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8.1.9.1 The applicant should verify that the stall identification system, considering system design features (e.g., filtering, phase advancing) and airplane and system production tolerances will not result in an unsafe diminishing of the margin between stall warning and stall identification, or between stall identification and any hazardous airplane flight characteristic in any foreseeable operating condition. This verification may be provided by a combination of analysis, simulation, and flight test. The following operating conditions should not result in unwanted activation of the stall identification system or in aerodynamic stall prior to, or close to, activation of the stall warning system: dynamic and accelerated stall entries, the effects of atmospheric turbulence, any foreseeable type of wing contamination (e.g., ice, frost, insects, dirt, anti-icing fluids), or wing leading edge damage within prescribed maintenance limits. Operation in windshear environments where the airplane will be flown at, or very near, stall warning, should also be considered, although, depending on the severity of the windshear, it may be impossible to ensure that there is no possibility of stall indication system operation. For wing contamination, the applicant should substantiate the critical height and density of the contaminant. Carborundum sandpaper No. 40 (that is, 40-grit carborundum sandpaper) has been used in past certification programs to represent residual ice or frost contamination.
8.1.9.2 Stall characteristics testing should be performed with the following airplane and stall identification system production tolerances set to achieve the most adverse stall identification system activation condition for stall characteristics:
8.1.9.2.1 Airframe build tolerances—the impact of wing angle of incidence variation relative to stall identification system vane angle; and
8.1.9.2.2 Stall identification system tolerances (e.g., activation vane angles).
8.1.9.3 If the combined root-sum-square (square root of the sum of the squares of each tolerance) effect of the tolerances identified above is less than ±1 knot, stall speeds testing can be performed and the stall speeds determined with the tolerances at their nominal values. If the combined root-sum-square effect is ±1 knot or greater, stall speed testing should be performed with the tolerances at the values that would result in the highest stall speeds.
8.1.10 Reliability of Artificial Stall Warning and Stall Identification Systems.
Additional guidance material related to the testing and approval of artificial stall warning and stall identification systems is presented in paragraph 42.1 of this AC.
9-1 CHAPTER 9. FLIGHT: GROUND AND WATER HANDLING CHARACTERISTICS
9.1 Part 25 Regulations.
The applicable regulations are §§ 25.231, 25.233, 25.235, 25.237, and 25.239.
9.2 Longitudinal Stability and Control—§ 25.231.
9.2.1 Explanation.
Test program objectives would not be expected to demonstrate taxiing over rough surfaces at speeds high enough to approach structural design limits, nor is it expected that in the test program the airplane be landed harder or at higher sink rates than it will ever encounter in service. However, new or modified landing gear systems should be evaluated on rough surfaces that are representative of normal service, and landings should be conducted at various sink rates sufficient to identify any dangerous characteristics or tendencies. Variables to be considered are CG and taxi speed. The cockpit motion dynamics during ground handling should not impede control of the airplane, and pitching motion during bounce should not create static pitch control problems or pilot induced oscillation tendencies.
9.2.2 Procedures.
Ground handling tests at speeds normally expected in service should be conducted on smooth and rough surfaces that are likely to be encountered under normal operating conditions. Particular attention should be paid to the following:
9.2.2.1 Brakes.
The adequacy of the brakes when maneuvering on the ground and the tendency of the brakes to cause nosing-over should be investigated. Any bad tendency will normally be exaggerated when taxiing in a strong cross or tail wind.
9.2.2.2 Seaplanes and Amphibians.
The most adverse water conditions safe for taxiing, takeoff, and landing must be established per § 25.231(b). Procedure and limitations for using reverse thrust should be determined.
9.3 Directional Stability and Control—§ 25.233.
9.3.1 Explanation.
None.
9.3.2 Procedures.
Taxi, takeoff, and landing should be conducted in all configurations under normal operating conditions.
9-2
9.3.2.1 There may be no uncontrollable ground-looping tendency in 90° crosswinds, up to a wind velocity of 20 knots or 0.2 VSR0, whichever is greater (except that the wind velocity need not exceed 25 knots) at any speed at which the airplane may be expected to be operated on the ground.
This may be shown while establishing the 90° crosswind component required by § 25.237.
9.3.2.2 Landplanes must be satisfactorily controllable, without exceptional piloting skill or alertness in power-off landings at normal landing speed, without using brakes or engine power or thrust to maintain a straight path.
This may be shown during power-off landings made in conjunction with other tests.
9.3.2.3 The airplane must have adequate directional control during taxiing. This may be shown during taxiing prior to takeoffs made in conjunction with other tests.
9.4 Taxiing Condition—§ 25.235. [Reserved]
9.5 Wind Velocities—§ 25.237.
9.5.1 Explanation.
9.5.1.1 Landplanes.
9.5.1.1.1 There must be a 90° crosswind component established that is shown to be safe for takeoff and landing on dry runways.
9.5.1.1.2 The airplane must exhibit satisfactory controllability and handling characteristics in 90° crosswinds at any ground speed at which the airplane is expected to operate.
9.5.1.2 Seaplanes and Amphibians.
9.5.1.2.1 There must be a 90° crosswind component established that is shown to be safe for takeoff and landing in all water conditions that may reasonably be expected in normal operation.
9.5.1.2.2 There must be a wind velocity established for which taxiing is safe in any direction under all water conditions that may reasonably be expected in normal operation.
9.5.1.3 Crosswind Demonstration.
A 90° crosswind component at 10 meters (as required by § 25.21(f)) of at least 20 knots or 0.2 VSR0 (where VSR0 is for the maximum design landing weight), whichever is greater, except that it need not exceed 25 knots,
9-3 must be demonstrated during type certification tests. There are two results possible:
9.5.1.3.1 A crosswind component value may be established that meets the minimum requirements but is not considered to be a limiting value for airplane handling characteristics. This demonstrated value should be included as information in the AFM.
9.5.1.3.2 A crosswind component value may be established that is considered to be a maximum limiting value up to which it is safe to operate for takeoff and landing. This limiting value should be shown in the operating limitations section of the AFM.
9.5.2 Procedures.
9.5.2.1 Configuration.
These tests should be conducted in the following configurations:
9.5.2.1.1 At light weight and aft CG. (This is desirable; however, flexibility should be permitted.)
9.5.2.1.2 Normal takeoff and landing flap configurations using the recommended procedures.
9.5.2.1.3 Normal usage of thrust reversers. Particular attention should be paid to any degradation of rudder effectiveness due to thrust reverser airflow effects.
9.5.2.1.4 Yaw dampers/turn coordinator On, or Off, whichever is applicable.
9.5.2.2 Test Procedures.
Three takeoffs and three landings, with at least one landing to a full stop, should be conducted in a 90° crosswind component of at least 20 knots or
0.2 VSR0, whichever is greater, except that 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 should be satisfactorily controllable without requiring exceptional piloting skill or strength. If thrust reversers are installed, these landings should be conducted with the thrust reversers deployed as per normal procedures and additional landings should be conducted at the critical reverse thrust/power level to verify that there are no unsatisfactory handling characteristics.
9.5.2.3 Test data.
Crosswind data may be obtained from a calibrated flight test wind measurement station, from an airfield wind reporting device, or from any other method acceptable to the FAA.
9-4
9.5.2.3.1 A calibrated flight test wind measurement station located in the vicinity of the liftoff or touchdown point generally provides the most accurate data and is preferable.
9.5.2.3.2 An airport wind reporting device may also be acceptable provided the device has been calibrated and is located near the runway being used for testing.
9.5.2.3.3 Crosswind data taken directly from a commercially available inertial or differential GPS based reference system may not be accurate in sideslips and is not accurate on the ground. During landing, filtering may introduce lags making the data incorrect due to wind shear with altitude (i.e., a higher wind value at altitude is “remembered”). Hence this method is considered unsuitable for accurately determining the crosswind during takeoff and landing.
9.5.2.3.4 Other methods based on the computation of the actual crosswind encountered by the airplane based on on-board measurements are also acceptable. For example, the crosswind can be computed by resolving the difference between true airspeed (from an ADC) and an accurate ground speed measurement (e.g., derived from IRS groundspeed) into the alongrunway and across-runway heading taking into account the airplane heading, track angle and sideslip.
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
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 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.
10-1 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 is 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 by the use of 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.
10-2
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 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.
10.1.1.7 Section 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 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
10-3 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.
10.1.2.6 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 31-1 provides guidance on stick force per g (FS/g) characteristics that would be considered acceptable or unacceptable.
10.1.2.7 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:
10.1.2.7.1 For all weight/altitude combinations where buffet onset occurs at various load factors between approximately +1 g and +2 g, the longitudinal
10-4 control force (FS) characteristics of § 25.255(b)(1) and (2) apply 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 paragraphs 10.1.2.7.1 above and 10.1.2.7.2 below.
Figure 10-1. Maneuvering Characteristics at Speeds up to VMO/MMO SAT.
SAT. SAT.
SAT. * SAT. * UNSAT.
UNSAT.
+2.5+1.0
DETERRENT
LOAD FACTOR (G) BUFFET ONSET
ELEVATOR PILOT EFFORT, F PUSH 0 PULL s
10.1.2.7.2 Under the airplane weight/altitude combinations of paragraph 10.1.2.7.1 above, but at load factors beyond buffet onset, the following FS characteristics apply (see figure 10-1):
1. 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.
2. Any pitching tendency (uncommanded changes in load factor) should be mild and readily controllable.
3. 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.
4. The airplane’s pitch response to primary longitudinal control should be predictable to the pilot.
10-5
10.1.2.7.3 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.
10.2.1.5.2 Effect of any reasonably probable mistrim on upset and recovery.
10.2.1.5.3 Dynamic and static stability.
10-6
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:
𝑀𝑀𝑂 ≤ 𝑀𝐷𝐹 − ∆𝑀 − 0.005𝑀 − 0.01𝑀 Or 𝑀𝑀𝑂 ≤ 𝑀𝐷𝐹 − 0.01𝑀