5-2 number effect depends on the design characteristics of the particular wing.
For wing designs with a large Mach number effect, the maximum bank angle that can be achieved while retaining an acceptable stall margin can be significantly reduced. Because the effect of Mach number can be significant, and because it can also vary greatly for different wing designs, the multiplying factors applied to VSR may be insufficient to ensure that adequate maneuvering capability exists at the minimum operating speeds.
To address this issue, § 25.143(h) was added by amendment 25-108 to require a minimum bank angle capability in a coordinated turn without encountering stall warning or any other characteristic (including the envelope protection features of fly-by-wire flight control systems or automatic power or thrust increases) that might interfere with normal maneuvering. The maneuvering requirements consist of the minimum bank angle capability the FAA deems adequate for the specified regimes of flight combined with additional bank angle capability to provide a safety margin for various operational factors. These operational factors include both potential environmental conditions (e.g., turbulence, wind gusts) and an allowance for piloting imprecision (e.g., inadvertent overshoots). The FAA considers the automatic application of power or thrust by an envelope protection feature to be a feature that might interfere with normal maneuvering because it will result in a speed increase and flight path deviation, as well as potentially increasing crew workload due to the unexpected power or thrust increase.
5.1.2 General Test Requirements.
5.1.2.1 Compliance with § 25.143 (a) through (g) is primarily a qualitative determination by the pilot during the course of the flight test program. The control forces required and airplane response should be evaluated during changes from one flight condition to another and during maneuvering flight. The forces required should be appropriate to the flight condition being evaluated. For example, during an approach for landing, the forces should be light and the airplane responsive in order that adjustments in the flight path can be accomplished with a minimum of workload. In cruise flight, forces and airplane response should be such that inadvertent control input does not result in exceeding limits or in undesirable maneuvers.
Longitudinal control forces should be evaluated during accelerated flight to ensure a positive stick force with increasing normal acceleration. Forces should be heavy enough at the limit load factor to prevent inadvertent excursions beyond the design limit. Sudden engine failures should be investigated during any flight condition or in any configuration considered critical, if not covered by another section of part 25. Control forces considered excessive should be measured to verify compliance with the maximum control force limits specified in § 25.143(d). Allowance should be made for delays in the initiation of recovery action appropriate to the situation.
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5.1.2.2 Since § 25.143(h) involves a target speed, bank angle, and maximum value of thrust/power setting, not all flight test conditions to demonstrate compliance will necessarily result in a constant-altitude, thrust-limited turn. In cases with positive excess power or thrust, a climbing condition at the target bank and speed is acceptable. Alternately, if desired, the power or thrust may be reduced to less than the maximum allowed, so that compliance is shown with a completely stabilized, constant-altitude turn.
With the airplane stabilized in a coordinated turn, holding power or thrust and speed, increase bank angle at constant airspeed until compliance is shown. For cases with negative excess power or thrust (e.g., the landing configuration case), a constant-altitude slow-down maneuver at the target bank angle has been shown to be a suitable technique. With the airplane descending at VREF in wings-level flight on a 3° glide path, trim and throttle position are noted. The airplane is then accelerated to VREF + 10 to 20 knots in level flight. The original trim and throttle conditions are reset as the airplane is rolled into a constant-altitude slow-down turn at the target bank angle. Throttles can be manipulated between idle and the marked position to vary slow-down rate as desired. Compliance is shown when the airplane decelerates through VREF in the turn without encountering a stall warning or other characteristic that might interfere with normal maneuvering.
5.1.2.3 If stall warning is provided by an artificial stall warning system, the effect of production tolerances on the stall warning system should be considered when evaluating compliance with the maneuvering capability requirements of § 25.143(h). See paragraph 8.1.6.2.6 of this AC for more information.
5.1.3 Controllability Following Engine Failure.
Section 25.143(b)(1) requires the airplane to be controllable following the sudden failure of the critical engine. To show compliance with this requirement, the demonstrations described in paragraphs 5.1.3.1 and 5.1.3.2 below, should be made with engine failure (simulated by fuel cuts) occurring during straight, wings level flight. To allow for likely in-service delays in initiating recovery action, no action should be taken to recover control for two seconds following pilot recognition of engine failure. The recovery action should not necessitate movement of the engine, propeller, or trim controls, and should not result in excessive control forces. Additionally, the airplane will be considered to have reached an unacceptable attitude if the bank angle exceeds 45° during the recovery. These tests may be conducted using throttle slams to idle, with actual fuel cuts repeated only for those tests found to be critical.
5.1.3.1 At each takeoff flap setting at the initial all-engine climb speed (e.g., V2 + 10 knots) with:
5.1.3.1.1 All engines operating at maximum takeoff power or thrust prior to failure of the critical engine;
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5.1.3.1.2 All propeller controls (if applicable) in the takeoff position;
5.1.3.1.3 The landing gear retracted; and
5.1.3.1.4 The airplane trimmed at the prescribed initial flight condition.
5.1.3.2 With the wing flaps retracted at a speed of 1.23 VSR with:
5.1.3.2.1 All engines operating at maximum continuous power or thrust prior to failure of the critical engine;
5.1.3.2.2 All propeller controls in the en route position;
5.1.3.2.3 The landing gear retracted; and
5.1.3.2.4 The airplane trimmed at the prescribed initial flight condition.
5.1.4 Pilot Induced Oscillations (PIO).
5.1.4.1 Explanation.
5.1.4.1.1 Section 25.143(a) and (b) require that the airplane be safely controllable and maneuverable without exceptional piloting skill and without danger of exceeding the airplane limiting load factor under any probable operating conditions. Service history events have indicated that modern transport category airplanes can be susceptible to airplane-pilot coupling under certain operating conditions and would not meet the intent of this requirement.
5.1.4.1.2 The classic PIO is considered to occur when an airplane’s response is approximately 180° out of phase with the pilot’s control input. However, PIO events with 180° phase relationships are not the only conditions in which the airplane may exhibit closed-loop (pilot-in-the-loop) characteristics that are unacceptable for operation within the normal, operational, or limit flight envelopes. Others include unpredictability of the airplane’s response to the pilot’s control input. This may be due to nonlinearities in the control system, actuator rate or position limiting not sensed by the pilot through the flight controls, or changing pitch response at high altitude as the airplane maneuvers into and out of Mach buffet.
Artificial trim and feel systems which produce controllers with too small a displacement and light force gradients may also lead to severe over control. This is especially true in a dynamic environment of high altitude turbulence or upsets in which the autopilot disconnects. This places the airplane in the hands of the unsuspecting pilot in conditions of only a small g or airspeed margin to buffet onset and with very low aerodynamic damping. These characteristics, while not a classic 180° out of phase PIO per se, may be hazardous and should be considered under the more general description of airplane-pilot coupling tendencies.
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5.1.4.1.3 Some of the PIO tendency characteristics described in paragraph 5.1.4.1.2 above are attributes of transport airplanes (e.g., low frequency short period, large response lags) that are recognized by part 25. Limits are placed on some of these individual attributes by part 25 (e.g., stick force per g, heavily damped short period) to assure satisfactory open-loop characteristics. However, service reports from recent years have indicated that certain operating envelope conditions, combined with triggering events, can result in airplane-pilot coupling incidents. Some of the conditions that have led to these PIOs include fuel management systems that permit extended operations with a CG at or near the aft limit, operating at weight/speed/altitude conditions that result in reduced margins to buffet onset combined with tracking tasks such as not exceeding speed limitations and severe buffet due to load factor following an upset, and control surface rate or position limiting.
5.1.4.1.4 This service experience has shown that compliance with only the quantitative, open-loop (pilot-out-of-the loop) requirements does not guarantee that the required levels of flying qualities are achieved.
Therefore, in order to ensure that the airplane has achieved the flying qualities required by § 25.143(a) and (b), the airplane should be evaluated by test pilots conducting high-gain (wide-bandwidth), closed-loop tasks to determine that the potential of encountering adverse PIO tendencies is minimal.
5.1.4.1.5 For the most part, these tasks should be performed in actual flight.
However, for conditions that are considered too dangerous to attempt in actual flight (i.e., certain flight conditions outside of the operational flight envelope, flight in severe atmospheric disturbances, flight with certain failure states, etc.), the closed loop evaluation tasks may be performed using a motion base high fidelity simulator if it can be validated for the flight conditions of interest.
5.1.4.2 Special Considerations.
5.1.4.2.1 The certification team should understand the flight control system and airplane design.
5.1.4.2.2 The applicant should explain why the design is not conducive to a PIO problem and how this is to be shown in both developmental and certification flight tests.
5.1.4.2.3 The applicant should explain what has been done during the development flight test experience and any design changes that were required for PIO problems.
5.1.4.2.4 The certification flight test program should be tailored to the specific airplane design and to evaluate the airplane in conditions that were found
5-6 to be critical during its development program and PIO analytical assessment.
5.1.4.2.5 The FAA flight test pilots should also continuously evaluate the airplane for PIO tendencies during the certification program in both the airplane and simulator. This evaluation should include both normal and malfunction states; all certification flight test points; transitions between and recoveries from these flight test points; and normal, crosswind, and offset landing task evaluations.
5.1.4.2.6 Since the evaluation of flying qualities under § 25.143(a) and (b) is basically qualitative, especially evaluations of PIO susceptibility, the high-gain tasks discussed herein should be accomplished by at least three test pilots. Use of other pilots can provide additional insights into the airplane handling qualities, but for the purpose of demonstrating compliance with this requirement the evaluation pilots should be trained test pilots.
5.1.4.3 Procedures: Flight Test.
5.1.4.3.1 Evaluation of the actual task performance achieved, e.g., flight technical error, is not recommended as a measure of proof of compliance. Only the pilot’s rating of the PIO characteristics is needed as described in paragraph 5.1.4.6. The tasks are used only to increase the pilot’s gain, which is a prerequisite for exposing PIO tendencies. Although task performance is not used as proof of compliance, task performance should be recorded and analyzed to insure that all pilots seem to be attempting to achieve the same level of performance.
5.1.4.3.2 Tasks for a specific certification project should be based on operational situations, flight testing maneuvers, or service difficulties that have produced PIO events. Task requirements for a specific project will be dictated by the particular airplane and its specific areas of interest as determined by the tailored flight test program mentioned above. Some of these include high altitude upset maneuvers, encounters with turbulence at high altitude in which the autopilot disconnects, crosswind/crossed control landings with and without one engine inoperative, and offset landings to simulate the operational case in which the airplane breaks out of instrument meteorological conditions (IMC) offset from the glideslope and/or localizer beam and the pilot makes a rapid alignment correction.
Tests should be conducted at or near the critical altitude/weight/CG combinations.
5.1.4.3.3 Tasks described here may be useful in any given evaluation and have proven to be operationally significant in the past. It is not intended that these are the only tasks that may be used or may be required depending on the scope and focus of the individual evaluation being conducted. Other
5-7 tasks may be developed and used as appropriate. For example, some manufacturers have used formation tracking tasks successfully in the investigation of these tendencies. For all selected tasks, a build-up approach should be used and all end points should be approached with caution. Capture tasks and fine tracking tasks share many common characteristics but serve to highlight different aspects of any PIO problem areas that may exist. In some cases, depending on individual airplane characteristics, it may be prudent to look at capture tasks first and then proceed to fine tracking tasks or combined gross acquisition (capture) and fine tracking tasks as appropriate.
5.1.4.4 Capture Tasks.
5.1.4.4.1 Capture tasks are intended to evaluate handling qualities for gross acquisition as opposed to continuous tracking. A wide variety of captures can be done provided the necessary cues are available to the pilot. Pitch attitude, bank angle, heading, flight path angle, angle-of-attack, and g captures can be done to evaluate different aspects of the airplane response.
These capture tasks can give the pilot a general impression of the handling qualities of the airplane, but because they do not involve closed-loop fine tracking, they do not expose all of the problems that may arise in fine tracking tasks. Capture tasks should not be used as the only evaluation tasks.
5.1.4.4.2 For pitch captures, the airplane is trimmed for a specified flight condition.
The pilot aggressively captures 5° pitch attitude (or 10° if the airplane is already trimmed above 5°). The pilot then makes a series of aggressive pitch captures of 5° increments in both directions, and then continues this procedure with 10° increments in both directions. An airplane with more capability can continue the procedure with larger pitch excursions. If possible, the initial conditions for each maneuver should be such that the airplane will remain within ±1,000 feet and ±10 knots of the specified flight condition during the maneuver; however, large angle captures at high-speed conditions will inevitably produce larger speed and altitude changes. If the airplane should get too far from the specified condition during a task, it should be re-trimmed for the specified condition before starting the next maneuver.
5.1.4.4.3 The other kinds of captures are usually done in a similar manner, with some minor differences. G captures can be done from a constant-g turn or pull ups and pushovers using ±0.2 g and ±0.5 g. Heading captures can be used to evaluate the yaw controller alone (usually small heading changes of 5° or less).
5.1.4.4.4 Bank angle captures are also commonly done using bank-to-bank rolls.
Starting from a 15° bank angle, the pilot aggressively rolls and captures the opposite 15° bank angle (total bank angle change of 30°). The pilot
5-8 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 flight 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
5-9 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 so as 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 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
5-10 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.
5-11 Table 5-1. PIO Rating Criteria and Comparison to MIL Standard FAA HQ Rating PIO Characteristics Description
MIL-STD- 1797A PIO Rating Scale SAT No tendency for pilot to induce undesirable motion. 1 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
5-12 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
5-13 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 impair significantly 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