Appendix 6 – Head-Up Display

ED Decision 2015/019/R

Contents

No table of contents entries found.

1.0     Introduction

1.1     Purpose

This Appendix provides additional guidance related to the unique aspects, characteristics, and functions of Head-Up Displays (HUDs) for transport category aeroplanes. 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 the Certification Review Items (CRIs) issued by the Agency until new CSs and AMCs are issued.

1.2     Definition of Head-Up Display (HUD)

An 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 of the windshield, without scanning the Head-Down Displays (HDDs). The flight information symbols should be presented as a virtual image focussed at optical infinity. Attitude and flight path symbology needs to be conformal (that is, aligned and scaled) with the outside view.

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. EASA Certifications Specifications for All Weather Operations (CS-AWO), and FAA Advisory Circular (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 Aeroplane 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.

2.0     Unique safety considerations

2.1     Aeroplane and systems safety

2.1.1  Systems

Installing HUD systems in flight decks may introduce complex functional interrelationships among the flight crew members and other display and control systems. Consequently, a functional hazard assessment which requires a top-down approach from an aeroplane-level perspective should be developed in accordance with CS 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 and availability of function, as well as the operational concept of the installation to be certified (e.g. dual-HUD versus single-HUD installation, and the type and amount of information displayed). Chapter 4 of this AMC provides material that may be useful in preparing the functional hazard assessment.

2.1.2  Aeroplane Flight Manual (AFM) procedures

All alleviating flight crew 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.

2.1.3  Availability of primary flight information

Requirements for the availability of primary flight information are provided in CS 25.1333.

2.2     Crew safety

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. An HUD combiner with a swing-arm deployment mechanism should be designed to avoid false detents and false latch indications between the fully stowed and deployed positions. A misstowed combiner could swing inadvertently into the path of the pilot’s head and cause injury. Additionally, the HUD installation, including the overhead unit and combiner, must comply with the occupant injury requirements of CS 25.785(d) and (k) and the retention requirements of CS 25.789(a).

2.2.2  Special considerations for dual-HUD installations

For dual-HUD installations, the applicant should 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 an emergency landing. The Agency may need to issue a Certification Review Item providing project-specific means of compliance if the installation geometry indicates that such events may produce occupant contact with the HUD installation.

2.2.3  Non-interference with emergency equipment

CS 25.803, CS 25.1411, and CS 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 should not adversely affect the emergency egress provisions for the flight crew, or significantly interfere with flight crew access. The system should not hinder the flight crew’s movement while conducting any flight procedures.

3.0     Design

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 the 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 manually flown approaches and/or for monitoring autopilot-coupled instrument approaches, display guidance for low-visibility take-off, and/or display enhanced vision imagery and synthetic vision video. See paragraph 11.c of this AMC for additional guidance.

3.1.1  General

In most applications, HUDs provide an indication of primary flight references, which allow the pilot to rapidly evaluate the aircraft 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 but will be addressed by associated CRIs until new CSs and AMCs are issued.

3.1.2  Display of primary flight information

3.1.2.1 HUD as de facto primary flight display

If an 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 recognise the primary flight information — it should not be ambiguous or confusing when taking into account information displayed on other flight deck displays.

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 CS-25 (e.g. CS 25.1303(b) for flight and navigation instruments that must be visible from each pilot station, and CS 25.1333(b) for the operational requirements of those systems). CS 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 1 of this AMC.

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 aeroplane by reference to the HUD.

3.1.3.1 Command guidance

When the HUD is used to monitor the autopilot, it should display the following information:

—          situation information based on independent raw data;

—          autopilot operating mode;

—          autopilot engage status; and

—          autopilot disconnect warning (visual).

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 aeroplane 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; or

—          heading or course references.

3.2     HUD controls

3.2.1  Control placement

For compliance with CS 25.777, the flight crew 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 CS 25.777 and CS 25.1301, the position and movement of the HUD controls must not lead to inadvertent operation.

3.2.2  Control illumination

To comply with CS 25.1381, the HUD controls must be adequately illuminated for all normal ambient lighting conditions and 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.

3.2.3  Control integration

To the greatest extent practicable, HUD controls should be integrated with other associated flight deck controls to minimise the flight crew workload and error associated with HUD operation and to enhance flight crew awareness of HUD modes.

3.2.4  Ease of use

HUD controls, including the controls to change or select HUD modes, should be implemented to minimise flight crew workload for data selection or data entry, and allow the pilot to easily view and perform all mode control selections from the seated position.

3.3     Visibility and Field-of-View (FOV)

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, aircraft 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.

3.3.2  Impact on pilot compartment view

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 CS 25.773. The HUD must also not restrict the view of any flight deck controls, indicators, or other flight instruments as required by CS 25.777 and CS 25.1321.

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 manoeuvres 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 flight crew member to spot traffic, 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 CS 25.773(a)(1).

3.3.2.3 HUD optical performance

As far as practicable, the optical performance of the HUD should not cause distortions that degrade or detract from the flight crew’s view of external references or of other aircraft. The optical performance should not degrade or detract from the flight crew’s ability to safely perform any manoeuvres within the operating limits of the aeroplane, as required by CS 25.773. Where the windshield optically modifies the pilot’s view of the outside world, the motions and positions of conformal HUD symbols should be optically consistent (i.e. aligned and scaled) with the perceived outside view. To avoid distortions, the optical qualities of the HUD should be uniform across the entire FOV. When the pilot views the HUD with both eyes from any off-centre position within the design eyebox, optical non-uniformities should not produce perceivable differences in the binocular view. SAE ARP 5288, Transport Category Aeroplane Head Up Display (HUD) Systems, provides additional guidance.

3.3.3  Conformal symbols with limited HUD Field-of-View

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.

4.0     HUD design eyebox criteria

4.1     Design eye position

The FAA AC 25.773-1, Pilot Compartment View Design Considerations, defines DEP as a single point that meets the requirements of CS 25.773 and CS 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 CS 25.773 and CS 25.1321. The HUD should be able to accommodate pilots, from 1 575 to 1 905 mm (5 ft 2 in to 6 ft 3 in) tall, while they are seated at the DEP with their shoulder harnesses and seat belts fastened, to comply with CS 25.777. The DEP should be centred within the minimum design eyebox dimensions found in paragraph 4.2.3 of this Appendix. Actual HUD eyeboxes are larger than these minimum dimensions and, if not centred around the DEP, they need only be large enough so that this minimum sub-volume is centred around the DEP.

4.2     Design eyebox

4.2.1  Display visibility requirements

The fundamental requirements for instrument arrangement and visibility in CS 25.773, CS 25.777, CS 25.1301, and CS 25.1321 apply to HUDs. Each flight instrument, including the flight information displayed on 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 DEP, the pilot must be able to see the flight information displayed on 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. Thus, whenever the pilot’s eyes are within the design eyebox, the required flight information must be visible on the HUD. The size of the design eyebox and the layout of flight information on 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 should 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 centre of the FOV and should be specified by the manufacturer. The HUD FOV should be designed by considering the intended operational environment and potential aeroplane configurations.

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 so that the required flight information is visible to the pilot at the minimum displacements from the DEP specified in paragraph 4.2.3 of this Appendix. The symbols should be laid out and positioned such that excessive eye movements are not required to scan elements of the display. The displayed symbols which are necessary to perform the required tasks should be visible to the pilot from the DEP. The DEP used for the evaluation of the eyebox location should be the same as that used for the basic flight deck in accordance with the FAA AC 25.773-1.

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 6.35 mm (0.25 in) 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 (refer to SAE AS 8055). 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.

4.2.3.1 Lateral: 38.1 mm (1.5 in) left and right from the DEP (76.2 mm (3.0 in) wide).

4.2.3.2 Vertical: 25.4 mm (1.0 in) up and down from the DEP (50.8 mm (2.0 in) high).

4.2.3.3 Longitudinal: 50.8 mm (2.0 in) fore and aft from the DEP (101.6 mm (4.0 in) deep).

4.3     Conformal display accuracy

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 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.

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, digitisation errors, electronic gain and offset errors, optical errors, combiner positioning errors, errors associated with the CRT and yoke (if applicable), misalignment errors, environmental conditions (e.g. temperature and vibration), and component variations.

4.3.2.1 Error sources

Optical errors are dependent upon both the head position and the field angle. Optical errors comprise three sources: uncompensated pupil and field errors originating in the optical system aberrations, image distortion errors, and manufacturing variations. 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).

4.3.2.2 Total accuracy

The optical errors should represent at least 95.4 % (2 sigma) of all sampled points. The display accuracy errors are characterised in both the horizontal and vertical planes. The total display accuracy should be characterised as the root-sum square errors of these two component errors.

4.3.2.3 Allowable margin for display errors

All display errors should be minimised across the display FOV consistent with the intended function of the HUD. Table A6-1 shows the allowable display accuracy errors for a conformal HUD as measured from the HUD eye reference point:

Table A6-1 — Allowable display accuracy errors

4.3.2.4 Maximum error

The HUD manufacturer should specify the maximum allowable installation error. In no case should the display accuracy error tolerances cause hazardously misleading data to be presented to the pilot viewing the HUD.

4.4     Symbol positioning alignment

The symbols intended for use in combination with other symbols and scales to convey meaning should be aligned and positioned precisely enough not to be misleading to the pilot.

4.5     Overlapping symbols

Symbols that share space with other symbols should not partially obscure or interfere with the appearance of other symbols in a way that misleads the pilot.

4.6     Alignment

4.6.1  Outside view

The HUD combiner should 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.

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 aircraft control. The HUD should 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.

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 Aeroplane Head Up Display (HUD) Systems and SAE AS8055, Minimum Performance Standard for Airborne Head Up Display (HUD), provide performance guidelines for an HUD. As stated in Chapter 3 of this AMC, the applicant should notify the Agency if any visual display characteristics do not meet the guidelines in SAE ARP5288 and AS8055.

4.7.1  Luminance

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-Lambert (fL), as specified in SAE AS8055), so that the HUD data are visible.

4.7.1.2 Luminance control

The HUD should 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 is provided in addition to the manual control. Manual control of the HUD brightness level should be available to the flight crew 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 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.

4.7.2  Reflections

The HUD must be free of glare and reflections that could interfere with the normal duties of the minimum flight crew, as required by CS 25.773 and CS 25.1523.

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 should be minimised so as not to impair the flight crews ability to use the display.

4.7.4  Accuracy and stability

4.7.4.1 Sensitivity to aircraft manoeuvring

The system operation should not be adversely affected by aircraft manoeuvring or changes in attitude encountered in normal service.

4.7.4.2 Motion of symbols

The accuracy of positioning of symbols should be commensurate with their intended use. Motion of non-conformal symbols should be smooth, not sluggish or jerky, and consistent with aircraft control response. Symbols should be stable with no discernible flicker or jitter.

5.0     Guidelines for presenting information

5.1     HUD and HDD compatibility

5.1.1  General

If the content, arrangement, or format of the HUD is dissimilar to the HDD, it can lead to flight crew confusion, misinterpretation, and excessive cognitive workload. During transitions between the HUD and HDDs (whether required by navigation duties, failure conditions, unusual aeroplane attitudes, or other reasons), dissimilarities could make it more difficult for the flight crew to manually control the aeroplane 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 colour on the HDD and a single colour (i.e. monochrome) on the HUD. The guidelines listed below are intended to minimise the potential for confusion, undue workload, and delays in flight crew task performance.

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 minimisation of display clutter, minimisation of excessive symbol flashing, and the presentation of certain information conformal to the outside scene. Deviations from these guidelines require additional pilot evaluation.

5.1.3  Guidelines for HUD–HDD compatibility

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 flight crew error (see paragraphs 31.b and 31.c(3) of this AMC). The layout and arrangement of HUD and HDD formats of the same information need to convey the same intended meanings (see paragraph 36.b of this AMC). 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.

5.1.3.2 Symbols

Table A6-2 provides the guidelines for symbols.

Table A6-2 — Symbol guidelines for HUD–HDD compatibility

5.1.3.3 Alphanumeric information

Alphanumeric (i.e. textual) information should have the same resolution, units, and labelling. For example, the command reference indication for vertical speed should be displayed in the same foot-per-minute increments and labelled 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 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 flight crew confusion or flight crew error.

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.

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 colour, 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).

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.

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.

5.1.4  Head-up to head-down transition

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.

5.1.4.2 Unambiguous information

While the HUD and HDD may display information (e.g. flight path, path deviation, or aircraft performance information) in a different manner, the meaning should 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 recognise and interpret information on the HUD. The information should not be ambiguous with similar information on other aircraft flight deck displays.

5.2     Indications and alerts

5.2.1  Monochrome attention-getting properties

To comply with CS 25.1322, and considering that most HUDs are predominantly monochrome devices, the HUD should emphasise 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 colour coding. For additional alerting guidance, see AMC 25.1322 ‘Flight Crew 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.

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 manoeuvre 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 on the HUD) to ensure that the pilot has equivalent and effective visual information for immediate awareness and response to the respective alerts.

5.2.3  Additional resources

Additional guidance on indications and alerts is contained in AMC No 1 to CS 25.1329, Flight Guidance System, in AMC No 2 to CS 25.1329, Flight Testing of Flight Guidance Systems, in AMC 25.1322, Flight Crew Alerting, and in the associated rules.

5.3     Display clutter

This AMC 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.

5.4     Display of information

5.4.1  General

The HUD information display requirements depend on the intended function of the HUD. Specific guidance for displayed information is contained within the main body and Appendix 1 of this AMC. 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.

5.4.2  Alternate formats for primary flight information

5.4.2.1 Phase of flight

There may be certain operations and phases of flight during which certain primary flight reference indications on 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 flight crew 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.

5.4.2.2 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 V_S, V_MO, and V_FE; and

—          a satisfactory means to avoid violating such limits.

5.4.2.3 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.

5.4.2.4 Minimise format changes

Changes in the display format and primary flight data arrangement should be minimised to prevent confusion and to enhance the flight crew’s ability to interpret vital data.

5.4.3  Aircraft 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 flight crew will operate primarily by reference to the HUD, the HUD should adequately provide the following information and cues.

5.4.3.1 Flight state and position

The HUD should provide information to permit the pilot to instantly evaluate the aeroplane’s flight state and position. This information should be adequate for manually controlling the aeroplane and for monitoring the performance of the automatic flight control system. Using the HUD for manual control of the aeroplane and for monitoring the automatic flight control system should not require exceptional pilot skill, excessive workload, or excessive reference to other flight displays.

5.4.3.2 Attitude cues

Attitude cues should enable the pilot to instantly recognise unusual attitudes. Attitude cues should 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 such as horizon pointers. This capability should be shown for all foreseeable modes of upset, including crew mishandling, autopilot failure (including ‘slow-overs’), and turbulence/gust encounters.

5.4.4  Airspeed considerations

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. CS 25.1541(a)(2) states that ‘The aeroplane must contain- Any additional information, instrument markings, and placards required for the safe operation if there are unusual design, operating, or handling characteristics.’.

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 aeroplane configuration (e.g. weight, flap setting, and landing gear position). Similarly, high-speed 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.

5.4.4.3 Format of low- and high-speed awareness cues

The low- and high-speed 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.

5.4.5  Flight path considerations

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 aeroplane and the phase of flight during which the flight path is to be displayed.

5.4.5.2 Velocity/flight path vector

An indication of the aeroplane’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 aeroplane is actually going. During an approach, this information can be used to indicate the aeroplane’s impact or touchdown point on the runway. The earth-referenced flight path shows the effects of wind on the motion of the aeroplane. 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 aeroplane’s track relative to the bore sight.

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 motion of the aeroplane. In this case, the lateral orientation of the flight path display represents the aeroplane’s sideslip, while the vertical position relative to the reference symbol represents the aeroplane’s angle of attack.

5.4.6  Attitude considerations

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.

5.4.6.2 Pitch

The pitch attitude display should be such that, during all manoeuvres, a horizon reference remains visible with enough margin to allow the pilot to recognise 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.

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 manoeuvring).

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.

5.4.6.4.1 Compensating features (e.g. characteristics of the aeroplane and the HUD system).

5.4.6.4.2 Immediate annunciation on the HUD to direct the pilot to use the head-down primary flight display for recovery.

5.4.6.4.3 Satisfactory demonstration of timely recognition and correct recovery manoeuvres.

5.4.6.5 Flight crew 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 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.

6.0     Dual HUDs

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) with regard to using and monitoring HDDs and HUDs during all phases of flight. It should specifically address the simultaneous use of the HUD by both pilots during each phase of flight, as well as cross-flight-deck transfer of control.

6.2     Flight crew awareness of other instruments and indications

With single-HUD installations, the PF likely uses the HUD as a primary flight reference and the PNF monitors the head-down instruments and alerting systems for failures of systems, modes, and functions that are not displayed on the primary flight displays or on the HUD. However, in the case where both flight crew members simultaneously use HUDs, they should be able to maintain an equivalent level of awareness of key information that is not displayed on the HUD (e.g. powerplant indications, alerting messages, and aircraft configuration indications).

6.3     Roles and responsibilities

The applicant should define the operational concept to account for the expected roles and responsibilities of the PF and the PNF. The concept should also take into account the following considerations.

6.3.1  Impact on head-down vigilance

When both pilots of the flight crew use an HUD as the primary flight display, the visual head-down indications may not receive the same level of vigilance (as compared to a pilot using the head-down primary flight display).

6.3.2  Assurance of head-down scan

The applicant should explain how the scan of the head-down instruments is ensured during all phases of flight and, if not, what compensating design features help the flight crew maintain awareness of key information that is only displayed on the HDDs (e.g. powerplant indications, alerting messages, and aircraft configuration indication). The applicant should describe which pilot scans the head-down instrument indications and how often. For any case in which at least one pilot is not scanning the head-down instruments full-time, the design should have compensating design features that ensure an equivalent level of timeliness and awareness of the information provided by the head-down visual indications.

6.3.3  Alerts

The design should effectively compensate for any cautions and warnings that do not have visual indications on the HUD that are equivalent to the head-down primary flight display. The purpose of the compensating design features is to make the pilot using the HUD aware of the alerts so there are no additional delays in awareness and response time. The flight crew should be able to respond to alerts without any reduction in task performance or degraded safety.

6.4     Reassessment

The applicant should globally reassess the alerting functions to ensure that the flight crew is aware of alerts and responds to them in a timely manner. The reassessment should review the design and techniques, the alerting attention-getting properties (e.g. visual master warning, master caution, and aural alerts), and other alerts in the flight deck. The flight crew’s awareness of alerts might differ between single- and dual-HUD installations. With a dual-HUD installation, there may be periods when neither pilot is scanning the instrument panel. With a single-HUD configuration, the PNF refers only to the head-down instrument panel and may have responsibility for monitoring indications on that panel. With dual-HUD configurations, both pilots’ attention may be turned to their HUDs, and they might miss an alert that would otherwise be plainly visible to a pilot not using an HUD.

7.0     Flight data recording

Flight data recorders must record the minimum data parameters required by the applicable operational regulations. In addition, flight data recorders should also record other parameters regarding unique operating characteristics of HUDs in compliance with CS 25.1459(e). For example, they may include information such as the mode in which the HUD was operating, the status (e.g. in use or inoperative), and if the display declutter mode was operating.

8.0     Continued airworthiness

CS 25.1309, CS 25.1529 and Appendix H to CS-25 require instructions for the continued airworthiness of a display system and its components. The content of the instructions depends on the type of operation and the intended function of the HUD.

[Amdt 25/17]