(e) For power-on stalls, power or thrust should be set to the value required to maintain level flight at a speed of 1.5 VSR at the maximum landing weight with flaps in the approach position, and the landing gear retracted. The approach flap position referred to is the maximum flap deflection used to show compliance with § 25.121(d), which specifies a configuration in which the reference stall speed does not exceed 110 percent of the reference stall speed for the related landing configuration.
(f) Stall characteristics testing is normally done at the aft c.g. limit, which is typically the most adverse; however, if the stall speed tests at forward c.g. indicate that marginal stall recovery characteristics may exist at forward c.g., compliance with § 25.203 should be shown for the most critical loading.
(g) In accordance with § 25.21(c), stalls must be demonstrated up to the maximum approved operating altitude to determine if there are any adverse compressibility effects on stall characteristics. These tests should be flown with gear and flaps up at the most adverse c.g.
Power or thrust may be set, as required, to maintain approximately level flight and a 1 knot/second deceleration. A slight descent rate is permissible as long as the stall occurs at approximately the maximum approved altitude. Characteristics should be checked during a wings level stall and in a 30-degree banked turn.
(h) For abnormal aerodynamic configurations covered by AFM procedures, high angle-of-attack characteristics should be evaluated down to the speed reached one second after stall warning in a one knot/second deceleration with the wings level and at idle power or thrust.
If there are no adverse characteristics and there is adequate controllability, it is not necessary to stall the airplane. Adequate controllability means that it is possible to produce and to correct pitch, roll, and yaw by unreversed use of the flight controls, and that there are no uncommanded airplane motions due to aerodynamic flow breakdown. The applicant should also demonstrate that the airplane is safely controllable and maneuverable when flown at the recommended operating speed.
(i) Stall characteristics should also be demonstrated with the maximum allowable asymmetric fuel loading. Requirements are as specified in § 25.203(a) and (c).
(3) Procedures.
(a) The airplane should be trimmed for hands-off flight at a speed 13 percent to 30 percent above the reference stall speed, with the appropriate power or thrust setting and configuration. Then, using only the primary longitudinal control, establish and maintain a deceleration (stall entry rate) consistent with that specified in § 25.201(c)(1) or (c)(2), as appropriate, until the airplane is stalled. Both power/thrust and pilot selectable trim should remain constant throughout the stall and recovery (to where the angle-of-attack has decreased to the point of no stall warning).
(b) The same trim reference (for example, 1.23 VSR) should be used for both the stall speeds and characteristics testing. For all stall testing, the trim speed is based on the stall speeds provided in the AFM.
(c) During the approach to the stall, the longitudinal control pull force should increase continuously as speed is reduced from the trimmed speed to the onset of stall warning.
Below that speed some reduction in longitudinal control force is acceptable, provided it is not sudden or excessive.
(d) Section 25.203(b) states that “the roll occurring between the stall and the completion of the recovery may not exceed approximately 20 degrees” for level wing stalls. In level wing stalls the bank angle may exceed 20 degrees occasionally, provided that lateral control is effective during recovery.
(e) Section 25.203(c) requires the action of the airplane, following the 30 degrees bank turning stalls, “not be so violent or extreme...” such that a prompt recovery would be difficult and require more than normal piloting skill. The maximum bank angle that occurs during the recovery should not exceed approximately 60 degrees in the original direction of the turn, or 30 degrees in the opposite direction.
(f) The intent of evaluating the 3 knot per second deceleration rate required under § 25.201(c)(2) is to demonstrate safe characteristics at higher rates of increase in angle-of-attack than are obtained from the 1 knot per second stalls. The specified airspeed deceleration rate, and associated angle-of-attack rate, should be maintained up to the point at which the airplane stalls.
The maximum bank angle that occurs during the recovery should not exceed approximately 90 degrees in the original direction of the turn, or 60 degrees in the opposite direction.
(g) For those airplanes where stall is defined by full nose-up longitudinal control for both forward and aft c.g., the time at full aft stick during characteristics testing should be not less than that used for stall speed determination. For turning flight stalls, however, recovery may be initiated once the pitch control reaches the aft stop when accompanied by a rolling motion that is not immediately controllable (provided the rolling motion complies with § 25.203(c)).
(h) As required by § 25.203(a), normal use of the lateral control must produce (or correct) a roll, and normal use of the directional control must produce (or correct) a yaw in the applied direction up to the point where the airplane is considered stalled. It must be possible to prevent or recover from a stall by normal use of the controls.
(i) If wind tunnel tests have indicated an airplane may be susceptible to deep stall penetration (i.e., that area beyond the stall angle-of-attack from which recovery may be difficult or impossible), substantiation should be provided that there is adequate recovery control available at, and sufficiently beyond, the stall angle-of-attack.
f. Stall Warning - § 25.207.
(1) Explanation. The purpose of these stall warning requirements is to provide an adequate spread between warning and stall to allow the pilot time to recover without inadvertently stalling the airplane.
(2) Background. To be acceptable, a stall warning must have the following features:
(a) Distinctiveness. The stall warning indication must be clear and distinct to a degree that will ensure positive pilot recognition of an impending stall.
(b) Timeliness. For one knot per second entry rate stalls, the stall warning must begin at a speed, VSW, not less than five knots or five percent (whichever is greater) above the speed at which the stall is identified in accordance with § 25.201(d). For straight flight stalls, at idle power or thrust and with the c.g. at the position specified in § 25.103(b)(5), the stall warning must begin at a speed not less than three knots or three percent (whichever is greater) above the reference stall speed. These speed margins should be in terms of the same units of measurement as VSR (i.e., calibrated airspeed).
(c) Consistency. The stall warning should be reliable and repeatable. The warning must occur with flaps and gear in all normally used positions in both straight and turning flight (§ 25.207(a)) and must continue throughout the stall demonstration until the angleof-attack is reduced to approximately that at which the stall warning was initiated (§ 25.207(c)).
The warning may be furnished naturally through the inherent aerodynamic characteristics of the airplane, or artificially by a system designed for this purpose. If artificial stall warning is provided for any airplane configuration, it must be provided for all configurations (§ 25.207(b)).
(d) An artificial stall warning indication that is a solely visual device which requires attention in the cockpit, inhibits cockpit conversation or, in the event of malfunction, causes distraction that would interfere with safe operation of the airplane, is not acceptable.
(e) For airplanes that use artificial stall warning systems, paragraph 228 of this AC presents guidance material for demonstrating compliance with the regulatory requirements of part 25.
(f) If the stall warning required by § 25.207 is provided by an artificial stall warning system (e.g., a stick shaker), the effect of production tolerances on the stall warning system should be considered when evaluating the stall warning margin required by § 25.207(c) through (f) and the maneuver capabilities required by § 25.143(h).
1 The stall warning margin required by § 25.207(c) through (f) should be available with the stall warning system set to the most critical setting expected in production.
Unless another setting would provide a lesser margin, the stall warning system should be operating at its high angle-of-attack limit. For airplanes where VSR is set by a device that abruptly pushes the nose down at a selected angle-of-attack (e.g., a stick pusher), the stall warning margin may be evaluated with both the stall warning and stall identification (e.g., stick pusher) systems at their nominal angle-of-attack settings unless a lesser margin can result from the various system tolerances.
2 The maneuver capabilities required by § 25.143(h) should be available assuming the stall warning system is operating on its nominal setting. When the stall warning system is operating at its low angle-of-attack limit, the maneuver capabilities should not be
138 reduced by more than 2 degrees of bank angle from those specified in § 25.143(h). A flight test, an acceptable analysis, or simulation can be used to make this assessment.
3 The stall warning margin and maneuver capabilities may be demonstrated by flight testing at the most critical settings specified above for the stall warning and, if so equipped, stall identification systems. Alternatively, compliance may be shown by applying adjustments to flight test data obtained at a different system setting if an acceptable method is used that takes into account all of the relevant variables.
(3) Procedures. Stall warning tests are normally conducted in conjunction with the stall testing required by § 25.103 (stall speeds), § 25.201 (stall demonstration), and § 25.203 (stall characteristics), including consideration of the prescribed bank angles, power or thrust settings, and c.g. position. The pilot technique in stalling the airplane should be consistent between the onset of stall warning and the point at which the stall is identified. That is, there should not be any deliberate attempt to reduce the load factor, change the deceleration, or use any other means to increase the stall warning margin. In addition, if the stall warning margin may be affected by a system (e.g., a stall warning or stick pusher system that modifies the stall warning or stall identification speed as a function of power or thrust, bank angle, angle-of-attack rate, etc.), compliance with § 25.207(c) should be demonstrated at the most critical conditions in terms of stall warning margin. However, for this case, bank angles greater than 40 degrees and power or thrust exceeding maximum continuous power or thrust need not be demonstrated. If the effect of the stall identification or stall warning system compensation is to increase the stall warning margin relative to the nominal values demonstrated during the testing required by §§ 25.103, 25.201, and 25.203, these additional stall warning margin demonstrations need not be done.
(4) Data Acquisition and Reduction. The stall warning speed and type and quality of warning should be noted. To determine if the required margin exits, compare the speed at which acceptable stall warning begins with (1) the stall identification speed, and (2) VSR (for the conditions under which VSR is defined). The stall warning margin comparisons should be made at a constant 1-g load factor when showing compliance with § 25.207(d).
g. Accelerated Stall Warning.
(1) Explanation. Section 25.207(f) requires that, in slow-down turns with at least a
1.5g load factor normal to the flight path and an airspeed deceleration rate greater than 2 knots per second, sufficient stall warning is provided to prevent stalling when the pilot takes recovery action not less than one second after recognition of stall warning. The purpose of the requirement is to ensure that adequate stall warning exists to prevent an inadvertent stall under the most demanding conditions that are likely to occur in normal flight. The elevated load factor will emphasize any adverse stall characteristics, such as wing drop or asymmetric wing flow breakdown, while also investigating Mach and potential aeroelastic effects on available lift.
(2) Procedures.
(a) Trim at 1.3 VSR. Once trimmed, accelerate to a speed that will allow enough time to set up and complete the maneuver at the specified load factor and airspeed deceleration rate. Set power or thrust appropriate to the power or thrust for level flight at 1.3 VSR and do not adjust it during the maneuver. In a level flight maneuver, 1.5g equates to a bank angle of 48 degrees. To prevent an excessive deceleration rate (e.g., greater than 3 knots per second), a descent may be used. Conversely, if the deceleration rate is too low, the maneuver should be conducted in a climbing turn.
(b) After the onset of stall warning, continue the maneuver without releasing stick force for one second before attempting recovery. Normal low speed recovery techniques should be used. If any of the indications of a stall prescribed in § 25.201(d) (see paragraph 29c(1) of this AC) occur during the accelerated stall warning demonstration, compliance with § 25.207(f) will not have been demonstrated.
h. Maneuver Margins. See paragraph 20 of this AC for guidance material associated with demonstrating compliance to the maneuvering capability requirements of § 25.143(h).
i. Additional Considerations for Airplanes Equipped with Stall Identification Systems. A stall identification system is any system that is used to show compliance with § 25.201(d), which requires the airplane to give the pilot a clear and distinctive indication of stall. The stall identification system consists of everything from the sensing devices that supply inputs to the system to the activation of the system response that provides stall identification to the flightcrew.
Section 25.1309(a) requires that such a system, when it is needed to show compliance with the stall-related requirements, must be designed to perform its intended function under any foreseeable operating condition.
(1) The applicant should verify that the stall identification system, considering system design features (e.g., filtering, phase advancing) and airplane and system production tolerances will not result in an unsafe diminishing of the margin between stall warning and stall identification, or between stall identification and any hazardous airplane flight characteristic in any foreseeable operating condition. This verification may be provided by a combination of analysis, simulation, and flight test. The following operating conditions should not result in unwanted activation of the stall identification system or in aerodynamic stall prior to, or close to, activation of the stall warning system: dynamic and accelerated stall entries, the effects of atmospheric turbulence, any foreseeable type of wing contamination (e.g., ice, frost, insects, dirt, anti-icing fluids), or wing leading edge damage within prescribed maintenance limits. Operation in windshear environments where the airplane will be flown at, or very near, stall warning, should also be considered, although, depending on the severity of the windshear, it may be impossible to ensure that there is no possibility of stall indication system operation. For wing contamination, the applicant should substantiate the critical height and density of the contaminant. Carborundum sandpaper no. 40 (that is, 40-grit carborundum sandpaper) has been used in past certification programs to represent residual ice or frost contamination.
(2) Stall characteristics testing should be performed with the following airplane and stall identification system production tolerances set to achieve the most adverse stall identification system activation condition for stall characteristics:
(a) Airframe build tolerances – the impact of wing angle of incidence variation relative to stall identification system vane angle; and
(b) Stall identification system tolerances (e.g., activation vane angles).
(3) If the combined root-sum-square (square root of the sum of the squares of each tolerance) effect of the tolerances identified above is less than ±1 knot, stall speeds testing can be performed and the stall speeds determined with the tolerances at their nominal values. If the combined root-sum-square effect is ±1 knot or greater, stall speed testing should be performed with the tolerances at the values that would result in the highest stall speeds.
j. Reliability of Artificial Stall Warning and Stall Identification Systems. Additional guidance material related to the testing and approval of artificial stall warning and stall identification systems is presented in paragraph 228 of this AC.
Section 7. Ground and Water Handling Characteristics
30. General.
a. Part 25 Regulations. The applicable regulations are §§ 25.231, 25.233, 25.235, 25.237, and 25.239 of the CFR.
b. Longitudinal Stability and Control - § 25.231.
(1) Explanation. Test program objectives would not be expected to demonstrate taxiing over rough surfaces at speeds high enough to approach structural design limits, nor is it expected that in the test program the airplane be landed harder or at higher sink rates than it will ever encounter in service. However, new or modified landing gear systems should be evaluated on rough surfaces that are representative of normal service, and landings should be conducted at various sink rates sufficient to identify any dangerous characteristics or tendencies. Variables to be considered are c.g. and taxi speed. The cockpit motion dynamics during ground handling should not impede control of the airplane, and pitching motion during bounce should not create static pitch control problems or pilot induced oscillation tendencies.
(2) Procedures. Ground handling tests at speeds normally expected in service should be conducted on smooth and rough surfaces that are likely to be encountered under normal operating conditions. Particular attention should be paid to the following:
(a) Brakes. The adequacy of the brakes when maneuvering on the ground and the tendency of the brakes to cause nosing-over should be investigated. Any bad tendency will normally be exaggerated when taxiing in a strong cross or tail wind.
(b) Seaplanes and Amphibians. The most adverse water conditions safe for taxiing, takeoff, and landing must be established per § 25.231(b). Procedure and limitations for using reverse thrust should be determined.
c. Directional Stability and Control - § 25.233.
(1) Explanation. None.
(2) Procedures. Taxi, takeoff, and landing should be conducted in all configurations under normal operating conditions.
(a) There may be no uncontrollable ground-looping tendency in 90-degree crosswinds, up to a wind velocity of 20 knots or 0.2 VSR0, whichever is greater (except that the wind velocity need not exceed 25 knots) at any speed at which the airplane may be expected to be operated on the ground. This may be shown while establishing the 90-degree crosswind component required by § 25.237.
(b) Landplanes must be satisfactorily controllable, without exceptional piloting skill or alertness in power-off landings at normal landing speed, without using brakes or engine power or thrust to maintain a straight path. This may be shown during power-off landings made in conjunction with other tests.
(c) The airplane must have adequate directional control during taxiing. This may be shown during taxiing prior to takeoffs made in conjunction with other tests.
d. Taxiing Condition - § 25.235. [Reserved]
e. Wind Velocities -§ 25.237.
(1) Explanation.
(a) Landplanes.
1 There must be a 90-degree crosswind component established that is shown to be safe for takeoff and landing on dry runways.
2 The airplane must exhibit satisfactory controllability and handling characteristics in 90-degree crosswinds at any ground speed at which the airplane is expected to operate.
(b) Seaplanes and Amphibians.
1 There must be a 90-degree crosswind component established that is shown to be safe for takeoff and landing in all water conditions that may reasonably be expected in normal operation.
142 2 There must be a wind velocity established for which taxiing is safe in any direction under all water conditions that may reasonably be expected in normal operation.
(c) Crosswind Demonstration. A 90-degree crosswind component at 10 meters (as required by § 25.21(f)) of at least 20 knots or 0.2 VSR0 (where VSR0 is for the maximum design landing weight), whichever is greater, except that it need not exceed 25 knots, must be demonstrated during type certification tests. There are two results possible:
1 A crosswind component value may be established that meets the minimum requirements but is not considered to be a limiting value for airplane handling characteristics. This demonstrated value should be included as information in the AFM.
2 A crosswind component value may be established that is considered to be a maximum limiting value up to which it is safe to operate for takeoff and landing. This limiting value should be shown in the operating limitations section of the AFM.
(2) Procedures.
(a) Configuration. These tests should be conducted in the following configurations:
1 At light weight and aft c.g. (This is desirable; however, flexibility should be permitted.)
2 Normal takeoff and landing flap configurations using the recommended procedures.
3 Normal usage of thrust reversers. Particular attention should be paid to any degradation of rudder effectiveness due to thrust reverser airflow effects.
4 Yaw dampers/turn coordinator On, or Off, whichever is applicable.
(b) Test Procedures. Three takeoffs and 3 landings, with at least one landing to a full stop, should be conducted in a 90-degree crosswind component of at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots. For each test condition, a qualitative evaluation by the pilot of airplane control capability, forces, airplane dynamic reaction in gusty crosswinds (if available), and general handling characteristics should be conducted. The airplane should be satisfactorily controllable without requiring exceptional piloting skill or strength. If thrust reversers are installed, these landings should be conducted with the thrust reversers deployed as per normal procedures and additional landings should be conducted at the critical reverse thrust/power level to verify that there are no unsatisfactory handling characteristics.
(c) Test data. Crosswind data may be obtained from a calibrated flight test wind measurement station, from an airfield wind reporting device, or from any other method acceptable to the FAA.
1 A calibrated flight test wind measurement station located in the vicinity of the liftoff or touchdown point generally provides the most accurate data and is preferable.
2 An airport wind reporting device may also be acceptable provided the device has been calibrated and is located near the runway being used for testing.
3 Crosswind data taken directly from a commercially available inertial or differential GPS based reference system may not be accurate in sideslips and is not accurate on the ground. During landing, filtering may introduce lags making the data incorrect due to wind shear with altitude (i.e., a higher wind value at altitude is “remembered”). Hence this method is considered unsuitable for accurately determining the crosswind during takeoff and landing.
4 Other methods based on the computation of the actual crosswind encountered by the airplane based on on-board measurements are also acceptable. For example, the crosswind can be computed by resolving the difference between true airspeed (from an ADC) and an accurate ground speed measurement (e.g., derived from IRS groundspeed) into the along runway and across runway heading taking into account the airplane heading, track angle and sideslip.
5 No matter which method is used, the wind should be continuously timerecorded throughout the takeoff from brake release (or any low speed above which all data necessary to the computation are available and of sufficient accuracy) to a height of 50 ft, and throughout the landing from a height of 50 ft to termination of the test event (e.g., full stop, touch-and-go, go-around) or any low speed above which all data necessary to the computation are available and of sufficient accuracy. The measured crosswind component should be corrected from the height of the measurement device to a height of 10 meters. The average crosswind at 90 degrees to the runway heading should then be calculated for the above time span. The maximum gust could also be derived during this process, based on the same time span.
6 With prior agreement from the FAA, it may also be permissible to obtain crosswind data from tower wind reports. However the use of this method should be carefully reviewed to ensure that the measurement sensor is properly calibrated to establish the measurement sensor reference height, to establish that the smoothing characteristics do not produce unacceptable filtering, and that the location of the measurement sensor is appropriate for the takeoff and landing runway(s). Such a method has the disadvantage of not being able to provide the gust value during takeoff and landing.
f. Spray Characteristics, Control, and Stability on Water - § 25.239.
(1) Explanation. These characteristics should be investigated at the most adverse weight/c.g. combinations.
(2) Procedures.
(a) The spray characteristics and, in particular, the pilot view during the initial takeoff run, should allow sufficient view in order to maintain a reasonable track over the water.
Since not all seaplane operations are on open lakes or bays, but can be on rivers or channels, the directional control and view should be sufficient enough to stay within the channel confines.
(b) The tendency of the wing floats or sponsons to submerge and/or cause waterloops should be evaluated during the crosswind testing. During the step taxiing evaluations, the floats should also be evaluated for any tendency to bury and either cause waterlooping or damage. The procedures used to avoid undesirable characteristics should be included in the AFM.
(c) During low speed taxi, the effectiveness of the water rudders and/or asymmetric power or thrust should be evaluated in view of the types of maneuvering to be expected in service. If reverse thrust is to be used, it too should be evaluated in terms of ease of accomplishment and crew coordination.
(d) If an amphibian is intended to be “beached” or run up a ramp, the handling characteristics and ability to maneuver onto the ramp should be evaluated. Forward c.g. is generally more critical. The procedures should be included in the AFM. There should be no undue tendency to damage the bow or other structure.
(e) Engine failure of the critical engine at any time during the takeoff run should be evaluated. No dangerous porpoising, swerving, or waterlooping should result.
(f) There should be no undue tendency to porpoise and no extraordinary skill or alertness should be required to control porpoising.
(g) Spray impingement on the airframe (control surfaces, etc.) should be evaluated to assure the resulting loads are within acceptable limits.
(h) The above evaluations should be performed in the airplane on the water rather than by analysis or model testing. Analysis and/or model testing may be used to point out the problem areas but should not be substituted for actual testing.
Section 8. Miscellaneous Flight Requirements
31. Vibration and Buffeting - § 25.251.
a. Explanation.
(1) The testing required by subpart C of part 25 covers the vibration extremes expected in service. The applicant’s flight tests should assure that the regulatory limits are not exceeded.
Flight testing should not be conducted beyond where structural (subpart C) tests and calculations have been completed.
(2) For § 25.251(b) and (c), vibration and buffeting are considered excessive when it is determined that it:
(a) May cause structural damage or, if sustained over an extended period of time, could lead to structural fatigue;
(b) May cause pilot fatigue or annoyance that interferes with operation of the airplane or management of the airplane systems; or
(c) Interferes with flight instrument readability.
(3) No perceptible buffeting is permitted in the cruise configuration as required by § 25.251(d). Weight and/or altitude AFM limitations may need to be imposed to comply with this criterion. Reasonable buffet during the deployment of spoilers and other high drag devices is permitted to the extent allowed under § 25.251(b) and (c), as described in paragraph (2) above.
(4) For airplanes with MD greater than 0.6 or with a maximum operating altitude greater than 25,000 feet, the buffet onset envelope must be established for the ranges of airspeed and/or Mach number, weight, altitude, and load factor for which the airplane is to be certificated.
This envelope must be provided in the AFM in accordance with § 25.1585(d). These AFM data should be valid criteria for forward c.g. conditions or correctable to forward c.g. by the use of AFM procedures. This boundary should be established by pilot qualitative evaluation or by correlation with pilot qualitative evaluation, as there is no predetermined criterion for buffet level at the pilot station. A normal acceleration of +0.05 g has been used in some cases; however, the appropriate acceleration level will vary from airplane to airplane and may also be affected by the dynamic response of the accelerometer. If a measured normal acceleration is to be used, the acceleration level and specific accelerometer should first be correlated against a pilot’s assessment of the onset of buffet.
(5) Modifications to airplanes, particularly modifications that may affect airflow about the wing, should be evaluated for their effect on vibration and buffeting characteristics, changes in the speeds for onset of buffet, and maneuvering characteristics beyond buffet onset. This change may not only impact the buffet boundary envelope, but may change the acceptability of the VMO/MMO or VDF/MDF speeds established on the unmodified airplane. If this occurs, the maximum operating speed and demonstrated flight diving speed may need to be reduced.
However, the regulations concerning the speed spread margin between VMOMMO and VDF/MDF remain in effect. Systems and flight characteristics affected by the reduced maximum speeds should also be reevaluated. Indicator markings, overspeed horns, etc. must be reset, as necessary, to remain in compliance with the applicable regulations.
(6) On swept-wing airplanes, undesirable pitch-up maneuvering characteristics can occur as the center of lift moves inboard and forward with increasing g, due to shock-wave induced separation and/or as wing load alleviation systems unload the wingtips. Straight-wing airplanes can also exhibit similar characteristics; therefore, new airplanes and those modified in a manner that may affect the spanwise lift distribution or produce undesirable pitching moment as
146 a function of g, or increase the exposure to high altitude buffet encounters, should be evaluated as described herein.
(7) Section 25.251(e) requires that “probable inadvertent excursions beyond the boundaries of buffet” may not result in “unsafe conditions.” In order to assure that no unsafe conditions are encountered in maneuvering flight, maneuvering flight evaluations to demonstrate satisfactory maneuvering stability are described below. A determination of the longitudinal maneuvering characteristics should be made to assure the airplane is safely controllable and maneuverable in the cruise configuration to assure there is no danger of exceeding the airplane limit load factor, and that the airplane’s pitch response to the primary longitudinal control is predictable to the pilot.
b. Procedures.
(1) Section 25.251(a). The test procedures outlined below will provide the necessary flight demonstrations for compliance with § 25.251(a).
(2) Section 25.251(b). The airplane should be flown at VDF/MDF at several altitudes from the highest practicable cruise altitude to the lowest practicable altitude. The test should be flown starting from trimmed flight at VMO/MMO at a power or thrust setting not exceeding maximum continuous power or thrust. The airplane gross weight should be as high as practicable for the cruise condition, with the c.g. at or near the forward limit. In addition, compliance with § 25.251(b) should be demonstrated with high drag devices (i.e., speed brakes) deployed at VDF/MDF. Thrust reversers, if designed for inflight deployment, should be deployed at their limit speed conditions.
(3) Section 25.251(c). The weight of the airplane should be as heavy as practical, commensurate with achieving the maximum certificated altitude.
(4) Section 25.251(d). It should be demonstrated in flight tests that perceptible buffeting does not occur in straight flight in the cruise configuration, at any speed up to VMO/MMO, to show compliance with § 25.251(d). This should be met from initial combinations of critical weight and altitude, if achievable, where the airplane has a 0.3 g margin to the buffet onset boundary developed under § 25.251(e). These initial conditions should be established using a nominal cruise Mach number (typically long-range cruise Mach, MLRC) with the c.g. at the forward limit. This flight condition is representative of practical operating criteria imposed by most operators. From these initial conditions, the airplane should be accelerated in 1 g flight to VMO/MMO using maximum continuous power or thrust. Descending flight is acceptable if needed to achieve VMO/MMO.
(5) Section 25.251(e). Section 25.251(e) requires the determination of the buffet onset envelope, in the cruise configuration, for airplanes with MD greater than 0.6 or maximum operating altitudes greater than 25,000 feet. This requirement also provides criteria for evaluation of maneuvering stability in cruise flight under load factor conditions up to and beyond the onset of buffet.