8-4 point, can advance or delay its activation without appreciably affecting the average stall entry airspeed rate. To minimize scatter in the stall speed versus entry rate data, the pilot should attempt to maintain a stable angleof-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 8.1.4.5.5, 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.
8.1.4.4 Thrust Effects on Stall Speed.
8.1.4.4.1 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.
8.1.4.4.2 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.
8.1.4.4.3 These data may then be extrapolated to a zero thrust condition to determine the effects of idle thrust on stall speeds. (See figure 8-1 below.)
If the difference between idle thrust and zero thrust stall speed is 0.5 knots or less, the effect may be considered insignificant.
8-5 Figure 8-1. Thrust Effect on Stall Speed
8.1.4.4.4 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 RPM, and estimated propeller efficiency can be used to predict the thrust associated with this configuration.
8.1.4.5 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.
8.1.4.5.1 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° to that axis, as well as a means to determine the angle between the flight path and the fuselage longitudinal axis.
8.1.4.5.2 Calculate the airplane lift coefficient (CL) from the equation given below and plot it as a time history throughout the stall maneuver:
𝐶𝐿 = 𝑛𝑧𝑤𝑊𝑞𝑆 = 295.37𝑛𝑧𝑤𝑊𝑉𝑒2𝑆
ENGINE THRUST
STALL SPEED
ZERO THRUST THRUST FOR 1.5 VSR
IN LEVEL FLIGHT IDLE THRUST
THRUST EFFECT ON
STALL SPEED
8-6 Where:
𝑛𝑍𝑊 = 𝐴𝑖𝑟𝑝𝑙𝑎𝑛𝑒 𝑙𝑜𝑎𝑑 𝑓𝑎𝑐𝑡𝑜𝑟 𝑛𝑜𝑟𝑚𝑎𝑙 𝑡𝑜 𝑡ℎ𝑒 𝑓𝑙𝑖𝑔ℎ𝑡 𝑝𝑎𝑡ℎ 𝑊 = 𝐴𝑖𝑟𝑝𝑙𝑎𝑛𝑒 𝑡𝑒𝑠𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑙𝑏𝑠) 𝑞 = 𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 (𝑙𝑏𝑠/𝑓𝑡2) 𝑆 = 𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑤𝑖𝑛𝑔 𝑎𝑟𝑒𝑎 (𝑓𝑡2) 𝑉𝑒 = 𝑘𝑛𝑜𝑡𝑠 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑎𝑖𝑟𝑠𝑝𝑒𝑒𝑑
8.1.4.5.3 The maximum lift coefficient CLMAX is defined as the maximum value of CL achieved during the stall test. Where the time history plot of CL exhibits multiple peak values, CLMAX normally corresponds to the first maximum. However, the peak corresponding to the highest CL achieved may be used for CLMAX, provided it represents usable lift, meaning that it does not occur after deterrent buffet or other stall identification cue (ref.
§ 25.201(d)). 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 CLMAX is reached. The analysis to determine CLMAXshould 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 CLMAX is reached. (See figure 8-2 below.)
Figure 8-2. CIMAX and Load Factor
2.2
1.8 CLHAT
1.4
1.0 @ BREAK
•
8.1.4.5.4 Correct the CLMAX Obtained for each stall, if necessary, from the test CG position to the targeted CG position, and for any thrust effects, using the equation:
CIMAX = CIMAX (test CG position) 1 + (MAC/Lt) (GGstd - CGtest)] - AGLT Where:
MAC = Wing mean aerodynamic chord length (inches) I = Effective tail length, measured between the wing 25 percent MAC and the stabilizer 25 percent MAC (inches) CGsta = CG position resulting in the highest value of reference stall speed, normally the forward CG limit at the pertinent weight (percent MAC/100) CGtest = Actual test CG position (percent MAC/ 100) AGI = Change in GI due to engine thrust (if effect of idle thrust is greater thatn 0.5 knots in stall speed 8-7
8-8
8.1.4.5.5 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 CLMAX is relatively insensitive to stall entry rate, a rigorous investigation of entry rate effects should not be necessary.
8.1.4.5.6 For each approved configuration, construct a plot of CLMAX versus weight.
(See figure 8-3 below.)
Figure 8-3. CLMAX versus Weight and Flap Setting
CLMAX
8.1.4.5.7 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 CLMAX usually increases as the Mach number is reduced, simple expansion of the flight test data could result in extrapolating to a higher CLMAX than tested. The expansion of CLMAX versus Mach number data is only permitted up to the highest CLMAX demonstrated within the range of W/’s tested, unless the continuation of the trend of higher CLMAX with decreasing Mach number is substantiated with other test data. For example, data obtained at a more aft CG position or with power on can be used for this purpose if CG and thrust effects can be accounted for. Data from another airplane in the same family with the
8-9 same wing and showing the same general trend of CLMAX versus Mach
(e.g., a lighter weight variant) may also be used if shown to be applicable.
8.1.4.5.8 The reference stall speed, VSR, is a calibrated airspeed defined by the applicant. VSR may not be less than the 1-g stall speed and is expressed as:
𝑉𝑆𝑅 ≥ 𝑉𝐶𝐿𝑀𝐴𝑋 √𝑛𝑧𝑤
Where:
𝐶𝐿𝑀𝐴𝑋 = √295.37(𝑛𝑧𝑤)(𝑊) 𝐶𝐿𝑀𝐴𝑋𝑆⁄ + ∆𝑉𝐶, 𝑖𝑓 𝑡ℎ𝑒 𝑠𝑡𝑎𝑙𝑙𝑖𝑛𝑔 𝑚𝑎𝑛𝑒𝑢𝑣𝑒𝑟 𝑖𝑠 𝑙𝑖𝑚𝑖𝑡𝑒𝑑 𝑏𝑦 𝑎 𝑑𝑒𝑣𝑖𝑐𝑒 𝑡ℎ𝑎𝑡 𝑐𝑜𝑚𝑚𝑎𝑛𝑑𝑠 𝑎𝑛 𝑎𝑏𝑟𝑢𝑝𝑡 𝑛𝑜𝑠𝑒 𝑑𝑜𝑤𝑛 𝑝𝑖𝑡𝑐ℎ (𝑒. 𝑔. , 𝑎 𝑠𝑡𝑖𝑐𝑘 𝑝𝑢𝑠ℎ𝑒𝑟), 𝐶𝐿𝑀𝐴𝑋 𝑚𝑎𝑦 𝑛𝑜𝑡 𝑏𝑒 𝑙𝑒𝑠𝑠 𝑡ℎ𝑎𝑛 𝑡ℎ𝑒 𝑠𝑝𝑒𝑒𝑑 𝑒𝑥𝑖𝑠𝑡𝑖𝑛𝑔 𝑎𝑡 𝑡ℎ𝑒 𝑖𝑛𝑠𝑡𝑎𝑛𝑡 𝑡ℎ𝑒 𝑖𝑛𝑠𝑡𝑎𝑛𝑡 𝑡ℎ𝑒 𝑑𝑒𝑣𝑖𝑐𝑒 𝑜𝑝𝑒𝑟𝑎𝑡𝑒𝑠.
∆𝑉𝐶 = 𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑏𝑖𝑙𝑖𝑡𝑦 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑖𝑜𝑛 (𝑖. 𝑒. , 𝑡ℎ𝑒 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 equivalent airspeed and calibrated airspeed) 𝑊 = 𝐴𝑖𝑟𝑝𝑙𝑎𝑛𝑒 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑙𝑏𝑠) 𝑛𝑧𝑤 = 𝐴𝑖𝑟𝑝𝑙𝑎𝑛𝑒 𝑙𝑜𝑎𝑑 𝑓𝑎𝑐𝑡𝑜𝑟 𝑛𝑜𝑟𝑚𝑎𝑙 𝑡𝑜 𝑡ℎ𝑒 𝑓𝑙𝑖𝑔ℎ𝑡 𝑝𝑎𝑡ℎ 𝐶𝐿𝑀𝐴𝑋 = 𝑉𝑎𝑙𝑢𝑒 𝑜𝑓 𝐶𝐿𝑀𝐴𝑋 𝑐𝑜𝑟𝑟𝑒𝑠𝑝𝑜𝑛𝑑𝑖𝑛𝑔 𝑡𝑜 𝑡ℎ𝑒 𝑐ℎ𝑜𝑠𝑒𝑛 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑠𝑒𝑒 Figure 8-4 below) 𝑆 = 𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑤𝑖𝑛𝑔 𝑎𝑟𝑒𝑎 (𝑓𝑡2)
8.1.4.5.9 Construct a plot of reference stall speed versus weight for each flap/gear configuration. (See figure 8-4 below).
8-10 Figure 8-4. Stall Speed versus Weight and Flap Setting A B C D
INCREASING
FLAP SETTING WEIGHT
STALL SPEED (KEAS)
8.1.4.5.10 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)).
8.1.4.5.11 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 8.1.4.3.1 to ensure that no abnormal or unusual pilot control input is used to obtain an artificially low speed at which the device first activates.
8.1.5 Stall Characteristics—§ 25.203.
8.1.5.1 Background.
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
8-11 during the stall and recovery must be easily controllable using normally expected pilot reactions.
8.1.5.2 Configuration.
8.1.5.2.1 Stall characteristics should be investigated with wings level and in a 30° banked turn, with both power or thrust on and power or thrust off in all configurations approved for normal operations.
8.1.5.2.2 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 or thrust off, except where deployment of the deceleration devices with power or thrust on would likely occur in normal operations (e.g., extended spoilers during landing approach).
8.1.5.2.3 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.
8.1.5.2.4 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 RPM) position.
8.1.5.2.5 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.
8.1.5.2.6 Stall characteristics testing is normally done at the aft CG limit, which is typically the most adverse; however, if the stall speed tests at forward CG indicate that marginal stall recovery characteristics may exist at forward CG, compliance with § 25.203 should be shown for the most critical loading.
8.1.5.2.7 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
8-12 be flown with gear and flaps up at the most adverse CG. 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° banked turn.
8.1.5.2.8 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.
8.1.5.2.9 Stall characteristics should also be demonstrated with the maximum allowable asymmetric fuel loading. Requirements are as specified in § 25.203(a) and (c).
8.1.5.3 Procedures.
8.1.5.3.1 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).
8.1.5.3.2 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.
8.1.5.3.3 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.
8.1.5.3.4 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° occasionally, provided that lateral control is effective during recovery.
8-13
8.1.5.3.5 Section 25.203(c) requires the action of the airplane, following the 30° 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° in the original direction of the turn, or 30° in the opposite direction.
8.1.5.3.6 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-ofattack 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° in the original direction of the turn, or 60° in the opposite direction.
8.1.5.3.7 For those airplanes where stall is defined by full nose-up longitudinal control for both forward and aft CG, 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)).
8.1.5.3.8 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.
8.1.5.3.9 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.
8.1.6 Stall Warning—§ 25.207.
8.1.6.1 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.
8.1.6.2 Background.
To be acceptable, a stall warning must have the following features:
8-14
8.1.6.2.1 Distinctiveness.
The stall warning indication must be clear and distinct to a degree that will ensure positive pilot recognition of an impending stall.
8.1.6.2.2 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 CG 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).
8.1.6.2.3 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 angle-of-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)).
8.1.6.2.4 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.
8.1.6.2.5 For airplanes that use artificial stall warning systems, paragraph 42.1 of this AC presents guidance material for demonstrating compliance with the regulatory requirements of part 25.
8.1.6.2.6 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).
8.1.6.2.7 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-ofattack 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
8-15 stall identification (e.g., stick pusher) systems at their nominal angle-ofattack settings unless a lesser margin can result from the various system tolerances.
8.1.6.2.8 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 reduced by more than 2° 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.
8.1.6.2.9 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.
8.1.6.3 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 CG 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° 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.
8.1.6.4 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).