Appendix F during all foreseeable operational conditions and against expected external scenes. Readability of the displays should be satisfactory in all foreseeable operating and ambient lighting conditions. SAE ARP5288 and SAE AS8055 provide guidelines for contrast and luminance control.
F.4.7.2 Reflections. The HUD must be free of glare and reflections that could interfere with the normal duties of the minimum flightcrew, as required by §§ 25.773 and 25.1523.
F.4.7.3 Ghost Images. A ghost image is an undesired image appearing at the image plane of an optical system. Reflected light may form an image near the plane of the primary image. This reflection may result in a false image of the object or an out-of-focus image of a bright source of light in the field of the optical system. The visibility of ghost images within the HUD of external surfaces must be minimized so as not to impair the pilot’s ability to use the display.
F.4.7.4 Accuracy and Stability.
F.4.7.4.1 Sensitivity to Airplane Maneuvering. The system operation should not be adversely affected by airplane maneuvering or changes in attitude encountered in normal service.
F.4.7.4.2 Motion of Symbols. The accuracy of positioning of symbols must be commensurate with their intended use. Motion of non-conformal symbols must be smooth, not sluggish or jerky, and consistent with airplane control response. Symbols must be stable with no discernible flicker or jitter.
F.5 Guidelines for Presenting Information.
F.5.1 HUD and HDD Display Compatibility.
F.5.1.1 General. If the content, arrangement, or format of the HUD is dissimilar to the HDD, it can lead to flightcrew confusion, misinterpretation, and excessive cognitive workload. During transitions between the HUD and HDDs (whether required by navigation duties, failure conditions, unusual airplane attitudes, or other reasons), dissimilarities could make it more difficult for the flightcrew to manually control the airplane or to monitor the automatic flight control system. Dissimilarities could also delay the accomplishment of time-critical tasks. Some differences may be unavoidable, such as the use of color on the HDD and a single color (i.e., monochrome) on the HUD. The guidelines listed below are intended to minimize the potential for confusion, undue workload, and delays in flightcrew task performance.
F.5.1.2 Exceptions. Deviation from the guidelines below may be unavoidable due to conflict with other information display characteristics or requirements unique to HUDs. These deviations may relate to the minimization of F-10
Appendix F display clutter, minimization of excessive symbol flashing, and the presentation of certain information conformal to the outside scene.
Deviations from these guidelines require additional pilot evaluation.
F.5.1.3 Guidelines for HUD-HDD Display Compatibility.
F.5.1.3.1 Consistent Displays and Format. The content, arrangement, symbology, and format of the information on the HUD should be sufficiently compatible with the HDDs to preclude pilot confusion, misinterpretation, increased cognitive workload, or flightcrew error. (See paragraphs 5.3 and
5.6 of this AC.) The layout and arrangement HUD and HDD formats of the same information need to convey the same intended meanings. (See paragraph 6.2 of this AC.) For example, the relative locations of barometric altitude, airspeed, and attitude should be similar. Likewise, the acronyms and relative locations of flight guidance mode annunciations for thrust and lateral and vertical flight path should be similar.
F.5.1.3.2 Symbols. Table F-2 provides the guidelines for symbols.
Table F-2. Symbol Guidelines for HUD-HDD Compatibility Symbol Guidelines Shape and HUD symbols that have similar shape and appearance as Appearance HDD symbols should have the same meaning. It is not acceptable to use similar symbols for different meanings.
Symbols that have the same meaning should have the same shape and appearance on the HUD and HDDs.
Special Symbolic Special display features or changes may be used to convey Features particular conditions, such as an overlaid “X” to mean failure of a parameter, a box around a parameter to convey that its value changed, a solid line/shape changing to a dashed line/shape to convey that its motion is limited, and so on. To the extent that it is practical and meaningful, the same display features should be used on the HUD as on the HDD.
Relative Location Information that relates to the symbols should appear in the same general location relative to other information.
F.5.1.3.3 Alphanumeric Information. Alphanumeric (i.e., textual) information 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 labeled with the same characters as on the HDDs. Likewise, the same terminology should be used for labels, modes, and alert messages on the HUD as on the HDDs. If the F-11
Appendix F design has exceptions to this principle, then they should be justified by necessity or impracticality, and shown not to increase workload or the potential for confusion or flightcrew error.
F.5.1.3.4 Analog Scales or Dials. 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 when it is displayed head-down, and the direction of movement should be consistent.
F.5.1.3.5 Flight Guidance Systems. Modes of flight guidance systems (e.g., autopilot, flight director, and autothrust) and state transitions (e.g., land 2 to land 3) should be displayed on the HUD. Except for the use of color, the modes should be displayed using consistent methods (e.g., the method used head-down to indicate a flight director mode transitioning from armed to captured should also be used head-up).
F.5.1.3.6 Command Information. When command information (e.g., flight director commands) is displayed on the HUD in addition to the HDDs, the HUD guidance cue and path deviation scaling (i.e., dots of lateral and vertical deviation) need to be consistent with that used on the HDDs. There may be cases when the other pilot is using the HDD of guidance and path deviations to monitor the flying pilot’s performance. Therefore, the HDD must have path deviation scaling that is sufficiently consistent with the HUD so as not to mislead the monitoring pilot.
F.5.1.3.7 Sensor Sources. Sensor system sources for instrument flight information
(e.g., attitude, direction, altitude, and airspeed) should be consistent between the HUD and the HDDs used by the same pilot.
F.5.1.4 Head-Up to Head-Down Transition.
F.5.1.4.1 Transition Scenarios. The applicant should identify conditions for which the pilot transitions between the HUD and the HDD and develop scenarios for evaluation (e.g., simulation or flight test). These scenarios should include systems failures and events leading to unusual attitudes. Transition capability should be shown for all foreseeable modes of upset.
F.5.1.4.2 Unambiguous Information. While the HUD and HDD may display information (e.g., flight path, path deviation, or airplane performance information) in a different manner, the meaning must be the same and any differences should not create confusion, misinterpretation, unacceptable delay, or otherwise hinder the pilot’s transition between the two displays.
The pilot should be able to easily recognize and interpret information on the HUD. The information should not be ambiguous with similar information on other airplane flight deck displays.
F.5.2 Indications and Alerts.
F.5.2.1 Monochrome Attention-Getting Properties. To comply with § 25.1322, and considering that most HUDs are predominantly monochrome devices, F-12
Appendix F the HUD should emphasize the display of caution and warning information with the appropriate use of attention-getting properties such as flashing, outline boxes, brightness, size, and/or location to compensate for the lack of color coding. For additional alerting guidance, see AC 25.1322-1, Flightcrew Alerting. The applicant should develop and apply a consistent documented philosophy for each alert level. These attention-getting properties should be consistent with those used on the HDDs. For example, flashing icons on the HUD should indicate situations with the same level of urgency as flashing icons on the HDDs.
F.5.2.2 Time-Critical Alerts on the HUD. For some phases of flight, airworthiness approval may be predicated on the use of the HUD. In these phases of flight, it can be reasonably expected that the pilot operates primarily by using the HUD, so the objective is to not redirect attention of the pilot flying (PF) to another display when an immediate maneuver is required
(e.g., resolution advisory or windshear). The applicant should provide in the HUD the guidance, warnings, and annunciations of certain systems, if installed, such as a Terrain Awareness and Warning System (TAWS), or a traffic alert and collision avoidance system (TCAS) and a windshear detection system. If the provision of TCAS or windshear guidance is not practical on the HUD, the applicant should provide compensating design features and pilot procedures (e.g., a combination of means such as control system protections and an unambiguous reversion message in the HUD) to ensure that the pilot has equivalent and effective visual information for immediate awareness and response to the respective alerts.
F.5.2.3 Additional Resources. Additional guidance on indications and alerts is in AC 25.1329-1B, Approval of Flight Guidance Systems, and AC 25.1322-1 Flightcrew Alerting, and the associated regulations.
F.5.3 Display Clutter.
This AC addresses display clutter for traditional displays on the instrument panel.
However, because the pilot must see through the HUD, special attention is needed to avoid display clutter that would otherwise unduly obscure the outside view.
F.5.4 Display of Information.
F.5.4.1 General. The HUD information display requirements depend on the intended function of the HUD. Specific guidance for displayed information is in the main body and appendix A of this AC. In addition, the following sections provide guidance related to unique characteristics of the HUD. As in the case of other flight deck displays, new and novel display formats may be subject to human factors evaluation of the pilot interface by an airworthiness authority.
F.5.4.2 Alternate Formats for Primary Flight Information.
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F.5.4.2.1
F.5.4.2.2 F.5.4.2.2.1 F.5.4.2.2.2 F.5.4.2.2.3 F.5.4.2.3 F.5.4.2.4 F.5.4.3 F.5.4.3.1 F.5.4.3.2 Appendix F Phase of Flight. There may be certain operations and phases of flight during which certain primary flight reference indications in the HUD do not need to have the analog cues for trend, deviation, and quick glance awareness that would normally be necessary. For example, during the precision approach phase, HUD formats have been accepted that provide a digital-only display of airspeed and altitude. Acceptance of these displays has been predicated on the availability of compensating features that provide clear and distinct warning to the flightcrew when these and certain other parameters exceed well-defined tolerances around the nominal approach state (e.g., approach warning). These warnings have associated procedures that require a missed approach.
Digital Displays. Formats with digital-only display of primary flight information (e.g., airspeed, altitude, attitude, and heading) should be demonstrated to provide at least one of the following:
A satisfactory level of task performance.
A satisfactory awareness of proximity to limit values, like VS, VMO, and VFE.
A satisfactory means to avoid violating such limits.
Go-Around and Missed Approach. If a different display format is used for go-around than that used for the approach, the format transition should occur automatically as a result of the normal go-around or missed approach procedure.
Minimize Format Changes. Changes in the display format and primary flight data arrangement should be minimized to prevent confusion and to enhance the flightcrew’s ability to interpret vital data.
Airplane Control Considerations. For those phases of flight where airworthiness approval is predicated on the use of the HUD, or when it can be reasonably expected that the flightcrew will operate primarily by reference to the HUD, the HUD should adequately provide the following information and cues:
Flight State and Position. The HUD should provide information to permit the pilot to instantly evaluate the airplane’s flight state and position. This information should be adequate for manually controlling the airplane and for monitoring the performance of the automatic flight control system.
Using the HUD for manual control of the airplane and monitoring of the automatic flight control system should not require exceptional pilot skill, excessive workload, or excessive reference to other flight displays.
Attitude Cues. Attitude cues must enable the pilot to instantly recognize unusual attitudes. Attitude cues must not hinder unusual attitude recovery.
If the HUD is designed to provide guidance or information for recovery from upsets or unusual attitudes, recovery steering guidance commands should be distinct from, and not confused with, orientation symbology F-14
Appendix F such as horizon pointers. This capability should be shown for all foreseeable modes of upset, including crew mishandling, autopilot failure (including “slowovers”), and turbulence/gust encounters.
F.5.4.4 Airspeed Considerations.
F.5.4.4.1 Airspeed Scale Range. As with other electronic flight displays, the HUD airspeed indications may not typically show the entire range of airspeed.
Section 25.1541(a)(2) states the airplane must contain “Any additional information, instrument markings, and placards required for the safe operation if there are unusual design, operating, or handling characteristics.”
F.5.4.4.2 Low- and High-Speed Awareness Cues. Low-speed awareness cues on the HUD should provide adequate visual cues to the pilot that the airspeed is below the reference operating speed for the airplane configuration
(e.g., weight, flap setting, and landing gear position). Similarly, highspeed awareness cues should provide adequate visual cues to the pilot that the airspeed is approaching an established upper limit that may result in a hazardous operating condition.
F.5.4.4.3 Format of Low- and High-Speed Awareness Cues. The low- and highspeed awareness cues should be readily distinguishable from other markings such as V-speeds and speed targets (e.g., bugs). The cues should indicate the boundary value of speed limit, and they should also clearly distinguish between the normal speed range and the unsafe speed range beyond those limiting values. Cross-hatching or other similar coding techniques may be acceptable to delineate zones of different meaning.
F.5.4.5 Flight Path Considerations.
F.5.4.5.1 General. The type of flight path information displayed (e.g., earth-referenced or air mass) may be dependent on the operational characteristics of a particular airplane and the phase of flight during which the flight path is to be displayed.
F.5.4.5.2 Velocity/Flight Path Vector. An indication of the airplane’s velocity vector, or flight path vector, is considered essential to most HUD applications. Earth-referenced flight path display information provides an instantaneous indication of where the airplane is actually going. During an approach, this information can be used to indicate the airplane’s impact or touchdown point on the runway. The earth-referenced flight path shows the effects of wind on the motion of the airplane. The flight path vector can be used by the pilot to set a precise climb or dive angle relative to the conformal outside scene or relative to the HUD’s flight path (pitch) reference scale and horizon displays. In the lateral axis, the flight path symbols should indicate the airplane track relative to the boresight.
F.5.4.5.3 Air-Mass-Derived Flight Path. Air-mass-derived flight path may be displayed as an alternative, but it does not show the effects of wind on the F-15
Appendix F motion of the airplane. In this case, the lateral orientation of the flight path display represents the airplane’s sideslip, while the vertical position relative to the reference symbol represents the airplane’s angle of attack.
F.5.4.6 Attitude Considerations.
F.5.4.6.1 General. For all unusual attitude situations and command guidance display configurations, the displayed attitude information should enable the pilot to make accurate, easy, quick glance interpretation of the attitude situation.
F.5.4.6.2 Pitch. The pitch attitude display should be such that, during all maneuvers, a horizon reference remains visible with enough margin to allow the pilot to recognize pitch and roll orientation. For HUDs that are capable of displaying the horizon conformally, the display of a non-conformal horizon reference should appear distinctly different than the display of a conformal horizon reference.
F.5.4.6.3 Display of Unusual Attitude Conditions. Extreme attitude symbology and automatically decluttering the HUD at extreme attitudes has been found acceptable (i.e., extreme attitude symbology should not be visible during normal maneuvering).
F.5.4.6.4 Unusual Attitude Recovery. When the HUD is not designed to be used for recovery from unusual attitude, the applicant should provide a satisfactory demonstration of the following:
• Compensating features (e.g., characteristics of the airplane and the HUD system).
• Immediate annunciation on the HUD to direct the pilot to use the head-down primary flight display for recovery.
• Satisfactory demonstration of timely recognition and correct recovery maneuvers.
F.5.4.6.5 Flightcrew Awareness of HUD Modes. The same information concerning current HUD system mode, reference data, status state transitions, and alert information that is displayed to the pilot using the HUD should also be displayed head-down to the other pilot. The display of this information for the other pilot should use consistent nomenclature to ensure unmistakable awareness of the HUD operation.
F.6 Dual HUDs.
F.6.1 Operational Concept for Dual HUDs.
The applicant should define the operational concept using dual HUDs. The operational concept should detail the tasks and responsibilities of both PF and pilot-not-flying (PNF) in regards to using and monitoring HDDs and HUDs during all phases of flight.
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Appendix F It should specifically address the simultaneous use of the HUD by both pilots during each phase of flight, as well as cross-flight-deck transfer of control.
F.6.2 Flightcrew Awareness of Other Instruments and Indications.
With single HUD installations, the PF likely uses the HUD as a primary flight reference and the PNF monitors the head-down instruments and alerting systems for failures of systems, modes, and functions that are not displayed on the primary flight displays or HUD. However, in the case where both flightcrew members simultaneously use HUDs, the flightcrew should be able to maintain an equivalent level of awareness of key information that is not displayed on the HUD (e.g., powerplant indications, alerting messages, and airplane configuration indications).
F.6.3 Roles and Responsibilities.
The applicant should define the operational concept to account for the expected roles and responsibilities of the PF and PNF. The concept should also take into account the following considerations:
F.6.3.1 Impact on Head-Down Vigilance. When both pilots of the flightcrew use a HUD as the primary flight display, the visual head-down indications may not receive the same level of vigilance (as compared to a pilot using the head-down primary flight display).
F.6.3.2 Assurance of Head-Down Scan. The applicant should explain how the scan of the head-down instruments is ensured during all phases of flight, and, if not, what compensating design features help the flightcrew maintain awareness of key information that is only displayed on HDDs
(e.g., powerplant indications, alerting messages, and airplane configuration indication). The applicant should describe which pilot scans the head-down instrument indications and how often. For any case in which at least one pilot is not scanning the head-down instruments fulltime, the design should have compensating design features that ensure an equivalent level of timeliness and awareness of the information provided by the head-down visual indications.
F.6.3.3 Alerts. The design should effectively compensate for any cautions and warnings that do not have visual indications in the HUD that are equivalent to the head-down primary flight display. The purpose of the compensating design features is to make the pilot using the HUD aware of the alerts so there are no additional delays in awareness and response time.
The flightcrew should be able to respond to alerts without any reduction in task performance or degraded safety.
F.6.3.4 Reassessment. The applicant should globally reassess the alerting functions to ensure that the flightcrew is aware of alerts and responds to them in a timely manner. The reassessment should review the design and techniques, the alerting attention-getting properties (e.g., visual master warning, master caution, and aural alerts), and other alerts in the flight F-17
Appendix F deck. The flightcrew’s awareness of alerts might differ between singleand dual-HUD installations. With a dual-HUD installation, there may be periods when neither pilot is scanning the instrument panel. With a single- HUD configuration, the PNF refers only to the head-down instrument panel and may have responsibility for monitoring indications on that panel. With dual-HUD configurations, both pilots’ attention may be turned to their HUDs, and they might miss an alert that would otherwise be plainly visible to a pilot not using a HUD.
F.7 Flight Data Recording.
Flight data recorders must record the minimum data parameters required by §§ 25.1459(e) and 121.344. Optionally, the flight data recorders may also record other information regarding unique operating characteristics of the HUDs. For example, they may include information such as the mode in which the HUD was operating, the status
(e.g., in use or inoperative), and if the display declutter mode was operating.
F.8 Continued Airworthiness.
Sections 25.1309 and 25.1529 and appendix H to part 25 require instructions for the continued airworthiness of a display system and its components. The content of the instructions depends on the type of operation and intended function of the HUD.
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Appendix G APPENDIX G. WEATHER DISPLAYS.
G.1 Introduction.
G.1.1 Purpose.
This appendix provides additional guidance for displaying weather information in the flight deck. Weather displays provide the flightcrew with additional tools to help make decisions based on weather information.
G.1.2 Examples.
Sources of weather information may include, but are not limited to, onboard weather sensors, data-linked weather information, and pilot/air traffic reports. The information from these sources can be displayed in a variety of graphical or text formats. Because many sources of weather information exist, it is important that the applicant identify the source of the information, assess its intended function, and apply the guidance contained within this AC.
G.2 Key Characteristics.
In addition to the general guidelines provided in the body of this AC, the following guidelines should be considered when establishing the intended functions of weather displays.
G.2.1 Unambiguous Meanings.
The meaning of the presentations (e.g., display format, colors, labels, data formats, and interaction with other display parameters) should be clear and unambiguous. The flightcrew should not misunderstand or misinterpret the weather information.
G.2.2 Color.
G.2.2.1 The use of color should be appropriate to its task and use.
G.2.2.2 The use of color must not adversely affect or degrade the attention-getting qualities of the information as required by Title 14, Code of Federal Regulations (14 CFR) 25.1322(f).
G.2.2.3 Color conventions should be followed (such as the conventions established in ARINC 708A-3, Airborne Weather Radar with Forward Looking Windshield Detection Capability; and AC 20-149A, Installation Guidance for Domestic Flight Information Services – Broadcast).
G.2.2.4 The use of red and yellow must be in compliance with § 25.1322(e).
Compliance can be shown using the guidance in AC 25.1322-1, Flightcrew Alerting, and this AC.
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Appendix G Note 1: AC 20-149A indicates an exclusion to the acceptability of RTCA DO-267A, Minimum Aviation System Performance Standards (MASPS) for Flight Information Services-Broadcast (FIS-B) Data Link, Sections 2.0 and 3.0, for part 25 airplanes.
Note 2: See paragraph 5.8 of this AC for information on guidelines on color progression.
G.2.3 Multiple Sources of Weather Information.
G.2.3.1 The weather display should enable the flightcrew to quickly, accurately, and consistently differentiate among sources of the displayed weather information. Time-critical information should be immediately distinguishable from dated, non-time-critical information.
G.2.3.2 If more than one source of weather information is available, the source of the weather information should be indicated on the selector and the resulting display.
G.2.3.3 When simultaneously displaying information from multiple weather sources (e.g., weather radar and data link weather), the display should clearly and unambiguously indicate the source of that information. In other words, the flightcrew should know the source of the symbol and whether it is coming from data-linked weather or real-time weather sources. These guidelines also apply to symbols (e.g., winds aloft and lightning) that have the same meaning but originate from different weather information sources.
G.2.3.4 If weather information is overlaid on an existing display, it should be easily distinguished from the existing display. It also should be consistent with the information it overlays in terms of position, orientation, range, and altitude.
G.2.3.5 When fusing or overlaying multiple weather sources, the resulting combined image should convey its intended meaning and meet its intended function, regardless of any differences in the sources in terms of image quality, projection, data update rates, data latency, or sensor alignment algorithms, for example.
G.2.3.6 If weather information is displayed on a head-up display, the guidance of this AC including appendix F of this AC should be followed.
G.2.3.7 When the source of the weather information source is not the onboard sensors, some means to identify its relevance (e.g., a time stamp or the age of the product) should be provided. Presenting the product age is particularly important when combining information from multiple weather products. In addition, the effective time of forecast weather should also be provided.
G-2
G.2.3.8 G.2.3.9 G.2.3.10 G.2.3.11 G.2.3.12 G.2.3.12.1 G.2.3.12.2 Appendix G If a weather-looping (animation) display feature is provided, the system should provide the means to readily identify the total elapsed time of the image compilation so the flightcrew does not misinterpret the movement of the weather cells.
For products that have the ability to present weather for varying altitudes
(e.g., potential or reported icing, radar, and lightning strikes), information should be presented that allows the flightcrew to distinguish or identify which altitude range applies to each feature.
Weather information may include a number of graphical and text information features or sets of information (e.g., text and graphical Aviation Routine Weather Reports and winds aloft). The display should provide a means to identify the meaning of each feature to ensure that the information is correctly used.
If the flightcrew or system has the ability to turn a weather information source on or off, the flightcrew should be able to easily determine if the source is on or off.
When weather information is presented in a vertical situation display, the lateral width of the weather swath (like that of the terrain swath) should be carefully considered to ensure that weather information that is relevant to the current phase of flight or flight path is displayed. An unsuitable lateral swath width could either mislead the flightcrew to abort an operation for weather that poses no hazard, or fail to abort an operation when the weather does pose a hazard. If swath dimensions are automatically controlled, then careful consideration should be given to include only the area that would be relevant to the operation. Means may be provided for the flightcrew to select the swath widths that they consider suitable for the phase of flight and prevailing weather conditions. The lateral width of the weather swath (like that of the terrain swath) should be made readily apparent to the flightcrew (e.g., use the same swath as is used for the terrain, or display its boundaries on the plan view weather display).
Generally, if the vertical situation displays terrain and weather at the same time, the choice of flight path-centered or track/heading-centered swath should be consistent. If the weather overlay is designed to show a smaller vertical swath than is represented by the altitude scale, then the boundaries of this swath should be clearly depicted on the display.
Weather information displayed on a vertical situation display should be accurately depicted with respect to the scale factors of the display
(i.e., vertical and horizontal).
Consideration should be given to making the width of the information on the weather display consistent with the width used by other systems, including the Terrain Awareness and Warning System, if displayed. This G-3