Appendix E Document Title Issued DO-289 and Change 1 Minimum Aviation System Performance Standards (MASPS) for Aircraft Surveillance Applications (ASA) December 9, 2003;
Change 1 on December 13, 2006 DO-296 Safety Requirements Standard for Aeronautical Operational Control (AOC) Datalink Messages October 19, 2004 DO-325 Minimum Operational Performance Standards (MOPS) for Automatic Flight Guidance and Control Systems and Equipment December 8, 2010 E.8.5 SAE International Documents.
You can purchase the following SAE International (formerly Society of Automotive Engineers) documents listed in table E-7 by mail from SAE Customer Service, 400 Commonwealth Drive, Warrendale, PA, 15096; by faxing your order information to
(724) 776-0790; or from the SAE website. Please note that the time of publication of this AC, the SAE documents have not been updated to reflect supercooled large drop analyses and testing but provide good reference material.
Note: In the event of conflicting information, this AC takes precedence as guidance for certification of transport category airplane installations.
Table E-7. SAE International Documents.
Document Title AIR818D Aircraft Instrument and Instrument System Standards: Wording, Terminology, Phraseology, and Environmental and Design Standards for AIR1093A Numeral, Letter and Symbol Dimensions for Aircraft Instrument Displays ARD50017 Aeronautical Charting ARD50062 Human Factors Issues Associated with Terrain Separation Assurance Display Technology ARP426A Compass System Installations ARP571C Flight Deck Controls and Displays for Communication and Navigation Equipment for Transport Aircraft E-16
Appendix E Document Title ARP926B Fault/Failure Analysis Procedure ARP1161A Crew Station Lighting—Commercial Aircraft ARP1782B Photometric and Colorimetric Measurement Procedures for Airborne Direct View CRT ARP1834A Fault/Failure Analysis for Digital Systems and Equipment ARP1874 Design Objectives for CRT Displays for Part 25 (Transport) Aircraft ARP4032A Human Engineering Considerations in the Application of Color to Electronic Aircraft Displays ARP4033 Pilot-System Integration ARP4101 Flight Deck Layout and Facilities ARP4102 Flight Deck Panels, Controls, and Displays ARP4102/7 Electronic Displays ARP4102/8A Flight Deck Head-Up Displays ARP4102/15A Electronic Data Management System (EDMS) ARP4103 Flight Deck Lighting for Commercial Transport Aircraft ARP4105C Abbreviations, Acronyms, and Terms for Use on the Flight Deck ARP4256A Design Objectives for Liquid Crystal Displays for Part 25 (Transport) Aircraft ARP4260A Photometric and Colorimetric Measurement Procedures for Airborne Electronic Flat Panel Displays ARP4754A Guidelines for Development of Civil Aircraft Systems ARP4761 Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment ARP5056 Flight Crew Interface Considerations in the Flight Deck Design Process for Part 25 Aircraft ARP5287 Optical Measurement Procedures for Airborne Head Up Display (HUD) 10/07/14 AC 25-11B E-17
Appendix E Document Title ARP5288 Transport Category Airplane Head Up Display (HUD) Systems ARP5289A Electronic Aeronautical Symbols ARP5364 Human Factor Considerations in the Design of Multifunction Display Systems for Civil Aircraft ARP5365 Human Interface Criteria for Cockpit Display of Traffic Information ARP5412A Aircraft Lightning Environment and Related Test Waveforms ARP5413A Certification of Aircraft Electrical/Electronic Systems for the Indirect Effects of Lightning ARP5414A Aircraft Lightning Zoning ARP5415A User’s Manual for Certification of Aircraft Electrical/Electronic Systems for the Indirect Effects of Lighting AS425C Nomenclature and Abbreviations for Use on the Flight Deck AS439A Stall Warning Instrument (Turbine Powered Subsonic Aircraft) AS8034B Minimum Performance Standard for Airborne Multipurpose Electronic Displays AS8055 Minimum Performance Standard f or Airborne Head Up Display (HUD) E.9 Other Documents.
You may purchase Colorimetry, 3rd edition, Commission Internationale de L’Eclairage, CIE publication 15:2004, from the CIE website.
E-18
Appendix F APPENDIX F. HEAD-UP DISPLAYS (HUDS) F.1 Introduction.
F.1.1 Purpose.
This appendix provides additional guidance related to the unique aspects, characteristics, and functions of head-up displays (HUDs) for transport category airplanes. This appendix also addresses issues related to the design, analysis, and testing of HUDs. It addresses HUDs that are designed for a variety of different operational concepts and functions. This guidance applies to HUDs that are intended to be used as a supplemental display in which the HUD contains the minimum information immediately required for the operational task associated with the intended function. It also applies to HUDs that are intended to be used effectively as primary flight displays.
This appendix addresses both the installation of a single HUD, typically used by the left-side pilot, as well as special considerations related to dual HUDs, one for each pilot.
This appendix does not provide the guidance for display of vision system (e.g., enhanced flight vision systems (EFVS) and synthetic vision systems (SVS)) video on the HUD. The airworthiness requirements and means of compliance criteria for display of video on the HUD may be found in special conditions issued by the FAA and the current version of AC 20-167, Airworthiness Approval of Enhanced Vision System, Synthetic Vision System, Combined Vision System, and Enhanced Flight Vision System Equipment.
F.1.2 Definition of Head-Up Display.
A HUD is a display system that projects primary flight information (for example, attitude, air data, and guidance) on a transparent screen (combiner) in the pilot’s forward field-of-view (FOV), between the pilot and the windshield. This allows the pilot to simultaneously use the flight information while looking along the forward path out the windshield, without scanning the head-down displays (HDDs). The flight information symbols should be presented as a virtual image focused at optical infinity.
Attitude and flight path symbology needs to be conformal (that is, aligned and scaled) with the outside view.
F.1.3 Other Resources.
For guidance associated with specific operations using HUDs, such as low visibility approach and landing operations, see the relevant requirements and guidance material
(e.g., European Aviation Safety Agency (EASA) Certifications Specifications for All Weather Operations (CS-AWO), and AC 120-28D, Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout). In addition, Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 5288, Transport Category Airplane Head Up Display (HUD) Systems; SAE Aerospace Standard (AS) 8055, Minimum Performance Standard for Airborne Head Up Display (HUD); and SAE ARP5287, Optical Measurement Procedures for Airborne Head Up Display; provide guidance for designing and evaluating HUDs.
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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