AC 25.1329-1C CHG 2 44 approach, resuming the Approach mode, and re-acquiring the defined vertical path may be an acceptable response, if the activation is sufficiently brief and not accompanied by large speed or path deviations. This response is considered consistent with criteria for Category III automatic landing systems in AC 20-191, paragraph 5.3.1, Automatic Flight Control Systems, which states that it should not be possible to change the flight path of the airplane with the autopilot(s) engaged, except by initiating an automatic go-around.
(4) Windshear recovery guidance. The interaction between low -speed protection and windshear recovery guidance is a special case. Windshear recovery guidance that meets the criteria found in AC 25-12 and AC 120-41 provides the necessary low -speed protection when it is activated and is considered acceptable for compliance with § 25.1329(h). The autopilot should be disengaged when the windshear recovery guidance activates, unless autopilot operation has been shown to be safe in these conditions and provides effect ive automatic windshear recovery that meets the criteria found in the ACs referenced above.
c. High-speed protection.
(1) General. Section 25.1329(h) states that means must be provided to avoid excursions beyond an acceptable margin from the speed range of the normal flight envelope .
VMO and MMO mark the upper speed/Mach limit of the normal flight envelope . This is not intended to require or preclude high-speed protection based on airplane configurations (for example, flaps extended).
(2) Factors to consider.
(a) Duration of airspeed excursions, rate of airspeed change, turbulence, and gust characteristics:
1 Operations at or near VMO/MMO in routine atmospheric conditions (for example, light turbulence) are safe . Small, brief excursions above V MO/MMO by themselves are not unsafe.
2 The FGS design should strive to strike a balance between providing adequate speed protection margin and avoiding nuisance activation of high-speed protection.
Note: The following factors apply only to designs that provide high-speed protection through FGS control of airspeed.
(b) High-speed protection while in altitude hold mode:
1 Climbing to control airspeed is not desirable because departing an assigned altitude can be disruptive to air traffic control (ATC) and potentially hazardous (for example, in RVSM airspace). As long as the speed does not exceed a certain margin beyond VMO/MMO (for example, six knots), it is better that the FGS remain in Altitude Hold mode.
2 The autothrust function, if operating normally, should affect high-speed protection by limiting its speed reference to the normal speed envelope (that is, at or below VMO/MMO).
3 The basic airplane high -speed alert should be sufficient for the pilot to recognize the overspeed condition and take corrective action to reduce thrust . However, if the airspeed exceeds a margin beyond V MO/MMO (for example, 6 knots), the FGS may transition from Altitude Hold to the Overspeed Protection mode and depart (that is, climb above) the selected altitude.
(c) High-speed protection during climbs and descents:
1 When the elevator channel of the FGS is not controlling airspeed, the autothrust function, if engaged, should reduce thrust, as needed, to prevent sustained airspeed excursions beyond VMO/MMO (for example, 6 knots) down to the minimum appropriate value.
2 When thrust is already the minimum appropriate value or the autothrust function is not operating, the FGS should begin using pitch control, as needed, for high-speed protection.
3 If conditions are encountered that result in airspeed excursions above VMO/MMO, it is preferable for the FGS to smoothly and positively guide or control the airplane back to within the speed range of the normal flight envelope.
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CHAP TER 6. CHARACTERISTICS OF SPECIFIC MODES
62. General. There are certain operational modes of the FGS that have been implemented in different ways in different airplanes and systems . The following paragraphs provide guidance and interpretative material that clarifies the operational intent of these modes, and criteria shown to be acceptable in current operations . The guidance in this chapter does not preclude other mode implementations. Pilot understanding of the mode behavior is especially important to avoid confusion. Therefore, mode behavior should be clearly annunciated, as described in Chapter 4 , paragraph 44, FGS Mode Selection, Annunciation, and Indication, of this AC .
63. Lateral Modes. This paragraph discusses modes that are implemented in many FGSs that are used primarily for lateral/directional control of the airplane . The criteria below identify acceptable mode operation, based on past operational experience gained from the use of these modes.
a. Heading or track hold. In the Heading or Track Hold mode, the FGS should maintain the airplane heading or track . When the airplane is in a bank when the Heading or Track Hold mode is engaged, the FGS should roll the airplane to a wings -level condition and maintain the heading or track when wings -level is achieved (typically less than 5 degrees of bank angle).
b. Heading or track select. In the Heading or Track Select mode, the FGS should expeditiously acquire and maintain a “selected” heading or track value consistent with occupant comfort. When the mode is initially engaged, the FGS should turn the airplane in a direction that is the shortest heading (or track) change to acquire the new heading (or track) . Once the Heading/Track Select mode is active, changes in the selected value should result in changes in heading/track. The FGS must turn the airplane in the same direction as the sense of the selected heading change. See § 25.779(a)(1). That is, if the pilot turns the heading select knob clockwise, the airplane must turn to the right, even if the shortest heading (or track) change is in the opposite direction. The target heading or track value should be presented to the flightcrew.
c. LNAV mode.
(1) Lateral flight path. In the LNAV mode, the FGS should acquire and maintain the lateral flight path commanded by a flight management function (that is, FMS or equivalent).
(2) Automatic mode transitions. If the airplane is not on the desired lateral path or within the designed path capture criteria when LNAV is selected, the FGS LNAV mode should enter an armed state. The FGS should transition from the armed state to an engaged state at a point where the lateral flight path can be smoothly acquired and tracked.
(3) Takeoff or go-around (TOGA). For an FGS incorporating the LNAV mode during the TOGA phase, the design should specify maneuvering capability immediately after takeoff and any limits that may exist. After TOGA, maneuvering should be based upon aircraft performance with the objective to prevent excessive roll attitudes where wingtip impact with the runway becomes probable, yet satisfy operational requirements where terrain and/or thrust limitations exist.
64. Vertical Modes. This paragraph discusses modes that are implemented in many FGSs that are used primarily for pitch control of the airplane . The criteria identified reflect operational experience gained from the use of these modes.
a. Target altitude selection. To avoid unconstrained climbs or descents for any altitude transitions when using applicable vertical modes, the altitude select controller should be set to a new target altitude before the vertical mode can be selected . If the design allows the vertical mode to be selected before setting the target altitude, then consideration should be given to the potential vulnerability of unconstrained climb or descent leading to an altitude violation or controlled flight into terrain. Consideration should also be given to appropriate annunciation of the deviation from previously selected altitude and/or subsequent required pilot action to reset the selected altitude.
b. Vertical speed. In the Vertical Speed mode, the FGS should smoothly acquire and maintain a selected vertical speed . Consideration should be given to the situation where the selected value is outside the performance capability of the airplane . Another situation to consider is use of Vertical Speed mode without autothrust . These situations could potentially lead to a low-speed or high-speed condition and corresponding pilot awareness vulnerabilities . For discussion of acceptable means of compliance for these situations , see Chapter 5, paragraph 57 of this AC.
c. Flight path angle. In the Flight Path Angle mode, the FGS should smoothly acquire and maintain the selected flight path angle. Consideration should be given to the situation where the selected value is outside the performance capability of the airplane . The use of Flight Path Angle mode without autothrust —potentially leading to a low-speed or high-speed condition and corresponding pilot awareness vulnerabilities —should also be considered. Acceptable means of compliance have included a reversion to an envelo pe protection mode or a timely annunciation of the situation.
d. Indicated Airspeed/Mach Hold (speed on elevator). In the Airspeed/Mach Hold mode, the FGS should maintain the airspeed or Mach at the time of engagement.
e. Indicated Airspeed/Mach Select (speed on elevator ). In the Airspeed/Mach Select mode, the FGS should acquire and maintain a selected airspeed or Mach . The selected airspeed or Mach may be either pre-selected or synchronized to the airspeed or Mach at the time of engagement.
f. Flight level (FL) change (speed on elevator). In the FL Change mode, the FGS should change altitude in a coordinated way with thrust control on the airplane . The autopilot/FD will typically maintain speed control through the elevator . The autothrust function, if engaged, should control the thrust to the appropriate value for climb or descent.
g. Altitude capture.
(1) Mode transition. The Altitude Capture mode should command the FGS to transition from a vertical mode to smoothly capture and maintain the selected target altitude with consideration of the rates of climb and descent experienced in service.
(2) Guidelines for Altitude Capture mode. In-service experience has shown that certain implementations have the potential to cause pilot confusion that may lead to altitude violations. Accordingly, the following are guidelines for the Altitude Capture mode.
(a) Automatic arming of mode. The Altitude Capture mode should be automatically armed to ensure capture of the selected altitude. Annunciation of the armed status is not required if the Altitude Capture mode is armed at all times . If the FGS is in the Altitude Capture mode, it should be annunciated.
(b) Engagement from any vertical mode. The Altitude Capture mode should e ngage from any vertical mode if the computed flight path will intercept the selected altitude and the altitude capture criteria are satisfied, except as specified during an approach (for example, when the glidepath for Approach mode is active).
(c) Changing climb/descent command references. Changes in the climb/descent command references (for example, the vertical speed command reference), with the exception of those made by the flightcrew using the altitude select controller, should not prevent capture of the target altitude.
(d) Capturing selected altitude. The Altitude Capture mode should smoothl y c apture the selected altitude, using an acceptable acceleration limit and pitch attitude with consideration for occupant comfort.
(e) Minimizing acceleration overshoot. The acceleration limit may, under certain conditions, result in an overshoot . To minimize the altitude overshoot, the normal acceleration limit may be increased, consistent with occupant safety.
(f) Selecting other vertical modes. Pilot selection of other vertical modes at the time of altitude capture should not prevent or adversely affect the level off at the target altitude .
One means of compliance is to inhibit transition to other pilot -selectable vertical modes (except Altitude Hold, Go-Around, and Approach modes) during altitude capture, unless the target altitude is changed. If glidepath capture criteria are satisfied during altitude capture, then the FGS should transition to glidepath capture.
(g) Changing target altitude. The FGS must be designed to minimize flightcrew confusion concerning the FGS operation when the target altitude is changed during altitude capture. See § 25.1329(i). It should be suitably annunciated and appropriate for the phase of flight.
(h) Barometric pressure a djustment. Adjusting the datum pressure at any time during altitude capture should not result in loss of the capture mode . The transition to the pressure altitude should be accomplished smoothly.
(i) Maintaining reference airspeed. If the autothrust function is active during a ltitude capture, the autopilot and autothrust functions should be designed such that the FGS maintains the reference airspeed during the level -off maneuver. For example, if the autopilot changes from a speed mode to an altitude capture or control mode, then the autothrust should transition to a speed mode to maintain the reference airspeed.
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h. Altitude hold.
(1) Entering mode. The Altitude Hold mode may be entered either by flightcrew selection or by transition from another vertical mode.
(a) Pilot selection.
1 Level flight. When initiated by pilot action in level flight, the Altitude Hold mode should provide guidance or control to maintain altitude at the time the mode is selected.
2 Climbing or descending. When initiated by pilot action when the airplane is either climbing or descending, the FGS should immediately initiate a pitch change to arrest the climb or descent and maintain the altitude when level flight (for example, less than 200 feet per minute) is reached. The intensity of the leveling maneuver should be consistent with occupant comfort and safety.
(b) Automatic transition. When initiated by an automatic transition from Altitude Capture, the Altitude Hold mode should provide guidance or contr ol to the selected altitude.
(2) Mode transition annunciation. Automatic transition into the Altitude Hold mode from another vertical mode should be clearly annunciated for flightcrew awareness.
(3) Barometric pressure adjustment. Any airplane response due to an adjustment of the datum pressure should be smooth.
i. VNAV.
(1) FMS path.
(a) Acquire vertical path. In the VNAV mode, the FGS should acquire a nd m aintain the vertical flight path commanded by a flight management function (that is, FMS or equivalent). If the airplane is not on the desired FMS path when the VNAV mode is selected, the FGS VNAV mode should go into an armed state or provide guidance to smoothly acquire the FMS path. The flightcrew should establish the airplane on a flight profile to intercept the desired FMS path. The FGS should transition from the armed state to an engaged state at a point where the FGS can smoothly acquire and track the FMS path.
(b) Deviation from vertical path. If the aircraft is flying a vertical path (for example, VNAV path), then the deviation from that path should be displayed in the primary field of view, such as the PFD, navigation display (ND), or other acceptable display.
(2) Climb or descent.
(a) Autothrust function. When VNAV is selected for climb or descent, the autothrust function (if installed and engaged) should maintain the appropriate thrust setting.
When leveling after a VNAV climb or descent, the autothrust function should maintain the target speed.
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(b) Preclude VNAV climb. The FGS should preclude a VNAV climb, unless the MSP altitude window is set to an altitude above the current altitude.
(c) Preclude VNAV descent. The FGS should preclude a VNAV descent, unless the MSP altitude window is set to an altitude below the current altitude, except when on a final approach to a runway.
(d) Preclude MSP altitude fly-through. The FGS should not allow the VNAV climb or descent to pass through a MSP altitude, except when on a final approach to a runway.
Note: See paragraph 67, Special Considerations for VNAV Approach Operations Related to Selecting a Target Altitude , of this AC.
65. Multi-Axis Modes. This paragraph discusses modes that are implemented in many FGSs that are used in an integrated manner for pitch, lateral/directional control, and thrust management of the airplane. The criteria identified reflect operational experience gained from the use of these modes.
a. Takeoff mode.
(1) Vertical guidance. In the Takeoff mode, the vertical element of the FGS shoul d pr ovide vertical guidance to acquire and maintain a safe climbout speed after initial rotation for takeoff.
(2) Lateral guidance. In the Takeoff mode, the lateral element of the FGS, if implemented, should maintain runway heading/track or wings level after liftoff . A separate lateral mode annunciation should be provided.
(3) Rotation guidance. If rotation guidance is provided, the use of the guidance shoul d n ot result in a tail strike. It should be consistent with takeoff methods necessary to meet takeoff performance requirements up to 35 feet above ground level (AGL). If no rotation guidance is provided, the pitch command bars may be displayed during takeoff roll , but should not be considered as providing rotation guidance, unless it is part of the intended function.
(4) Autothrust. The autothrust function should increase and maintain engine thrust t o th e selected thrust limits (for example, full takeoff thrust, de -rate).
(5) Operation and performance. The FGS design should address all engine an d e ngine-inoperative conditions consistent with the following takeoff system performance characteristics after liftoff.
(a) Transitions between flight phases. Takeoff system operation should be continuous and smooth through transition from the runway portion of the takeoff to the airborne portion and reconfiguration for en route climb. The pilot should be able to continue the use of the same primary display(s) for the airborne portion as for the runway portion. Changes in guidance modes and display formats should be automatic.
(b) Pitch attitude and climb speed/normal operation. The vertical axis guidance of the takeoff system during normal operation should result in the appropriate pitch attitude and climb speed for the airplane considering the following factors:
1 Normal rate rotation of the airplane to the commanded pitch attitude at V R (takeoff rotation speed) - 10 knots for all engines operative and V R - 5 knots for engine out should not result in a tail strike.
2 The system should provide commands that lead the airplane to smoothly acquire a pitch attitude that results in capture and tracking of the all -engine takeoff climb speed, V2 (takeoff safety speed) + X . X is the all-engine speed add itive from the AFM (normally 10 knots or higher). If pitch limited conditions are encountered, a higher climb airspeed may be used to achieve the required takeoff path without exceeding the pitch limit.
(c) Engine-out operation. For engine-out operation, the system should provide commands that lead the airplane to smoothly acquire a pitch attitude that results in capture and tracking of the following reference speeds:
1 V2, for engine failure at or below V2. This speed should be attained by the time the airplane has reached 35 feet altitude.
2 Airspeed at engine failure for failures between V 2 and V2 + X.
3 V2 + X, for failures at or above V 2 + X. Alternatively, the airspeed at engine failure may be used, provided it has been shown that the minimum takeoff climb gradient can still be achieved at that speed.
(d) Lateral commands during takeoff mode. If implemented, the lateral element of the Takeoff mode should maintain runway heading/track or wings level after liftoff and a separate lateral mode annunciation should be provided.
b. Go-around mode. Characteristics of the Go-Around mode and resulting flight path should be consistent with a manually flown go-around.
(1) Vertical elements. The vertical element of the FGS Go -Around mode should i nitially rotate the airplane or provide guidance to rotate the airplane to arrest the rate of descent.
(2) Speed. The FGS should acquire and maintain a safe speed during climbout a nd a irplane configuration changes . Typically, a safe speed for go -around climb is V2, but a different speed may be found safe for windshear recoveries. See AC 25-12.
(3) Autothrust.
(a) Thrust during climb. The autothrust function, if installed, should increase thrust and either maintain thrust to specific thrust limits or maintain thrust for an adequate, safe climb. The autothrust function should not exceed thrust limits (for example, full go-around thrust or de-rated go-around thrust limits). Furthermore, the autothrust function should not reduce 51
AC 25.1329-1C CHG 2 52 thrust for wind below the minimum value required for an adequate, safe climb or reduce the thrust lever position below a point that would cause a warning system to activate.
(b) Pitch attitude during go-around. The initial go-around maneuver may require a significant change in pitch attitude . It is acceptable to reduce thrust to lower the pitch attitude for comfort of the occupants when a safe climb gradient has been established. It should be possible for the pilot to re-select the full thrust value, if needed.
(4) Engagement. The Go-Around mode should engage when go-around is selected by the pilot, even if the MSP selected altitude is at or below the go -around initiation point. The airplane should climb until another vertical mode is selected or the MSP altitude is adjusted to an altitude above the present aircraft altitude.
(5) All engine and engine out capability. The FGS design of the Go-Around mode should address all engine and engine- out operation. The design should consider an engine failure resulting in a go-around and the engine failure occurring during an all engine go-around.
c. Approach mode.
(1) Final approach path. In the Approach mode, the FGS should capture and track a final approach lateral and vertical path, if applicable, fr om a NAV or landing system, for example, ILS, GLS, RNP, area navigation (RNAV) . See the current editions of AC 20-191 and AC 20-138.
(2) Mode annunciations. The FGS should annunciate all operationally relevant approach modes. Modes that are armed and waiting for capture criteria to be satisfied should be indicated in addition to the active pre -capture mode. A positive indication of the capture of the previously armed mode should be provided.
(3) Sub-modes. The FGS may have sub -modes that become active without additional crew selection. An assessment of the significance of these sub -mode transitions to the flightcrew should be made. If assessed to be significant (for example, flare), positive annunciation of the transition should be provided.
(4) Mode engagement sequence. Glideslope capture mode engagement may occur prior to localizer capture. However, it is the flightcrew’s responsibility to ensure proper safe obstacle/terrain clearance when following vertical guidance when the airplane is not established on the final lateral path.
Note: Additional guidance and criteria are con tained in AC 20-191 and AC 20-138D or latest.
66. Autothrust Modes. This paragraph discusses modes that are implemented in many FGSs that are used primarily for controlling the engines on the airplane . The criteria identified reflect operational experience gained from the use of these modes.
a. Thrust mode. In the Thrust mode, the FGS should command the autothrust function to achieve a selected target thrust value.
b. Speed mode. In the Speed mode, the FGS should command the autothrust function t o a cquire and maintain the selected target speed value, assuming that the selected speed is within the speed range of the normal flight envelope. The autothrust system may fly a higher airspeed than the selected target speed during an approach when operating in winds or turbulent conditions.
c. Retard mode. If a Retard mode is implemented in the FGS, it should work in the same manner for both automatic and manual landings when the autothrust function is engaged.
67. Special Considerations for VNAV Approach Operations Related to Selecting a Target Altitude.
a. Approach operations. The FGS vertical modes should allow the pilot to set the targe t a ltitude to a missed approach value prior to capturing the final approach segment . This should be possible for capturing from both above and below the final approach segment.
b. VNAV path operations. It should be possible to define a descent path to the final approach fix and another path from the final approach fix to the runway, with the target altitude set for the missed approach altitude . Appropriate targets and descent points should be identified by the FMS.
68. Control Wheel Steering (Control Steering through the A utopilot).
a. General. In the control wheel steering ( CWS) mode, the FGS allows the flightcrew t o m aneuver the airplane manually through the autopilot control path. CWS is considered an FGS mode, as it is a specific function of the FGS . However, during CWS operation, the pilot is in control of the aircraft, rather than the FGS . Operationally, CWS is identical to the pilot flying the airplane during manual flight . In both cases, the pilot is in control of the flight path and speed of the airplane. The only difference is the mechanization of how the actual flight control surfaces are moved. No automatic FGS commands are involved during CWS operation, with the possible exception of automatic aircraft trim function(s) . Therefore, paragraphs in this AC, such as those that discuss speed protection and performance objectives, should be applied only to those autopilot modes with which the FGS is in control of the flight path of the airplane. Such paragraphs in this AC should not be applied to CWS, unless such features are specifically provided while the FGS is in the CWS mode.
b. Design. If provided, a CWS mode should meet the following:
(1) Control forces. It should be possible for the pilot to maneuver the airplane usi ng t he normal flight controls with the CWS mode engaged and to achieve the maximum available control surface deflection without using forces so high that the controllability requirements of § 25.143(d) are not met. Excessive discontinuities in control force that might adversely affect the flight path should not be encountered.
(2) Bank and pitch attitudes. The maximum bank and pitch attitudes that can be achieved during CWS operations should be limited to those necessary for the normal operation of the airplane.
Note 1: Normal operational limits are typically 35 degrees in roll and +20 degrees to – 10 degrees in pitch.
Note 2: Limits that can be physically overcome with forceful c ontrol commands by the pilot are sometimes referred to as “soft limits.”
(3) Control behavior. It should be possible to perform all normal maneuvers smoothly and accurately without nuisance oscillation. It should also be possible to counter all normal changes of trim due to change of configuration or power, within the range of flight conditions in which CWS may be used.
(4) Stall and stall recovery. The stall and stall recovery characteristics of the airplane should remain acceptable . It should be assumed that recovery from such a stall is made with CWS in use, unless automatic disengagement of the autopilot is provided.
(5) Adjustments to trim. In showing compliance with § 25.143(f), consideration should be given to adjustments to trim that may be made by the autopilot in the course of maneuvers that can reasonably be expected . Some alleviation may be acceptable in the case of unusually prolonged maneuvers, provided that the reduced control forces would not be potentially hazardous.
(6) Takeoff and landing. If the use of CWS for takeoff and landing is to be per mitted, it should be shown that—
(a) Sufficient control, in both amplitude and rate, is available without encountering force discontinuities.
(b) Reasonable mishandling is not hazardous (for example, engaging the autopilot while the elevators or ailerons are held in an out -of-trim position).
(c) Runaway rates and control forces are such that the pilot can readily overpower the autopilot with no significant deviation in flight path .
(d) Any lag in aircraft response induced by the CWS mode is acceptable for the intended maneuver.
(7) Automatic reversion to CWS mode. The autopilot, when engaged, should not automatically revert to the CWS mode by applying an input to the control column or wheel, unless the autopilot is in a capture mode (for example, Altitude Capture or Localizer Capture) .
When the force is released, the autopilot should return to the previously engaged capture mode or to the Track/Heading Hold mode.
c. Terminology and application of advisory material. The term “CWS” is currently used by industry to refer to several different types of systems . Several other types of systems that are similar in nature to CWS, but functionally different from it, are described below .
Paragraph 68.b, Design, of this AC applies to systems as described in paragraph 68.a, General, of this AC. It does not apply to those systems described in paragraphs 68.c(1) and 68.c(2) below.
These descriptions are intended to be generic. Implementation may vary from airplane to airplane.
(1) Touch control steering (TCS). A TCS system is available on many business a nd c ommuter aircraft. With a TCS system, a pilot is able to physically disengage the autopilot servos from the flight control system, usually by pushing and holding a button on the control wheel, without causing the autopilot system itself to disengage or lose its currently sel ected modes. The pilot may then maneuver the airplane as desired using the aircraft’s flight control system (that is, the autopilot servos are not part of the control loop) . The pilot is then able to reconnect the autopilot servos to the flight control system by releasing the TCS button. Using the new orientation of the aircraft as a basis, the autopilot will then reassume control of the airplane using the same mode selections as were present before the selection of TCS . This type of system on some aircraft is also sometimes referred to as CWS.
(2) Supervisory override. A function available on some aircraft is referred to as a “supervisory override” of an engaged autopilot . With this function, a pilot is able to physically overpower an engaged autopilot servo by applying force to the flight deck controls. With a supervisory override, the autopilot does not automatically disengage due to the pilot input . This allows the pilot to position the airplane as desired using the flight deck controls without first disengaging the autopilot. When the pilot releases the controls, the autopilot reassumes control of the airplane using the same mode selections as were present before the supervisory override.
69. Special Considerations for the Integration of Fly-by -Wire Flight Control Systems and FGS.
a. Speed protection. Speed protection features may be implemented in the fly-by-wire flight control system. However, if speed protection is also implemented within the FGS, it should be compatible with the envelope protection features of the fly-by-wire flight control system . The FGS speed protection (normal flight envelope) should operate to or within the limits of the flight control system (limit flight envelope).
b. Syst em degradation. Information should be provided to the flightcrew about the impact on the FGS following degradation of the fly-by-wire flight control systems.
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