05/04/18
AC 25-7D Where:
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 i s reached. (See figure 8-2 below.)
8-6
Figure 8-2. CIMAX and Load Factor
2.2
1.8 CLHAX
1.4-293388/2988808|8288|833|33888 FACTOR АТ 1.0 g BREAK § 0.6
8.1.4.5.4 Correct the CIMAX 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 (Pest CG position) [1 + (MAC/ L) (CGstd - CGeest)] - ACL Where:
MAC = Wing mean aerodynamic chord length (inches) It = Effective tail length, measured between the wing 25 percent MAC and the stabilizer 25 percent MAC (inches) CGstd = 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) ACp = Change in C1 due to engine thrust (if effect of idle thrust is greater than 0.5 knots in stall speed 8-7
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 CL MAX
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 CL LMAX than tested. The expansion of C MAX 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 8-8
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. Vsk may not be less than the 1-g stall speed and is expressed as:
VSR 2 VGLMAX VRzw Where:
CIMAX = 295.37 (nzw)(W) /CIMAYS + AVc.
if the stalling maneuver is limited by a device that commands an abrupt nose down pitch (e.g., a stick pusher), CLux may not be less than the speed existing at the instant the instant the device operates.
AVc = Compressibility correction (i.e., the difference between equivalent airspeed and calibrated airspeed) W = Airplane weight (Ibs) Nzw = Airplane load factor normal to the flight path CLMAX = Value of CLMAX Corresponding to the chosen weight (see Figure 8-4 below) 5 = Reference wing area (ft?)
8.1.4.5.9 Construct a plot of reference stall speed versus weight for each flap/gear configuration. (See figure 8-4 below).
8-9
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 8-10
05/04/18
AC 25-7D 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 8-11
adverse compressibility effects on stall characteristics. These tests should 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 8-12
05/04/18
AC 25-7D
level wing stalls. In level wing stalls the bank angle may exceed 20° occasionally, provided that lateral control is effective during recovery.
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.
8-13
05/04/18
AC 25-7D
8.1.6.2 Background.
To be acceptable, a stall warning must have the following features:
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 8-14
05/04/18
AC 25-7D 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 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.
8-15
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).
8.1.7 Accelerated Stall Warning.
8.1.7.1 Explanation.
Section 25.207(f) requires that, in slow-down turns with at least a 1.5 g 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.
8.1.7.2 Procedures.
8.1.7.2.1 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, 1.5 g equates to a bank angle of 48°.
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.
8.1.7.2.2 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 8.1.3.1 of this AC) occur during the accelerated stall warning demonstration, compliance with § 25.207(f) will not have been demonstrated.
8.1.8 Maneuver Margins.
See paragraph 5.1 of this AC for guidance material associated with demonstrating compliance to the maneuvering capability requirements of § 25.143(h).
8-16
8.1.9 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, be designed to perform its intended function under any foreseeable operating condition.
8.1.9.1 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.
8.1.9.2 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:
8.1.9.2.1 Airframe build tolerances—the impact of wing angle of incidence variation relative to stall identification system vane angle; and
8.1.9.2.2 Stall identification system tolerances (e.g., activation vane angles).
8.1.9.3 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 8-17