AC 25-7D CHG 1
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
AC 25-7D CHG 1
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
AC 25-7D CHG 1
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
AC 25-7D CHG 1
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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AC 25-7D CHG 1
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
CHAPTER 12. DESIGN AND CONSTRUCTION: GENERAL [RESERVED] 12-1
CHAPTER 13. DESIGN AND CONSTRUCTION: CONTROL SURFACES [RESERVED] 13-1
CHAPTER 14. DESIGN AND CONSTRUCTION: CONTROL SYSTEMS
14.1 General—§ 25.671.
14.1.1 Explanation.
This material deals with the all-engines out case of § 25.671(d). The intent of this rule is to assure that in the event of failure of all engines, the airplane will be controllable, and an approach and landing flare possible. This may be done by analysis where the method is considered reliable.
14.1.2 Procedures.
14.1.2.1 In accordance with § 25.671(d), the airplane must be controllable when all engines fail. Compliance should be shown for each approved configuration. The airplane should remain controllable following the failure of the engines. Reconfiguration, if possible with all engines failed, is permitted. Any such reconfiguration should be included in the AFM operating procedures for an all engines failed condition.
14.1.2.2 The effectiveness of the emergency power to drive the airplane control system, whether generated from a windmilling engine or an auxiliary power supply, should be demonstrated in flight.
14.1.2.3 Past approaches to showing compliance with § 25.671(d) have been to show that the airplane is controllable following the failure of all engines in the climb, cruise, descent, approach, and holding configurations and can be flared to a landing attitude from a reasonable approach speed.
14.1.2.4 For airplanes with fully powered or electronic flight control systems, the non-normal procedures section of the approved AFM should contain appropriate operating procedures and a statement similar to the following:
“The airplane has a fully powered (or electronic) control system that is dependent upon engine windmill RPM, or an auxiliary power supply, to provide the necessary source of control system power in the event all engines fail in flight. A minimum airspeed of XXX knots IAS will provide adequate hydraulic or electrical power for airplane controllability in this emergency condition.”
14.2 Flap and Slat Interconnections—§ 25.701.
14.2.1 Explanation.
In accordance with § 25.701(a), if the wing flaps are not mechanically interconnected, tests or analysis should be conducted to show that the airplane has safe flight characteristics with asymmetric flap or slat deployment.
14-1
14.2.2 Reference.
See AC 25-22 for additional guidance.
14.2.3 Procedures.
Simulate appropriate flap and slat malfunctioning during takeoffs, approaches, and landings to demonstrate that the airplane is safe under these conditions. To be considered safe, adequate stall margins and controllability should be retained without requiring exceptional piloting skill or strength. Additionally, there should be no hazardous change in altitude or attitude during transition to the asymmetric condition considering likely transition rates.
14.3 Takeoff Warning System—§ 25.703. [Reserved] 14-2
CHAPTER 15. DESIGN AND CONSTRUCTION: LANDING GEAR
15.1 Retracting Mechanism—§ 25.729.
15.1.1 Explanation.
None.
15.1.2 Procedures.
15.1.2.1 In accordance with the provisions of § 25.729, flight tests should be conducted to demonstrate the ability of the landing gear and associated components, in their heaviest configuration, to properly extend and retract at:
15.1.2.1.1 VLO (the placard airspeed) in the cruise configuration at near 1 g flight and normal yaw angles; and
15.1.2.1.2 Airspeeds and flap settings corresponding to typical landings. The landing gear operating placard airspeed, VLO, established in accordance with § 25.1515(a), should not exceed the 1.6 VS1 design value of § 25.729(a)(1)(ii).
Note: “Normal” yaw angles are those associated with engine-out flight and counteracting crosswinds of up to 20 knots.
15.1.2.2 The alternate extend system should be demonstrated at airspeeds up to VLO at near 1 g flight and normal yaw angles. An envelope of emergency extension capability should be established and presented in the emergency operating procedures section of the AFM. (Refer to the note under paragraph 15.1.2.1.2 above.)
15.1.2.3 Operation Test—§ 25.729(d).
15.1.2.3.1 The engine-out gear retraction time should be determined from flight tests with one engine at idle power or thrust and the operating engine(s) adjusted to provide the lowest thrust-to-weight ratio to be certificated. The hydraulic system should be in the critical configuration corresponding to an actual engine failure condition. The airplane should be stabilized on a steady heading before gear retraction is initiated. The resulting gear retraction time will be used in developing AFM takeoff flight path performance information in accordance with § 25.111.
15.1.2.3.2 Gear retraction time is the time from landing gear lever movement to the “UP” position until the last landing gear, including doors, is in the retracted configuration. Allowance should be made for any delays associated with the landing gear indication system.
15-1