 takeoff. The recommended technique that is published in the AFM and used to achieve the performance should be one that the operational pilot can duplicate using the minimum amount of type design cockpit instrumentation and the minimum crew.
(iv) Tire Speed Limits. If tires approved through the technical standard order (TSO) process are used, it should be determined, within the weight, altitude, and temperature for which takeoff performance is shown in § 23.1587, that the TSO tire speed ratings are not exceeded at VL0F. If the tire speed rating would be exceeded under some combinations of weight, altitude, and temperature, then the tire speed limit should be established as an operating limitation. In compliance with § 23.1581(a)(2), a maximum takeoff weight limited by tire speed chart should also be included in the AFM performance section.
b. Procedures.
(1) Takeoff Distance Tests. The takeoff distance should be established by test. It may be obtained either by takeoffs conducted as a continuous operation from start to the 50-foot height or synthesized from acceleration segments and climb segment(s) determined separately. Recording theodolite or electronic equipment that is capable of providing horizontal distance and velocity, and height above the takeoff surface is highly desirable for takeoff distance tests. Additional required special ground equipment includes a sensitive anemometer capable of providing wind velocity and direction, a thermometer capable of providing accurate free-air temperature under all conditions, and an altimeter or barograph to provide pressure altitude.
(2) Segment Technique. For the segment technique, the airplane should be accelerated on the surface from brake release to rotation speed VR and on to the speed selected for the 50-foot height point. Six acceptable runs are recommended to establish the takeoff acceleration segment. VR should be selected so that the 50-foot speed can be achieved. A climb segment based on the rate of climb, free of ground effect, is added to the acceleration segment. See paragraph 25 of this AC and Appendix 2 for climb performance methods. Total distance is the sum of the acceleration segment plus the climb segment. For AFM presentation, the ground run would be the ground acceleration distance to VL0F, and the air distance would be the horizontal distance to climb at the 50-foot speed for 50 feet plus the ground acceleration distance from VL0F to the 50-foot speed. For those airplanes with retractable gear, the landing gear should be extended throughout or an alternative method of retraction may be initiated at a speed corresponding to a safe speed for gear retraction following liftoff in normal operations. If takeoff distance is determined using the "segmented" method, actual takeoffs using the AFM takeoff speed schedule should be conducted to verify that the actual takeoff distance to the 50-foot height does not exceed the calculated takeoff distance to the 50-foot height.
(3) Weight. Takeoff distance tests should be conducted at the maximum weight, and at a lesser weight if takeoff distance data for a range of weights is to be approved. The test results may be considered acceptable without correction for weight if a ±0.5 percent weight tolerance is observed.
(4) Nose wheel/tail wheel. In the absence of evidence to the contrary, the "critical" c.g.
position for takeoff distance tests may be assumed to be forward.
(5) Wind. Wind velocity and direction should be measured adjacent to the runway during the time interval of each test run. See paragraph 6.a.(5) of this AC for wind velocity and direction tolerances. For the ground run portion of the segment technique, the following relationship was developed empirically and is an acceptable method for correction of lowwind conditions:
SG = SGW 1 18 5 ±⎛⎝⎜ ⎞⎠⎟VV WTOW .
Where: S G = no-wind takeoff ground distance (feet) S GW = takeoff ground distance at a known wind velocity (feet) V W = wind velocity (feet second) V TOW = true ground speed, at lift-off with a known wind velocity (feet/second)
Notes: 1. + is used for headwind and - for tailwind
2. Wind, then slope corrections should be applied before further data reduction.
(6) Runway Slope. The effect of runway gradient can be significant for heavy airplanes or for low thrust-to-weight ratio airplanes, even if the gradient of the runway is small. Gradient should be controlled by proper runway selection. The correction is as follows:
SG = θsin21 1 1 ⎟⎟⎠⎞⎜⎜⎝⎛± TOGS GS V gS S
Where: S GS1 = ground distance on a sloping runway g = acceleration of gravity, 32.17 ft./sec2 V TO = airplane velocity at lift-off in ft./sec. (true) θ = angle of the slope in degrees (not percent)
+ for upslope and - for downslope
c. Commuter Category Airplanes.
(1) Objective of Takeoff Requirement. Section 23.53(c) requires that performance be determined that provides accountability for the selected operating weights, altitudes, ambient temperatures, configurations, and corrected for various wind and runway gradient conditions.
(2) Takeoff Profile. Tests are required to determine the performance throughout the takeoff path as specifically defined by § 23.55 through § 23.59, and as discussed in paragraphs 21 through 23 of this AC.
(3) Expansion of Takeoff Data for a Range of Airport Elevations.
(i) These guidelines are applicable to expanding takeoff data above the altitude at which the basic or verifying tests were obtained.
(ii) In general, takeoff data may be extrapolated above and below the altitude at which the basic test data was obtained without additional conservatism within the following constraints.
(iii) When the basic takeoff tests are accomplished between sea level and approximately 3,000 feet, the maximum allowable extrapolation limits are 6,000 feet above and 3,000 feet below the test field elevation. If it is desired to extrapolate beyond these limits, one of two procedures may be employed.
(A) Extrapolation of Performance Data for a Range of Altitudes When Verifying Tests are Not Conducted. The approval of performance data for airport elevations beyond the maximum elevation permitted by basic tests may be allowed without conducting verifying tests if the calculated data include a conservative factor. This conservatism should result in an increase of the calculated takeoff distance at the desired airport elevation by an amount equal to zero percent for the highest airport elevation approved on the results of the basic tests and an additional cumulative 2 percent incremental factor for each 1,000 feet of elevation above the highest airport elevation approved for zero percent conservatism. The 2 percent incremental factor should have a straight line variation with altitude. When performance data are calculated for the effects of altitude under this procedure, the following provisions are applicable:
(1) Previously established calculation procedures should be used, taking into account all known variables.
(2) The calibrated installed engine power for the pertinent speed and altitude ranges should be used.
(3) The brake kinetic energy limits established by airplane ground tests should not be exceeded.
(B) Extrapolation of Performance Data When Verifying Tests are Conducted.
(1) If data approval is desired for a greater range of airport elevations, the performance may be calculated from the basic test data up to the maximum airport elevation, provided verifying tests are conducted at appropriate elevations to substantiate the validity of the calculations. The actual airplane performance data from the verifying tests should correspond closely to the calculated performance values.
(2) For the verifying tests, it has been found that normally three takeoffs at maximum weights for the elevations tested will provide adequate verification.
(3) If verifying tests substantiate the expanded takeoff data, the data may be further expanded up to 6,000 feet above the altitude at which the verifying tests were conducted. At altitudes higher than 6,000 feet above the verifying test altitude, the 2 percent per 1,000 feet cumulative factor discussed in paragraph (i) above should be applied starting at zero percent at the verifying test altitude plus 6,000 feet.
20. RESERVED
21. SECTION 23.55 ACCELERATE-STOP DISTANCE.
a. Explanation. This section describes test demonstrations necessary to determine accelerate-stop distances for airplane performance required to be published in the Performance Section of the AFM.
b. Procedures.
(1) Accelerate-stop tests should be determined in accordance with the provisions of this paragraph.
(i) Number of Test Runs. A sufficient number of test runs should be conducted for each airplane configuration desired by the applicant, in order to establish a representative distance that would be required in the event of a rejected takeoff at or below the takeoff decision speed V1.
(ii) Time Delays. The procedures outlined in paragraph 21.b.(12), as required by § 23.45(f)(5), apply appropriate time delays for the execution of retarding means related to the accelerate-stop operational procedures and for expansion of accelerate-stop data to be incorporated in the AFM.
(iii) Reverse Thrust. The stopping portion of the accelerate-stop test may not utilize propeller reverse thrust unless the thrust reverser system is shown to be safe, reliable, and capable of giving repeatable results. See subparagraph c.
(2) Airport Elevation. Accelerate-stop runs at different airport elevations can be simulated at one airport elevation provided the braking speeds used include the entire energy range to be absorbed by the brakes. In scheduling the data for the AFM, the brake energy assumed should not exceed the maximum demonstrated in these tests.
(3) Braking Speeds. The braking speeds referred to herein are scheduled test speeds and need not correspond to the values to be scheduled in the AFM, since it is necessary to increase or decrease the braking speed to simulate the energy range and weight envelope.
(4) Number of Runs. At least two test runs are necessary for each configuration when multiple aerodynamic configurations are being shown to have the same braking coefficient of friction, unless sufficient data is available for the airplane model to account for variation of braking performance with weight, kinetic energy, lift, drag, ground speed, torque limit, and so forth. These runs should be made with the airplane weight and kinetic energy varying throughout the range for which takeoff data is scheduled. This will usually require at least six test runs. These tests are usually conducted on hard surfaced, dry runways.
(5) Alternate Approvals. For an alternate approval with antiskid inoperative, nose wheel brakes or one main wheel brake inoperative, auto braking systems, and so forth, a full set of tests, as mentioned in paragraph 21.b.(4), should normally be conducted. A lesser number of tests may be accepted for "equal or better" demonstrations, or to establish small increments, or if adequate conservatism is used during testing.
(6) Maximum Energy Stop. A brake energy demonstration is needed to show compliance with the brake energy requirements. A maximum energy stop (or some lesser brake energy) is used to establish a distance that can be associated with the demonstrated kinetic energy.
An applicant can choose any level of energy for demonstration providing that the AFM does 28