d. Stall Characteristics .
(1) Background . Since operational pilots may not be required, or trained, to fly to an angle of attack beyond that for stall warning, any exposure to the behavior of the airplane in an actual stall would be both unexpected and unfamiliar. Therefore, to assure a safe and expeditious recovery from an unintentional stall, it should not require any unusual piloting technique to successfully demonstrate compliance with § 25.203, nor should it require exceptional skill or repeated practice by the test pilot. The behavior of the airplane during the stall and recovery must be easily controllable using normally expected pilot reactions.
(2) Configuration .
(i) Stall characteristics should be investigated with wings level and in a 30-degree banked turn, with both power on and power off in all configurations approved for normal operations.
(ii) The test configurations for stall characteri stics should include deployed deceleration devices for all flap positions, unless limitations against the use of those devices with particular flap positions are imposed.
‘Deceleration devices’ include spoilers used as airbrakes, and thrust reversers approved for inflight use. Stall demonstrations with deceleration devices deployed should normally be carried out with power off, except where deployment of the deceleration devices with power on would likely occur in normal operations (e.g., extended spoilers during landing approach).
(iii) Stall characteristics should be investigated with any systems or devices that may alter the stalling behavior of the airplane in their normal functioning mode. Unless the design of the airplane’s automatic flight control system precludes its ability to operate beyond the stall warning angle of attack, stall characteristics and the adequacy of stall warning should be evaluated when the airplane is stalled under the control of the automatic flight control system.
(iv) Power-off stalls should be conducted at flight idle for the appropriate configuration. For propeller-driven airplanes, the propeller should be set in the normal low pitch (high r.p.m.) position.
(v) For power-on stalls, thrust should be set to the v alue required to maintain level flight at a speed of 1.6 VS with flaps in the approach position, landing gear retracted, and at maximum landing weight. The approach flap position referred to is the maximum flap deflection used to show compliance with § 25.121(d), Approach Climb.
(vi) 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.
(vii) 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 center of gravity. 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.
(viii) For airplanes that are certificated for flight into known icing conditions, stall character istics should be demonstrated with simulated ice shapes symmetrically attached to all surfaces that are not protected by anti-ice or de-icing systems. (For further guidance on approval for flight into known icing conditions, see paragraphs 231 and 232 of this AC.)
(ix) For abnormal aerodynamic configurations covered by AFM procedures, high angle of attack characteristics should be evaluated down to either stall warning, or to an angle of attack equivalent to the AFM recommended landing approach speed divided by 1.3. If adequate controllability is present at either of these conditions, 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. If stall warning is used as the end point, then it should be
demonstrated that the airplane is safely controllable and maneuverable when flown at the recommended operating speed.
(x) 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 .
(i) The airplane should be trimmed for ha nds-off flight at a speed 20 percent to 40 percent above the stall speed, with the appropriate power setting and configuration. Then, using only the primary longitudinal control, establish and maintain a deceleration (entry rate) consistent with that specified in § 25.201(c)(1) or
25.201(c)(2), as appropriate, until the airplane is stalled. Both power and pilot selectable trim should remain constant throughout the stall and recovery (angle of attack has decreased to the point of no stall warning).
(ii) The same trim reference (for example, 1.3 V S) should be used for both the stall speeds and characteristics testing. For all stall testing, the trim speed is based on the performance stall speeds that are (or will be) shown in the AFM.
(iii) During the approach to the stall, the longitudinal control pull force should increase continuously as speed is reduced from the trimmed speed to approximately 10 percent above the stall speed; below that speed some reduction in longitudinal control force is acceptable, provided it is not sudden or excessive.
(iv) 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.
(v) 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.
(vi) 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.
(vii) 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)).
(viii) 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.
(ix) 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.
e. (Reserved)
f. Stall Warning.
(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:
(i) Distinctiveness. The stall warning indication must be clear and distinct to a degree that will ensure positive pilot recognition of an impending stall.
(ii) Timeliness. The stall warning should normally begin at a speed not less than 7 percent above the stall speed. A lesser margin may be acceptable, depending on the probability of an inadvertent stall following stall warning recognition, and how much difference there is between the speed at which the airplane stalls (stall identification), and the minimum speed allowed under § 25.103(a).
(iii) Consistency. The stall warning must be reliable and repeatable. The warning must occur with flaps and gear in all normally used positions in both straight and turning flight. 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, and continue throughout the stall until the angle of attack is reduced to approximately that at which stall warning was initiated.
(iv) 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.
(v) For airplanes that use artificial stall warning systems, paragraph 228 of this AC pres ents guidance material for demonstrating compliance with the regulatory requirements of Part 25 of the FAR.
(3) Procedures . Stall warning tests are normally conducted in conjunction with the stall testing required by §§ 25.103 (stall speeds) and 25.203 (stall characteristics).
(4) Data Acquisition and Reduction . The stall warning speed and type and quality of warning should be noted. The speed at which acceptable stall warning begins should then be compared to the stall speed as defined in paragraph (3) above to determine if the required margin exists.
g. Accelerated Stall Warning . The applicant should determine that adequate stall warning occurs in turning flight under expected conditions of flight for takeoff, en route, and approach/landing configurations at aft c.g. and heavy weight. It should be demonstrated in slow-down turns at 1.5g normal load factor, with entry rates of at least 2 knots per second, that sufficient stall warning is provided to prevent stalling when the pilot takes recovery action not less than one second after recognition of stall warning. When stall warning is provided by an artificial system, that system’s activation point should be set to the high side of its angle of attack tolerance band for this testing.
h. Maneuver Margins . The applicant should determine that adequate maneuvering capability exists prior to stall warning at V2, all-engines-operating takeoff speed (typically defined as V 2+XX kts.), final takeoff speed (§ 25.121(c)), and VREF at forward c.g. and heavy weight for each appropriate flap setting. When stall warning is provided by an artificial system, that system’s activation point should be set to the low side of its angle of attack tolerance band for this testing.
NOTE: If it can be shown that the angle of attack tolerance band of an artificial stall warning system results in no more than a ±1.0 knot variation about the stall warning speed obtained at the nominal AOA setting, that nominal setting may be used for the maneuver margin testing specified in paragraph 29h, above.
i. Reliability of Artificial Stall Warning and Stall Identification Systems . Guidance material related to the testing and approval of artificial stall warning and stall identification systems is presented in paragraph 228 of this AC.
FIGURE 29-1. STALL TEST TIME HISTORY TRIM SPEED
1.1 Vs INOSE DOWN PITCH (NOT ELEVATOR LIMITED)
SENTRY RATE * SLOPE TRIM SPEED
1.1 Vs
ELEVATOR LIMITED
TEST AIRSPEED ELEVATOR
REACHES STOP TRIM SPEED
ISTALL IDENTIFICATION
1.1 Vs SYSTEM
I SYSTEM FIRES
ENTRYRATE SLOPE / TIME FIGURE 29-2. STALL CLCG VS ENTRY RATE
FIGURE 29-3. STALL C L vs WEIGHT AND FLAP SETTING FIGURE 29-4. STALL SPEEDS vs WEIGHT AND FLAP SETTING
A B C D
INCREASING
FLAP SETTING WEIGHT FIGURE 29-5. THRUST EFFECT ON STALL C L
THRUST EFFECT
ON STALL CL
ENGINE THRUST
Section 7. GROUND AND WATER HANDLING CHARACTERISTICS
30. GENERAL.
a. Applicable Federal Aviation Regulations (FAR). The applicable regulations are §§ 25.231, 25.233, 25.235, 25.237, and 25.239 of the FAR.
b. Section 25.231 - Longitudinal Stability and Control.
(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 center of gravity 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:
(i) 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 tailwind.
(ii) Seaplanes and Amphibians. The most adverse water conditions safe for taxiing, takeoff, and landing must be established. The use and limitations of reverse thrust must be determined.
c. Section 25.233 - Directional Stability and Control.
(1) Explanation. None.
(2) Procedures. Taxi, takeoff, and landing should be conducted in all configurations under normal operating conditions.
(i) There may be no uncontrollable ground-looping tendency in 90-degree crosswinds, up to a wind velocity of 20 knots or 0.2VSO, 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.
(ii) 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 to maintain a straight path. This may be shown during power-off landings made in conjunction with other tests.
(iii) 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. Section 25.235 - Taxiing Condition. [RESERVED]
e. Section 25.237 - Wind Velocities.
(1) Explanation.
(i) Landplanes.
(A) There must be a 90-degree crosswind component established that is shown to be safe for takeoff and landing on dry runways.
(B) 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.
(ii) Seaplanes and Amphibians.
(A) 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.
(B) 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.
(iii) Crosswind Demonstration. A 90-degree crosswind component at 10 meters (as required by § 25.21(f)) of at least 20 knots or 0.2 VSO (where VSO 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:
(A) 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.
(B) 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.
(i) Configuration. These tests should be conducted in the following configurations:
(A) At light weight and aft c.g. (this is desirable; however, flexibility should be permitted).
(B) Normal takeoff and landing flap configurations using the recommended procedures.
(C) Normal usage of thrust reversers. Particular attention should be paid to any degradation of rudder effectiveness due to thrust reverser airflow effects.
(D) Yaw dampers/turn coordinator On, or Off, whichever is applicable.
(ii) Test Procedure and Required Data. 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 0.2VSO, whichever is greater, except that for airplanes whose certification basis includes Amendment 25-42, 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 must be satisfactorily controllable without requiring exceptional piloting skill or strength. Wind data from Inertial Navigation Systems (INS), tower, or portable ground recording stations should be corrected to a 90-degree crosswind component and to a height of 10 meters.
f. Section 25.239 - Spray Characteristics, Control, and Stability on Water.
(1) Explanation. These characteristics should be investigated at the most adverse weight/c.g. combinations.
(2) Procedures.
(i) 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.
(ii) 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.
(iii) During low speed taxi, the effectiveness of the water rudders and/or asymmetric 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.
(iv) 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.
(v) Engine failure of the critical engine at any time during the takeoff run should be evaluated. No dangerous porpoising, swerving, or waterlooping should result.
(vi) There should be no undue tendency to porpoise and no extraordinary skill or alertness should be required to control porpoising.
(vii) Spray impingement on the airframe (control surfaces, etc.) should be evaluated to assure the resulting loads are within acceptable limits.
(viii) 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 is considered excessive when it is determined that it:
(i) May cause structural damage or, if sustained over an exte nded period of time, could lead to structural fatigue;
(ii) May cause pilot fatigue or annoyance that interferes with operation of the airplane or management of the airplane systems; or,
(iii) 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 Airplane Flight Manual (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) A determination of a buffet onset envelope is to 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 should be made a part of the AFM in accordance with § 25.1585(c). 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 event, as there is no established criterion for buffet level at the pilot station. A normal acceleration of +.05g has been proposed; however, that will vary from airplane to airplane and may also be affected by the dynamic response of the accelerometer.
(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 V MO/MMO or VDF/MDF derived from the unmodified airplane. If this occurs, the maximum operating speed and dive speed may be reduced. However, the applicable speed spread margin regulations remain in effect. Systems and flight characteristics affected by the reduced maximum speeds should also be reevaluated. Indicator markings, overspeed horns, etc. should be reset, as necessary, if the airplane is to remain in the transport category.
(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 a function of g, or increase the exposure to high altitude buffet encounters, should be evaluated as described herein.
(7) It has been determined that a pos itive slope of the stick force versus g (F Ss/g) relationship is not necessarily required at speeds and g combinations beyond V FC/MFC and/or buffet onset. However, § 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 herein. 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 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 V DF/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 thrust setting not exceeding maximum continuous power. The airplane gross weight should be as high as practicable for the cruise condition, with the c.g. at or near the forward limit.
(i) High drag devices should also be deployed at V DF/MDF (spoilers and speed brakes); thrust reversers, if designed for inflight deployment, should be deployed at their limit speed conditions.
(ii) Airplanes equipped with pneumatic de- icer boots should be evaluated to V MO/MMO with deicing on and off (if automatic) and not operating. If the applicant desires to restrict the maximum operating speed (VMO/MMO) to a lower value with de-icing on, it should be shown that excessive vibration or buffeting does not occur at speeds of the new V MO/MMO plus the speed spread (V MO/MMO to VDF/MDF ) determined with anti icing off.
(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 V MO/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
0.3g 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, M LRC) 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 1.0g flight to V MO/MMO using maximum continuous thrust. Descending flight is acceptable if needed to achieve V MO/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 M D 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.
(i) The determination of compliance with § 25.251(e), using flight test data from maneuvers conducted well into buffet, is extremely difficult due to the dynamics of this type of maneuver and the establishment of the FSs/g relationship from such data. The pilot flying the airplane needs to evaluate the airplane characteristics under such conditions. In this regard, Figures 31-1 and 31-2 illustrate general airplane responses and do not necessarily indicate required quantitative flight test results.
(A) Airplanes should be evaluated at the most aft c.g. in accordance with the following criteria:
(1) For all weight/altitude combinations where buffet onset occurs at various load factors between approximately +1g and +2g, the longitudinal control force (F Ss) characteristics of §§ 25.255(b)(1) and (2) apply prior to encountering that buffet onset (see buffet free regions of Figures 31-1 and 31-2).
(2) Under the airplane weight/altitude/speed combinations of ( 1), above, but at load factors beyond buffet onset, the following F S characteristics apply (see buffet regions of Figures 31-1 and 31-2):