(d) The activation point of a stall identification device that provides one of the characteristics listed above. Paragraph 228 of this AC presents guidance material for demonstrating compliance with the regulatory requirements of part 25 for stall identification systems.
(2) It should be recognized that the point at which the airplane is considered stalled may vary, depending on the airplane configuration (e.g., flaps, gear, center of gravity, and gross weight). In any case, the angle-of-attack should be increased until one or more of these characteristics is reached for all likely combinations of variables.
d. Stall Speeds .
(1) Background .
(a) Since many of the regulations pertaining to performance and handling qualities specify trim speeds and other variables that are functions of stall speeds, it is desirable to accomplish the stall speed testing early in the program, so the data are available for subsequent testing. Because of this interrelationship between the stall speeds and other critical performance parameters, it is essential that accurate measurement methods be used. Most standard airplane pitot-static systems are unacceptable for stall speed determination. These tests require the use of properly calibrated instruments and usually require a separate test airspeed system.
(b) Prior to amendment 25-108, the stall speed defined in § 25.103 was the minimum speed attained in the stalling maneuver. For many high speed swept wing transport category airplanes, the resulting stall speed often occurs at a load factor normal to the flight path considerably less than one, which leads to inconsistent and unrepeatable reference stall speeds.
Pilot technique can also significantly influence the rate and magnitude of any nose down pitch occurring at the stall, thereby contributing to inconsistencies in the determination of the minimum speed obtained in the stalling maneuver. Since part 25 defines operating speeds as multiples of the stall speed, the resulting operating speed margins to stall may not be representative of the actual lift margin available (i.e., the margin to the speed at which wing lift alone can support the weight of the airplane in 1-g flight). To ensure that operating speed and maneuvering margins are directly related to wing lift margin, amendment 25-108 redefined the reference stall speed as the 1-g stall speed, which is the speed at which the wing is generating maximum usable lift in a 1-g flight condition.
(c) Since the 1-g stall speed is generally higher than the minimum speed obtained in the stalling maneuver, retaining the existing multiplying factors for determining the minimum operating speeds would have resulted in higher minimum operating speeds. To retain approximately equivalent operating speeds that have been found acceptable, the multiplying factors applied to the reference stall speed were reduced by approximately six percent.
(d) Although the reduced multiplying factors were intended to result in roughly equivalent operating speeds, there is one class of airplanes for which a significantly lower operating speed would be obtained. Airplanes equipped with a device that abruptly pushes the
121 nose down (e.g., a stick pusher) near the angle-of-attack for maximum lift would be operated at speeds and angles-of-attack closer to the pusher activation point than has been experienced in operational service. Therefore, to maintain equivalency in operating speeds for these airplanes, a supplementary margin has been established such that VSR must not be less than the greater of 2 knots or 2 percent above the speed at which the device activates (§ 25.103(d)). In addition, see paragraph 228 of this AC for guidance material regarding the means of compliance criteria for the design and function of such systems.
(2) Configuration .
(a) Stall speeds should be determined for all aerodynamic configurations to be certificated for use in the takeoff, en route, approach, and landing configurations.
(b) The center of gravity positions to be used should be those that result in the highest stall speeds for each weight (forward c.g. in most cases).
(c) Sufficient testing should be conducted to determine the effects of weight on stall speed. Altitude effects (compressibility, Reynolds Number) may also be considered if credit for variations in these parameters is sought by the applicant. If stall speeds are not to be defined as a function of altitude, then all stall speed testing should be conducted at a nominal altitude no lower than 1,500 ft. above the maximum approved takeoff and landing altitude. (See paragraph 29d(5)(g).)
(3) Procedures .
(a) The airplane should be trimmed for hands-off flight at a speed 13 percent to 30 percent above the anticipated VSR, with the engines at idle and the airplane in the configuration for which the stall speed is being determined. Then, using only the primary longitudinal control for speed reduction, maintain a constant deceleration (entry rate) until the airplane is stalled, as defined in § 25.201(d) and paragraph 29c(1) of this AC. Following the stall, engine thrust may be used as desired to expedite recovery.
(b) A sufficient number of stalls (normally four to eight) should be accomplished at each critical combination of weight, altitude, c.g., and external configuration. The intent is to obtain enough data to define the stall speed at an entry rate of 1.0 knot/second.
(c) During the stall speed testing, the stall characteristics of the airplane must also satisfy the requirements of § 25.203(a) and (b).
(d) For airplanes that have stall identification devices for which the angle-ofattack for activation is biased by angle-of-attack rate, some additional considerations are necessary. The stall speeds are normalized against an average airspeed deceleration rate, as described in paragraph 29d(5)(e). However, stall identification systems generally activate at a specific angle-of-attack, biased by an instantaneous angle-of-attack rate. Therefore, longitudinal control manipulation by the pilot during the stall maneuver, close to the stall identification system activation point, can advance or delay its activation without appreciably affecting the
122 average stall entry airspeed rate. To minimize scatter in the stall speed versus entry rate data, the pilot should attempt to maintain a stable angle-of-attack rate or pitch rate (not necessarily a fixed airspeed deceleration rate), until the stall identification system activates. The resulting timehistory of angle-of-attack data should be smooth and without discontinuities. A cross plot of airspeed deceleration rate, as defined in paragraph 29d(5)(e), versus angle-of-attack rate for all related test points, will show the general trend of this relationship for each flap setting. Any points that do not follow this general trend should not be used in establishing the stall speed.
(4) Thrust Effects on Stall Speed .
(a) Stall speeds are typically determined with the thrust levers at idle; however, it is necessary to verify by test or analysis that engine idle thrust does not result in appreciably lower stall speeds than would be obtained at zero thrust. Prior to amendment 25-108, a negative idle thrust at the stall, which slightly increases stall speeds, was considered acceptable, but applicants were not required to base stall speeds on idle thrust. With the adoption of amendment 25-108, it became a requirement to base stall speeds on idle thrust, except where that thrust level results in a significant decrease in stall speeds. If idle thrust results in a significant decrease in stall speeds, then stall speeds cannot be based on more than zero thrust.
(b) To determine whether thrust effects on stall speed are significant, at least three stalls should be conducted at one flap setting, with thrust set to approximately the value required to maintain level flight at 1.5 VSR in the selected configuration.
(c) These data may then be extrapolated to a zero thrust condition to determine the effects of idle thrust on stall speeds (see figure 29-1). If the difference between idle thrust and zero thrust stall speed is 0.5 knots or less, the effect may be considered insignificant.
Figure 29-1. Thrust Effect On Stall Speed
(d) The effects of engine power on stall speeds for a turbopropeller airplane can be evaluated in a similar manner. Stall speed flight tests should be accomplished with engines idling and the propellers in the takeoff position. Engine torque, engine r.p.m., and estimated propeller efficiency can be used to predict the thrust associated with this configuration.
(5) Data Reduction and Presentation . The following is an example of how the data obtained during the stall speed testing may be reduced to standard conditions. Other methods may be found acceptable.
(a) Record the indicated airspeed from the flight test airspeed system throughout the stall, and correct these values to equivalent airspeed. Also record load factor normal to the flight path. Typically, the load factor data would be obtained from a sufficient number of accelerometers capable of resolving the flight path load factor. It may be possible to obtain acceptable data using one accelerometer aligned along the expected 1-g stall pitch angle. More likely, it will take at least two accelerometers, one aligned along the fuselage longitudinal axis and one aligned at 90 degrees to that axis, as well as a means to determine the angle between the flight path and the fuselage longitudinal axis.
(b) Calculate the airplane lift coefficient (C L) from the equation given below and plot it as a time history throughout the stall maneuver.
C L = nWqSzw = SV Wne zw237.295
ENGINE THRUST
STALL SPEED
ZERO THRUST THRUST FOR 1.5 VSR
IN LEVEL FLIGHT IDLE THRUST
THRUST EFFECT ON
STALL SPEED
Where: n zw = airplane load factor normal to the flight path W = airplane test weight - lbs.
q = dynamic pressure - lbs./ft. 2 S = reference wing area - ft. 2 Ve = knots equivalent airspeed.
(c) The maximum lift coefficient is defined as the maximum value of C L achieved during the stall test. Where the time history plot of CL exhibits multiple peak values, corresponds to the first maximum. There should also typically be a noticeable break in a plot of the load factor normal to the flight path near the point at which is reached. The analysis to determine should disregard any transient or dynamic increases in recorded load factor, such as might be generated by abrupt control inputs that do not reflect the lift capability of the airplane. The load factor normal to the flight path should be maintained at nominally 1.0 until is reached. (See figure 29-2.)
Figure 29-2. CLMAX and Load Factor
CLMAX (CLMAX) CLMAX CLMAX CLMAX
(d) Correct the obtained for each stall, if necessary, from the test c.g.
position to the targeted c.g. position, and for any thrust effects, using the equation:
C LMAX = C LMAX (test c..g. position)[1 + (MAC/lt)(CGstd - CGtest)] - CLT
Where: MAC = Wing mean aerodynamic chord length -` inches.
l t = Effective tail length, measured between the wing 25 percent MAC and the stabilizer 25 percent MAC - inches.
C G std = C.G. position resulting in the highest value of reference stall speed (normally the forward c.g. limit at the pertinent weight)
- percent MAC/100
C G test = Actual test c.g. position - percent MAC/100
CLT = Change in C L due to engine thrust (if effect of idle thrust is greater than 0.5 knots in stall speed).
(e) Determine the stall entry rate, which is defined as the slope of a straight line connecting the stall speed and an airspeed 10 percent above the stall speed, for each stall test.
Because is relatively insensitive to stall entry rate, a rigorous investigation of entry rate effects should not be necessary. Test data should bracket a 1.0 knot/second entry rate such that the value of corresponding to an entry rate of 1.0 knot/second can be determined. (See figure 29-3.)
CLMAX CLMAX CLMAX
126 Figure 29-3. CLMAX vs Entry Rate
CLMAX
(f) For each approved configuration, construct a plot of versus weight (see figure 29-4.)
Figure 29-4. CLMAX vs Weight and Flap Setting
CLMAX
CLMAX
(g) Flight test safety concerns usually dictate the lowest test altitude for determining stall speeds. The test data should then be expanded to lower altitudes, and hence lower Mach numbers, to cover the operational envelope of the airplane. Since usually increases as the Mach number is reduced, simple expansion of the flight test data could result in extrapolating to a higher than tested. The expansion of versus Mach number data is only permitted up to the highest demonstrated within the range of W/’s tested.
(h) Determine the reference stall speed defined in § 25.103(a) from . The reference stall speed is a calibrated airspeed not less than the 1-g stall speed and is expressed as:
zw CL SR n V MAX V
Where: V CLMAX = S))(W)/(C295.37(n MAXLZW +VC. If the stalling maneuver is limited by a device that commands an abrupt nose down pitch (e.g., a stick pusher), may not be less than the speed existing at the instant the device operates.
VC = compressibility correction (i.e., the difference between equivalent airspeed and calibrated airspeed).
W = a i r p l a n e w e i g h t - l b s .
n zw = airplane load factor normal to the flight path.
C LMAX = value of corresponding to the chosen weight (see figure 29-4).
S = reference wing area - ft. 2
(i) Construct a plot of reference stall speed versus weight for each flap/gear configuration. (See figure 29-5).
CLMAX
CLMAX CLMAX CLMAX CLMAX CLMAX CLMAX
Figure 29-5. Stall Speed vs Weight and Flap Setting
A B C D
INCREASING
FLAP SETTING WEIGHT
STALL SPEED (KEAS)
(j) For airplanes equipped with a device that abruptly pushes the nose down at a selected angle-of-attack (e.g., a stick pusher), VSR must not be less than the greater of 2 knots or 2 percent above the speed at which the device activates (§ 25.103(d)).
(k) In showing compliance with § 25.103(d) for airplanes equipped a device that abruptly pushes the nose down at a selected angle-of-attack (e.g., a stick pusher), the speed at which the device operates need not be corrected to 1 g. Requiring a load factor correction of the device activation speed to the 1-g condition would unnecessarily increase the stringency of § 25.103(d). For example, it would be possible for the device activation speed to be assessed as higher than VSR (or at least closer to VSR than would be obtained without correcting to the 1 g condition). Test procedures should be in accordance with paragraph 29d(3)(a) to ensure that no abnormal or unusual pilot control input is used to obtain an artificially low speed at which the device first activates.
e. Stall Characteristics - § 25.203 .
(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
129 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 .
(a) 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.
(b) The test configurations for stall characteristics 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).
(c) 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.
(d) 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.
(e) For power-on stalls, 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 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.
(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 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 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 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 V SR) 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
131 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.
e. (Reserved).
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 thrust and with the center-of-gravity 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