c. Level of Performance.
(1) Performance Standard. For rare normal conditions, the FGS should be designed to provide guidance or control, as appropriate, for the intended function of the active mode(s) in a safe and predictable manner, both within the normal flight envelope and for momentary excursions outside the normal flight envelope. If a determination is made that there are environmental conditions in which the FGS cannot be safely operated, appropriate operational limitations should be placed in the AFM.
(2) Autopilot Authority Limit. Operations in rare normal conditions may result in automatic or pilot-initiated autopilot disengagement close to the limit of autopilot authority. Autopilot disengagement in rare normal conditions should meet the safety criteria for autopilot disengagement found in Chapter 3, FGS Engagement, Disengagement, Indications, and Override, paragraph 27b, Autopilot Disengagement, and the criteria for flight guidance alerting in Chapter 4, Controls, Indications, and Alerts, paragraph 45d, Awareness of Potential Significant Transient Condition (“Bark Before Bite”).
(3) Masking of Potential Hazard. It is not necessary that the FGS always be disengaged when rare normal conditions that may degrade its performance or capability are encountered. The FGS may significantly help the flightcrew during such conditions.
However, the design should address the potential for the FGS to mask a condition from the flightcrew or otherwise delay appropriate flightcrew action. See Chapter 4, paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status, for discussion of alerting under such conditions.
55. Icing coniseration in normal and rare normal conditions.
a. Effect on Performance. The FGS typically will be designed to provide acceptable performance in all standard airplane configurations. Operating an airplane in icing conditions can have significant implications on the aerodynamic characteristics of the airplane (for example, ice accretion on wings, tail, and engines) and, consequently, on FGS performance. Ice accretion may be slow, rapid, symmetric, or asymmetric. During autopilot operation, the flightcrew may not be aware of the gradual onset of icing conditions or the effect that the accumulation of ice is having on the handling qualities of the airplane.
b. Alerts. Means should be provided to alert the flightcrew when icing conditions begin to have an effect upon FGS performance, as described in Chapter 4, paragraph 45.
c. Effect On Speed Protection. The implication of icing conditions on speed protection should be assessed. If the threshold of the stall warning system is adjusted due to icing conditions, appropriate adjustments should also be made to the FGS low speed protection threshold, which is discussed in paragraph 57b, Low Speed Protection, of this chapter.
56. Performance in non-normal conditions.
a. General. The FGS will occasionally be operating when the airplane transitions outside of its normal flight envelope, due to failures or non-standard ferry configurations.
Under such circumstances, the FGS characteristics and flightcrew interaction with the FGS should be shown to be safe.
b. Conditions to Consider. The following table shows examples of non–normal conditions. The table does not fully define every condition that may be encountered during an airplane’s life and unequivocally categorize what can be considered to be a “non–normal condition.” Rather, the table is intended to give examples to be used during system development and testing. By the very nature of the phenomena involved, there will always be some subjectivity to these categorizations. Also, the same conditions may affect different airplane models in very different ways. These differences should be considered in determining how to characterize the severity of the conditions discussed in the following table.
Table 5-3 Examples of Non-Normal Conditions
Significant f uel imbalance Large variation of the amount of fuel between the two wing tanks (and center and tail tanks, if so equipped) that causes the FGS, if engaged, to counter the aerodynamic effect of the fuel imbalance with a pitch, roll, or yaw command that is approaching maximum system authority.
Non-standard ferry flight configurations Possible aerodynamic drag (both symmetrical and unsymmetrical) caused by non-standard airplane ferry flight conditions such as locked high lift devices, landing gear locked in the deployed position, or an extra engine carried underneath one wing in an inoperative position.
Inoperative engine(s) Loss of one or more engines that causes the FGS, if engaged, to counter the aerodynamic effect of the difference in thrust with a pitch, roll, or yaw command that is approaching maximum system authority.
Loss of one or more hydraulic systems Loss of one or more hydraulic systems down to the minimum amount of remaining operational systems which the FGS is certified to operate.
Inoperative Ice Detection/ Protection System Loss of ice detection/protection system on an airplane so equipped where the FGS is certified for operation in icing conditions with that failure present.
c. Level of Performance.
(1) Performance Standard. For non- normal conditions, the FGS should be designed to provide guidance or control, as appropriate, for the intended function of the active mode(s) in a safe and predictable manner, both within the normal flight envelope and for momentary excursions outside the normal flight envelope. If a determination is made that there are non-normal conditions in which the FGS cannot be safely operated, appropriate operational limitations should be placed in the AFM.
(2) Autopilot Authority Limit. Operations in non-normal conditions may result in automatic or pilot-initiated autopilot disengagement close to the limit of autopilot authority. Autopilot disengagement in non-normal conditions should meet the safety criteria for autopilot disengagement found in Chapter 3, FGS Engagement, Disengagement, Indications, and Override, paragraph 27b, Autopilot Disengagement, and the criteria for flight guidance alerting in Chapter 4, Controls, Indications, and Alerts, paragraph 45d, Awareness of Potential Significant Transient Condition (“Bark Before Bite”).
(3) Masking of Potential Hazard. It is not necessary that the FGS always be disengaged when non-normal conditions that may degrade its performance or capability are encountered. The FGS may significantly help the flightcrew during such conditions.
However, the design should address the potential for the FGS to mask a condition from the flightcrew or otherwise delay appropriate flightcrew action. See Chapter 4, paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status, for discussion of alerting under such conditions.
57. Speed protection.
a. General.
(1) Speed Excursions. The requirement for speed protection is based on the premise that reliance on flightcrew attentiveness to airspeed indications alone during FGS operation is not adequate to avoid unacceptable speed excursions outside the speed range of the normal flight envelope. Many existing FGS systems have no provisions to avoid speed excursions outside the normal flight envelope. Some FGSs will remain engaged until the aircraft slows to stall conditions and also to speeds well above maximum operating limit speed/maximum operating limit Mach (VMO/MMO).
(2) Compliance with § 25.1329(h). Standard stall warning and high-speed alerts are not always timely enough for the flightcrew to intervene to prevent unacceptable speed excursions during FGS operation. The intent of § 25.1329(h) is for the FGS to provide a speed protection function for all operating modes, so that the airspeed can be safely maintained within an acceptable margin of the speed range of the normal flight envelope. The FGS design may use any of the following ways or a combination of ways to provide acceptable speed protection:
(a) The FGS Provides Speed Protection. In this case, the following are acceptable means to comply with this rule:
1 The FGS may detect the speed protection condition, alert the flightcrew, and provide speed protection control or guidance.
2 The FGS may detect the speed protection condition, alert the flightcrew, and then disengage the FGS.
3 The FGS may detect the speed protection condition, alert the flightcrew, and remain engaged in the active mode without providing speed protection control or guidance.
(b) Other Systems Provide Speed Protection. Other systems, such as the primary flight control system or the FMS (when in a vertical navigation (VNAV) mode) may be used to provide equivalent speed protection.
Note: If compliance with this requirement is based on use of alerting alone, the alerts should be shown to be appropriate and timely to ensure flightcrew awareness and enable the pilot to keep the airplane within an acceptable margin from the speed range of the normal flight envelope.
See Chapter 4, Controls, Indications, and Alerts, paragraph 45b, Speed Protection Alerts, for additional discussion of speed protection alerting.
(3) Design Standard.
(a) Interaction of FGS Elements. The design should consider how and when the speed protection is provided for combinations of autopilot, FDs, and autothrust operation. Care should be taken to set appropriate values for transitioning into and out of speed protection such that the flightcrew does not consider the transitions a nuisance.
(b) Integration of Pitch and Thrust Control. The speed protection function should integrate pitch and thrust control. Consideration should be given to automatically activating the autothrust function when speed protection is invoked. If an autothrust function is either not provided or is unavailable, speed protection should be provided through pitch control alone.
(c) Interaction of Systems. The role and interaction of autothrust with elements of the FMS, the primary flight control system, and the propulsion system, as applicable, should be accounted for in the design for speed protection.
(d) Engine Inoperative. Consideration should be given to the effects of an engine inoperative condition on the performance of speed protection.
Change 1
b. Low Speed Protection.
(1) General. When the FGS is engaged in any modes (with the possible exception of Approach, as discussed in paragraph (3) below) for which the available thrust is insufficient to maintain a safe operating speed, the low speed protection function should be invoked to avoid unsafe speed excursions.
(2) Factors to Consider. Activation of speed protection should take into account such factors as the phase of flight, turbulence and gusty wind conditions, and compatibility with the speed schedules. The low speed protection function should activate at a suitable margin to stall warning that will not result in nuisance alerts.
Consider the operational speeds, as specified in the AFM, for all-engine and engine-inoperative cases during the following phases of flight:
(a) Takeoff.
(b) Departure, climb, cruise, des cent, and terminal area operations. During these flight phases, airplanes are normally operated at or above the minimum maneuvering speed for the given flap configuration.
Note: For high altitude operations, it may be desirable to incorporate low speed protection at the appropriate engine out drift–down speed schedule, if the FGS (or other integrated sensors/systems) can determine that the thrust deficiency is due to an engine failure.
(c) Approach.
(d) Transition from approach to go–around and go–around climb.
Note: A low speed alert and a transition to the Speed Protection mode at approximately 1.13 VSR (reference stall speed) for the landing flap configuration has been found to be acceptable.
(3) Low Speed Protection During Approach Operations.
(a) Non–Interference. Speed protection should not interfere with the approach and landing phases of flight.
(b) Autothrust Operation. It is assumed that with autothrust operating normally, the combination of thrust control and pitch control during the approach will be sufficient to maintain speed and desired vertical flight path. In cases where it is not sufficient, an alert should be provided in time for the flightcrew to take appropriate corrective action.
(c) Defined Vertical Path. For approach operations with a defined vertical path (for example, ILS, microwave landing system (MLS), GBAS landing system (GLS), 57
Lateral Navigation (LNAV) mode, VNAV mode), if the thrust is insufficient to maintain both the desired flight path and the desired approach speed, there are several ways to meet the intent of low speed protection:
1 The FGS may maintain the define d vertical path as the airplane decelerates below the desired approach speed until the airspeed reaches the low speed protection value. At that time, the FGS would provide guidance to maintain the low speed protection value as the airplane departs the defined vertical path. The FGS mode reversion and low speed alert should be activated to ensure pilot awareness.
Note: The pilot is expected to take corrective action to add thrust and return the airplane to the defined vertical path or go–around, as necessary.
2 The FGS may maintain the define d vertical path as the airplane decelerates below the desired approach speed to the low speed protection value. The FGS will then provide a low speed alert while remaining in the existing FGS approach mode.
Note: The pilot is expected to take corrective action to add thrust to cause the airplane to accelerate back to the desired approach speed while maintaining the defined vertical path or go–around, as necessary.
3 The FGS may maintain the define d vertical path as the airplane decelerates below the desired approach speed until the airspeed reaches the low speed protection value. The FGS will then provide a low speed alert and disengage.
Note: The pilot is expected to take corrective action when alerted to the low speed condition and the disengagement of the autopilot, to add thrust, and to manually return the airplane to the desired vertical path or go– around.
(d) Vertical Flight Path Not Protected. If the speed protection is invoked during approach such that vertical flight path is not protected, the subsequent behavior of the FGS after speed protection should be carefully considered. Activating low speed protection during the 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 120-28D, Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout, appendix 3, section 8.1, 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 58
necessary lo w 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 effective 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 V MO/MMO in routine atmospheric conditions (for example, light turbulence) are safe. Small, brief excursions above VMO/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 highspeed 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 effect highspeed 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 VMO/MMO (for example, six knots), 59
the FGS m ay 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, six 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.
58 — 61 [Reserved]
Chapter 6 Characteristics of Specific Modes
62. Genera
l. 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, Controls, Indications, and Alerts, paragraph 44, FGS Mode Selection, Annunciation, and Indication.
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 five 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. Lateral Navigation (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 61
engaged state at a po int 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. See Chapter 5, paragraph 57, Speed Protection, for discussion of acceptable means of compliance for these situations.
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 lowspeed or high-speed condition and corresponding pilot awareness vulnerabilities—should also be considered. Acceptable means of compliance have included a reversion to an envelope protection mode or a timely annunciation of the situation.
d. Indicated Airspeed (IAS)/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 (IAS)/Mach Select [speed on elevator]. In the Airspeed/Mach Select mode, the FGS should acquire and maintain a selected airspeed or 62
Mach. The selected airspeed or Mach m ay 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 engage 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 smoothly capture 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 63