132 airplane, or artificially by a system designed for this purpose. If artificial stall warning is provided for any airplane configuration, it must be provided for all configurations (§ 25.207(b)).
(d) An artificial stall warning indication that is a solely visual device which requires attention in the cockpit, inhibits cockpit conversation or, in the event of malfunction, causes distraction that would interfere with safe operation of the airplane, is not acceptable.
(e) For airplanes that use artificial stall warning systems, paragraph 228 of this AC presents guidance material for demonstrating compliance with the regulatory requirements of part 25.
(f) If the stall warning required by § 25.207 is provided by an artificial stall warning system (e.g., a stick shaker), the effect of production tolerances on the stall warning system should be considered when evaluating the stall warning margin required by § 25.207(c) through (f) and the maneuver capabilities required by § 25.143(h).
1 The stall warning margin required by § 25.207(c) through (f) should be available with the stall warning system set to the most critical setting expected in production.
Unless another setting would provide a lesser margin, the stall warning system should be operating at its high angle-of-attack limit. For airplanes where VSR is set by a device that abruptly pushes the nose down at a selected angle-of-attack (e.g., a stick pusher), the stall warning margin may be evaluated with both the stall warning and stall identification (e.g., stick pusher) systems at their nominal angle-of-attack settings unless a lesser margin can result from the various system tolerances.
2 The maneuver capabilities required by § 25.143(h) should be available assuming the stall warning system is operating on its nominal setting. When the stall warning system is operating at its low angle-of-attack limit, the maneuver capabilities should not be reduced by more than 2 degrees of bank angle from those specified in § 25.143(h). A flight test, an acceptable analysis, or simulation can be used to make this assessment.
3 The stall warning margin and maneuver capabilities may be demonstrated by flight testing at the most critical settings specified above for the stall warning and, if so equipped, stall identification systems. Alternatively, compliance may be shown by applying adjustments to flight test data obtained at a different system setting if an acceptable method is used that takes into account all of the relevant variables.
(3) Procedures . Stall warning tests are normally conducted in conjunction with the stall testing required by § 25.103 (stall speeds), § 25.201 (stall demonstration), and § 25.203 (stall characteristics), including consideration of the prescribed bank angles, power settings, and center-of-gravity position. The pilot technique in stalling the airplane should be consistent between the onset of stall warning and the point at which the stall is identified. That is, there should not be any deliberate attempt to reduce the load factor, change the deceleration, or use any other means to increase the stall warning margin. In addition, if the stall warning margin may be affected by a system (e.g., a stall warning or stick pusher system that modifies the stall warning or stall identification speed as a function of thrust, bank angle, angle-of-attack rate,
133 etc.), compliance with § 25.207(c) should be demonstrated at the most critical conditions in terms of stall warning margin. However, for this case, bank angles greater than 40 degrees and power or thrust exceeding maximum continuous power or thrust need not be demonstrated. If the effect of the stall identification or stall warning system compensation is to increase the stall warning margin relative to the nominal values demonstrated during the testing required by §§ 25.103, 25.201, and 25.203, these additional stall warning margin demonstrations need not be done.
(4) Data Acquisition and Reduction . The stall warning speed and type and quality of warning should be noted. To determine if the required margin exists, compare the speed at which acceptable stall warning begins with (1) the stall identification speed, and (2) VSR (for the conditions under which VSR is defined). The stall warning margin comparisons should be made at a constant 1-g load factor when showing compliance with § 25.207(d).
g. Accelerated Stall Warning .
(1) Explanation. Section 25.207(f) requires that, in slow-down turns with at least a
1.5g load factor normal to the flight path and an airspeed deceleration rate greater than 2 knots per second, sufficient stall warning is provided to prevent stalling when the pilot takes recovery action not less than one second after recognition of stall warning. The purpose of the requirement is to ensure that adequate stall warning exists to prevent an inadvertent stall under the most demanding conditions that are likely to occur in normal flight. The elevated load factor will emphasize any adverse stall characteristics, such as wing drop or asymmetric wing flow breakdown, while also investigating Mach and potential aeroelastic effects on available lift.
(2) Procedures.
(a) Trim at 1.3 V SR. Once trimmed, accelerate to a speed that will allow enough time to set up and complete the maneuver at the specified load factor and airspeed deceleration rate. Set power or thrust appropriate to the power for level flight at 1.3 VSR and do not adjust it during the maneuver. In a level flight maneuver, 1.5g equates to a bank angle of 48 degrees. To prevent an excessive deceleration rate (e.g., greater than 3 knots per second), a descent may be used. Conversely, if the deceleration rate is too low, the maneuver should be conducted in a climbing turn.
(b) After the onset of stall warning, continue the maneuver without releasing stick force for one second before attempting recovery. Normal low speed recovery techniques should be used. If any of the indications of a stall prescribed in § 25.201(d) (see paragraph 29c(1) of this AC) occur during the accelerated stall warning demonstration, compliance with § 25.207(f) will not have been demonstrated.
h. Maneuver Margins . See paragraph 20 of this AC for guidance material associated with demonstrating compliance to the maneuvering capability requirements of § 25.143(h).
i. Additional Considerations for Airplanes Equipped with Stall Identification Systems . A stall identification system is any system that is used to show compliance with § 25.201(d), which
134 requires the airplane to give the pilot a clear and distinctive indication of stall. The stall identification system consists of everything from the sensing devices that supply inputs to the system to the activation of the system response that provides stall identification to the flightcrew.
Section 25.1309(a) requires that such a system, when it is needed to show compliance with the stall-related requirements, must be designed to perform its intended function under any foreseeable operating condition.
(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), 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.
(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:
(a) Airframe build tolerances – the impact of wing angle of incidence variation relative to stall identification system vane angle; and
(b) Stall identification system tolerances (e.g., activation vane angles).
(3) If the combined root-sum-square (squa re 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 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.
j. 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 228 of this AC.
Section 7. Ground and Water Handling Characteristics
30. General.
a. Part 25 Regulations . The applicable regulations are: §§ 25.231, 25.233, 25.235, 25.237, and 25.239 of the CFR.
b. Longitudinal Stability and Control - § 25.231 .
(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 center of gravity 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.
(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:
(a) 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.
(b) Seaplanes and Amphibians. The most adverse water conditions safe for taxiing, takeoff, and landing must be established (§ 25.231(b)). Procedure and limitations for using reverse thrust should be determined.
c. Directional Stability and Control - § 25.233 .
(1) Explanation . None.
(2) Procedures . Taxi, takeoff, and landing should be conducted in all configurations under normal operating conditions.
(a) There may be no uncontrollable ground-looping tendency in 90-degree 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-degree crosswind component required by § 25.237.
(b) Landplanes must be satisfactorily controllable, without exceptional piloting skill or alertness in power-off landings at normal landing speed, without using brakes or engine
136 power to maintain a straight path. This may be shown during power-off landings made in conjunction with other tests.
(c) The airplane must have adequate directional control during taxiing. This may be shown during taxiing prior to takeoffs made in conjunction with other tests.
d. Taxiing Condition - § 25.235 . [Reserved]
e. Wind Velocities -§ 25.237 .
(1) Explanation .
(a) Landplanes.
1 There must be a 90-degree crosswind component established that is shown to be safe for takeoff and landing on dry runways.
2 The airplane must exhibit satisfactory controllability and handling characteristics in 90-degree crosswinds at any ground speed at which the airplane is expected to operate.
(b) Seaplanes and Amphibians.
1 There must be a 90-degree 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.
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.
(c) Crosswind Demonstration. A 90-degree 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, must be demonstrated during type certification tests. There are two results possible:
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.
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.
(2) Procedures .
(a) Configuration. These tests should be conducted in the following configurations:
1 At light weight and aft c.g. (This is desirable; however, flexibility should be permitted.)
2 Normal takeoff and landing flap configurations using the recommended procedures.
3 Normal usage of thrust reversers. Particular attention should be paid to any degradation of rudder effectiveness due to thrust reverser airflow effects.
4 Yaw dampers/turn coordinator On, or Off, whichever is applicable.
(b) Test Procedure and Data. Three takeoffs and three 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 0.2 VSR0, 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 should be satisfactorily controllable without requiring exceptional piloting skill or strength. Wind data from an inertial navigation system (INS), tower, or portable ground recording station should be corrected to a 90-degree crosswind component and to a height of 10 meters.
f. Spray Characteristics, Control, and Stability on Water - § 25.239 .
(1) Explanation . These characteristics should be investigated at the most adverse weight/c.g. combinations.
(2) Procedures .
(a) 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.
(b) 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.
(c) 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
138 service. If reverse thrust is to be used, it too should be evaluated in terms of ease of accomplishment and crew coordination.
(d) 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.
(e) Engine failure of the critical engine at any time during the takeoff run should be evaluated. No dangerous porpoising, swerving, or waterlooping should result.
(f) There should be no undue tendency to porpoise and no extraordinary skill or alertness should be required to control porpoising.
(g) Spray impingement on the airframe (control surfaces, etc.) should be evaluated to assure the resulting loads are within acceptable limits.
(h) 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:
(a) May cause structural damage or, if sustained over an extended period of time, could lead to structural fatigue;
(b) May cause pilot fatigue or annoyance that interferes with operation of the airplane or management of the airplane systems; or,
(c) 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 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(d).
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 VMO/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 positive slope of the stick force versus g (F S/g) relationship is not necessarily required at speeds and g combinations beyond VFC/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 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 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. 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.
(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 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 0.3g 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, MLRC) 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 VMO/MMO using maximum continuous thrust. Descending flight is acceptable if needed to achieve VMO/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 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.
(a) 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. In this regard, figures 31-1 and 31-2 illustrate general airplane responses and do not necessarily indicate required quantitative flight test results.
1 Airplanes should be evaluated at the most aft c.g. in accordance with the following criteria:
(aa) For all weight/altitude combinations where buffet onset occurs at various load factors between approximately +1g and +2g, the longitudinal control force (FS) 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).
141 (bb) Under the airplane weight/altitude/speed combinations of (aa), above, but at load factors beyond buffet onset, the following FS 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.
( i i ) Any pitching tendency (uncommanded changes in load factor) should be mild and readily controllable.
( i i i ) Sufficient control should be available to the pilot, through unreversed use of only the primary longitudinal control, to affect a prompt recovery to +1g flight from the load factors described herein.
( i v ) The airplane’s pitch response to primary longitudinal control should be predictable to the pilot.
2 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 2g at Mach numbers below MMO 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 VEc or Mrc or Buffet Onset at 1G SAT.
ELEVATOR PILOT EFFORT, F SAI* DETERRENT SAT.
UNSAT SAT* PUSH O PULL +10 LOAD FACTOR (G) UNS +2.5 * These characteristics are satisfactory only in accordance with paragraphs 31b(5)(a)(1) (aa) and (bb).
Figure 31-2. Maneuvering Characteristics Speeds Between figure 31-1 and VDF Or MDF ELEVATOR PILOT EFFORT, F S SAT* DETERRENT SAT.
UNSAT 0 PULL +1.0 LOAD FACTOR (G) +2.5 PUSH * These characteristics are satisfactory only in accordance with paragraphs 31b(5)(a)(1)(aa) and (bb).
32. High Speed Characteristics - § 25.253.
a. Explanation .
(1) The maximum flight demonstrated dive speed, V DF/MDF, is used when establishing VMO/MMO and the associated speed margins under the provisions of § 25.1505. Both VMO 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. 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.
(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:
(a) Effectiveness of longitudinal control at V MO/MMO and up to VDF/MDF.
(b) Effect of any reasonably probable mistrim on upset and recovery.
(c) Dynamic and static stability.
(d) The speed increase that results from likely passenger movement when trimmed at any cruise speed to VMO/MMO.
(e) Trim changes resulting from compressibility effects.
(f) Characteristics exhibited during recovery from inadvertent speed increase.
(g) Upsets due to vertical and horizontal gusts (turbulence).
(h) Speed increases due to horizontal gusts and temperature inversions.