Appendix F F.2 Unique Safety Considerations.
F.2.1 Airplane and Systems Safety.
F.2.1.1 Systems. Installing HUD systems in flight decks may introduce complex functional interrelationships among the flightcrew members and other display and control systems. Consequently, a functional hazard assessment that requires a top-down approach from an airplane-level perspective should be developed in accordance with Title 14, Code of Federal Regulations (14 CFR) 25.1309. Developing a functional hazard assessment for a particular installation requires careful consideration of the role that the HUD plays within the flight deck in terms of integrity of function and availability of function, as well the operational concept of the installation to be certified (e.g., dual- versus single-HUD installation and the type and amount of information displayed). Chapter 4 of this AC provides material that may be useful in preparing the functional hazard assessment.
F.2.1.2 Airplane Flight Manual (AFM) Procedures. All alleviating flightcrew actions that are considered in the HUD safety analysis need to be validated for incorporation into the AFM procedures section or for inclusion in type-specific training.
F.2.1.3 Availability of Primary Flight Information. There might be failure conditions that result in a loss of all but one display of primary flight information. For such a condition, one HUD as the only remaining display could not comply with § 25.1333(b), since the HUD is visible only to one pilot. The rule requires that, after the loss of other flight information displays, the “one display of the information essential to the safety of flight” remain available to both pilots, not just one pilot.
F.2.2 Crew Safety.
F.2.2.1 Prevention of Head Injury. HUD equipment introduces potential hazards that are not traditionally associated with head-down electronic flight deck displays. The HUD system must be designed and installed to prevent the possibility of pilot injury in the event of an accident or any other foreseeable circumstance such as turbulence, hard landing, or bird strike.
For airplanes with § 25.562 in the certification basis, the HUD installation, including the overhead unit and combiner, must comply with the head injury criteria defined in § 25.562(c)(5). A HUD combiner with a swing-arm deployment mechanism should be designed to avoid false detents and flash latch indications between the fully stowed and deployed positions. A mis-stowed combiner could swing inadvertently into the path of the pilot’s head and cause injury. Additionally, the HUD installation must comply with the occupant injury requirements of § 25.785(d) and (k) and the retention requirements of § 25.789(a).
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Appendix F F.2.2.2 Special Considerations for Dual-HUD Installations. For dual-HUD installations, the applicant must address single events that could simultaneously incapacitate both pilots and, therefore, become safety-of-flight issues. Examples of such single events are flight or gust loads, a hard landing, or emergency landing. The FAA may need to provide an issue paper providing project-specific means of compliance if the installation geometry indicates that such events may produce occupant contact with the HUD installation.
F.2.2.3 Noninterference with Emergency Equipment. Sections 25.803, 25.1411, and 25.1447 require that the HUD installation must not interfere with, or restrict the use of, other installed equipment such as emergency oxygen masks, headsets, or microphones. The installation of the HUD must not adversely affect the emergency egress provisions for the flightcrew, or significantly interfere with flightcrew access. The system must not hinder the flightcrew’s movement while conducting any flight procedures.
F.3 Design.
F.3.1 Intended Function of HUDs.
The applicant is responsible for identifying the intended function of the HUD. The description of the intended function should include the operational phases of flight and concept of operation, including how, when, and for what purpose(s) the HUD is to be used. For example, the HUD may display situational information and/or guidance information, be a supplemental display of primary flight information in all phases of flight, display command guidance for Category II, manual, and/or monitoring autopilot-coupled instrument approaches, display guidance for visibility takeoff, and/or display enhanced vision imagery and synthetic vision video. See paragraph 2.4 of the AC for additional guidance.
F.3.1.1 General. In most applications, HUDs provide an indication of primary flight references that allow the pilot to rapidly evaluate the airplane attitude, energy status, and position during the phases of flight for which the HUD is designed. HUDs are usually designed to present information to enhance pilot performance in such phases of flight as during the transition between instrument and visual flight conditions with variable outside visibility conditions. While HUDs may be designed to display enhanced and synthetic visual imagery, particular means of compliance guidance for this purpose is not found in this appendix. See AC 20-167 for guidance.
F.3.1.2 Display of Primary Flight Information.
F.3.1.2.1 HUD as de facto Primary Flight Display. If a HUD displays primary flight information, it is considered a de facto primary flight display while the pilot is using it, even if it is not the pilot’s sole display of this information.
The pilot should be able to easily recognize the primary flight information;
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Appendix F it should not be ambiguous or confusing when taking into account information on other flight deck displays.
F.3.1.2.2 Applicable Instrument Requirements for HUD. Primary flight information displayed on the HUD should comply with all the requirements associated with such information in part 25 (e.g., § 25.1303(b) for flight and navigation instruments that must be visible from each pilot station and § 25.1333(b) for the operational requirements of those systems).
Section 25.1321(b) specifies the requirements for arranging primary flight information. For specific guidance regarding the display of primary flight information, see the main body and appendix A of this AC.
F.3.1.3 Display of Other Flight Information. Additional information may be related to the display of command guidance or specific flight parameter information needed for operating the airplane by reference to the HUD.
F.3.1.3.1 Command Guidance. When the HUD is used to display flight guidance, either for manual control or for monitoring the autopilot, it should display the following information:
• Path deviation indications based on sources of raw data that are independent from those used by the autopilot.
• Autopilot operating mode.
• Autopilot engage status.
• Autopilot disconnect warning (visual).
F.3.1.3.2 Flight Parameter Information. The HUD should also display additional flight parameter information if required to enable the pilot to operate the airplane during phases of flight for which the HUD is approved. This additional information may include:
• Flight path indication.
• Target airspeed references and speed limit indications.
• Target altitude references and altitude awareness (e.g., decision height and minimum descent altitude) indications.
• Heading or course references.
F.3.2 HUD Controls.
F.3.2.1 Control Placement. For compliance with § 25.777, the flightcrew must be able to see, identify, and reach the means of controlling the HUD, including its configuration and display modes, from the normal seated position. To comply with §§ 25.777 and 25.1301, the position and movement of the HUD controls must not lead to inadvertent operation.
F.3.2.2 Control Illumination. To comply with § 25.1381, the HUD controls must be adequately illuminated for all normal ambient lighting conditions and F-4
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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