+-----------------------------------------------------------------------+
| **REG.REF.:** §§ 25.562\*, 25.773, 25.777, 25.785, 25.789, 25.803,    |
| 25.1301, 25.1303, 25.1307, 25.1309, 25.1316, 25.1321, 25.1323,        |
| 25.1331, 25.1333, 25.1335, 25.1381, 25.1441, 25.1447, 25.1541,        |
| 25.1543                                                               |
|                                                                       |
| (\* Except or airplanes without 25.562 in their certification basis.) |
+-----------------------------------------------------------------------+
| **NATIONAL POLICY REF.:** AC 20-115B, AC 25.1309-1A, AC 25-15, AC     |
| 25.773-1, AC 120-28D, AC 120-29, and AC 25-11 with two related policy |
| memoranda, dated 2/92 and 9/96.                                       |
+-----------------------------------------------------------------------+
| **INDUSTRY DOCUMENTS:** Society of Automotive Engineers (SAE)         |
| Aerospace Standard (AS) 8055 *Minimum Performance Standard for        |
| Airborne Head Up Display (HUD)*, March 1999; SAE Aerospace            |
| Recommended Practice (ARP) 5287 *Optical Measurement Procedures for   |
| Airborne Head Up Display (HUD)*, March 1999; and ARP5288 *Transport   |
| Category Airplane Head Up Display (HUD) Systems*, May 2001.           |
+-----------------------------------------------------------------------+

**1. [Display Requirements]{.underline}**

a.  **\[Applicability of existing display criteria\]** This HUD display
    should be evaluated and found to meet the guidelines contained in AC
    25-11, Section 7, *Information Display*. For a monochrome HUD, the
    criteria for information separation may be applied by using
    distinctive coding of display parameters like symbol shape, size and
    location to compensate for the lack of color (see AC 25-11,
    paragraph 5 b.). 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 pilot will operate primarily by
    reference to the HUD the criteria contained in Section 7.d. *Primary
    Flight Displays* apply.

b\. **\[Required content - flight references\]** 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 pilot will operate
primarily by reference to the HUD, it is not reasonable to expect the
pilot to scan the instrument panel for primary flight information (i.e.,
Basic T). Therefore, to perform its intended function in compliance with
§ 25.1301, the HUD must provide adequate information to permit instant
evaluation of the airplane\'s flight state and position during all
applicable phases of flight. This display of information should be shown
to be adequate for manually controlling the airplane, and for monitoring
the performance of the automatic flight control system. Use of the HUD
for manual control of the airplane and monitoring of the automatic
flight control system, should not require exceptional skill, excessive
workload, or excessive reference to other flight displays.

1)  The flight reference HUD should always display attitude (artificial
    horizon, bank angle and pitch angle), airspeed (including mach
    number, if required for the airplane), barometric altitude,
    skid/slip, gyroscopically stabilized heading, and vertical speed.

2)  For instrument approaches, the HUD should also display radio
    altitude, vertical and lateral path deviation (e.g., glide slope,
    localizer), visual marker annunciation, DME, selected navigation
    source (such as ILS-1, VOR/DME, FMS, etc.), and flight director, as
    applicable. Note that operational approval requires that the
    reference scale (dots) be displayed for the glideslope and
    localizer.

3)  Specific considerations regarding AC 25-11, section 7.d. *Primary
    Flight Displays*:

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a)  **\[Flight references presentation - analog scales\]** For dynamic
    flight phases, such as takeoff or go-around, the airspeed and
    altitude displays should be evaluated against the full criteria of
    Section 7.d., including scale length, reference marks, and labeling.
    The overriding consideration is that the scales should provide the
    pilot with a quick-glance (instant) sense of present speed and
    altitude for those flight phases where it cannot be reasonably
    expected for the pilot to transition to the HDD, or to transfer
    control to the non-flying pilot.

b)  **\[Flight references presentation- digital/analog\]** 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 (approach
    warn), and these warnings have associated procedures that require
    the termination of the approach.

c)  **\[Flight references - automatic format/mode changes\]** During
    time-critical, high-workload phases of flight, such as the
    initiation of go-around or takeoff rotation, operation of the HUD
    should not distract the pilot. For example, if a different display
    format is used for takeoff/go-around than that used for the
    approach, the format transition should be automatic.

d)  **\[Flight references - automatic format/mode changes\]** Changes in
    the display format and arrangement of primary flight data should be
    minimized to prevent pilot confusion and to enhance the pilots'
    ability to interpret vital data.

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4)  **\[Special function - Engine failure\]** To avoid delays in the
    pilot's recognition and response to an engine failure, the HUD
    system should be adequate for airplane *control* and guidance during
    an engine failure during any phase of flight where airworthiness
    approval is predicated on the use of the HUD, or where it can be
    reasonably expected that pilot will operate primarily by reference
    to the HUD.

5)  **\[Special function - Unusual Attitude Recognition/Recovery
    (UAR)\]** To avoid a delay in the pilot's recognition and initial
    response, the HUD should provide adequate cues for the pilot to
    satisfactorily respond to unusual attitudes. It should not be
    assumed that the pilot will continuously scan primary flight
    references on the instrument panel while using the HUD. The pilot
    should neither be required to nor prohibited from immediately
    transitioning to the primary flight references on the instrument
    panel once an unusual attitude is detected. The HUD display should
    be demonstrated for timely pilot recognition and proper recovery
    from unusual attitudes in all foreseeable modes of upset, including
    crew mishandling, autopilot failure (including \"slowovers\"), and
    turbulence/gust encounters.

6)  **\[Special Function - Visual Takeoff Display Formats\]** A HUD
    display format intended for use during visual takeoffs should not
    distract the pilot, increase pilot workload, nor degrade lateral
    tracking performance and speed awareness during the takeoff roll.

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a)  When symbolic cues are provided to assist the lateral tracking task,
    they should be consistent with the external visual references of
    runway centerline, edges, lights, and direction. Cues intended to
    mimic certain runway features, such as the centerline, the symbology
    should be positioned to overlay those runway features and move with
    the same rate and dynamics. Formats intended for use during takeoff
    roll should provide lateral acceleration in multi-engine airplanes
    and essential airspeed information, to include V~1~, V~R~, and V~2~
    as appropriate for the airplane.

b)  If the takeoff format provides cues for rotation, then following
    these cues should not degrade takeoff performance nor cause the tail
    to strike the runway.

c)  At night and even in the lower visibility conditions for which a
    takeoff guidance system is not required (e.g. 600RVR), the HUD
    indications may become extremely dominant in comparison to external
    visual references. The guidance and other indications displayed in
    the HUD, if heeded by the pilot without regard to external visual
    references, should not lead the pilot to take unsafe actions.
    Extremely active dynamics and gains could lead the pilot to take
    excessively strong heading corrections, when visual references are
    relatively weak.

d)  Some flight test cases should be conducted in simulated zero
    visibility conditions. Some degradation of task performance is
    acceptable, compared to visual takeoffs without the HUD, provided
    the performance is found to be safe and satisfactory.

e)  External visual references at night and in the lower visibility's
    (at or above 600RVR) may not be strong enough for the pilot to
    visually detect errant HUD cues during malfunctions. For
    malfunctions which could cause misleading indications, the
    indications should be removed and the condition annunciated, if
    possible. For system safety considerations, the effects of
    misleading cues are more hazardous that loss of function.

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7)  **\[Evaluation of Tasks Performed with HUD Reference\]** The HUD
    system adequacy for use while manually controlling the airplane
    should be demonstrated and evaluated according to the following the
    rating levels:

-   Satisfactory: Full performance criteria can be met with routine
    pilot effort and attention.

-   Adequate: Adequate for continued safe flight and landing; full or
    specified reduced performance can be met, but with heightened pilot
    effort and attention.

-   Controllable: Inadequate for continued safe flight and landing, but
    controllable for return to a safe flight condition, safe flight
    envelope and/or reconfiguration so that the handling qualities are
    at least adequate.

a)  This task oriented evaluation should consider all normal, abnormal
    and emergency operations, with single and multiple failures not
    shown to be extremely improbable and is extended to all HUD display
    formats, unless use of specific formats is prohibited for specific
    phases of flight.

b)  The pilot workload and compensation is allowed to progressively vary
    with failure state, atmospheric disturbance level and flight
    envelope. Specifically, within the normal flight envelope, ratings
    of Adequate are acceptable in moderate atmospheric disturbance for
    probable failures, and in light atmospheric disturbance for
    improbable failures.

c\. **\[Required content- Airspeed Awareness\]** As with other
electronic flight displays, the HUD airspeed indications do not
typically show the entire range of airspeed. Section 25.1541(b)(2) of
the Federal Aviation Regulations states: \"The airplane must contain -
Any additional information, instrument markings, and placards required
for the safe operation if there are unusual design, operating, or
handling characteristics.\"

1)  For compliance with § 25.1541, FAA policy states that the airspeed
    indications need to provide pilots the equivalent \"quick glance\"
    airspeed awareness that has been intrinsic on round dial
    instruments. See FAA Policy Memoranda dated February 25, 1992 and
    September 12, 1996.

    a.  Low speed awareness cues should provide adequate warning to the
        pilot that the airspeed is below the reference operating speed
        for the airplane configuration (i.e., weight, flap setting,
        landing gear position, etc.).

    b.  Similarly, high speed awareness cues should provide adequate
        warning to the pilot that the airspeed is approaching an
        established upper limit that may result in a hazardous operating
        condition.

    c.  The cues should be readily distinguishable from other markings
        such as V-speeds and speed targets (bugs). The cues should
        indicate not only the boundary value of speed limit, but also
        the speed range which is beyond those limiting values.

2)  The display requirements for airspeed awareness cues are in addition
    to other alerts associated with exceeding high and low speed limits,
    such as the stick shaker and aural overspeed warning.

d\. **\[Required content - indications of system modes, failures\]** 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 pilot
will operate primarily by reference to the HUD, the pilot should have
awareness of flight guidance system operations, modes, and status.

1)  The current mode of the flight guidance/automatic flight control
    system should be clearly annunciated in the HUD, unless another
    location in close proximity to the HUD is shown to be equivalently
    conspicuous.

2)  Other essential information and alerts which may require immediate
    pilot action should be displayed for instant recognition. Such
    information includes malfunctions of primary data sources, guidance
    and control systems, or excessive deviations which require a
    go-around.

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e.  **\[Required content -- support for immediate actions\]** 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 pilot
    will operate primarily by reference to the HUD, such use should not
    cause a delay in the pilot's performance of immediate safety-related
    actions required in response to warnings.

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1)  Since the transition from the HUD to the head down instruments would
    delay the pilot's immediate intervention, there should be no event
    which requires the pilot to make an immediate transition from the
    HUD to the HDD in order to perform an immediately required action
    (e.g. response to a warning, initiate a missed approach), except for
    loss of the HUD system itself.

2)  Certain alerts and guidance information which are displayed head
    down to support immediate pilot intervention should be displayed on
    the HUD. For example, if a wind shear detection system, a ground
    proximity warning system (GPWS), a traffic alert and collision
    avoidance system (TCAS), or go-around guidance flight director mode
    is installed, it is essential that the guidance, warnings, and
    annunciations which are required to support pilot intervention by
    display in the pilot's primary field of view, also be displayed on
    the HUD.

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f.  **\[Required content/layout - cockpit integration/compatibility\]**
    The content, arrangement and format of the information should be
    sufficiently compatible with the head down displays to preclude
    pilot confusion, misinterpretation, or excessive cognitive workload.
    Transitions between the two displays, whether for navigation duties,
    failure conditions, unusual airplane attitudes, or other reasons,
    should not present difficulties in data interpretation, delays or
    interruptions in the crew\'s ability to manually control the
    airplane or to monitor the automatic flight control system.

g\. **\[Evaluation - transitions between Head Up and Head Down\]**
Events that may require a transition between the HUD and the HDD should
be identified. Test scenarios should be developed to evaluate workload
and pilot performance. These scenarios would include systems failures,
as well as events leading to unusual attitudes.

h\. **\[Format, Style, Clutter, Obstruction of outside view\]** The HUD
display should present all information in a clear and unambiguous
manner. Display clutter should be minimized. For compliance with
§ 25.773, HUD symbology must not excessively interfere with pilots'
forward view, ability to visually maneuver the airplane, acquire
opposing traffic, and see the runway environment. For compliance with
§§ 25.1301 and 25.1303, data elements of primary flight displays which
are required by the rules or are essential or critical to the phase of
flight must be displayed, regardless of the display mode.

i\. **\[Symbol Characteristics\]** For compliance with § 25.1301,
symbols, lines and characters must appear clean-shaped, clear, and
explicit with no distracting visual effects, ambiguities, halo,
aliasing, flicker or jitter.

j\. **\[Symbology - Control Laws, Dynamics, Latency\]** For compliance
with § 25.1301 in all phases of flight, the HUD must update the
positions and motions of primary control symbols with sufficiently high
rates and short latencies to support satisfactory manual control
performance.

**2. [Installation Requirements]{.underline}**

a\. **\[Controls\]**

1)  For compliance with § 25.777, HUD controls must be visible to, and
    within reach of, the pilot from any normal seated position.

2)  For compliance with §§ 25.77, 25. 789 and 25.1301, the position and
    movement of the HUD controls must not lead to inadvertent operation.

3)  For compliance with §25.1381, the HUD controls must be adequately
    illuminated for all normal background lighting conditions and must
    not create any objectionable reflections on the HUD or other flight
    instruments. Unless a fixed illumination of the HUD controls is
    shown to be satisfactory under all lighting conditions, there should
    be a means to adjust it.

4)  To the greatest extent practicable, the HUD controls should be
    integrated with other controls, including the flight director, to
    minimize the crew workload associated with HUD operation and to
    ensure flightcrew awareness of engaged flight guidance modes.

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b.  **\[Control of HUD brightness\]** The HUD display brightness is a
    key factor in the pilot's ability to use the display and to see
    through it to view the outside scene.

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1)  The control and dynamic range of HUD brightness should provide
    suitable display readability in ambient lighting conditions from 0
    foot-Lamberts (fL) to 10,000 fL. When certain extreme lighting
    conditions make use of the HUD impractical (for example, flying
    directly toward the setting sun), a suitable alternative head down
    flight display must be available for the crew to make a ready
    transition to the HDD.

2)  Background (ambient) lighting conditions can change quickly while
    the airplane is maneuvering, so, for compliance to §§ 25.773,
    25.1301 and to avoid high workload, automatic brightness control
    should be provided, unless it can be shown that a single manual
    setting is satisfactory for the duration of a high workload phase of
    flight (e.g., instrument approach).

3)  When the brightness level is altered, the relative luminance of each
    displayed symbol, character, or data should vary smoothly without
    allowing any information to become invisible while other data
    remains discernible.

4)  For compliance with § 25.773, there should be no objectionable
    brightness transients when transitioning between manual and
    automatic control.

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c.  **\[Physical Interference\]** For compliance with §§ 25.803,
    25.1307, 25.1411, and 25.1447, the installation of the HUD system
    must:

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1)  Not interfere with or restrict other installed equipment such as
    emergency oxygen masks, headsets, or microphones.

2)  Not adversely affect the emergency egress provisions for the flight
    crew, or significantly interfere with crew access.

3)  Not hinder the crew\'s movement while conducting any required flight
    procedures.

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d.  **\[Physical Hazard\]** For compliance with §§ 25.562, 25.785 and
    25.789, 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, bird
    strike, etc. The installation of the HUD, including overhead unit
    and combiner, must be accounted for when demonstrating compliance
    with the head injury criteria (HIC) of § 25.562 (c)(5).
    Additionally, the HUD installation must comply with the retention
    requirements of § 25.789(a) and occupant injury requirements of
    § 25.785 (b), (d) and (k).\
    \
    (Note: § 25.562 applies only for airplanes with that rule in their
    certification basis.)

e.  **\[Glare, Interference with Pilot Compartment View\]** For
    compliance with § 25.773, the installation of the HUD system must
    not present the crew with any objectionable glare or reflection in
    any lighting conditions. This is equally applicable from glare or
    reflections visible on the HUD system itself, or that originating
    from the HUD system and visible in other areas such as the
    windshield.

f.  **\[Pilot Compartment View\]** For compliance with § 25.773, the
    installation of the HUD system must not significantly obstruct
    either pilot\'s external field of view with or without the
    combiner(s) deployed.

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1)  The external view requirements of § 25.773 for the airplane must be
    retained with the combiner deployed.

2)  The HUD system combiner must not create any adverse distortion of
    the pilot\'s external view.

3)  The optical qualities (accommodation, luminance, vergence) of the
    HUD which could contribute to distortion should be uniform across
    the entire field of view. When viewed by both eyes from any
    off-center position within the eyebox, non-uniformities should not
    produce perceivable differences in binocular view.

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g.  **\[Visibility, Eyebox, Design Eye Point, Motion Tolerance\]** The
    visibility of the HUD and the primary flight information it displays
    is paramount to the HUD's intended function as a flight reference
    display. The fundamental requirements for instrument arrangement and
    visibility found in §§ 25.1321, 25.773 and 25.777 apply to these
    devices.

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1)  Section 25.1321 requires that each flight instrument for use by any
    pilot be plainly visible at that pilot's station, with minimum
    practicable deviation from the normal position and forward line of
    vision.

> Advisory Circular (AC) 25.773-1 defines the Design Eye Position (DEP)
> as a single point, selected by the applicant, that meets the
> requirements of §§ 25.773 and 25.777. For certification purposes, the
> DEP is the pilot's normal seated position, and fixed markers or other
> guides must be installed 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.

2)  For compliance with §§ 25.773 and 25.1301, the pilot must be able to
    view all of the flight essential information displayed in the HUD
    while located at the DEP. The optical characteristics of the HUD
    make the ability to fully view information on the combiner much more
    sensitive to the pilot's eye position than for displays on the
    instrument panel.

3)  Additionally, the flight essential information should remain in view
    with the small, involuntary motions of the pilot's head around the
    DEP . HUD manufacturers typically define a three-dimensional viewing
    volume called the "eyebox" within which certain visual performance
    requirements are met, but may not always provide a view of all
    flight essential information. The portion of the eyebox from which
    the pilot can view all of the flight essential information is
    defined as the Cockpit Head Motion Box (CHMB), (ref. SAE ARP 5288).

4)  The Cockpit Head Motion Box (CHMB) is centered on the Design Eye
    Position (DEP) and its minimum dimensions are:

> Lateral: 1.5 inches left and right from the DEP
>
> Vertical: 1.0 inches up and down from the DEP
>
> Longitudinal: 2.0 inches fore and aft from the DEP
>
> 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,
> larger minimum CHMB dimensions, than those shown above, may be
> necessary.
>
> In addition to meeting the minimum CHMB dimensions, described above,
> the HUD should provide adequate tolerance for variant head
> displacements, and for "relaxed" pilot sitting positions, commonly
> used during long flights. The location of the HUD combiner and
> dimensions of the CHMB eyebox should not place physiologically
> burdensome fatigue, visual stress, or limitations on head position,
> particularly considering long duration flights.

5)  The applicant should specify the minimum monocular field of view
    (FOV), which will include the center of the HUD FOV, that provides a
    full display of the minimum required flight information. By
    definition, whenever at least one of the pilot's eyes are within the
    CHMB, the required flight information will be visible in the HUD.
    The lateral and vertical dimensions of the CHMB represent the total
    movement of a monocular viewing instrument (e.g., pilot's eye) with
    a 1/4 in. (6.35 mm) entrance aperture (pupil). The eyebox
    longitudinal dimension represents the total fore-aft movement over
    which the requirement of this specification are met.

6)  The HUD installation must comply with §§ 25.1321, 25.773 and 25.777
    at the Design Eye Position. The HUD installation must accommodate
    pilots from 5'2" to 6'3" tall, seated with seat belts fastened and
    positioned at the DEP.

7)  HUD installations with overhead projectors should be evaluated for
    the potential of the pilot's head, while located within the CHMB, to
    partially block the optical path between the projector and the
    combiner, and prevent the display of some flight essential
    information, under any foreseeable conditions, including unusual
    attitudes.

8)  Operational suitability of the CHMB, the minimum monocular field of
    view (FOV), and specified set of minimum required flight information
    will be evaluated by the FAA.

**3. [System Requirements]{.underline}**

a.  For compliance with 25.1301, the HUD system must perform its
    intended function, whether as a primary flight display or as a
    supplemental display, for each operation and phase of flight that
    its use will be permitted. The normal operation of the HUD system
    cannot adversely affect, or be adversely affected by other airplane
    systems.

b.  For compliance with §§ 25.1309 and 25.1333, the malfunctions of the
    HUD system which cause loss of all primary flight information,
    including that displayed on the HUD and head down instrument panel,
    must be extremely improbable.

c.  For compliance with § 25.1309, misleading guidance displayed on the
    HUD which is not detected and annunciated in a timely manner, and,
    if followed, can result in a catastrophic event, during the takeoff,
    go-around, or landing phases of flight, must be extremely improbable
    for each takeoff roll, go-around, or landing. See AC 120-28D,
    Appendices 2 and 3 for airworthiness criteria specific to low
    visibility guidance systems, if applicable. See SAE ARP5288, section
    4.2.1 for other recommended system safety guidelines.

d.  The criticality of the HUD system's function to display flight and
    navigation data, including the potential to display hazardously
    misleading information, must comply with §§ 25.1309, 25.1333, and
    should be assessed according to Advisory Circular (AC) 25-11
    paragraph 4.a., and AC 25.1309-1A. All alleviating flight crew
    actions that are considered in the HUD system safety analysis must
    be validated during testing for incorporation in the airplane flight
    manual procedures section or for inclusion in type-specific
    training.

e.  For compliance with § 25.1309, the safety analysis must be conducted
    to show that the integrated system, consisting of the HUD
    components, sensors, ILS receivers, Radio Altimeter, airplane IRU or
    equivalent, and Air Data, meet critical signal integrity
    requirements for the airplane and HUD. System and subsystem
    malfunctions which are not shown to be extremely improbable should
    be demonstrated in a simulation or in flight. The malfunction
    annunciation and fault detection schemes should demonstrate
    operation to the designed level of integrity.

f.  The safety analysis should be conducted by qualified specialists,
    knowledgeable with the preparation, documentation and requirements
    of § 25.1309. If a qualified Designated Engineering Representative
    (DER) is employed, the FAA Aircraft Certification Office will still
    retain the compliance finding. The safety analysis is a significant
    project milestone. The Failure Hazard Analysis (FHA) and Preliminary
    System Safety Analysis should be submitted to the FAA for
    concurrence with the basic assumptions, early in the project
    schedule.

g.  AC 25-11 paragraph 4.a., provides guidance that the display of
    hazardously misleading information on more than one primary flight
    display should be extremely improbable. Accordingly, HUD system
    software which generates, displays or affects the generation or
    display of primary flight information should be developed to Level A
    requirements, as specified by RTCA Document DO‑178B, "Software
    Considerations in Airborne Systems and Equipment Certification," or
    similar processes which provide equivalent product and compliance
    data.

h.  Monitoring software shown to have no ability to generate, display or
    affect the generation or display of primary flight information, and
    with the capability to command shutdown of the HUD system, should be
    developed to the appropriate software level criteria as determined
    by a safety assessment of the HUD system.

> Alternatively, the monitoring software may be developed to Level A
> criteria and the information display software to Level B or Level C
> criteria, provided it can be shown, based on system and software
> architecture and other means, including redundancy, independence,
> partitioning, dissimilarity and other protection of software and
> hardware functions, that such a design provides an equivalent level of
> system integrity as would be provided by a Level A HUD display system
> with a Level B or Level C (as required) monitoring function.

i.  The HUD system must provide a means to alert the crew when the
    combiner is deployed, but not in its normal operating position, such
    that conformal symbols are hazardously misaligned.

j.  For the takeoff, approach, and go-around phases of flight where
    airworthiness approval is predicated on the use of the HUD, or for
    these and other phases of flight where it can be reasonably expected
    that the pilot will operate primarily by reference to the HUD, the
    HUD display should not be unuseable or unstable for more than one
    second after the normally expected electrical bus transients due to
    a power transfer. Transfer to standby attitude or transfer of
    control of the airplane to the other pilot cannot be reasonably
    accomplished under these conditions in a timely enough manner to
    prevent an unsafe condition.

k.  Provisions for reversion from a failed HUD to the HDD should be
    appropriate to system design, intended function, and intended
    operational use. The applicant should clarify whether the HUD is
    intended as a reversionary backup to failed HDD. Any automatic
    display reversions should be readily apparent and allow continued
    safe flight using alternative flight displays appropriate to the
    phase of flight.

l.  **\[Dual HUD\]** To adequately understand and evaluate the intended
    function of a dual HUD installation for compliance with § 25.1301,
    the applicant should provide a HUD Concept of Operations that fully
    details Pilot Flying/Pilot Not Flying (PF/PNF) tasks and
    responsibilities in regards to using and monitoring head down
    instruments and HUD's during all phases of flight.

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1)  The dual HUD Concept of Operations should specifically address
    flight crew awareness of essential information during the
    simultaneous use of the HUD in cruise, terminal, approach, landing,
    and takeoff phases of flight.

2)  The applicant should clarify whether the HUD is intended as a
    revsionary backup to failed head down instruments. Provisions for
    reversion from a failed HUD to the head down instruments should be
    appropriate to system design, intended function, and intended
    operational use. Any automatic display reversions should be readily
    apparent and allow continued safe flight using alternative flight
    displays appropriate to the phase of flight

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m.  The high intensity radiated fields (HIRF) protection criteria,
    electromagnetic interference (EMI) and system lightning protection
    criteria apply to the HUD installation. The HUD installation must
    comply with § 25.1316, Amendment 25-80.