(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 (head up display)). 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 Flight Path 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 (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 Windshear Guidance System. For airplanes equipped with a FD windshear guidance system, FD engagement should be provided consistent with the criteria in AC 25–12, Airworthiness Criteria for the Approval of Airborne Windshear Warning Systems in Transport Category Airplanes; and AC 120–41, Criteria for Operational Approval of Airborne Windshear Alerting and Flight Guidance.
b. FD Disengagement. There should be a means for each pilot to readily de-select his or her on–side FD function. Flightcrew awareness of disengagement and de-selection 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 57b, Low Speed Protection.) If such automatic engagement occurs, it should be clear to the flightcrew that automatic engagement has occurred. The automatic engagement may not 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 29
flight envelope or norm al 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 that inadvertent disengagement of the autothrust function is unlikely.
Positioning the control on the outboard side has been shown to be acceptable for multi– engine 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 F GS.
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 Autom atic 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.
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 99f, Flightcrew Override of the FGS, 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 (CWS) [control steering through the autopilot].
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 safely remain 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 Engagmenet mode comp atibility.
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.
b. Engagement Mode Compatibility. It should not be possible to select incompatible FGS command or guidance functions, such as commanding speed through elevator and autothrust at the same time.
32 — 41 [Reserved]
This page left blank.
Chapter 4 Controls, Indication s, and Alerts
42. General.
a. Human-Machine Interface. The human-machine interface with the FGS is crucial to ensuring safe, effective, and consistent FGS operation. The manner in which FGS information is depicted to the flightcrew is vital to flightcrew awareness and, therefore, to safe operation of the FGS.
b. Design of Controls, Indications, and Alerts. These features must be designed to minimize flightcrew errors and confusion. [See § 25.1329(i)] Indications and alerts should be presented in a manner compatible with the procedures and assigned tasks of the flightcrew and provide the necessary information to perform those tasks. The indications must be grouped and presented in a logical and consistent manner and must be visible from each pilot’s station under all expected lighting conditions. [See § 25.1329(i)] The choice of colors, fonts, font size, location, orientation, movement, graphical layout, and other characteristics—such as steady or flashing—should all contribute to the effectiveness of the system. Controls, indications, and alerts should be implemented in a consistent manner.
c. Evaluation of Information Provided by FGS. It is recommended that the applicant evaluate the adequacy and effectiveness of the information provided by the FGS interface, that is the controls, indications, alerts, and displays, to ensure flightcrew awareness of FGS behavior and operation. See Chapter 9, Compliance Demonstration Using Flight Test and Simulation, for more discussion of appropriate analyses which may include, for example, cognitive task analysis as a basis for evaluation.
43. FGS Controls.
a. General. The FGS controls must be designed and located to prevent crew errors, confusion, and inadvertent operation. [See § 25.1329(i)] They should be designed to provide convenience of operation to each crewmember. The function and direction of motion of each control must be readily apparent or plainly indicated on or adjacent to each control, if needed to prevent inappropriate use or confusion. [See § 25.1329(f)] Sections 25.777(b) and 25.779(a) provide requirements regarding direction of motion of flight deck controls. These requirements apply for FGS command reference controls which select target values, such as heading select or vertical speed. Section 25.781 also provides requirements for the shapes of the knobs.
b. Design Considerations. The design of the FGS should address the following specific considerations:
(1) Differentiation of Knob Shape and Position. Errors by the flightcrew have included confusing speed and heading knobs on the MSP.
(2) Design to Support Correct Selection of Target Values. Use of a single control, such as concentric controls, for selecting multiple command reference targets has resulted in erroneous target value selection.
(3) Commonality Across Aircraft Types. Use of uniform control design in different aircraft prevents negative transfer of learning with respect to operation of the controls. Using knowledge about the operation of the controls on one type of airplane has resulted in activation of the incorrect control when pilots move from that type of airplane to another type of airplane. An example of this is confusion between the takeoff/go-around button and the autothrust disengagement button when transitioning between aircraft types with differing designs and locations for these controls.
(4) Positioning of Controls and Related Indications. Individual FGS controls, flight mode annunciator (FMA), and related PFD information should be positioned so that, as much as reasonably practical, items of related function have similarly related positions. Misinterpretation and confusion have occurred due to the inconsistent arrangement of FGS controls with the annunciations on the FMA.
(5) Inadvertent Operation. The FGS controls must be located to discourage or avoid inadvertent operation, such as inadvertent engagement or disengagement. [See § 25.777(a)] Further design aspects of the FGS controls, other than location of the controls, should also be used discourage or avoid inadvertent operation. Examples include the shape of the control and the force required for operation.
44. FGS Mode selection, annunciation, and indication.
a. Annunciation of Engagem ent of the FGS. Engagement of the FGS functions must be suitably annunciated to each pilot. [See § 25.1329(i)] Guidance is provided in Chapter 3, FGS Engagement, Disengagement, Indications, and Override.
b. Description of FGS Modes. The operator should be provided with appropriate descriptions of the FGS modes and their behaviors.
c. FGS Mode Annunciations. Mode annunciation must indicate the state of the system. [See § 25.1329(i)] Mode annunciation should be presented in a manner compatible with flightcrew procedures/tasks and be consistent with the mode annunciation design for the specific aircraft type. That is, the mode annunciation should be compatible with other flight deck systems mode annunciations. Mode selector switch position or status is not acceptable as the sole means of mode annunciation. [See § 25.1329(i)] Modes and mode changes should be depicted in a manner that achieves flightcrew attention and awareness.
(1) Active and Armed Modes. The FGS mode annunciations must effectively and unambiguously indicate the active and armed modes of operation. [See § 25.1329(i)]
Change 1
The m ode annunciation should convey explicitly, and as simply as possible, what the FGS is doing (for active modes), what it will be doing (for armed modes), and target information (such as selected speed, heading, and altitude) for satisfactory flightcrew awareness.
(2) Location. Mode annunciations should be located in the forward field of view. For example, a suitable location is on the PFD. Engaged modes should be annunciated at different locations than armed modes to assist in mode recognition.
(3) Discriminators. Color, font type, font size, location, highlighting, and symbol flashing have historical precedent as good discriminators, when implemented appropriately. The fonts and font size should be chosen so that annunciation of FGS mode and status information is readable and understandable without eyestrain when viewed by the pilot seated at the design eye position. The use of graphical or symbolic (that is, non-textual) indications is acceptable. Implementation of such discriminators should follow accepted guidelines, as described in applicable international standards (such as AC 25-11A, Transport Category Airplane Electronic Display Systems), and should be evaluated for their consistency with and integration with the flight deck design.
(4) Color. Color should be used in a consistent manner and assure compatibility with the overall use of color on the flight deck. Specific colors should be used such that the FGS displays are consistent with other flight deck systems, such as a flight management system (FMS). The use of monochrome displays is not precluded, provided that the aspects of flightcrew attention and awareness are satisfied. Engaged modes should be annunciated with different colors than armed modes to assist in mode recognition.
d. Mode Changes.
(1) Operationally Relevant Mode Changes. Mode changes that are operationally relevant—especially mode reversions and sustained speed protection— should be clearly and positively annunciated to ensure flightcrew awareness. Altitude Capture is an example of an operationally relevant mode that should be annunciated, because pilot actions during that brief time the mode is operational may have different effects on the airplane. Annunciation of sustained speed protection should be clear and distinct to ensure flightcrew awareness. The FGS sub–modes that are not operationally relevant (for example, sub-modes as the FGS transitions from instrument landing system Localizer (LOC) Capture to Localizer Track) need not be annunciated.
(2) Attention-Getting Features. The transition from an armed mode to an engaged mode should provide an additional attention–getting feature, such as boxing and flashing on an electronic display (see AC 25-11A) for a suitable, but brief, period (for example, ten seconds) to assist in flightcrew awareness. Aural notification of mode changes should be limited to special considerations.
(3) Use Of Alerts. In–service experience has shown that mode annunciation alone may be insufficient—unclear or not compelling enough—to communicate mode changes to the flightcrew, especially in high workload situations. Therefore, the safety consequences of the flightcrew not recognizing mode changes should be considered. If necessary, an appropriate alert should be used.
e. Failure to Engage or Arm. The failure of a mode to engage or arm when selected by the pilot should be apparent.
f. FGS Mode Display and Indications. Mode information provided to the flightcrew should be sufficiently detailed, so that the consequences of the interaction between the FGS and the flightcrew can be determined unambiguously. The FGS interface should provide timely and positive indication when the FGS deviates from the pilot's direct commands (for example, a target altitude or speed setting) or from the pilot's pre-programmed set of commands (for example, waypoint crossing). The interface should also provide clear indication when there is a difference or conflict between pilot-initiated commands. An example would be when a pilot engages positive vertical speed and then selects an altitude that is lower than the aircraft altitude. The default action taken by the FGS should be made apparent.
45. FGS Alerting, warning, caution, advisory, and status.
a. General.
(1) Alerting Provisions. Alerting information must follow the provisions of § 25.1322. The advisory material associated with § 25.1322 should be consulted. Alerts for FGS engagement and disengagement are described in Chapter 3, FGS Engagement, Disengagement, Indications, and Override.
(2) Determination of FGS Capability. There should be some method for the flightcrew to determine and monitor the availability or capability of the FGS (for example, prior to dispatch), where the intended operation is predicated on the use of the FGS. The method of monitoring provided should take account of the hazard resulting from the loss of the autopilot function for the intended operation.
b. Speed Protection Alerts.
(1) Alerts to Crew. To assure crew awareness, an alert should be provided when a sustained speed protection condition is detected. This is in addition to any annunciations associated with mode reversions that occur as a consequence of invoking speed protection (See Chapter 5, Performance of Function, paragraph 57, Speed Protection).
(2) Alert Specifications.
(a) Low Speed Protection. Low speed protection alerts should include both an aural and a visual component.
(b) High Speed Protection. High-speed protection alerts need include only a visual alert component because of existing high–speed aural alert requirements.
However, giving an alert prior to that required aural alert is not precluded. [Refer to § 25.1303(c)(1) for overspeed alerting regulations.]
(3) Consistency. Alerts for speed protection should be consistent with the protection provided and with the other alerts in the flight deck.
(4) Nuisance Alerts. Care should be taken to set appropriate values for indicating speed protection that would not be considered a nuisance for the flightcrew.
c. Loss of Autopilot Approach Mode. The loss of the Approach mode requires immediate flightcrew awareness. This may be accomplished through autopilot disengagement described in AC 120-28D, Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout. If the autopilot remains engaged and reverts to a non-approach mode, an appropriate aural warning and/or visual alert should be provided.
d. Awareness of Potential Significant Transien t Condition (“Bark Before Bite”).
(1) General. There have been situations where an autopilot was engaged, operating normally and controlling up to the limit of its authority for an extended period of time, and the flightcrew was unaware of the potential hazard. This service experience has shown that, without timely flightcrew awareness and action, such a situation can progress to a loss of control after autopilot disengagement, particularly in rare normal or non-normal conditions. However, with adequate flightcrew awareness and pilot action, loss of control may be prevented.
(2) Flightcrew Alert(s). To help ensure flightcrew awareness and timely action, appropriate alert(s)—normally a caution or warning—should be provided to the flightcrew for conditions that could require exceptional piloting skill or alertness for manual control following autopilot disengagement (for example, significantly out of trim conditions). The number and type of alerts required should be determined by the unique situations that are being detected and by the crew procedures required to address those situations. Any alert should be clear and unambiguous and should be consistent and compatible with other flight deck alerts. Care should be taken to set appropriate thresholds for these alerts so that they are not considered a nuisance for the flightcrew.