AC 25-7D CHG 1 then rolls back and captures 15° bank in the original direction. This procedure should continue for a few cycles. The procedure is then repeated using 30° bank angles, and then repeated again using 45° bank angles. A variation of this is to capture wings-level from the initial bank condition.
5.1.4.4.5 Where suitable, combined conditions could be used as described in the task shown in paragraph 5.1.4.4.6 below, in which a target g and bank angle are tightly tracked until the target pitch attitude and heading are captured.
5.1.4.4.6 The following upset and/or collision avoidance maneuvers have been found to be effective in evaluating PIO susceptibility when the airplane is flying at high altitude under conditions of low g to buffet onset, typically
0.3 g. This emphasis on cruise susceptibility stems from operational experiences, but should not be interpreted as placing less emphasis on other flight phases.
1. Trim for level f light at long range cruise Mach number. Initiate a slight climb and slow the aircraft while leaving power/thrust set. Push the nose over and set up a descending turn with 30° to 40° of bank and approximately 10° nose below the horizon, or as appropriate, to accelerate to the initial trim speed. At the initial trim airspeed initiate a
1.5 g to 1.67 g (not to exceed deterrent buffet) pull up and establish a turn in the opposite direction to a heading which will intercept the initial course on which the airplane was trimmed. Establish a pitch attitude which will provide a stabilized climb back to the initial trim altitude. The pilot may use the throttles as desired during this maneuver and should pick a target g, bank angle, heading, and pitch attitude to be used prior to starting the maneuver. The target g and bank angle should be set and tightly tracked until the target pitch attitude and heading are obtained respectively. The stabilized steady heading climb should be tightly tracked for an adequate amount of time to allow the pilot to assess handling qualities, even through the initial trim altitude and course if required. The pilot should qualitatively evaluate the airplane during both the gross acquisition and fine tracking portions of this task while looking for any tendency towards PIO in accordance with the criteria in paragraph 5.1.4.6.
2. This maneuver should be repeated in the nose-down direction by accelerating to MMO from the trim condition 10° nose down and then recover as above.
3. Trim for level flight as above. Initiate a 1.5 g to 1.67 g (not to exceed deterrent buffet) pull-up and approximately a 30° bank turn. Once the target g is set, transition the aircraft to approximately a 0.5 g pushover and reverse the turn to establish an intercept heading to the initial course. Using power or thrust as required, set up a stabilized steady 5-8
AC 25-7D CHG 1 heading descent to intercept the initial course and altitude used for the trimmed condition. The pilot may continue the heading and descent through the initial conditions to allow more tracking time if needed. Attempt to precisely set and track bank angle, g, heading, and pitch attitude as appropriate. The pilot should qualitatively evaluate the airplane during both the gross acquisition and fine tracking portions of this task while looking for a PIO tendency in accordance with paragraph 5.1.4.6.
5.1.4.5 Fine Tracking Tasks.
5.1.4.5.1 These tasks may be used to assess the airplane’s PIO susceptibility when flying in turbulent atmospheric conditions. In this task, a tracking target is displayed which commands pitch and roll changes for the evaluation pilot to follow. Whatever visual cue is used (e.g., head up display (HUD), flight director, etc.), it should present the tracking task without filtering, smoothing, or bias. The pitch and roll commands should be combinations of steps and ramps. The sequence of pitch and roll commands should be designed to keep the airplane within ±1,000 feet of the test altitude and within ±10 knots of the test airspeed. The sequence should be long enough and complex enough that the pilot cannot learn to anticipate the commands. The unfamiliarity is intended to help keep the test pilot’s gain high and to preclude inadvertent pilot compensation while accomplishing the task. Such compensation, along with reduced gains, could mask any PIO tendencies.
5.1.4.5.2 Even though these fine tracking tasks will provide insight into PIO susceptibility of a conventional airplane when flying in turbulence, other considerations apply to augmented airplane types. For example, structural load alleviation systems that use the same flight control surface as the pilot will limit the pilot’s control authority in turbulent atmospheric conditions.
Under these circumstances of rate or position limiting, PIO tendencies will be more critical as previously discussed. Therefore, specific evaluations for turbulent atmospheric conditions with these systems operating are necessary for these airplane types.
5.1.4.5.3 For single axis tasks, it has been found that aural commands given in a timed sequence provide an adequate cue in the event it is not possible to modify the flight director to display the pitch commands.
5.1.4.5.4 Based on PIO events seen in service, high altitude tracking tasks (with up to approximately ±4° pitch excursions from trim occurring at varying intervals of approximately 2 to 5 seconds) have been effective in evaluating PIO susceptibility. These tasks have been used where the 5-9
AC 25-7D CHG 1 airplane is flying under conditions of low g margin to buffet onset. The time history in figure 5-1 below is a pictorial representation of a sample task in MIL-STD-1797A that has the desired attributes for high altitude PIO evaluations.
Figure 5-1. Sample Pitch Tracking Task
time (seconds) Δ pitch angle (degrees) -4 -3 -2 -1 0 1 2 3 4 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
5.1.4.6 PIO Assessment Criteria.
5.1.4.6.1 The evaluation of an airplane for PIO susceptibility will be conducted using the FAA handling qualities rating method (HQRM). (See appendix E of this AC for more information on the HQRM). Tasks should be designed to focus on any PIO tendencies that may exist. Table 5-1 below contains the descriptive material associated with PIO characteristics and its relationship to the PIO Rating Scale called out in the U.S. Military Standard.
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Table 5-1. PIO Rating Criteria and Comparison to MIL Standard
FAA HQ Rating PIO Characteristics Description
MIL-STD- 1797A PIO Rating Scale No tendency for pilot to induce undesirable motion. 1 SAT Undesirable motions (overshoots) tend to occur when pilot initiates abrupt maneuvers or attempts tight control. These motions can be prevented or eliminated by pilot technique. (No more than minimal pilot compensation is required.)
2 ADQ Undesirable motions (unpredictability or over control) easily induced when pilot initiates abrupt maneuvers or attempts tight control.
These motions can be prevented or eliminated but only at sacrifice to task performance or through considerable pilot attention and effort. (No more than extensive pilot compensation is required.)
3 CON Oscillations tend to develop when pilot initiates abrupt maneuvers or attempts tight control. Adequate performance is not attainable and pilot has to reduce gain to recover. (Pilot can recover by merely reducing gain.)
4 UNSAT Divergent oscillations tend to develop when pilot initiates abrupt maneuvers or attempts tight control. Pilot has to open control loop by releasing or freezing the controller.
5 Disturbance or normal pilot control may cause divergent oscillation. Pilot has to open control loop by releasing or freezing the controller.
6 SAT = Satisfactory CON = Controllable ADQ = Adequate UNSAT = Unsatisfactory or Failed
5.1.4.6.2 Table 5-1 above provides the FAA handling qualities (HQ) rating descriptions of airplane motions that may be seen during the conduct of specific PIO tasks or during tests throughout the entire certification flight test program. The italicized phrases highlight major differences between rating categories in the table.
5.1.4.6.3 The acceptable HQ ratings for PIO tendencies is shown in table E-2 of appendix E. As described in that appendix, the minimum HQ rating, and consequently the pass/fail criteria, varies with the flight envelope, atmospheric disturbance considered, and failure state. For example, table 5-2 below shows a handling qualities matrix for a tracking task with 5-11
the airplane at aft CG trimmed in flight conditions giving 1.3 g to buffet onset.
Table 5-2. Example of Acceptable HQ Rating for PIO Tendencies
Airspeed MLRC MLRC MLRC MLRC Load Factor Range
0.8 to 1.3 -1.0 to 2.5 0.8 to 1.3 -1.0 to 2.5 Buffet Level Onset Deterrent Onset Deterrent Turbulence Light Light Light Light Failure None None Improbable failure of SAS Improbable failure of SAS Flight Envelope NFE LFE NFE LFE Minimum Permitted HQ Rating
SAT ADQ ADQ CON SAT = Satisfactory ADQ = Adequate CON = Controllable NFE = Normal flight envelope LFE = Limit flight envelope SAS = Stability augmentation system MLRC = Long range cruise Mach number
5.1.5 Maneuvering Characteristics—§ 25.143(g).
5.1.5.1 General.
An acceptable means of compliance with the requirement that stick forces may not be excessive when maneuvering the airplane is to demonstrate that, in a turn for 0.5 g incremental normal acceleration (0.3 g above 20,000 ft) at speeds up to VFC/MFC, the average stick force gradient does not exceed 120 lbs per g.
5.1.5.2 Interpretive Material.
The objective of § 25.143(g) is to ensure that the limit strength of any critical component on the airplane would not be exceeded in maneuvering flight. In much of the structure, the load sustained in maneuvering flight can be assumed to be directly proportional to the load factor applied.
However, this may not be the case for some parts of the structure (e.g., the tail and rear fuselage). Nevertheless, it is accepted that the airplane load factor will be a sufficient guide to the possibility of exceeding limit 5-12
strength on any critical component if a structural investigation is undertaken whenever the design positive limit maneuvering load factor is closely approached. If flight testing indicates that the positive design limit maneuvering load factor could be exceeded in steady maneuvering flight with a 50-lb stick force, the airplane structure should be evaluated for the anticipated load at a 50-lb stick force. The airplane will be considered to have been overstressed if limit strength has been exceeded in any critical component. For the purposes of this evaluation, limit strength is defined as the lesser of either the limit design loads envelope increased by the available margins of safety, or the ultimate static test strength divided by
1.5.
5.1.5.3 Minimum Stick Force to Reach Limit Strength.
5.1.5.3.1 A stick force of at least 50 lbs to reach limit strength in steady maneuvers or wind-up turns is considered acceptable to demonstrate adequate minimum force at limit strength in the absence of deterrent buffeting. If heavy buffeting occurs before the limit strength condition is reached, a somewhat lower stick force at limit strength may be acceptable. The acceptability of a stick force of less than 50 lbs at the limit strength condition will depend upon the intensity of the buffet, the adequacy of the warning margin (i.e., the load factor increment between the heavy buffet and the limit strength condition), and the stick force characteristics. In determining the limit strength condition for each critical component, the contribution of buffet loads to the overall maneuvering loads should be taken into account.
5.1.5.3.2 This minimum stick force applies in the en route configuration with the airplane trimmed for straight flight, at all speeds above the minimum speed at which the limit strength condition can be achieved without stalling. No minimum stick force is specified for other configurations, but the requirements of § 25.143(g) are applicable in these conditions.
5.1.5.4 Stick Force Characteristics.
5.1.5.4.1 At all points within the buffet onset boundary determined in accordance with § 25.251(e), but not including speeds above VFC/MFC, the stick force should increase progressively with increasing load factor. Any reduction in stick force gradient with change of load factor should not be so large or abrupt as to significantly impair the ability of the pilot to maintain control over the load factor and pitch attitude of the airplane.
5.1.5.4.2 Beyond the buffet onset boundary, hazardous stick force characteristics should not be encountered within the permitted maneuvering envelope as limited by paragraph 5.1.5.4.3. It should be possible, by use of the primary longitudinal control alone, to rapidly pitch the airplane nose down so as to 5-13
regain the initial trimmed conditions. The stick force characteristics demonstrated should comply with the following:
1. For normal acceleration increments of up to 0.3 g beyond buffet onset, where these can be achieved, local reversal of the stick force gradient may be acceptable, provided that any tendency to pitch up is mild and easily controllable.
2. For normal acceleration increments of more than 0.3 g beyond buffet onset, where these can be achieved, more marked reversals of the stick force gradient may be acceptable. It should be possible to contain any pitch-up tendency of the airplane within the allowable maneuvering limits, without applying push forces to the control column and without making a large and rapid forward movement of the control column.
5.1.5.4.3 In flight tests to satisfy paragraphs 5.1.5.4.1 and 5.1.5.4.2 above, the load factor should be increased until:
1. The level of buffet becomes sufficient to provide a strong and effective deterrent to any further increase of the load factor;
2. Further increase of the load factor requires a stick force in excess of 150 lbs (or in excess of 100 lbs when beyond the buffet onset boundary) or is impossible because of the limitations of the control system; or
3. The positive limit maneuvering load factor established in compliance with § 25.337(b) is achieved.
5.1.5.5 Negative Load Factors.
It is not intended that a detailed flight test assessment of the maneuvering characteristics under negative load factors should necessarily be made throughout the specified range of conditions. An assessment of the characteristics in the normal flight envelope involving normal accelerations from 1 g to zero g will normally be sufficient. Stick forces should also be assessed during other required flight testing involving negative load factors. Where these assessments reveal stick force gradients that are unusually low, or that are subject to significant variation, a more detailed assessment, in the most critical of the specified conditions, will be required. This may be based on calculations, provided they are supported by adequate flight test or wind tunnel data.
5.1.6 Thrust or Power Setting for Maneuver Capability Demonstrations.
The effect of thrust or power on maneuver capability is normally a function of only the thrust-to-weight ratio. Therefore, for those configurations in which the WAT-limited thrust or power setting is prescribed, it is usually acceptable to use the thrust or power setting that is consistent with a WAT-limited climb gradient at the test conditions of weight, altitude, and temperature. However, if the maneuver margin to stall warning (or other characteristic that might interfere with normal maneuvering) is reduced with 5-14
AC 25-7D CHG 1
increasing thrust or power, the critical conditions of both thrust or power and thrust-to-weight ratio should be taken into account when demonstrating the required maneuvering capabilities.
5.2 Longitudinal Control—§ 25.145.
5.2.1 Explanation.
5.2.1.1 Section 25.145(a) requires that there be adequate longitudinal control to promptly pitch the airplane nose down from at or near the stall to return to the original trim speed. The intent is to ensure that there is sufficient pitch control for a prompt recovery if inadvertently slowed to the point of stall.
Although this requirement must be met with power off and at maximum continuous thrust or power, there is no intention to require stall demonstrations with thrust or power above that specified in § 25.201(a)(2). Instead of performing a full stall at maximum continuous power or thrust, compliance may be assessed by demonstrating sufficient static longitudinal stability and nose down control margin when the deceleration is ended at least one second past stall warning during a one knot per second deceleration. The static longitudinal stability during the maneuver and the nose down control power remaining at the end of the maneuver must be sufficient to assure compliance with the requirement.
5.2.1.2 Section 25.145(b) requires changes to be made in flap position, power or thrust, and speed without undue effort when re-trimming is impractical.
The purpose is to ensure that any of these changes are possible assuming that the pilot finds it necessary to devote at least one hand to the initiation of the desired operation without being overpowered by the primary airplane controls. The objective is to show that an excessive change in trim does not result from the application or removal of power or thrust or the extension or retraction of wing flaps. The presence of gated positions on the flap control does not affect the requirement to demonstrate full flap extensions and retractions without changing the trim control. Compliance with § 25.145(b) also requires that the relation of control force to speed be such that reasonable changes in speed may be made without encountering very high control forces.
5.2.1.3 Section 25.145(c) contains requirements associated primarily with attempting a go-around maneuver from the landing configuration.
Retraction of the high-lift devices from the landing configuration should not result in a loss of altitude if the power or thrust controls are moved to the go-around setting at the same time that flap/slat retraction is begun.
The design features involved with this requirement are the rate of flap/slat retraction, the presence of any flap gates, and the go-around power or thrust setting. The go-around power or thrust setting should be the same as is used to comply with the approach and landing climb performance 5-15
AC 25-7D CHG 1 requirements of §§ 25.121(d) and 25.119, and the controllability requirements of §§ 25.145(b)(3), 25.145(b)(4), 25.145(b)(5), 25.149(f), and 25.149(g). The controllability requirements may limit the go-around power or thrust setting.
5.2.1.4 Section 25.145(d) provides requirements for demonstrating compliance with § 25.145(c) when gates are installed on the flap selector. Section
25.145(d) also specifies gate design requirements. Flap gates, which prevent the pilot from moving the flap selector through the gated position without a separate and distinct movement of the selector, allow compliance with these requirements to be demonstrated in segments. High lift device retraction must be demonstrated beginning from the maximum landing position to the first gated position, between gated positions, and from the last gated position to the fully retracted position.
5.2.1.4.1 If gates are provided, § 25.145(d) requires the first gate from the maximum landing position to be to be located at a position corresponding to a go-around configuration. If there are multiple go-around configurations, the following criteria should be considered when selecting the location of the gate:
1. The expected relative frequency of use of the available go-around configurations.
2. The effects of selecting the incorrect high-lift device control position.
3. The potential for the pilot to select the incorrect control position, considering the likely situations for use of the different go-around positions.
4. The extent to which the gate(s) aid the pilot in quickly and accurately selecting the correct position of the high-lift devices.
5.2.1.4.2 Regardless of the location of any gates, initiating a go-around from any of the approved landing positions should not result in a loss of altitude.
Therefore, § 25.145(d) requires that compliance with § 25.145(c) be demonstrated for retraction of the high-lift devices from each approved landing position to the control position(s) associated with the high-lift device configuration(s) used to establish the go-around procedure(s) from that landing position. A separate demonstration of compliance with this requirement should only be necessary if there is a gate between an approved landing position and its associated go-around position(s). If there is more than one associated go-around position, conducting this test using the go-around configuration with the most retracted high-lift device position should suffice, unless there is a more critical case. If there are no gates between any of the landing flap positions and their associated go-around positions, the demonstrations discussed in paragraph 5.2.1.4 5-16
AC 25-7D CHG 1
above should be sufficient to show compliance with this provision of § 25.145(d).
5.2.2 Procedures.
The following test procedures outline an acceptable means for demonstrating compliance with § 25.145. These tests may be conducted at an optional altitude in accordance with § 25.21(c). Where applicable, the conditions should be maintained on the engines throughout the maneuver.
5.2.2.1 Longitudinal Control Recovery—§ 25.145(a).
5.2.2.1.1 Configuration.
Maximum weight, or a lighter weight if more critical.
Critical CG position.
Landing gear extended.
Wing flaps retracted and extended to the maximum landing position.
Engine power or thrust at idle and maximum continuous.
5.2.2.1.2 Test Procedure.
The airplane must be trimmed at the speed for each configuration as prescribed in § 25.103(b)(6). The airplane should then be decelerated at 1 knot per second with wings level. For tests at idle power or thrust, the applicant must demonstrate that the nose can be pitched down from any speed between the trim speed and the stall. Typically, the most critical point is at the stall when in stall buffet. The rate of speed increase during the recovery should be adequate to promptly return to the trim point. Data from the stall characteristics testing can be used to evaluate this capability at the stall. For tests at maximum continuous power or thrust, the maneuver need not be continued for more than one second beyond the onset of stall warning. However, the static longitudinal stability characteristics during the maneuver, and the nose down control power remaining at the end of the maneuver, must be sufficient to assure that a prompt recovery to the trim speed could be attained if the airplane is slowed to the point of stall.
5.2.2.2 Longitudinal Control, Flap Extension—§ 25.145(b)(1).
5.2.2.2.1 Configuration.
Maximum landing weight or a lighter weight if considered more critical.
Critical CG position.
Wing flaps retracted.
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AC 25-7D CHG 1
Landing gear extended.
Engine power or thrust at flight idle.
5.2.2.2.2 Test procedure.
The airplane must be trimmed at a speed of 1.3 VSR. The flaps must be extended to the maximum landing position as rapidly as possible while maintaining approximately 1.3 VSR for the flap position existing at each instant throughout the maneuver. The control forces must not exceed 50 lbs (the maximum force for short term application that can be applied readily by one hand) throughout the maneuver without changing the trim control.
5.2.2.3 Longitudinal Control, Flap Retraction—§ 25.145(b)(2) and (3).
5.2.2.3.1 Configuration.
Maximum landing weight or a lighter weight if considered more critical.
Critical CG position.
Wing flaps extended to the maximum landing position.
Landing gear extended.
Engine power or thrust at flight idle and the go-around power or thrust setting.
5.2.2.3.2 Test procedure.
With the airplane trimmed at 1.3 VSR, the flaps must be retracted to the full up position while maintaining approximately 1.3 VSR for the flap position existing at each instant throughout the maneuver. The longitudinal control force must not exceed 50 lbs throughout the maneuver without changing the trim control.
5.2.2.4 Longitudinal Control, Power or Thrust Application—§ 25.145(b)(4) and (5).
5.2.2.4.1 Configuration.
Maximum landing weight or a lighter weight if considered more critical.
Critical CG position.
Wing flaps retracted and extended to the maximum landing position.
Landing gear extended.
Engine power or thrust at flight idle.
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AC 25-7D CHG 1
5.2.2.4.2 Test procedure.
The airplane must be trimmed at a speed of 1.3 VSR. Quickly set goaround power or thrust while maintaining the speed of 1.3 VSR. The longitudinal control force must not exceed 50 lbs throughout the maneuver without changing the trim control.
5.2.2.5 Longitudinal Control, Airspeed Variation—§ 25.145(b)(6).
5.2.2.5.1 Configuration.
Maximum landing weight or a lighter weight if considered more critical.
Most forward CG position.
Wing flaps extended to the maximum landing position.
Landing gear extended.
Engine power or thrust at flight idle.
5.2.2.5.2 Test Procedure.
The airplane must be trimmed at a speed of 1.3 VSR. The speed should then be reduced to VSW and then increased to 1.6 VSR, or the maximum flap extended speed, VFE, whichever is lower. The longitudinal control force must not be greater than 50 lbs. Data from the static longitudinal stability tests in the landing configuration at forward CG, § 25.175(d), may be used to show compliance with this requirement.
5.2.2.6 Longitudinal Control, Flap Retraction and Power or Thrust Application—§ 25.145(c).
5.2.2.6.1 Configuration.
Critical combinations of maximum landing weights and altitudes.
Critical CG position.
Wing flaps extended to the maximum landing position and gated position, if applicable.
Landing gear extended.
Engine power or thrust for level flight at a speed of 1.08 VSR for propeller driven airplanes, or 1.13 VSR for turbojet powered airplanes.
5.2.2.6.2 Test procedure.
With the airplane stable in level flight at a speed of 1.08 VSR for propeller driven airplanes, or 1.13 VSR for turbojet powered airplanes, retract the flaps to the full up position, or the next gated position, while simultaneously setting go-around power or thrust. Use the same power or thrust as is used to comply with the performance requirement of 5-19