Appendix F must not create any objectionable reflections on the HUD or other flight instruments. Unless a fixed level of illumination is satisfactory under all lighting conditions, there should be a means to control its intensity.
F.3.2.3 Control Integration. To the greatest extent practicable, HUD controls should be integrated with other associated flight deck controls to minimize the flightcrew workload and error associated with HUD operation and to enhance flightcrew awareness of HUD modes.
F.3.2.4 Ease of Use. HUD controls, including the controls to change or select HUD modes, should be implemented to minimize flightcrew workload for data selection or data entry and allow the pilot to easily view and perform all mode control selections from the seated position.
F.3.3 Visibility and Field-of-View (FOV).
F.3.3.1 Field-of-View. The design of the HUD installation should provide adequate display FOV in order for the HUD to function as intended in all anticipated flight attitudes, airplane configurations, and environmental conditions, such as crosswinds, for which it is approved. The AFM should specify all airworthiness and operational limitations related to these factors.
F.3.3.2 Impact on Pilot Compartment View.
F.3.3.2.1 Interior View. Whether or not the combiner is deployed and the HUD is in use, it must not create additional significant obstructions to either pilot’s compartment view as required by § 25.773. The HUD must also not restrict the view of any flight deck controls, indicators, or other flight instruments as required by §§ 25.777 and 25.1321.
F.3.3.2.2 External View. The HUD should not significantly obscure the necessary pilot compartment view of the outside world for normal, non-normal, or emergency flight maneuvers during any phase of flight for a pilot seated at the design eye position (DEP). The HUD should not significantly affect the ability of any flightcrew member to spot traffic or distinctly see approach lights, runways, signs, markings, or other aspects of the external visual scene. The combination of the windshield and the HUD must meet the requirements of § 25.773(a)(1).
F.3.3.2.3 HUD Optical Performance. As far as practicable, the optical performance of the HUD must not cause distortions that degrade or detract from the flightcrew’s view of external references or of other aircraft. The optical performance should not degrade or detract from the flightcrew’s ability to safely perform any maneuvers within the operating limits of the airplane, as required by § 25.773. Where the windshield optically modifies the pilot’s view of the outside world, the motions and positions of conformal HUD symbols must be optically consistent (i.e., aligned and scaled) with the perceived outside view. To avoid distortions, the optical qualities of F-5
Appendix F the HUD should be uniform across the entire FOV. When the pilot views the HUD with both eyes from any off-center position within the design eyebox, optical non-uniformities shall not produce perceivable differences in the binocular view. SAE ARP5288, Transport Category Airplane Head Up Display (HUD) Systems, provides additional guidance.
F.3.3.3 Conformal Symbols with Limited HUD FOV. The range of motion of conformal symbology 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 FOV. 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. It should also be shown that there is no interference between the indications of primary flight information and the flight guidance cues.
F.4 HUD Design Eyebox Criteria.
F.4.1 Design Eye Position.
AC 25.773-1, Pilot Compartment View Design Considerations, January 8, 1993, defines the design eye position as a single point that meets the requirements of §§ 25.773 and
25.777. For certification purposes, the DEP is the pilot’s normal seated position. Fixed markers or some other means should be provided at each pilot station to enable the pilots to position themselves in their seats at the DEP for an optimum combination of outside visibility and instrument scan. The HUD installation must comply with §§ 25.773 and 25.1321. The HUD must be able to accommodate pilots, from 5′2″ to 6′3″ tall, while they are seated at the DEP with their shoulder harnesses and seat belts fastened, to comply with § 25.777. The DEP must be centered within the minimum design eyebox dimensions found in paragraph F.4.2.3 of this appendix. Actual HUD eyeboxes are larger than these minimum dimensions and, if not centered around the DEP, they need only be large enough that this minimum sub-volume is centered around the DEP.
F.4.2 Design Eyebox.
F.4.2.1 Display Visibility Requirements. The fundamental requirements for instrument arrangement and visibility in §§ 25.773, 25.777, 25.1301, and
25.1321 apply to HUDs. Each flight instrument, including the flight information displayed in the HUD, must be plainly visible to the pilot at that pilot’s station with minimum practicable deviation from the normal position and forward line of vision. While seated at the design eye position, the pilot must be able to see the flight information displayed in the HUD. The optical characteristics of the HUD—particularly the limits of its design eyebox—cause the pilot’s ability to fully view essential flight information to be more sensitive to the pilot’s eye position, as compared to HDDs. The HUD design eyebox is a three-dimensional volume, specified by the manufacturer, within which display visibility requirements are met.
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Appendix F Thus, whenever the pilot’s eyes are within the design eyebox, the required flight information must be visible in the HUD. The size of the design eyebox and the layout of flight information in the HUD should be designed so that visibility of the displayed symbols is not unduly sensitive to pilot head movements in all expected flight conditions. In the event that the pilot’s view of displayed information is totally lost as a result of a head movement, the pilot must be able to regain the view of the display rapidly and without difficulty. The minimum monocular FOV required to display this required flight information should include the center of the FOV and must be specified by the manufacturer. The HUD FOV should be designed by considering the intended operational environment and potential airplane configurations.
F.4.2.2 Design Eyebox Position. The HUD design eyebox should be laterally and vertically positioned around the respective pilot’s DEP. It should be large enough that the required flight information is visible to the pilot at the minimum displacements from the DEP specified by paragraph F.4.2.3 of this appendix. The symbols must be laid out and positioned such that excessive eye movements are not required to scan elements of the display.
The displayed symbols that are necessary to perform the required tasks must be visible to the pilot from the DEP. The DEP used for evaluation of the eyebox location must be the same as that used for the basic flight deck in accordance with AC 25.773-1.
F.4.2.3 Design Eyebox Dimensions. The lateral and vertical dimensions of the design eyebox represent the total movement of a monocular viewing instrument with a 0.25 inches (6.35 mm) entrance aperture (pupil). The longitudinal dimension of the design eyebox represents the total fore-aft movement over which the requirement of this specification is met (see SAE AS8055). When the HUD is a primary flight display, when airworthiness approval is predicated on the use of the HUD, or when the pilot can be reasonably expected to operate primarily by reference to the HUD, dimensions larger than the minimums shown below may be necessary.
F.4.2.3.1 Lateral: 1.5 inches left and right from the DEP (3.0 inches wide).
F.4.2.3.2 Vertical: 1.0 inches up and down from the DEP (2.0 inches high).
F.4.2.3.2 Longitudinal: 2.0 inches fore and aft from the DEP (4.0 inches deep).
F.4.3 Conformal Display Accuracy.
F.4.3.1 Symbol Positioning. The accuracy of symbol positioning relative to the external references, or display accuracy, is a measure of the relative conformality of the HUD display with respect to the pilot’s view of the real world through the combiner and windshield from any eye position within the HUD design eyebox. The display accuracy is a monocular measurement. For a fixed field point, the display accuracy is numerically F-7
Appendix F equal to the angular difference between the position of a real-world feature (as seen through the combiner and windshield) and the HUD projected symbology.
F.4.3.2 Error Budget. The total error budget for the display accuracy of the HUD system (excluding sensor and windshield errors) includes installation errors, digitization errors, electronic gain and offset errors, optical errors, combiner positioning errors, errors associated with the cathode ray tube (CRT) and yoke (if applicable), misalignment errors, environmental conditions (e.g., temperature and vibration), and component variations.
F.4.3.2.1 Error Sources. Optical errors are dependent upon both the head position and the field angle. Optical errors are comprised of three sources:
uncompensated pupil and field errors originating in the optical system aberrations, image distortion errors, and manufacturing variations. The optical errors are statistically determined by sampling the HUD FOV and the design eyebox. (See 4.2.10 of SAE AS8055 for a discussion of FOV and design eyebox sampling).
F.4.3.2.2 Total Accuracy. The optical errors shall represent at least 95.4% (2 sigma) of all sampled points. The display accuracy errors are characterized in both the horizontal and vertical planes. The total display accuracy shall be characterized as the root-sum square errors of these two component errors.
F.4.3.2.3 Allowable Margin for Display Errors. All display errors shall be minimized across the display FOV consistent with the intended function of the HUD. Table F-1 shows the allowable display accuracy errors for a conformal HUD as measured from the HUD eye reference point.
Table F-1. Display Error Tolerances Location on the HUD Combiner Error Tolerance in milliradians (mrad) At HUD boresight ≤ 5.0 mrad ≤ 10° diameter ≤ 7.5 mrad (2 sigma) ≤ 30° diameter ≤ 10.0 mrad (2 sigma) < 10 mrad + kr[(FOV)(in degrees) - 30)] > 30° diameter (2 sigma) where, kr = 0.2 mrad of error per degree of FOV F.4.3.2.4 Maximum Error. The HUD manufacturer shall specify the maximum allowable installation error. In no case shall the display accuracy error tolerances cause hazardously misleading data to be presented to the pilot viewing the HUD.
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Appendix F F.4.4 Symbol Positioning Alignment.
The symbols intended for use in combination with other symbols and scales to convey meaning must be aligned and positioned precisely enough not to be misleading to the pilot.
F.4.5 Overlapping Symbols.
Symbols that share space with other symbols must not partially obscure or interfere with the appearance of other symbols in a way that misleads the pilot.
F.4.6 Alignment.
F.4.6.1 Outside View. The HUD combiner must be properly aligned so that display elements such as attitude scales and flight path vector symbology are conformal (i.e., the position and motion are aligned and scaled). Proper combiner alignment is needed to match conformal display parameters as close as possible to the outside real world, depending on the intended function of those parameters.
F.4.6.2 Combiner. If the HUD combiner is stowable, means should be provided to ensure that it is in its fully deployed and aligned position before using the symbology for airplane control. The HUD shall alert the pilot if the position of the combiner causes normally conformal data to become misaligned in a manner that may result in the display of misleading information.
F.4.7 Visual Display Characteristics.
The following paragraphs highlight some areas related to performance aspects that are specific to the HUD. SAE ARP5288, Transport Category Airplane Head Up Display (HUD) Systems, and SAE AS8055, Minimum Performance Standard for Airborne Head Up Display (HUD), provide performance guidelines for a HUD. As stated in chapter 3 of this AC, the applicant should notify the airworthiness authority if any visual display characteristics do not meet the guidelines in SAE ARP5288 and SAE AS8055.
F.4.7.1 Luminance.
F.4.7.1.1 Background Light Conditions. The display luminance (brightness) should be satisfactory in the presence of dynamically changing background (ambient) lighting conditions (5 to 10,000 foot Lamberts (fL) as specified in SAE AS8055), so that the HUD data are visible.
F.4.7.1.2 Luminance Control. The HUD must have adequate means to control luminance so that displayed data is always visible to the pilot. The HUD may have both manual and automatic luminance control capabilities. It is recommended that automatic control be provided in addition to the manual control. Manual control of the HUD brightness level should be available to the flightcrew to set a reference level for automatic brightness control. If the HUD does not provide automatic control, a single manual setting should be satisfactory for the range of lighting conditions encountered F-9
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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