AC 25-7D CHG 1
d. The airplane’s pitch response to primary longitudinal control should be predictable to the pilot.
10.1.2.5.3 Experience has shown that maneuvering evaluations conducted at the highest Mach and the highest weight and altitude (W/δ) combination may not necessarily produce the most critical results. Equally important is the character of the buffet buildup (e.g., slowly increasing or rapid rise, and the g at which it starts). Conditions associated with buffet onset near +2 g at Mach numbers below MMO have sometimes yielded the most critical characteristics. Therefore, a sufficient spread of conditions should be evaluated.
10.2 High Speed Characteristics—§ 25.253.
10.2.1 Explanation.
10.2.1.1 The maximum flight demonstrated dive speed, VDF/MDF, selected by the applicant, is used along with VD/MD when establishing VMO/MMO in accordance with the associated speed margins under the provisions of § 25.1505. Both VMO/MMO and VDF/MDF are then evaluated during flight tests for showing compliance with § 25.253.
10.2.1.2 The pitch upset defined in § 25.335(b), as amended by amendment 25-23, or defined in § 25.1505, prior to amendment 25-23, provides a means for determining the required speed margin between VMO/MMO and both VD/MD and VDF/MDF. The operational upsets expected to occur in service for pitch, roll, yaw, and combined axis upsets are evaluated when showing compliance to § 25.253 and must not result in exceeding VD/MD or VDF/MDF.
10.2.1.3 In general, the same maneuvers should be accomplished in both the dynamic pressure and Mach critical ranges. All maneuvers in either range should be accomplished at power/thrust and trim points appropriate for the specific range. Some maneuvers in the Mach range may be more critical for some airplanes due to drag rise characteristics, and at high altitudes a lower gross weight may be required to achieve the maximum approved operating altitude and Mach/airspeed conditions.
10.2.1.4 The airplane’s handling characteristics in the high-speed range should be investigated in terms of anticipated action on the part of the flightcrew during normal and emergency conditions.
10.2.1.5 At least the following factors should be considered in determining the necessary flight tests:
10.2.1.5.1 Effectiveness of longitudinal control at VMO/MMO and up to VDF/MDF.
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AC 25-7D CHG 1
10.2.1.5.2 Effect of any reasonably probable mistrim on upset and recovery.
10.2.1.5.3 Dynamic and static stability.
10.2.1.5.4 The speed increase resulting from likely passenger movement when trimmed at any cruise speed to VMO/MMO.
10.2.1.5.5 Trim changes resulting from compressibility effects.
10.2.1.5.6 Characteristics exhibited during recovery from inadvertent speed increase.
10.2.1.5.7 Upsets due to vertical and horizontal gusts (turbulence).
10.2.1.5.8 Speed increases due to horizontal gusts and temperature inversions.
10.2.1.5.9 Effective and unmistakable aural speed warning at VMO plus 6 knots, or MMO plus 0.01 M.
10.2.1.5.10 Speed and flight path control during application of deceleration devices.
10.2.1.5.11 Control forces resulting from the application of deceleration devices.
10.2.1.6 Section 25.1505 states that the speed margin between VMO/MMO, and VD/MD or VDF/MDF, as applicable, “may not be less than that determined under § 25.335(b) or found necessary during the flight tests conducted under § 25.253.” Note that one speed margin must be established that complies with both § 25.335(b) and § 25.253. Therefore, if the applicant chooses a VDF/MDF that is less than VD/MD, then VMO/MMO must be reduced by the same amount (i.e., compared to what it could be if VDF/MDF were equal to VD/MD) in order to provide the required speed margin to VDF/MDF. In determining the speed margin between VMO/MMO and VDF/MDF during type certification programs, the factors outlined in paragraph 10.2.1.5 above should also be considered in addition to the items listed below:
10.2.1.6.1 Increment for production tolerances in airspeed systems (0.005 M), unless larger differences are found to exist.
10.2.1.6.2 Increment for production tolerances of overspeed warning error (0.0l M).
10.2.1.6.3 Increment ΔM due to speed overshoot from MMO, established during flight tests in accordance with § 25.253, should be added to the values for production differences and equipment tolerances. The value of MMO may not be greater than the lowest value obtained from each of the following equations, which reflect the requirements of §§ 25.253 and 25.1505:
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AC 25-7D CHG 1
Or Note: The combined minimum increment may be reduced from 0.07 M to as small as 0.05 M if justified by the rational analysis used to show compliance with § 25.335(b)(2).
10.2.1.6.4 At altitudes where VMO is limiting, the increment for production differences of airspeed systems and production tolerances of overspeed warning errors are 3 and 6 knots, respectively, unless larger differences or errors are found to exist.
10.2.1.6.5 Increment ΔV due to speed overshoot from VMO, established during flight tests in accordance with § 25.253, should be added to the values for production differences and equipment tolerances. The value of VMO should not be greater than the lowest obtained from the following equation, and from § 25.1505:
Where:
10.2.1.6.6 For an airplane with digital interface between the airspeed system and the overspeed warning system, the production tolerance for the warning system may be deleted when adequately substantiated.
10.2.2 Affected Regulations.
These criteria refer to certain provisions of part 25. They may also be used in showing compliance with the corresponding provisions of the former Civil Air Regulations (CARs) in the case of airplanes for which these regulations apply. Other affected CFR are as follows:
Section 25.175(b)regarding demonstration of static longitudinal stability in cruise condition.
Section 25.251, Vibration and buffeting.
Section 25.253, High-speed characteristics.
Section 25.335(b) regarding design dive speed (VD).
Section 25.1303(b)(1) and (c) regarding required flight and navigation instruments.
Section 25.1505, Maximum operating limit speed.
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AC 25-7D CHG 1
10.2.3 Procedures.
Using the speeds VMO/MMO and VDF/MDF determined in accordance with §§ 25.1505 and 25.251, respectively, and the associated speed margins, the airplane should be shown to comply with the high-speed characteristics of § 25.253. Unless otherwise stated, the airplane characteristics should be investigated beginning at the most critical speed up to and including VMO/MMO, and the recovery procedures used should be those selected by the applicant, except that the normal acceleration during recovery should be no more than 1.5 g (total). Testing should be conducted with the CG at the critical position and generally perpendicular to local wind aloft.
10.2.3.1 CG Shift.
The airplane should be upset by the CG shift corresponding to the forward movement of a representative number of passengers (and/or serving carts) depending upon the airplane interior configuration. The airplane should be permitted to accelerate until 3 seconds after VMO/MMO.
10.2.3.2 Inadvertent Speed Increase.
Simulate an evasive control application when trimmed at VMO/MMO, by applying sufficient forward force to the pitch control to produce 0.5 g (total) for a period of 5 seconds, after which recovery should be initiated at not more than 1.5 g (total).
10.2.3.3 Gust Upset.
In the following three upset tests, the values of displacement should be appropriate to the airplane type and should depend upon airplane stability and inertia characteristics. The lower and upper limits should be used for airplanes with low and high maneuverability, respectively.
10.2.3.3.1 With the airplane trimmed in wings-level flight, simulate a transient gust by rapidly rolling to the maximum bank angle appropriate for the airplane, but not less than 45° nor more than 60°. The rudder and longitudinal control should be held fixed during the time that the required bank is being attained. The rolling velocity should be arrested at this bank angle.
Following this, the controls should be abandoned for a minimum of 3 seconds after VMO/MMO or 10 seconds, whichever occurs first.
10.2.3.3.2 Perform a longitudinal upset from normal cruise. Airplane trim is determined at VMO/MMO using power/thrust required for level flight but with not more than maximum continuous power/thrust. (If VMO/MMO cannot be reached in level flight with maximum continuous power or thrust, then the airplane should be trimmed at VMO/MMO in as shallow a descent as practicable that allows VMO/MMO to be reached.) This is followed by a decrease in speed, after which a pitch attitude of 6-12° nose down, as appropriate for the airplane type, is attained using the same power/thrust and trim. The airplane is permitted to accelerate until 3 10-8
AC 25-7D CHG 1
seconds after VMO/MMO. The force limits of § 25.143(d) for short term application apply.
10.2.3.3.3 Perform a two-axis upset, consisting of combined longitudinal and lateral upsets. Perform the longitudinal upset, as in paragraph 10.2.3.3.2 above, and when the pitch attitude is set, but before reaching VMO/MMO, roll the airplane 15-25°. The established attitude should be maintained until 3 seconds after VMO/MMO.
10.2.3.4 Leveling Off from Climb.
Perform transition from climb to level flight without reducing power or thrust below the maximum value permitted for climb until 3 seconds after VMO/MMO. Recovery should be accomplished by applying not more than
1.5 g (total).
10.2.3.5 Descent from Mach Airspeed Limit Altitude.
A descent should be performed at the airspeed schedule defined by MMO and continued until 3 seconds after VMO/MMO occurs, at which time recovery should be accomplished without exceeding 1.5 g (total).
10.2.3.6 Roll Capability, § 25.253(a)(4).
10.2.3.6.1 Configuration.
Wing flaps retracted.
Speedbrakes retracted and extended.
Landing gear retracted.
Trim: The airplane trimmed for straight flight at VMO/MMO. The trimming controls should not be moved during the maneuver.
Power: (1) All engines operating at the power required to maintain level flight at VMO/MMO, except that maximum continuous power need not be exceeded; and (2) if the effect of power is significant, with the throttles closed.
10.2.3.6.2 Test Procedure.
An acceptable method of demonstrating that roll capability is adequate to assure prompt recovery from a lateral upset condition is as follows:
1. Establish a steady 20° banked turn at a speed close to VDF/MDF limited to the extent necessary to accomplish the following maneuver and recovery without exceeding VDF/MDF. Using lateral control alone, it should be demonstrated that the airplane can be rolled to a 20° bank angle in the opposite direction in not more than 8 seconds. The demonstration should be made in the most adverse direction. The maneuver may be unchecked.
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2. For airplanes that exhibit an adverse effect on roll rate when rudder is used, it should also be demonstrated that use of rudder to pick up the low wing in combination with the lateral control will not result in a roll capability significantly below that specified above.
10.2.3.7 Extension of Speedbrakes.
10.2.3.7.1 The following guidance is provided to clarify the meaning of the words “the available range of movements of the pilot’s control” in § 25.253(a)(5) and to provide guidance for demonstrating compliance with this requirement. Normally, the available range of movements of the pilot’s control includes the full physical range of movements of the speedbrake control (i.e., from stop to stop). Under some circumstances, however, the available range of the pilot’s control may be restricted to a lesser range associated with in-flight use of the speedbrakes. A means to limit the available range of movement to an in-flight range may be acceptable if it provides an unmistakable tactile cue to the pilot when the control reaches the maximum allowable in-flight position and compliance with § 25.697(b) is shown for positions beyond the in-flight range.
Additionally, the applicant’s recommended procedures and training must be consistent with the intent to limit the in-flight range of movements of the speedbrake control.
10.2.3.7.2 Section 25.697(b) requires that lift and drag devices intended only for ground operation have means to prevent the inadvertent operation of their controls in flight if that operation could be hazardous. If speedbrake operation is limited to an in-flight range, operation beyond the in-flight range of available movement of the speedbrake control must be shown to be not hazardous. Two examples of acceptable, unmistakable tactile cues for limiting the in-flight range are designs incorporating either a gate or both a detent and a substantial increase in force to move the control beyond the detent. It is not an acceptable means of compliance to restrict the use of or available range of the pilot’s control solely by means of an AFM limitation or procedural means.
10.2.3.7.3 The effect of extension of speedbrakes may be evaluated during other high speed testing (for example, paragraph 10.1.2.2 and paragraphs 10.2.3.1 through 10.2.3.5 of this AC) and during the development of emergency descent procedures. It may be possible to infer compliance with § 25.253(a)(5) by means of this testing. To aid in determining compliance with the qualitative requirements of this rule, the following quantitative values may be used as a generally acceptable means of compliance. A positive load factor should be regarded as excessive if it exceeds 2 g. A nose-down pitching moment may be regarded as small if it necessitates an incremental force of less than 20 lbs to maintain 1 g flight. These values may not be appropriate for all airplanes and will depend on the characteristics of the particular airplane design in high speed flight. Other 10-10
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means of compliance may be acceptable, provided that compliance has been shown to the qualitative requirements specified in § 25.253(a)(5).
10.3 Out-of-Trim Characteristics—§ 25.255.
10.3.1 Explanation.
Certain early, trimmable stabilizer equipped jet transports experienced “jet upsets” that resulted in high-speed dives. When the airplane was mistrimmed in the nose-down direction and allowed to accelerate to a high airspeed, it was found that there was insufficient elevator power to recover. Also, the stabilizer could not be trimmed in the nose-up direction, because the stabilizer motor stalled due to excessive airloads imposed on the horizontal stabilizer. As a result, a special condition was developed and applied to most part 25 airplanes with trimmable stabilizers. With certain substantive changes, it was adopted as § 25.255, effective with amendment 25-2. While these earlier problems seem to be generally associated with airplanes having trimmable stabilizers, it is clear from the preamble discussions to amendment 25-42 that § 25.255 applies “regardless of the type of trim system used in the airplane.” Section 25.255 is structured to give protection against the following unsatisfactory characteristics during mistrimmed flight in the higher speed regimes:
10.3.1.1 Changes in maneuvering stability leading to overcontrolling in pitch.
10.3.1.2 Inability to achieve at least 1.5 g for recovery from upset due to excessive control forces.
10.3.1.3 Inability of the flightcrew to apply the control forces necessary to achieve recovery.
10.3.1.4 Inability of the pitch-trim system to provide necessary control force relief when high control force inputs are present.
10.3.2 Discussion of § 25.255.
10.3.2.1 Section 25.255(a) is the general statement of purpose. Maneuvering stability may be shown by a plot of applied control force versus normal acceleration at the airplane CG. Mistrim must be set to the greater of the following:
10.3.2.1.1 Section 25.255(a)(l).
A 3-second movement of the longitudinal trim system at its normal rate for the particular flight condition with no aerodynamic load. Since many modern trim systems are variable rate systems, this subsection requires that the maneuver condition be defined and that the no-load trim rate for that condition be used to set the degree of mistrim required. For airplanes that do not have power-operated trim systems, experience has shown a suitable amount of longitudinal mistrim to be applied is that necessary to 10-11
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produce a 30-lb control force, or reach the trim limit, whichever occurs first.
10.3.2.1.2 Section 25.255(a)(2).
The maximum mistrim that can be sustained by the autopilot while maintaining level flight in the high-speed cruising condition. The highspeed cruising condition corresponds to the speed resulting from maximum continuous power or thrust, or VMO/MMO, whichever occurs first. Maximum autopilot mistrim may be a function of several variables, and the degree of mistrim should therefore correspond to the conditions of test. In establishing the maximum mistrim that can be sustained by the autopilot, the normal operation of the autopilot and associated systems should be taken into consideration. If the autopilot is equipped with an auto-trim function, then the amount of mistrim that can be sustained, if any, will generally be small. If there is no auto-trim function, consideration should be given to the maximum amount of out-of-trim that can be sustained by the elevator servo without causing autopilot disconnect.
10.3.2.2 Section 25.255(b) establishes the basic requirement to show positive maneuvering stability throughout a specified acceleration envelope at all speeds to VFC/MFC, and the absence of longitudinal control force reversals throughout that acceleration envelope at speeds between VFC/MFC and VDF/MDF. (Later subsections (d) and (e) recognize that buffet boundary and control force limits will limit the acceleration actually reached; this does not account for Mach trim gain, etc.)
10.3.2.2.1 The out-of-trim condition for which compliance must be shown with § 25.255(b) is specified in § 25.255(a). For the initial trimmed condition before applying the mistrim criteria, the airplane should be trimmed at:
For speeds up to VMO/MMO, the particular speed at which the demonstration is being made; and For speeds higher than VMO/MMO, VMO/MMO.
10.3.2.2.2 Section 25.255(b)(2) appears to indicate that unstable airplane characteristics would be satisfactory, regardless of the character of the primary longitudinal control force as load factor is increased, as long as the force did not reverse (e.g., from a pull to a push). While such criteria may have merit for evaluating airplanes when starting the maneuver from a trimmed condition, it can be shown that this provides a poor specification for evaluating an airplane’s maneuvering characteristics when starting the test from the specified mistrimmed condition. For example, an airplane would be deemed to have unacceptable characteristics with a nose-up mistrim, if while relaxing the large initial elevator push force to increase the load factor to the specified value, the elevator force just happened to cross through zero to a slight pull force at 10-12
AC 25-7D CHG 1 one load factor, and then back through zero to a push force at a higher load factor. Such an airplane’s characteristics are clearly superior to one that has a severe elevator force slope reversal, during the same maneuver, but never reaches a zero elevator force condition as the load factor is increased. A literal interpretation of § 25.255(b)(2) would find this airplane to be compliant, while finding the preceding airplane non-compliant because it had a slight reversal of the primary longitudinal control force.
10.3.2.2.3 Section 25.255(b)(2) should be interpreted to mean that the primary longitudinal control force, for load factors greater than 1.0, may not be less than that used to obtain the initial 1 g flight condition. This is illustrated in figure 10-2 below. Slight control force reversals, as discussed in paragraph 10.3.2.2.1 above will be permitted for speeds between VFC/MFC and VDF/MDF only if:
No severe longitudinal control force slope reversals exist;
Any pitching tendency (uncommanded changes in load factor) should be mild and readily controllable; and The airplane’s pitch response to primary longitudinal control should be predictable to the pilot.
Figure 10-2. Mistrimmed Maneuvering Characteristics: Speeds Between VFC/MFC and VDF/MDF 10-13
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10.3.2.3 Section 25.255(c) requires that the investigation of maneuvering stability (§ 25.255(b)) include all attainable acceleration values between –l g and +2.5 g. Sections 25.333(b) and 25.337, to which it refers, limit the negative g maximum to 0 g at VD. Section 25.251 further limits the g to that occurring in probable inadvertent excursions beyond the buffet onset boundary at those altitudes where buffet is a factor.
10.3.2.4 Section 25.255(c)(2) allows for extrapolation of flight test data by an acceptable method. For example, if the stick force gradient between 0 and +2 g agrees with predicted data, extrapolation to -1 g and 2.5 g should be allowed.
10.3.2.5 Section 25.255(d) requires flight tests to be accomplished from the normal acceleration at which any marginal stick force reversal conditions are found to exist to the applicable limits of § 25.255(b)(1). This requirement takes precedence over the extrapolation allowance described in paragraph 10.3.2.4 above.
10.3.2.6 Section 25.255(e), limits the investigation to the required structural strength limits of the airplane and maneuvering load factors associated with probable inadvertent excursions beyond the boundary of the buffet onset envelope. It also accounts for the fact that speed may increase substantially during test conditions in the -1 g to +1 g range. It limits the entry speed to avoid exceeding VDF/MDF.
10.3.2.7 Section 25.255(f) requires that, in the out-of-trim condition specified in § 25.255(a), it be possible to produce at least 1.5 g during recovery from the overspeed condition of VDF/MDF. If adverse flight characteristics preclude the attainment of this load factor at the highest altitude reasonably expected for recovery to be initiated at VDF/MDF following an upset at high altitude, the flight envelope (CG, VDF/MDF, altitude, etc.) of the airplane should be restricted to a value where 1.5 g is attainable. If trim must be used for the purpose of obtaining 1.5 g, it must be shown to operate with the primary control surface loaded to the least of three specified values.
10.3.2.7.1 The force resulting from application of the pilot limit loads of § 25.397 (300 lbs).
10.3.2.7.2 The control force required to produce 1.5 g (between 125 and 300 lbs).
10.3.2.7.3 The control force corresponding to buffeting or other phenomena of such intensity that it is a strong deterrent to further application of primary longitudinal control force.
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10.3.3 Procedures.
10.3.3.1 Compliance is determined by the characteristics of FS/g (normally a plot).
Any standard flight test procedure that yields an accurate evaluation of FS/g data in the specified range of speeds and acceleration should be considered for acceptance. Bounds of investigation and acceptability are set forth in the rule and in discussion material above, and broad pilot discretion is allowed in the selection of maneuvers.
10.3.3.2 Investigation Range.
Out-of-trim testing should be done at the most adverse loading for both high and low control forces. Testing should be accomplished both at the dynamic pressure (q) and Mach limits.
10.3.3.3 The ability to move the primary controls (including trim), when loaded, should be considered prior to the tests.
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CHAPTER 11. STRUCTURE [RESERVED] 11-1