(A) At light weight and aft c.g. (this is desirable; however, flexibility should be permitted).
(B) Normal takeoff and landing flap configurations using the recommended procedures.
(C) Normal usage of thrust reversers. Particular attention should be paid to any degradation of rudder effectiveness due to thrust reverser airflow effects.
(D) Yaw dampers/turn coordinator On, or Off, whichever is applicable.
(ii) Test Procedure and Required Data. Three takeoffs and 3 landings, with at least one landing to a full stop, should be conducted in a 90-degree crosswind component of at least 20 knots or 0.2VSO, whichever is greater, except that for airplanes whose certification basis includes Amendment 25-42, it need not exceed 25 knots. For each test condition, a qualitative evaluation by the pilot of airplane control capability, forces, airplane dynamic reaction in gusty crosswinds (if available), and general handling characteristics should be conducted. The airplane must be satisfactorily controllable without requiring exceptional piloting skill or strength. Wind data from Inertial Navigation Systems (INS), tower, or portable ground recording stations should be corrected to a 90-degree crosswind component and to a height of 10 meters.
f. Section 25.239 - Spray Characteristics, Control, and Stability on Water.
(1) Explanation. These characteristics should be investigated at the most adverse weight/c.g. combinations.
(2) Procedures.
(i) The spray characteristics and, in particular, the pilot view during the initial takeoff run, should allow sufficient view in order to maintain a reasonable track over the water.
Since not all seaplane operations are on open lakes or bays, but can be on rivers or channels, the directional control and view should be sufficient enough to stay within the channel confines.
(ii) The tendency of the wing floats or sponsons to submerge and/or cause waterloops should be evaluated during the crosswind testing. During the step taxiing evaluations, the floats should also be evaluated for any tendency to bury and either cause waterlooping or damage. The procedures used to avoid undesirable characteristics should be included in the AFM.
(iii) During low speed taxi, the effectiveness of the water rudders and/or asymmetric thrust should be evaluated in view of the types of maneuvering to be expected in service. If reverse thrust is to be used, it too should be evaluated in terms of ease of accomplishment and crew coordination.
(iv) If an amphibian is intended to be "beached" or run up a ramp, the handling characteristics and ability to maneuver onto the ramp should be evaluated. Forward c.g. is generally more critical. The procedures should be included in the AFM. There should be no undue tendency to damage the bow or other structure.
(v) Engine failure of the critical engine at any time during the takeoff run should be evaluated. No dangerous porpoising, swerving, or waterlooping should result.
(vi) There should be no undue tendency to porpoise and no extraordinary skill or alertness should be required to control porpoising.
(vii) Spray impingement on the airframe (control surfaces, etc.) should be evaluated to assure the resulting loads are within acceptable limits.
(viii) The above evaluations should be performed in the airplane on the water rather than by analysis or model testing. Analysis and/or model testing may be used to point out the problem areas but should not be substituted for actual testing.
Section 8. MISCELLANEOUS FLIGHT REQUIREMENTS
31. VIBRATION AND BUFFETING - § 25.251 .
a. Explanation.
(1) The testing required by Subpart C of Part 25 covers the vibration extremes expected in service. The applicant's flight tests should assure that the regulatory limits are not exceeded. Flight testing should not be conducted beyond where structural (Subpart C) tests and calculations have been completed.
(2) For §§ 25.251(b) and (c), vibration and buffeting is considered excessive when it is determined that it:
(i) May cause structural damage or, if sustained over an exte nded period of time, could lead to structural fatigue;
(ii) May cause pilot fatigue or annoyance that interferes with operation of the airplane or management of the airplane systems; or,
(iii) Interferes with flight instrument readability.
(3) No perceptible buffeting is permitted in the cruise configuration as required by § 25.251(d). Weight and/or altitude Airplane Flight Manual (AFM) limitations may need to be imposed to comply with this criterion.
Reasonable buffet during the deployment of spoilers and other high drag devices is permitted to the extent allowed under §§ 25.251(b) and (c), as described in paragraph (2) above.
(4) A determination of a buffet onset envelope is to be established for the ranges of airspeed and/or Mach number, weight, altitude, and load factor for which the airplane is to be certificated. This envelope should be made a part of the AFM in accordance with § 25.1585(c). These AFM data should be valid criteria for forward c.g.
conditions or correctable to forward c.g. by the use of AFM procedures. This boundary should be established by pilot event, as there is no established criterion for buffet level at the pilot station. A normal acceleration of +.05g has been proposed; however, that will vary from airplane to airplane and may also be affected by the dynamic response of the accelerometer.
(5) Modifications to airplanes, particularly modifications that may affect airflow about the wing, should be evaluated for their effect on vibration and buffeting characteristics, changes in the speeds for onset of buffet, and maneuvering characteristics beyond buffet onset. This change may not only impact the buffet boundary envelope, but may change the acceptability of the V MO/MMO or VDF/MDF derived from the unmodified airplane. If this occurs, the maximum operating speed and dive speed may be reduced. However, the applicable speed spread margin regulations remain in effect. Systems and flight characteristics affected by the reduced maximum speeds should also be reevaluated. Indicator markings, overspeed horns, etc. should be reset, as necessary, if the airplane is to remain in the transport category.
(6) On swept-wing airplanes, undesirable pitch-up maneuvering characteristics can occur as the center of lift moves inboard and forward with increasing g, due to shock-wave induced separation and/or as wing load alleviation systems unload the wingtips. Straight-wing airplanes can also exhibit similar characteristics; therefore, new airplanes and those modified in a manner that may affect the spanwise lift distribution or produce undesirable pitching moment as a function of g, or increase the exposure to high altitude buffet encounters, should be evaluated as described herein.
(7) It has been determined that a pos itive slope of the stick force versus g (F Ss/g) relationship is not necessarily required at speeds and g combinations beyond V FC/MFC and/or buffet onset. However, § 25.251(e) requires that "probable inadvertent excursions beyond the boundaries of buffet" may not result in "unsafe conditions." In order to assure that no "unsafe conditions" are encountered in maneuvering flight, maneuvering flight evaluations to demonstrate satisfactory maneuvering stability are described herein. A determination of the longitudinal maneuvering characteristics should be made to assure the airplane is safely controllable and
maneuverable in the cruise configuration to assure there is no danger of exceeding the airplane limit load factor, and that the airplane's pitch response to primary longitudinal control is predictable to the pilot.
b. Procedures .
(1) Section 25.251(a). The test procedures outlined below will provide the necessary flight demonstrations for compliance with § 25.251(a).
(2) Section 25.251(b). The airplane should be flown at V DF/MDF at several altitudes from the highest practicable cruise altitude to the lowest practicable altitude. The test should be flown starting from trimmed flight at VMO/MMO at a thrust setting not exceeding maximum continuous power. The airplane gross weight should be as high as practicable for the cruise condition, with the c.g. at or near the forward limit.
(i) High drag devices should also be deployed at V DF/MDF (spoilers and speed brakes); thrust reversers, if designed for inflight deployment, should be deployed at their limit speed conditions.
(ii) Airplanes equipped with pneumatic de- icer boots should be evaluated to V MO/MMO with deicing on and off (if automatic) and not operating. If the applicant desires to restrict the maximum operating speed (VMO/MMO) to a lower value with de-icing on, it should be shown that excessive vibration or buffeting does not occur at speeds of the new V MO/MMO plus the speed spread (V MO/MMO to VDF/MDF ) determined with anti icing off.
(3) Section 25.251(c). The weight of the airplane should be as heavy as practical, commensurate with achieving the maximum certificated altitude.
(4) Section 25.251(d). It should be demonstrated in flight tests that perceptible buffeting does not occur in straight flight in the cruise configuration, at any speed up to V MO/MMO, to show compliance with § 25.251(d).
This should be met from initial combinations of critical weight and altitude, if achievable, where the airplane has a
0.3g margin to the buffet onset boundary developed under § 25.251(e). These initial conditions should be established using a nominal cruise Mach number (typically long-range cruise Mach, M LRC) with the c.g. at the forward limit. This flight condition is representative of practical operating criteria imposed by most operators.
From these initial conditions, the airplane should be accelerated in 1.0g flight to V MO/MMO using maximum continuous thrust. Descending flight is acceptable if needed to achieve V MO/MMO.
(5) Section 25.251(e). Section 25.251(e) requires the determination of the buffet onset envelope, in the cruise configuration, for airplanes with M D greater than 0.6 or maximum operating altitudes greater than 25,000 feet. This requirement also provides criteria for evaluation of maneuvering stability in cruise flight under load factor conditions up to and beyond the onset of buffet.
(i) The determination of compliance with § 25.251(e), using flight test data from maneuvers conducted well into buffet, is extremely difficult due to the dynamics of this type of maneuver and the establishment of the FSs/g relationship from such data. The pilot flying the airplane needs to evaluate the airplane characteristics under such conditions. In this regard, Figures 31-1 and 31-2 illustrate general airplane responses and do not necessarily indicate required quantitative flight test results.
(A) Airplanes should be evaluated at the most aft c.g. in accordance with the following criteria:
(1) For all weight/altitude combinations where buffet onset occurs at various load factors between approximately +1g and +2g, the longitudinal control force (F Ss) characteristics of §§ 25.255(b)(1) and (2) apply prior to encountering that buffet onset (see buffet free regions of Figures 31-1 and 31-2).
(2) Under the airplane weight/altitude/speed combinations of ( 1), above, but at load factors beyond buffet onset, the following F S characteristics apply (see buffet regions of Figures 31-1 and 31-2):
(i) The evaluation should proceed to a g level that will allow recovery to be accomplished near +2.5g, unless sufficient buffet or other phenomena (natural, artificial, or a combination) of such intensity exists that is a strong and effective deterrent to further pilot application of nose-up longitudinal control force (as in § 25.201(d)(2)) so that there is no danger of exceeding the airplane limit load factor (Ref. § 25.143(b)).
NOTE: A strong and effective deterrent is analogous to that required for stall identification; stick shaker or stall warning buffet are not considered to be an adequate end point for these tests.
(ii) Any pitching tendency ( uncommanded changes in load factor) shall be mild and readily controllable.
(iii) Sufficient control s hall be available to the pilot, through unreversed use of only the primary longitudinal control, to effect a prompt recovery to +1g flight from the load factors described herein.
(iv) The airplane's pitch response to primary longitudinal control sha ll be predictable to the pilot.
(B) Experience has shown that maneuvering evaluations conducted at the highest Mach and the highest weight and altitude (W/delta) 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 2g at Mach numbers below M MO have sometimes yielded the most critical characteristics. Therefore, a sufficient spread of conditions should be evaluated.
FIGURE 31-1. MANEUVERING CHARACTERISTICS Speeds Up to the Lesser of V FC or MFC or Buffet Onset at 1G * These characteristics are satisfactory only in accordance with paragr aphs b(5)( i)(A)(1) and (2).
FIGURE 31-2. MANEUVERING CHARACTERISTICS Speeds Between Figure 31-1 and V DF or MDF * These characteristics are satisfactory only in accordance with paragraphs b(5)( i)(A)(1) and (2).
32. HIGH SPEED CHARACTERISTICS - § 25.253 .
a. Explanation.
(1) The maximum flight demonstrated dive speed, V DF/MDF, is used when establishing V MO/MMO and the associated speed margins under the provisions of § 25.1505. Both V MO and MMO are then evaluated during flight tests for showing compliance with § 25.253.
(2) The pitch upset described in § 25.335(b), as amended by Amendment 25-23, or § 25.1505, prior to Amendment 25-23, is for design criteria only. The operational upsets expected to occur in service for pitch, roll, yaw, and combined axis upsets are covered under § 25.253.
(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 thrust and trim points appropriate for the specific range. It must be realized that some maneuvers in the Mach range may be more critical for some airplanes due to drag rise characteristics, and that at high altitudes a lower gross weight may be required to achieve the maximum approved operating altitude and Mach/airspeed conditions.
(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.
(5) At least the following factors should be considered in determining the necessary flight tests:
(i) Effectiveness of longitudinal control at V MO/MMO and up to VDF/MDF.
(ii) Effect of any reasonably probable mistrim on upset and recovery.
(iii) Dynamic and static stability.
(iv) The speed increase that results from likely passenger movement when trimmed at any cruise speed to VMO/MMO.
(v) Trim changes resulting from compressibility effects.
(vi) Characteristics exhibited during recovery from inadvertent speed increase.
(vii) Upsets due to vertical and horizontal gusts (turbulence).
(viii) Speed increases due to horizontal gusts and temperature inversions.
(ix) Effective and unmistakabl e aural speed warning at V MO plus 6 knots, or MMO plus 0.0lM.
(x) Speed and flightpath control during application of deceleration devices.
(xi) The manageability of control forces resulting from the application of deceleration devices.
(6) Section 25.1505 states that the speed margin between V MO/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." Historically, this speed margi n allowance for atmospheric variations (e.g., horizontal gusts, penetration of jet streams and cold fronts), instrument errors, and airframe production tolerances has been interpreted as .05M in the Mach limited portion of the flight envelope. However, studies conducted by industry during the 1980s have shown that for a conventional aircraft, atmospheric disturbances resulted in larger airspeed variations than accounted for by the .05M minimum margin. Consequently, this speed margin was increased to .07 Mach by Amendment 25-91, with an allowance for reducing the margin to the old value of .05M if justified by a rational analysis. In establishing the minimum speed margins during type certification programs, the factors outlined in paragraph (5), above, should also be considered in addition to the items listed below:
(i) Increment allowance for production tolerances in airspeed systems (0.005M), unless larger differences are found to exist.
(ii) Increment allowance for production tolerances of overs peed warning error (0.0lM), unless larger tolerances or errors are found to exist.
(iii) Increment allowance ∆M due to speed overshoot from M MO, established by upset during flight tests in accordance with § 25.253, should be added to the values for production differences and equipment tolerances, and the minimum acceptable combined value should not be less than .07M between M MO and MD/MDF. The value of MMO then should not be greater than the lowest value obtained from each of the following equations and from § 25.l505:
MMO ≤ MD/MDF - ∆M - .005M - .0lM or MMO ≤ MD/MDF - .07M NOTE: The combined minimum increment may be reduced from .07M to as small as .05M if justified by the rational analysis used to show compliance with § 25.335(b)(2).
(iv) At altitudes where V MO is limiting, the increment allowance 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.
(v) Increment allowance ∆V due to speed overshoot from V MO, established by upset during flight tests in accordance with § 25.253, should be added to the values for production differences and equipment
tolerances. The value of V MO should not be greater than the lowest obtained from the following equation, and from § 25.l505:
VMO ≤ VD/VDF - ∆V - 3 knots - 6 knots (production differences) (equipment tolerances)
(vi) For an airplane with digital interface between the airspeed system and the ove rspeed warning system, the production tolerance for the warning system may be deleted when adequately substantiated.
b. Regulations Affected . These criteria refer to certain provisions of Part 25 of the Federal Aviation Regulations (FAR). They may also be used in showing compliance with the corresponding provisions of the former Civil Air Regulations (CAR) in the case of airplanes to which those regulations are applicable. Other affected FAR are as follows:
Section 25.175(b) Demonstration of stati c longitudinal stability.
Section 25.251 Vibration and buffeting.
Section 25.253 High-speed characteristics.
Section 25.335(b) Design dive speed, V D.
Sections 25.1303(b)(1) and (c) Flight and navigation instruments.
Section 25.1505 Maximum operating limit speed.
c. 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 at the most critical speed up to and including V MO/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 l.5g (total). Testing should be conducted with the c.g. at the critical position and generally perpendicular to local wind aloft.
(1) Center of Gravity Shift . The airplane should be upset by the center of gravity 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 effective overspeed warning.
(2) Inadvertent Speed Increase . Simulate an evasive control application when trimmed at V MO/MMO, by applying sufficient forward force to the pitch control to produce 0.5g (total) for a period of 5 seconds, after which recovery should be effected at not more than l.5g (total).
(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.
(i) 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 degrees nor more than 60 degrees. 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 overspeed warning or 10 seconds, whichever occurs first.
(ii) Perform a longitudinal upset from normal cruise. Airplane trim is determined at V MO/MMO using power/thrust required for level flight but with not more than maximum continuous power/thrust. This is followed by a decrease in speed after which an attitude of 6 -l2 degrees nose down, as appropriate for the airplane type, is attained with the power/thrust and trim initially required for V MO/MMO in level flight. The airplane is
permitted to accelerate until 3 seconds after overspeed warning. The force limits of § 25.143(c) for short term application apply.
(iii) Perform a two-axis upset, consisting of combined longitudinal and lateral upset s. Perform the longitudinal upset, as in paragraph (ii) above, and when the pitch attitude is set, but before reaching V MO/MMO, roll the airplane l5-25 degrees. The established attitude should be maintained until 3 seconds after overspeed warning.
(4) Leveling Off from Climb . Perform transition from climb to level flight without reducing power below the maximum value permitted for climb until 3 seconds after overspeed warning. Recovery should be accomplished by applying not more than 1.5g (total).
(5) Descent from Mach Airspeed Limit Altitude . A descent should be performed at the airspeed schedule defined by M MO and continued until 3 seconds after overspeed warning occurs, at which time recovery should be accomplished without exceeding 1.5g (total).
(6) Application of Deceleration Devices . It should be demonstrated that extension of the airbrakes at high speeds will not result in excessive flight path deviation. With the airplane trimmed for level flight at VMO/MMO, extension of the airbrakes to any position that may be selected by the pilot's control, at speeds above VMO/MMO but not so high that V DF/MDF would be exceeded during the maneuver, should not result in:
(i) A positive normal load factor greater than 2.0 with the stick free; or
(ii) A nose down pitching moment that requires more than a 20 pound longitudinal control force to maintain 1g flight.
33. OUT-OF-TRIM CHARACTERISTICS - § 25.255 .
a. 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:
(1) Changes in maneuvering stability leading to overcontrolling in pitch.
(2) Inability to achieve at least l.5g for recovery from upset due to excessive control forces.
(3) Inability of the flightcrew to apply the control forces necessary to achieve recovery.
(4) Inability of the pitch trim system to provide necessary control force relief when high control force inputs are present.
b. Reference Regulation . Section 25.255
c. Discussion of The Regulation .
(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 center of gravity. Mistrim must be set to the greater of the following:
(i) Section 25.255(a)(l). A 3-second move ment 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 produce a 30 pound control force, or reach the trim limit, whichever occurs first.
(ii) 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 high speed cruising condition corresponds to the speed resulting from maximum continuous power or thrust, or V MO/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.
(2) Section 25.255(b) establishes the basic requirement to show positive maneuvering stability throughout a specified acceleration envelope at all speeds to V FC/MFC, and the absence of longitudinal control force reversals throughout that acceleration envelope at speeds between V FC/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.)
(i) Section 25.255(b)(2), taken at face value, indicates 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 an anomalous specification for evaluation of an airplane's maneuvering characteristics when starting the test from the specified mistrimmed condition. For example, an airplane would 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 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 pass this airplane while failing the preceding airplane because it had a slight reversal of the primary longitudinal control force.
(ii) 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 1g flight condition. This is illustrated in Figure 33-1 and is consistent with the criteria depicted in Figure 31-2 for in-trim maneuver stability.
Slight control force reversals, as discussed in paragraph ( i), above, will be permitted for speeds between V FC/MFC and VDF/MDF only if:
(A) No severe longitudinal control force slope reversals exist;
(B) Any pitching tendency ( uncommanded changes in load factor) shall be mild and readily controllable; and (C) The airplane's pitch response to prima ry longitudinal control shall be predictable to the pilot.
FIGURE 33-1. MISTRIMMED MANEUVERING CHARACTERISTICS Speeds Between V FC/MFC and VDF/MDF
(3) Section 25.255(c) requires that the investigation of maneuvering stability (§ 25.255(b)) includ e all attainable acceleration values between - lg and +2.5g. Sections 25.333(b) and 25.337, to which it refers, limit the negative g maximum to 0g at V D. Section 25.25l further limits the g to that occurring in probable inadvertent excursions beyond the buffet onset boundary at those altitudes where buffet is a factor.
(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 +2g agrees with predicted data, extrapolation to -1g and 2.5g should be allowed.
(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 (4), above.
(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 -1g to +1g range. It limits the entry speed to avoid exceeding V DF/MDF.
(7) Section 25.255(f) requires that in the out-of-trim condition of paragraph (a) it must be possible to produce at least 1.5g during recovery from the overspeed condition of V DF/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 ( c.g., VDF/MDF, altitude, etc.) of the airplane should be restricted to a value where 1.5g is attainable. If trim must be used for the purpose of obtaining 1.5g, it must be shown to operate with the primary control surface loaded to the least of three specified values.
(i) The force resulting from application of the pilot limit loads of § 25.397 ( 300 lbs.).
(ii) The control force required to produce 1.5g (between 125 and 300 lbs.).
(iii) 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.
d. Procedures .
(1) Basic compliance is determined by the characteristics of F S/g (normally a plot). Any standard flight test procedure that yields an accurate evaluation of F S/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.
(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 "Q" and Mach limits.
(3) The ability to actuate the primary controls (including trim), when loaded, should be considered prior to the tests.