AC 25-7D CHG 1
9.5.2.3.5 No matter which method is used, the wind should be continuously timerecorded throughout the takeoff from brake release (or any low speed above which all data necessary to the computation are available and of sufficient accuracy) to a height of 50 feet, and throughout the landing from a height of 50 feet to termination of the test event (e.g., full stop, touchand-go, go-around) or any low speed above which all data necessary to the computation are available and of sufficient accuracy. The measured crosswind component should be corrected from the height of the measurement device to a height of 10 meters. The average crosswind at 90° to the runway heading should then be calculated for the above time span. The maximum gust could also be derived during this process, based on the same time span.
9.5.2.3.6 With prior agreement from the FAA, it may also be permissible to obtain crosswind data from tower wind reports. However, the use of this method should be carefully reviewed to ensure that the measurement sensor is properly calibrated to establish the measurement sensor reference height, to establish that the smoothing characteristics do not produce unacceptable filtering, and that the location of the measurement sensor is appropriate for the takeoff and landing runway(s). Such a method has the disadvantage of not being able to provide the gust value during takeoff and landing.
9.5.2.3.7 For all methods of gathering test data (with the exception of the method described in paragraph 9.5.2.3.6), the following is applicable:
The averaged value of the crosswind should be understood as a mathematical average obtained from:
Where Vwy is the crosswind component corrected to 10m height, per § 25.21(f), and perpendicular to the runway axis.
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AC 25-7D CHG 1
1. The maximum gust derived from test analysis should be of a duration sufficient to interfere with airplane handling characteristics in crosswind. It should be obtained from a centered moving average applied to the test data. The centered moving average should not be of less than 3 seconds, in order to be consistent with the filtering standard applied by airports to communicate the gust value. Note, however, that a 3 second moving average applied on the data collected during the tests may not necessarily provide the same result as airport reported gusts, due to differences in data acquisition and reduction methodologies between flight tests systems and airport systems. This filtering also introduces some conservatism in the gust determination to account for the variability of gusts profiles, as compared to the ones encountered in flight tests.
2. If an applicant wants to use a centered moving average below 3 seconds, this should be substantiated on a case-by-case basis. The moving average should not go below 1 second.
3. Using a smaller portion of the takeoff or landing run (i.e. computing the average starting below 50 ft and/or above the first/last valid data) or only relevant portions of the takeoff or landing run to compute the average crosswind may be acceptable if properly justified by the applicant.
9.5.2.3.8 If demonstrated crosswind is not considered limiting, an applicant may use a separate and distinctively identified portion of the AFM to provide “unapproved” data, regarding airplane crosswind capability beyond the demonstrated level.
9.6 Spray Characteristics, Control, and Stability on Water—§ 25.239.
9.6.1 Explanation.
These characteristics should be investigated at the most adverse weight/CG combinations.
9.6.2 Procedures.
9.6.2.1 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.
9.6.2.2 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 9-6
AC 25-7D CHG 1 tendency to bury and either cause waterlooping or damage. The procedures used to avoid undesirable characteristics should be included in the AFM.
9.6.2.3 During low-speed taxi, the effectiveness of the water rudders and/or asymmetric power or 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.
9.6.2.4 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 CG 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.
9.6.2.5 Engine failure of the critical engine at any time during the takeoff run should be evaluated. No dangerous porpoising, swerving, or waterlooping should result.
9.6.2.6 There should be no undue tendency to porpoise and no extraordinary skill or alertness should be required to control porpoising.
9.6.2.7 Spray impingement on the airframe (control surfaces, etc.) should be evaluated to assure the resulting loads are within acceptable limits.
9.6.2.8 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.
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CHAPTER 10. FLIGHT: MISCELLANEOUS REQUIREMENTS
10.1 Vibration and Buffeting—§ 25.251.
10.1.1 Explanation.
10.1.1.1 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.
10.1.1.2 For § 25.251(b) and (c), vibration and buffeting are considered excessive when it is determined that it:
10.1.1.2.1 May cause structural damage or, if sustained over an extended period of time, could lead to structural fatigue;
10.1.1.2.2 May cause pilot fatigue or annoyance that interferes with operation of the airplane or management of the airplane systems; or
10.1.1.2.3 Interferes with flight instrument readability.
10.1.1.3 No perceptible buffeting is permitted in the cruise configuration as required by § 25.251(d). Weight and/or altitude 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 10.1.1.2 above.
10.1.1.4 For airplanes with MD greater than 0.6 or with a maximum operating altitude greater than 25,000 feet, the buffet onset envelope must 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 must be provided in the AFM in accordance with § 25.1585(d).
These AFM data should be valid criteria for forward CG conditions or correctable to forward CG using AFM procedures. This boundary should be established by pilot qualitative evaluation or by correlation with pilot qualitative evaluation, as there is no predetermined criterion for buffet level at the pilot station. A normal acceleration of ±0.05 g has been used in some cases; however, the appropriate acceleration level will vary from airplane to airplane and may also be affected by the dynamic response of the accelerometer. If a measured normal acceleration is to be used, the acceleration level and specific accelerometer should first be correlated against a pilot’s assessment of the onset of buffet.
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10.1.1.5 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 VMO/MMO or VDF/MDF speeds established on the unmodified airplane. If this occurs, the maximum operating speed and demonstrated flight diving speed may need to be reduced. However, the regulations concerning the speed spread margin between VMOMMO and VDF/MDF remain in effect. Systems and flight characteristics affected by the reduced maximum speeds should also be reevaluated. Indicator markings, overspeed horns, etc. must be reset, as necessary, to remain in compliance with the applicable regulations.
10.1.1.6 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 shockwave-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.
10.1.1.7 Section 25.251(e) requires that “probable inadvertent excursions beyond the boundaries of buffet” may not result in “unsafe conditions.” To assure that no unsafe conditions are encountered in maneuvering flight, maneuvering flight evaluations to demonstrate satisfactory maneuvering stability are described below. 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 the primary longitudinal control is predictable to the pilot.
10.1.2 Procedures.
10.1.2.1 Section 25.251(a).
The test procedures outlined below will provide the necessary flight demonstrations for compliance with § 25.251(a).
10.1.2.2 Section 25.251(b).
The airplane should be flown at VDF/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 power or thrust setting not exceeding maximum continuous power or thrust. The airplane gross weight should be as high as practicable for the cruise 10-2
AC 25-7D CHG 1 condition, with the CG at or near the forward limit. In addition, compliance with § 25.251(b) should be demonstrated with high drag devices (i.e., speed brakes) deployed at VDF/MDF. Thrust reversers, if designed for inflight deployment, should be deployed at their limit speed conditions.
10.1.2.3 Section 25.251(c).
The weight of the airplane should be as heavy as practical, commensurate with achieving the maximum certificated altitude.
10.1.2.4 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 VMO/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 0.3 g 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, MLRC) with the CG 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 g flight to VMO/MMO using maximum continuous power or thrust. Descending flight is acceptable if needed to achieve VMO/MMO.
10.1.2.5 Section 25.251(e).
Section 25.251(e) requires the determination of the buffet onset envelope, in the cruise configuration, for airplanes with MD 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.
10.1.2.5.1 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 FS/g relationship from such data. The pilot flying the airplane needs to evaluate the airplane characteristics under such conditions. Figure 10-1 provides guidance on stick force per g (FS/g) characteristics that would be considered acceptable or unacceptable.
10.1.2.5.2 For determination of the buffet onset envelope, the flight tests should be conducted at forward CG. For maneuvering characteristics, airplanes should be evaluated at the most aft CG in accordance with the following criteria:
4. For all weight/altitude combinations where buffet onset occurs at various load factors between approximately +1 g and +2 g, the longitudinal control force (FS) vs. g curve should have a positive slope 10-3
AC 25-7D CHG 1 at any speed up to VMO/MMO prior to encountering that buffet onset.
See figure 10-1 of this AC.
Note: The characteristics shown in figure 10-1 are satisfactory only in accordance with item #1 above and item #2 below.
Figure 10-1. Maneuvering Characteristics at Speeds up to VMO/MMO
5. Under the airplane weight/altitude combinations of item #1 above but at load factors beyond buffet onset, the following FS characteristics apply (see figure 10-1):
a. The evaluation should proceed to a g level that will allow recovery to be accomplished near +2.5 g, 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. (See § 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.
b. Any pitching tendency (uncommanded changes in load factor) should be mild and readily controllable.
c. Sufficient control should be available to the pilot, through unreversed use of only the primary longitudinal control, to affect a prompt recovery to +1 g flight from the load factors described herein.
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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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