(3) Situations to Consider for an Alert.
( a ) Sustained Lateral Control Command. If the autopilot is holding a sustained lateral control command, it could be indicative of an unusual operating condition for which the autopilot is compensating. Examples of such unusual operating conditions are asymmetric lift and/or drag due to asymmetric icing, fuel imbalance, or asymmetric thrust. In the worst case, the autopilot may be operating at or near its full authority in one direction. If the autopilot were to disengage while holding this lateral trim, the result would be that the airplane could undergo a rolling moment that could possibly take the pilot by surprise. Therefore, a timely alert should be considered to permit the crew to manually disengage the autopilot and take control prior to any automatic disengagement that might result from the condition.
(b) Sustained Pitch Command. If the autopilot is holding sustained pitch command, it could be indicative of an unusual operating condition (for example, inoperative automatic horizontal trim) for which the autopilot is compensating. If the autopilot were to disengage while holding this pitch command, the result would be that the airplane could undergo an abrupt change in pitch that could possibly take the pilot by surprise. Therefore, a timely alert should be considered to permit the crew to manually disengage the autopilot and take control prior to any automatic disengagement that might result from the condition.
(c) Bank and Pitch Angles. Most autopilots are designed with operational limits in both the pitch and roll axes, such that those predetermined limits will not be purposely exceeded. If the airplane exceeds those limits, it could be indicative of a situation that requires the pilot to intervene. Such a situation may not be covered by paragraphs (a) or (b) above. Therefore, a timely alert should bring this condition to the attention of the flightcrew and permit the crew to manually disengage the autopilot and take control prior to any automatic disengagement which might result.
(4) Automatic Disengagement. It is preferable that the autopilot remains engaged during out–of–trim conditions. However, if there is an automatic disengagement feature due to excessive out–of–trim, an alert should be generated and should precede any automatic disengagement with sufficient margin to permit timely flightcrew recognition and manual disengagement. See also Chapter 3, FGS Engagement, Disengagement, Indications, and Override, paragraph 30, Override of the FGS, for related material.
Note 1: This paragraph is not intended to require alerting for all instances of automatic autopilot disengagement. It is intended only for conditions that would, if not addressed, lead to disengagement that could result in a significant transient (or greater) for which the pilot may be unprepared. The intent is to provide crew awareness that would allow the flightcrew to be prepared with hands on controls and take appropriate corrective action before the condition results in a potentially hazardous airplane configuration or state.
Note 2: This paragraph describes alerting requirem ents for conditions resulting in unintended out–of–trim operation. There are FGS functions that can intentionally produce out-of-trim conditions. Examples would be parallel rudder operation in align or engine failure compensation modes, pitch trim operation during the approach/landing to provide trim up/flare spring bias, and pitch trim operation for certain types of speed/Mach trim systems. It is not the intent of this paragraph to require alerts for functions producing intentional out–of–trim conditions. Other system indications (for example, mode and status annunciations) should be provided to make the crew aware of the operation of these functions, where appropriate.
46. FGS Considerations for head up displays (HUD).
a. General. HUDs have unique characteristics compared to flight displays installed on the instrument panel. Most of these HUD differences are addressed during HUD certification whether or not the HUD provides flight guidance functions. The intent of this chapter is to address how such HUD differences may affect FGS functions. (See SAE ARP5288, Transport Category Airplane Head Up Display Systems, for further information on this subject. An address for obtaining SAE ARP documents is provided in paragraph 4 of this AC.)
b. Characteristics of HUD Guidance.
(1) Supplemental Use Display System. If the HUD is designed as a supplemental use display system, it does not replace the requirement for standard head down display (HDD) of flight instrument data. The HUD is intended for use during takeoff, climb, cruise, descent, approach, and landing under day, night, visual meteorological conditions (VMC), and instrument meteorological conditions (IMC) conditions. When it can be reasonably expected that the pilot will operate primarily by reference to the HUD, it should be shown that the HUD is satisfactory for manually controlling the airplane and for monitoring the performance of the FGS.
(2) External Visual References. During takeoff and landing in certain light and visibility conditions, HUD symbology can be extremely dominant in comparison to external visual references. When visual references are relatively dim, extremely active symbology dynamics and guidance cue gains can lead the pilot to make excessively strong corrections. It should be shown that—if HUD guidance cues are followed regardless of the appearance of external visual references—they do not cause the pilot to take unsafe actions.
(3) Interference and Occlusion. It should be shown that there is no interference between the indications of primary flight information and the flight guidance cues. In takeoff, approach, and landing FGS modes, the flight guidance symbology should have occlusion priority. That is, the flight guidance symbology should not be obscured or covered by the primary flight information. The HUD guidance symbology should not 41
excessiv ely interfere with pilots’ forward view, ability to visually maneuver the airplane, acquire opposing traffic, and see the runway environment.
(4) Display Criteria. Generally, the criteria for the mechanization of guidance displayed on the HUD should be no different than for guidance displayed on the HDD.
See Chapter 5, Performance of Function, for FD performance criteria.
(5) Conformal Symbology. Unlike HDDs, HUDs are capable of displaying certain symbology conformal to the outside scene, including guidance cues.
Consequently, the range of motion of this conformal symbology, including the guidance cues, can present certain challenges in rapidly changing and high crosswind conditions.
In certain cases, the motion of the guidance and the primary reference cue may be limited by the field of view. It should be shown that, in such cases, the guidance remains usable and that there is a positive indication that it is no longer conformal with the outside scene.
(6) Low Visibility Approach. The HUD guidance is often used in cases, like the low visibility approach, where the pilot will need to reference both the information displayed on the HUD and outside references. Consequently, it should be shown that the location and presentation of the HUD information does not distract the pilot or obscure the pilot’s outside view. For example, it would be necessary for the pilot to track the guidance to the runway without having the view of runway references or hazards along the flight path obscured by the HUD symbology.
(7) Clutter. The HUD display should present flight guidance information in a clear and unambiguous manner. Display clutter should be minimized. Some flight guidance data elements are essential or critical and should not be removed by any declutter function.
c. HUD/Head Down Display (HDD) Compatibility.
(1) General. The HUD FGS symbology should be compatible and consistent with symbology on other FGS displays such as head down electronic flight instruments.
The FGS-related display parameters should be consistent to avoid misinterpretation of similar information, but the display presentations need not be identical. The HUD and head down PFD formats and data sources need to be compatible to ensure that the same FGS-related information presented on both displays have the same intended meaning.
(2) Consistency and Compatibility. While not all information displayed on the HUD is directly related to the FGS, the pilot is likely to use most of the displayed information while using the HUD-displayed guidance and FGS annunciations.
Therefore, when applicable, the guidelines below for the presentation of FGS-related display information should be followed as much as possible. Certain deviations from these guidelines may be appropriate due to conflict with other information display characteristics or requirements unique to HUDs. These may include minimization of display clutter, minimization of excessive symbol flashing, and the presentation of certain information conformal to the outside scene.
(a) Symbology. Symbols should be the same format. For example, a triangle-shaped pointer head down should appear as a triangle pointer head up. However, some differences in HUD symbology, such as the FD “circle” versus head down FD “bars” or “wedge,” have been found acceptable.
(b) Location. Information (symbols) should appear in the same general location relative to other information.
(c) Alphanumeric Data. Alphanumeric readouts should have the same resolution, units, and labeling. For example, the command reference indication for “vertical speed” should be displayed in the same foot-per-minute increments and be labeled with the same characters as the HDDs.
(d) Analog Scales. Analog scales or dials should have the same range and dynamic operation. For example, a glideslope deviation scale displayed head up should have the same displayed range as the glideslope deviation scale displayed head down, and the direction of movement should be consistent.
(e) Display of FGS Modes. The FGS modes and status state transitions should be displayed on the HUD using consistent methods, except for the use of color.
That is, the method used head down to indicate a FD mode transitioning from armed to captured should also be used head up.
(f) Information Sources. Information sources should be consistent between the HUDs and the HDDs used by the same pilot.
(g) Display of FGS Command Information. When FGS command information (for example, a FD command) is displayed on the HUD in addition to the HDDs, the HUD depiction and guidance cue deviation “scaling” needs to be consistent with that used on the HDDs. This is intended to provide comparable pilot performance and workload when using either HUDs or HDDs.
(h) Display for Pilot Not Flying. The same information concerning current HUD system mode, reference data, status state transitions, and alert information that is displayed to the pilot flying on the HUD should also be displayed to the pilot not flying on the HDDs. Consistent nomenclature should be used to ensure unambiguous awareness of the HUD operation.
d. Alerting Issues.
(1) Alerts Displayed on HUDs. Although HUDs are typically not intended to be classified as integrated caution and warning systems, they may display cautions, warnings, and advisories as part of their FGS function. In this regard, HUDs should provide the equivalent alerting functionality as the head down PFD(s). Warnings that require continued flightcrew attention on the PFD also should be presented on the HUD
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(for exam ple, TCAS, windshear, and ground proximity warning (GPWS) annunciations).
If master alerting indications are not provided within the peripheral field of view of the pilot while using the HUD, the HUD should provide annunciations that inform the pilot of caution and/or warning conditions.
(2) Monochrome HUDs. For monochrome HUDs, appropriate use of attentiongetting properties, such as flashing, outline boxes, brightness, size, and/or location are necessary to adequately compensate for the lack of color normally assigned to distinguish and call attention to cautions and warnings.
(3) Multi–Color HUDs. For multi-color HUDs, the use of red, amber, or yellow for symbols not related to caution and warning functions should be avoided, so that the effectiveness of distinguishing characteristics of true cautions and warnings is not reduced.
(4) Single HUD Installation. Single HUD installations rely on the fact that the pilot not flying (PNF) will monitor the head down instruments and alerting systems for failures of systems, modes, and functions not associated with PFDs.
(5) Dual HUD Installations. Dual HUD installations require special consideration for alerting systems. It should be assumed that both pilots will be head up simultaneously (full or part-time), especially when the HUD is being used as the primary flight reference or when the HUD is required equipment for the operation being conducted. If master alerting indications are not provided within the peripheral field of view of each pilot while using the HUD, then each HUD should provide annunciations that direct the pilot’s attention to head down alerting displays. The types of information that should trigger the HUD master alerting display are any cautions or warnings not already duplicated on the HUD from head down primary display as well as any caution level or warning level engine indications or system alerts.
Note: The objective is to avoid redirecting attention of the pilot flying (PF) to another display when an immediate maneuver is required (for example, TCAS resolution advisory, windshear).
(6) Ground Proximity Warning System (GPWS)/Traffic Alert and Collision Avoidance System (TCAS) on HUD. If a GPWS, wind shear detection system, wind shear escape guidance system, or a TCAS is installed, then the guidance, warnings and annunciations required to be a part of these systems and normally required to be in the pilot’s primary field of view should be displayed on the HUD.
e. Upset/Unusual Attitude Recovery Guidance.
(1) Guidance Cues. Upsets due to wake turbulence or other environmental conditions may result in near instantaneous excursions in pitch and bank angles and a subsequent unusual attitude. If the HUD is designed to provide guidance for recovery from upsets or unusual attitudes, recovery steering guidance commands should be distinct 44
from and not confused with orientation symbology such as horizon “pointers.” For example, a cue for left control wheel/stick input should not be confused with a cue indicating direction to the nearest horizon. Guidance should be removed if cues become invalid at extreme attitudes, such as zenith, nadir, overbanked (for example, greater than 90 degrees bank angle), or inverted. For extreme attitudes, it is acceptable for the pilot to transition to the HDD, provided that the cues to transition from the HUD are clear and unambiguous.
(2) Orientation Cues. If the HUD is designed to provide only orientation during upsets or unusual attitudes (that is, no guidance cues are provided), those orientation cues should be designed to prevent them from being mistaken as flight control input commands.
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Chapter 5 Performance of Function
52. Genera
l..
a. Intended Function. The FGS should provide guidance or control, as appropriate, for the intended function of the active mode(s) in a safe and predictable manner within the airplane’s normal flight envelope. There are special considerations when the FGS is operating at the limits of its performance capabilities or under significant environmental conditions. The following paragraphs provide criteria for acceptable means of compliance and interpretive material pertaining to these considerations.
b. Effect of System Tolerances. Where system tolerances have a significant effect on autopilot authority limits, consideration should be given to the effect on autopilot performance. Factors to be considered include, but are not limited to, tolerances of servo authority, servo clutch setting, “cam-out” settings, control friction, and sensor tolerances.
53. Nornal performance.
a. General. 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 within the airplane’s normal flight envelope.
b. Conditions to Consider. The design of the FGS should consider the normal conditions listed in the following table. This 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 “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 table.
Table 5-1 Examples of Normal Conditions
No f ailure conditions All airplane systems that are associated with airplane performance are fully operational. Failures of those systems could impair the flight guidance system's (FGS) ability to perform its functions.
Light to moderate winds Constant wind in a specific direction that may cause a slight deviation in intended flight path or a small difference between airspeed and groundspeed.
Light to moderate wind gradients Variation in wind velocity—as a function of altitude, position, or time—which may cause slight erratic or unpredictable changes in intended flight path.
Light to moderate gusts Non–repetitive momentary changes in wind velocity that can cause changes in altitude and/or attitude to occur, but the aircraft remains in positive control at all times.
Light turbulence Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude (pitch, roll, or yaw).
Moderate turbulence Similar to light turbulence but of greater intensity. Changes in altitude and/or attitude occur, but the aircraft remains in positive control at all times.
Light chop Turbulence that causes slight, rapid, and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude.
Moderate chop Similar to light chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude.
Icing All icing conditions covered by 14 CFR Part 25, Appendix C with the exception of “asymmetric icing” discussed under “Rare Normal Conditions” in Table 5-2.
Note: Representative levels of the environmental effects should be established consistent with the airplane’s intended operation.
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c. Level of Performance.
(1) Significant Characteristics. Any performance characteristics that are operationally significant or operationally limiting must be identified with an appropriate statement or limitation in the airplane flight manual (AFM). [See § 25.1581]
(2) Configuration Changes. The FGS should perform its intended function during routine airplane configuration or power changes, including the operation of secondary flight controls and landing gear.
(3) Compliance Evaluation. Evaluation of FGS performance for compliance should be based on the minimum level of performance needed for its intended functions.
Subjective judgment may be applied to account for experience acquired from similar equipment and levels that have been established as operationally acceptable by the end-user.
(4) Prescribed Levels of Performance. There are certain operations that dictate a prescribed level of performance. When the FGS is intended for operations that require specific levels of performance, the use of FGS should be shown to meet those specific levels of performance (for example, low visibility operations – Category II and III operations, reduced vertical separation minimums (RVSM), required navigation performance (RNP)).
(5) Equivalence of Performance. The FGS performance of intended functions should at least be equivalent to that expected of a pilot for a similar task. AC 25-7C, Flight Test Guide for Certification of Transport Category Airplanes, and the SAE ARP5366, Autopilot, Flight Director and Autothrust Systems, may prove useful for establishing the general behavior of the FGS. When integrated with navigation (NAV) sensors or the FMS, the FGS should satisfy the flight technical error tolerances expected for the use of those systems in performing their intended functions.
(6) Autopilot. The autopilot should provide smooth and accurate control without divergent or perceptible sustained nuisance oscillation.
(7) Flight Director (FD). The FD, in each available display presentation (for example, single cue, cross–pointer, flight path director) should provide smooth and accurate guidance and be appropriately damped, to achieve satisfactory control task performance without pilot compensation or excessive workload.
(8) Autothrust. The autothrust function should provide smooth and accurate control of thrust without significant or sustained oscillatory power changes or excessive overshoot of the required power setting.
(9) Automatic Trim. The automatic pitch trim function should operate at a rate sufficient to mitigate excessive control surface deflections or limitations of control 49
authority without introducing adverse interactions with autom atic control of the aircraft.
Automatic roll and yaw trim functions, if installed, should operate without introducing adverse interactions with automatic control of the aircraft.
54. Performance in rare normal conditions.
a. General.
(1) Range of Conditions. The FGS will encounter a wide range of conditions in normal operations, some of which may be infrequent but levy a greater than average demand on the FGS capabilities. Certain environmental conditions, as discussed in paragraph b, below, are prime examples. The FGS performance during such rare normal conditions should be assessed.
(2) Effect on Performance. Rare normal conditions may degrade the performance of the FGS but should be safe for operation of the FGS. The relative infrequency of such conditions may also be a factor in the flightcrew’s ability to detect and mitigate in a timely manner any limited capability of the FGS to cope with them.
b. Conditions to Consider. The design of the FGS should consider the rare normal conditions listed in the following table. This 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 “rare 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 table.
Table 5-2 Examples of Rare Normal Conditions
Significant winds Constant wind in a specific direction that may cause a large change in intended flight path or groundspeed or cause a large difference between airspeed and groundspeed.
Significant wind gradients Variation in wind velocity—as a function of altitude, position, or time—which may cause large changes in intended flight path.
Windshear/microburst A wind gradient of such magnitude that it may cause damage to the aircraft.
Large gusts Non-repetitive momentary changes in wind velocity that can cause large changes in altitude and/or attitude to occur. Aircraft may be momentarily out of control.
Severe turbulence Turbulence that causes large, abrupt changes in altitude or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control.
Asymmetric icing Icing conditions that result in ice accumulations that causes the FGS, if engaged, to counter the aerodynamic effect of the icing conditions with a sustained pitch, roll, or yaw command that approaches its maximum authority.
Note: Airplanes intended to meet § 121.358 for windshear warning and guidance need FD windshear guidance. The FGS may also provide suitable autopilot control during windshear. Refer to AC 25-12 and AC 120-41 for wind shear guidance system requirements.