11 14 CFR Section AC Chapter/Paragraph Reference § 25.1329(h) When the flight guidance system is in use, a means must be provided to avoid excursions beyond an acceptable margin from the speed range of the normal flight envelope. If the airplane experiences an excursion outside this range, a means must be provided to prevent the flight guidance system from providing guidance or control to an unsafe speed.
Chapter 5 Performance of Function Paragraph 57, Speed Protection § 25.1329(i) The flight guidance system functions, controls, indications, and alerts must be designed to minimize flightcrew errors and confusion concerning the behavior and operation of the flight guidance system. Means must be provided to indicate the current mode of operation, including any armed modes, transitions, and reversions. Selector switch position is not an acceptable means of indication. The controls and indications must be grouped and presented in a logical and consistent manner. The indications must be visible to each pilot under all expected lighting conditions.
Chapter 4 Controls, Indications, and Alerts § 25.1329(j) Following disengagement of the autopilot, a warning (visual and auditory) must be provided to each pilot and be timely and distinct from all other cockpit warnings.
Chapter 3 Flight Guidance System Engagement, Disengagement, Indications, and Override Paragraph 27, Autopilot Engagement, Disengagement, and Indications Chapter 8 Safety Assessment
12 14 CFR Section AC Chapter/Paragraph Reference § 25.1329(k) Following disengagement of the autothrust function, a caution must be provided to each pilot.
Chapter 3 Flight Guidance System Engagement, Disengagement, Indications, and Override Paragraph 27, Autopilot Engagement, Disengagement, and Indications Chapter 8 Safety Assessment § 25.1329(l) The autopilot may not create a potential hazard when the flightcrew applies an override force to the flight controls.
Chapter 3 Flight Guidance System Engagement, Disengagement, Indications, and Override Paragraph 30, Override of the FGS Chapter 4 Controls, Indications, and Alerts Paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status Chapter 8 Safety Assessment § 25.1329(m) During autothrust operation, it must be possible for the flightcrew to move the thrust levers without requiring excessive force. The autothrust may not create a potential hazard when the flightcrew applies an override force to the thrust levers.
Chapter 3 Flight Guidance System Engagement, Disengagement, Indications, and Override Paragraph 30, Override of the FGS Chapter 8 Safety Assessment
13 14 CFR Section AC Chapter/Paragraph Reference § 25.1329(n) For purposes of this section, a transient is a disturbance in the control or flight path of the airplane that is not consistent with response to flightcrew inputs or environmental conditions.
(1) A minor transient would not significantly reduce safety margins and would involve flightcrew actions that are well within their capabilities. A minor transient may involve a slight increase in flightcrew workload or some physical discomfort to passengers or cabin crew.
(2) A significant transient may lead to a significant reduction in safety margins, an increase in flightcrew workload, discomfort to the flightcrew, or physical distress to the passengers or cabin crew, possibly including non-fatal injuries. Significant transients do not require, in order to remain within or recover to the normal flight envelope, any of the following:
(i) Exceptional piloting skill, alertness, or strength.
(ii) Forces applied by the pilot which are greater than those specified in § 25.143(c).
(iii) Accelerations or attitudes in the airplane that might result in further hazard to secured or non-secured occupants.
Appendix B—Definitions Note: The text for each paragraph in § 25.1329 is included in this AC for reference only. Please refer to the actual text in part 25.
21—26 [RESERVED]
14 CHAPTER 3. FLIGHT GUIDANCE SYSTEM ENGAGEMENT, DISENGAGEMENT,
INDICATIONS, AND OVERRIDE
27. Autopilot Engagement, Disengagement, and Indications. Autopilot engagement and disengagement should be accomplished in a manner consistent with other flightcrew procedures and tasks and should not require undue attention.
a. Autopilot engagement.
(1) Single-switch action. Each pilot should be able to select the autopilot function of the FGS with a single-switch action. The single-switch action should engage both the pitch and roll axes.
(2) Engagement status of autopilot. The autopilot system should provide positive indication to the flightcrew that the system has been engaged. The selector switch position is not acceptable as a means of indication. See § 25.1329(i). Regardless of the method used, the engagement status (and changes in status) of the autopilot(s) should be clearly indicated and should not require undue attention or recall.
Note: If an operational need is identified for split-axis engagement, then annunciation or indication should be provided for each axis.
(3) Multiple autopilots. If a single autopilot within a multiple autopilot installation can be individually selected by the flightcrew, the engagement annunciation should reflect the flightcrew selection. It should not be possible for multiple autopilots to be engaged in different modes. For modes that use multiple autopilots, the additional autopilots may engage automatically at selection of the mode or after arming the mode. A means should be provided to determine that adequate autopilot capability exists to support the intended operation (for example, “Land 2” and “Land 3” are used in some aircraft).
(4) Acceptable transients. The engagement of the autopilot should be free of perceptible transients. Under dynamic conditions, including maneuvering flight, minor transients are acceptable. However, with normal operating conditions, significant transients may not occur.
See § 25.1329(d).
(5) FD not engaged. Without a FD engaged, the initial lateral and vertical modes should be consistent with minimal disturbance from the flightpath. For example, the lateral mode at engagement may roll the airplane to wings level, and then hold the airplane heading/track or maintain the existing bank angle (if in a normal range). A heading/track preselect at engagement function may be provided, if precautions are taken to ensure that selection reflects the current intent of the flightcrew. The modes at engagement should be annunciated, and any associated selected target values should be displayed.
(6) FD engaged. With a FD engaged, the autopilot should engage in a mode consistent with (that is, the same as, or if that is not possible, then compatible with) the active FD mode of operation. Consideration should be given to the mode in which the autopilot will engage when large commands are present on either or both FDs. The following examples list some of these considerations:
(a) Should the autopilot retain the active FD mode or should it engage in the basic default mode?
(b) Are there any implications for current flight path references and targets?
(c) Is there any potential for flightcrew confusion and unintended changes in flightpath or modes?
Note: The design should consider the possibility that the pilot may attempt to engage the autopilot outside of the normal flight envelope. It is not required that the autopilot compensate for unusual attitudes or other situations outside the normal flight envelope, unless that is part of the autopilot’s intended function.
b. Autopilot disengagement.
(1) General.
(a) Normal conditions. Under normal conditions, automatic or manual disengagement of the autopilot should be free of perceptible transients or out-of-trim forces that are not consistent with the maneuvers being conducted by the airplane at the time of disengagement. A disengagement in normal conditions may not result in a transient any greater than a minor transient. If multiple autopilots are engaged, any disengagement of an individual autopilot may not result in a transient any greater than a minor transient. See § 25.1329(d). The disengagement of an individual autopilot should not adversely affect the operation of the remaining engaged autopilot(s).
(b) Other than normal conditions. Under other than normal conditions (that is, rare normal or non-normal conditions), disengagement of the autopilot may not result in a transient any greater than a significant transient. See § 25.1329(e). (For a discussion of rare normal and non-normal conditions, see Chapter 5, Performance of Function, paragraph 54, Performance in Rare Normal Conditions, and paragraph 56, Performance in Non-Normal Conditions, of this AC.) The flightcrew should be able to respond to a significant transient without using any of the following:
1 Exceptional piloting skill, alertness, or strength;
2 Forces greater than those given in § 25.143(d); or 3 Accelerations or attitudes in the airplane that might result in a hazard to secured or nonsecured occupants.
(c) Potential for significant transient. The flightcrew should be made aware (via a suitable alerting or other indication) of conditions or situations (for example, continued out-of-trim) that could result in a significant transient at disengagement. (See Chapter 4, Controls, Indications, and Alerts, paragraph 45.d, Awareness of Potential Significant Transient Condition (“Bark before Bite”).)
16 Note: See Appendix B for definitions of significant transient and minor transient.
For a discussion of normal conditions, rare normal conditions, and non-normal conditions, see Chapter 5, paragraphs 53, 54, and 56, respectively, of this AC.
(2) Autopilot disengagement alerts.
(a) Alert type. Since it is necessary for a pilot to immediately assume manual control following disengagement (manual or automatic) of the autopilot, a warning (both visual and aural) must be given. See § 25.1329(j).
(b) Aural warning specifications. A timely aural warning must be provided and must be distinct from all other cockpit warnings. See § 25.1329(j). It should sound long enough to ensure that it is heard and recognized by the pilot and other flightcrew members, but not so long that it adversely affects communication between crewmembers or is a distraction.
The aural warning should continue until silenced by one of the following means:
1 Activation of the autopilot quick disengagement control.
2 Reengagement of the autopilot.
3 Another acceptable means.
(c) Multiple autopilot system.
1 Disengagement of an autopilot channel within a multiple-channel autopilot system, downgraded system capability, or a reduction in the level of system redundancy that requires immediate flightcrew awareness and possible timely action should cause a caution level alert to be issued to the flightcrew.
2 Disengagement of an autopilot channel within a multiple-channel autopilot system that requires only flightcrew awareness should cause a suitable advisory level alert to be issued to the flightcrew.
(3) Quick disengagement control.
(a) Purpose. The purpose of the quick disengagement control is to ensure the capability for each pilot to manually disengage the autopilot quickly with a minimum of pilot hand/limb movement.
(b) Location. The quick disengagement control must be located on each control wheel or equivalent. See § 25.1329(a). It should be within easy reach of one or more fingers/thumb of the pilot’s hand when the hand is in a position for normal use on the control wheel or equivalent.
(c) Criteria. The quick disengagement control:
17 1 Must be operable with one hand on the control wheel or equivalent and the other hand on the thrust levers. See § 25.1329(a).
2 Should be accessible and operable from a normal hands-on position without requiring a shift in hand position or grip on the control wheel or equivalent.
3 Should be easily locatable by the pilot without having to first locate the control visually.
4 Should be designed so that any action to operate the quick disengagement control should not cause an unintended input to the control wheel (or equivalent).
5 Should be designed to minimize inadvertent operation and interference from other nearby control wheel (or equivalent) switches/devices, such as radio control or trim.
Note: When establishing location of the quick disengagement control, consideration should be given to its accessibility with large displacements of or forces on the control wheel (or equivalent) and the possible need to operate the quick disengagement control with the other hand.
(4) Alternative means of autopilot disengagement.
(a) Factors to consider. When a § 25.1309 assessment shows a need for an alternative means of disengagement, the following factors should be addressed:
1 Independence from primary quick disengagement control.
2 Whether alternative means are readily accessible to each pilot.
3 Reliability and the possibility of latent failure of those alternative means.
(b) Acceptable means. The following means of providing an alternative disengagement have been found to be acceptable:
1 Selecting the engagement control to the “off” position.
2 Disengaging the bar on mode select panel (MSP).
3 Activating the trim switch on the yoke.
Note: Use of circuit breakers as a means of disengagement is not considered acceptable.
(5) Flightcrew pitch trim input. If the autopilot is engaged and the pilot applies manual pitch trim input, and the autopilot is designed to disengage because of that flightcrew action, the autopilot must disengage with no more than a minor transient. See § 25.1329(c).
Alternatively, pitch trim changes may be inhibited, such that the potential for a transient is removed.
28. FD Engagement, Disengagement, and Indications. Engagement and disengagement of the FD should be accomplished consistent with other flightcrew procedures and tasks and should not require undue attention.
a. FD engagement.
(1) General.
(a) Selection. A means may be provided for each pilot to select (that is, turn on) and deselect (that is, turn off) the FD for display on his or her primary flight display (PFD).
(b) Engagement. A FD is considered “engaged” if it is selected and displaying guidance cues.
Note: The distinction is made between “engaged” and “selected” because the FD might be selected but not displaying guidance cue(s) (for example, the cue(s) are biased out of view).
(c) Engagement status of FD. The selection status of the FD and the source of FD guidance should be clear and unambiguous. Failure of a selected FD should be clearly annunciated.
(d) Multiple FDs. If there are multiple FDs and if necessary for crew awareness, indications should be provided to denote which FD is engaged (for example, FD1, FD2, HUD). For airplanes with multiple FDs installed, all engaged FDs should always be in the same armed and active FGS modes. The selection status of each FD should be clear and unambiguous for each pilot. In addition, indications should be provided to denote loss of FD independence (for example, first officer selection of captain’s FD).
(e) Autopilot engaged. A FD should engage into the current modes and targets of an already engaged autopilot or FD, if any. With no autopilot engaged, the basic modes at engagement of the FD functions should be established consistent with typical flight operations.
Note: The engagement of the pitch axis in Vertical Speed or Flightpath Angle and engagement of the lateral axis in Heading Hold, Heading Select, or Bank Angle Hold have been found to be acceptable.
(f) HUD. Since the HUD can display flight guidance, the HUD guidance mode should be indicated to both pilots and should be compatible with the active head down FD mode.
(g) Maneuvering flight. Engagement during maneuvering flight should be considered.
Note: The design should consider the safety consequences if it is possible for the FD to engage outside of the normal flight envelope. It is not required that the FD should compensate for unusual attitudes or other situations outside the normal flight envelope, unless that is part of the FD’s intended function.
(2) Guidance cue(s).
(a) Display. The FD command guidance cue(s) will typically be displayed under the following conditions:
1 When the FD is selected and valid command guidance is available, or 2 When the FD is automatically providing guidance. (See paragraph 28.a(3) below.)
(b) Engagement indication. The display of guidance cue(s) (for example, FD bars) is sufficient indication that the FD is engaged.
(c) Invalid guidance. The FD guidance cue(s) should be removed when guidance is determined to be invalid.
(3) Reactive wind shear guidance system. For airplanes equipped with a FD wind shear guidance system, FD engagement should be provided consistent with the criteria in the current editions of AC 25-12 and AC 120-41.
b. FD disengagement. There should be a means for each pilot to readily deselect his or her on-side FD function. Flightcrew awareness of disengagement and deselection is important.
Removal of guidance cue(s) alone is not sufficient indication of deselection because the guidance cue(s) may be removed from view for a number of reasons, including invalid guidance or autopilot engagement. Therefore, the FD function should provide clear and unambiguous indication to the flightcrew that the function has been deselected.
29. Autothrust Engagement, Disengagement, and Indications. Engagement and disengagement should be accomplished in a manner consistent with other flightcrew procedures and tasks and should not require undue attention.
a. Autothrust engagement.
(1) Indication. The autothrust function must provide the flightcrew positive indication that the system has been engaged. See § 25.1329(i).
(2) Accessibility of controls. The autothrust engagement controls should be accessible to each pilot.
(3) Inadvertent engagement/disengagement. The autothrust function should normally be designed to prevent inadvertent engagement and inadvertent application of thrust for both on-ground and in-air operations. For example, separate arm and engage functions may be provided.
(4) Automatic engagement. Intended automatic engagement, such as a “wake up” mode to protect for unsafe speeds, may be acceptable. (See Chapter 5, paragraph 57.b, Low-Speed Protection, of this AC.) If such automatic engagement occurs, it should be clear to the flightcrew that automatic engagement has occurred. The automatic engagement may not
20 cause a transient larger than a minor transient. See § 25.1329(c). The transition to the engaged state should be smooth, and should not cause large, unexpected changes in pitch attitudes or pitching moments. The reason for automatic engagement should be clear and obvious to the flightcrew.
Note: The design should consider the possibility that the pilot may attempt to engage the autothrust function outside of the normal flight envelope or at excessive or too low engine thrust. It is not expected that the autothrust feature should compensate for situations outside the normal flight envelope or normal engine operation range, unless that is part of the intended function of the autothrust system.
b. Autothrust disengagement.
(1) Indication of disengagement. Positive indication of disengagement of the autothrust function must result in a caution level alert to the flightcrew. See § 25.1329(k). The autothrust “engage” status annunciations should be deleted.
(a) Visual indication.
1 Following automatic disengagement: Visual indication of disengagement should persist until canceled by flightcrew action.
2 Following manual disengagement: If an aural alert is provided, visual indication of disengagement should persist for some minimum period. If an aural alert is not provided, visual indication of disengagement should persist until canceled by flightcrew action.
(b) Aural alert. If provided, the aural disengagement alert should be of sufficient duration and volume to assure that the flightcrew has been alerted that disengagement has occurred. However, an extended cycle of an aural alert is not acceptable following disengagement, if such an alert can significantly interfere with flightcrew coordination or radio communication.
(2) Inadvertent disengagement. The autothrust normally should be designed to preclude inadvertent disengagement during activation of autothrust modes of operation.
(3) Consequence of disengagement. Under normal conditions, autothrust disengagement may not cause a transient any greater than a minor transient. See § 25.1329(d).
The transition to the disengaged state should be smooth and not cause unexpected changes in pitch attitude or pitching moment or a significant thrust transient. The disengagement should not preclude, inhibit, or interfere with timely thrust changes for go-around, landing, or other maneuvers requiring manual thrust changes.
(4) Autothrust quick disengagement control. Autothrust quick disengagement controls must be provided for each pilot on the respective thrust control (thrust lever or equivalent). A single-action, quick disengagement switch must be incorporated on the thrust control, so that switch activation can be executed when the pilot’s other hand is on the control wheel (or equivalent). See § 25.1329(a). The disengagement control should be positioned such
21 that inadvertent disengagement of the autothrust function is unlikely. Positioning the control on the outboard side has been shown to be acceptable for multiengine aircraft. Thrust lever knob, end-mounted disengagement controls available on both sides to facilitate use by either pilot have been shown to be preferable to those positioned to be accessible by the pilot’s palm.
30. Override of the FGS.
a. General. An override of an engaged FGS function is defined as an action taken by the flightcrew intended to prevent, oppose, or alter an operation being conducted by the FGS function without first disengaging that function.
b. Autopilot override.
(1) With automatic disengagement.
(a) Override force. The autopilot should disengage when the flightcrew applies a significant override force to the controls. The applicant should interpret “significant” as a force that is consistent with an intention to overpower the autopilot by either or both pilots. The autopilot should not disengage by minor application of force to the controls, such as a pilot gently bumping the control column while entering or exiting a pilot seat during cruise.
Note: Twenty-five pounds (25 lbs) of force at the control column or wheel has been determined to be a significant override force level for other than approach operations on some aircraft types. To reduce nuisance disengagement, higher forces have been found acceptable for certain approach, landing, and go-around operations on some aircraft. The force to disengage an autopilot is not necessarily the force required at the column to oppose autopilot control, that is, to cause elevator movement. The corresponding forces for a sidestick controller may be different.
(b) Transients resulting from override. In normal operating conditions, a transient larger than a minor transient may not result from autopilot disengagement when the flightcrew applies an override force to the controls. See § 25.1329(d). Mitigation may be accomplished through provision of an appropriate alert and flightcrew procedure.
(c) Sustained override force below level required for automatic disconnect.
Sustained application of force below the disengagement threshold may not result in a potential hazard. See § 25.1329(l). For example, the automatic trim should not run to oppose the override of the autopilot by the flightcrew that would result in unacceptable airplane motion, if the autopilot were to automatically disengage or be manually disengaged.
(2) Without automatic disengagement.
(a) Potential hazard. If the FGS is not designed to disengage in response to any override force, then the response to an override may not result in a potential hazard.
Sustained application of an override force may not result in a potential hazard, such as when the flightcrew abruptly releases the force on the controls. See § 25.1329(l). Mitigation may be accomplished through provision of an appropriate alert and flightcrew procedure.
22 Note: The term “sustained application of override force” is intended to describe a force that is applied to the controls, which may be small, slow, and sustained for some period of time. This may be due to an inadvertent crew action or may be an intentional crew action meant to “assist” the autopilot in a particular maneuver.
(See Chapter 9, Compliance Demonstration Using Flight Test and Simulation, paragraph 99.f, Flightcrew Override of the FGS, of this AC for more information.)
(b) Transients resulting from override. In normal operating conditions, a transient larger than a minor transient may not result from manual autopilot disengagement after the flightcrew has applied an override force to the controls. See § 25.1329(d). Mitigation may be accomplished through provision of an appropriate alert and flightcrew procedure.
Note 1: The term “override force” is intended to describe a pilot action that is intended to prevent, oppose, or alter an operation being conducted by a flight guidance function without first disengaging that function. One possible reason for this action could be an avoidance maneuver, such as responding to a traffic alert and collision avoidance system (TCAS) resolution advisory that requires immediate action by the flightcrew and would typically involve a rapid and forceful input from the flightcrew.
Note 2: For control wheel steering considerations, see Chapter 6, Characteristics of Specific Modes, paragraph 68, Control Wheel Steering (Control Steering through the Autopilot), of this AC.
c. Autothrust override.
(1) Force required. It should be possible for the pilot to readily override the autothrust function and set thrust by moving the thrust levers (or equivalent) with one hand.
(2) Response to override. The autothrust response to a flightcrew override may not create a potential hazard. See § 25.1329(m). The autothrust response to the flightcrew override should not result in an abrupt change of pitch attitude, an abrupt pitching moment, or an uncontrolled change of thrust.
(3) Engagement status with override. Autothrust functions may be designed to remain safely engaged during pilot override. Alternatively, autothrust functions may disengage as a result of pilot override, provided that the design prevents unintentional autothrust disengagement and adequately alerts the flightcrew to ensure pilot awareness.
31. FGS Engagement Mode Compatibility.
a. FGS mode engagement philosophy. A description of the philosophy used for the mode at engagement of the autopilot, FD, and autothrust functions should be provided in flightcrew training material.