(a) 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.
(b) The classic PIO is considered to occur when an airplane’s response is approximately 180 degrees out of phase with the pilot’s control input. However, PIO events with 180 degrees 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
(c) Some of the PIO tendency characteristics described in paragraph (b) 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 c.g. 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.
(d) 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.
(e) 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.
(2) Special Considerations.
(a) The certification team should understand the flight control system and airplane design.
(b) 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.
(c) The applicant should explain what has been done during the development flight test experience and any design changes that were required for PIO problems.
(d) The certification flight test program should be tailored to the specific airplane design and to evaluate the airplane in conditions that were found to be critical during its development program and PIO analytical assessment.
(e) 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.
(f) 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.
(3) Procedures (Flight Test).
(a) 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 20d(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.
(b) 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
91 and the pilot makes a rapid alignment correction. Tests should be conducted at or near the critical altitude/weight/c.g. combinations.
(c) 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 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.
(4) Capture Tasks.
(a) 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.
(b) For pitch captures, the airplane is trimmed for a specified flight condition.
The pilot aggressively captures 5 degrees pitch attitude (or 10 degrees if the airplane is already trimmed above 5 degrees). The pilot then makes a series of aggressive pitch captures of 5 degree increments in both directions, and then continues this procedure with ten degree 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.
(c) 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 degrees or less).
(d) Bank angle captures are also commonly done using bank-to-bank rolls.
Starting from a 15 degree bank angle, the pilot aggressively rolls and captures the opposite 15 degree bank angle (total bank angle change of 30 degrees). The pilot then rolls back and captures 15 degrees bank in the original direction. This procedure should continue for a few
92 cycles. The procedure is then repeated using 30 degree bank angles, and then repeated again using 45 degree bank angles. A variation of this is to capture wings-level from the initial bank condition.
(e) Where suitable, combined conditions could be used as described in the task shown in paragraph (f), below, in which a target g and bank angle are tightly tracked until the target pitch attitude and heading are captured.
(f) 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 0.3g. 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 degrees of bank and approximately 10 degrees 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 20d(6).
2 This maneuver should be repeated in the nose-down direction by accelerating to MMO from the trim condition 10 degrees 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 degree 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 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 20d(6).
(5) Fine Tracking Tasks.
(a) 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.
(b) 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.
(c) 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.
(d) 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 following time history is a pictorial representation of a sample task in MIL-STD-1797A that has the desired attributes for high altitude PIO evaluations:
Figure 20-1. Sample Pitch Tracking Task
FIGURE 20-1. 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
(6) PIO Assessment Criteria.
(a) The evaluation of an airplane for PIO susceptibility will be conducted using the FAA handling qualities rating method (HQRM) (See Appendix 5 for more information on the HQRM.). Tasks should be designed to focus on any PIO tendencies that may exist. Figure 20-2 contains the descriptive material associated with PIO characteristics and its relationship to the PIO Rating Scale called out in the U.S. Military Standard.
(b) Figure 20-2 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.
95 Figure 20-2. PIO Rating Criteria and Comparison To MIL Standard
FAA HQ
RATING PIO CHARACTERISTICS DESCRIPTION MIL 1797A STD.
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.
SAT = Satisfactory CON = Controllable ADQ = Adequate UNSAT = Unsatisfactory or Failed
(c) The acceptable HQ ratings for PIO tendencies is shown in Figure 9 of Appendix 5. 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, Figure 20-3 below shows a handling qualities matrix for a tracking task with the airplane at aft c.g. trimmed in flight conditions giving 1.3 g to buffet onset.
Figure 20-3. Example of Acceptable HQ Rating For PIO Tendencies
Airplane at aft c.g. trimmed in conditions giving 1.3 g to buffet onset
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
e. Maneuvering Characteristics - § 25.143(g).
(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 0.5g incremental normal acceleration (0.3g above 20,000 feet) at speeds up to VFC/MFC, the average stick force gradient does not exceed 120 pounds per g.
(2) Interpretive Material.
(a) 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 strength on any critical component if a
97 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 pound stick force, the airplane structure should be evaluated for the anticipated load at a 50 pound 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.
(b) Minimum Stick Force to Reach Limit Strength.
1 A stick force of at least 50 pounds 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 pounds 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.
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.
(c) Stick Force Characteristics.
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.
2 Beyond the buffet onset boundary, hazardous stick force characteristics should not be encountered within the permitted maneuvering envelope as limited by paragraph 20e(2)(c)3. It should be possible, by use of the primary longitudinal control alone, to rapidly pitch the airplane nose down so as to regain the initial trimmed conditions. The stick force characteristics demonstrated should comply with the following:
(aa) For normal acceleration increments of up to 0.3g 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.
(bb) For normal acceleration increments of more than 0.3g beyond buffet onset, where these can be achieved, more marked reversals of the stick force gradient may be
98 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.
3 In flight tests to satisfy paragraphs 20e(2)(c)(1) and (2), the load factor should be increased until either:
(aa) The level of buffet becomes sufficient to provide a strong and effective deterrent to any further increase of the load factor; or
(bb) Further increase of the load factor requires a stick force in excess of 150 pounds (or in excess of 100 pounds when beyond the buffet onset boundary) or is impossible because of the limitations of the control system; or
(cc) The positive limit maneuvering load factor established in compliance with § 25.337(b) is achieved.
(d) 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 1g 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.
f. 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 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.
21. Longitudinal Control - § 25.145.
a. Explanation.
(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.
(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.
(3) Section 25.145(c) contains requirements associated primarily with attempting a goaround 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 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.
(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.
(a) 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.
(b) 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 21a(4) above should be sufficient to show compliance with this provision of § 25.145(d).
b. 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.
(l) Longitudinal control recovery, § 25.145(a).
(a) Configuration:
1 Maximum weight, or a lighter weight if more critical.
2 Critical c.g. position.
3 Landing gear extended.
4 Wing flaps retracted and extended to the maximum landing position.