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(for exam ple, TCAS, windshear, and ground proximity warning (GPWS) annunciations).
If master alerting indications are not provided within the peripheral field of view of the pilot while using the HUD, the HUD should provide annunciations that inform the pilot of caution and/or warning conditions.
(2) Monochrome HUDs. For monochrome HUDs, appropriate use of attentiongetting properties, such as flashing, outline boxes, brightness, size, and/or location are necessary to adequately compensate for the lack of color normally assigned to distinguish and call attention to cautions and warnings.
(3) Multi–Color HUDs. For multi-color HUDs, the use of red, amber, or yellow for symbols not related to caution and warning functions should be avoided, so that the effectiveness of distinguishing characteristics of true cautions and warnings is not reduced.
(4) Single HUD Installation. Single HUD installations rely on the fact that the pilot not flying (PNF) will monitor the head down instruments and alerting systems for failures of systems, modes, and functions not associated with PFDs.
(5) Dual HUD Installations. Dual HUD installations require special consideration for alerting systems. It should be assumed that both pilots will be head up simultaneously (full or part-time), especially when the HUD is being used as the primary flight reference or when the HUD is required equipment for the operation being conducted. If master alerting indications are not provided within the peripheral field of view of each pilot while using the HUD, then each HUD should provide annunciations that direct the pilot’s attention to head down alerting displays. The types of information that should trigger the HUD master alerting display are any cautions or warnings not already duplicated on the HUD from head down primary display as well as any caution level or warning level engine indications or system alerts.
Note: The objective is to avoid redirecting attention of the pilot flying (PF) to another display when an immediate maneuver is required (for example, TCAS resolution advisory, windshear).
(6) Ground Proximity Warning System (GPWS)/Traffic Alert and Collision Avoidance System (TCAS) on HUD. If a GPWS, wind shear detection system, wind shear escape guidance system, or a TCAS is installed, then the guidance, warnings and annunciations required to be a part of these systems and normally required to be in the pilot’s primary field of view should be displayed on the HUD.
e. Upset/Unusual Attitude Recovery Guidance.
(1) Guidance Cues. Upsets due to wake turbulence or other environmental conditions may result in near instantaneous excursions in pitch and bank angles and a subsequent unusual attitude. If the HUD is designed to provide guidance for recovery from upsets or unusual attitudes, recovery steering guidance commands should be distinct 44
from and not confused with orientation symbology such as horizon “pointers.” For example, a cue for left control wheel/stick input should not be confused with a cue indicating direction to the nearest horizon. Guidance should be removed if cues become invalid at extreme attitudes, such as zenith, nadir, overbanked (for example, greater than 90 degrees bank angle), or inverted. For extreme attitudes, it is acceptable for the pilot to transition to the HDD, provided that the cues to transition from the HUD are clear and unambiguous.
(2) Orientation Cues. If the HUD is designed to provide only orientation during upsets or unusual attitudes (that is, no guidance cues are provided), those orientation cues should be designed to prevent them from being mistaken as flight control input commands.
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Chapter 5 Performance of Function
52. Genera
l..
a. Intended Function. The FGS should provide guidance or control, as appropriate, for the intended function of the active mode(s) in a safe and predictable manner within the airplane’s normal flight envelope. There are special considerations when the FGS is operating at the limits of its performance capabilities or under significant environmental conditions. The following paragraphs provide criteria for acceptable means of compliance and interpretive material pertaining to these considerations.
b. Effect of System Tolerances. Where system tolerances have a significant effect on autopilot authority limits, consideration should be given to the effect on autopilot performance. Factors to be considered include, but are not limited to, tolerances of servo authority, servo clutch setting, “cam-out” settings, control friction, and sensor tolerances.
53. Nornal performance.
a. General. The FGS should be designed to provide guidance or control, as appropriate, for the intended function of the active mode(s) in a safe and predictable manner within the airplane’s normal flight envelope.
b. Conditions to Consider. The design of the FGS should consider the normal conditions listed in the following table. This table does not fully define every condition that may be encountered during an airplane’s life and unequivocally categorize what can be considered to be a “normal condition.” Rather, the table is intended to give examples to be used during system development and testing. By the very nature of the phenomena involved, there will always be some subjectivity to these categorizations. Also, the same conditions may affect different airplane models in very different ways. These differences should be considered in determining how to characterize the severity of the conditions discussed in the table.
Table 5-1 Examples of Normal Conditions
No f ailure conditions All airplane systems that are associated with airplane performance are fully operational. Failures of those systems could impair the flight guidance system's (FGS) ability to perform its functions.
Light to moderate winds Constant wind in a specific direction that may cause a slight deviation in intended flight path or a small difference between airspeed and groundspeed.
Light to moderate wind gradients Variation in wind velocity—as a function of altitude, position, or time—which may cause slight erratic or unpredictable changes in intended flight path.
Light to moderate gusts Non–repetitive momentary changes in wind velocity that can cause changes in altitude and/or attitude to occur, but the aircraft remains in positive control at all times.
Light turbulence Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude (pitch, roll, or yaw).
Moderate turbulence Similar to light turbulence but of greater intensity. Changes in altitude and/or attitude occur, but the aircraft remains in positive control at all times.
Light chop Turbulence that causes slight, rapid, and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude.
Moderate chop Similar to light chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude.
Icing All icing conditions covered by 14 CFR Part 25, Appendix C with the exception of “asymmetric icing” discussed under “Rare Normal Conditions” in Table 5-2.
Note: Representative levels of the environmental effects should be established consistent with the airplane’s intended operation.
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c. Level of Performance.
(1) Significant Characteristics. Any performance characteristics that are operationally significant or operationally limiting must be identified with an appropriate statement or limitation in the airplane flight manual (AFM). [See § 25.1581]
(2) Configuration Changes. The FGS should perform its intended function during routine airplane configuration or power changes, including the operation of secondary flight controls and landing gear.
(3) Compliance Evaluation. Evaluation of FGS performance for compliance should be based on the minimum level of performance needed for its intended functions.
Subjective judgment may be applied to account for experience acquired from similar equipment and levels that have been established as operationally acceptable by the end-user.
(4) Prescribed Levels of Performance. There are certain operations that dictate a prescribed level of performance. When the FGS is intended for operations that require specific levels of performance, the use of FGS should be shown to meet those specific levels of performance (for example, low visibility operations – Category II and III operations, reduced vertical separation minimums (RVSM), required navigation performance (RNP)).
(5) Equivalence of Performance. The FGS performance of intended functions should at least be equivalent to that expected of a pilot for a similar task. AC 25-7C, Flight Test Guide for Certification of Transport Category Airplanes, and the SAE ARP5366, Autopilot, Flight Director and Autothrust Systems, may prove useful for establishing the general behavior of the FGS. When integrated with navigation (NAV) sensors or the FMS, the FGS should satisfy the flight technical error tolerances expected for the use of those systems in performing their intended functions.
(6) Autopilot. The autopilot should provide smooth and accurate control without divergent or perceptible sustained nuisance oscillation.
(7) Flight Director (FD). The FD, in each available display presentation (for example, single cue, cross–pointer, flight path director) should provide smooth and accurate guidance and be appropriately damped, to achieve satisfactory control task performance without pilot compensation or excessive workload.
(8) Autothrust. The autothrust function should provide smooth and accurate control of thrust without significant or sustained oscillatory power changes or excessive overshoot of the required power setting.
(9) Automatic Trim. The automatic pitch trim function should operate at a rate sufficient to mitigate excessive control surface deflections or limitations of control 49
authority without introducing adverse interactions with autom atic control of the aircraft.
Automatic roll and yaw trim functions, if installed, should operate without introducing adverse interactions with automatic control of the aircraft.
54. Performance in rare normal conditions.
a. General.
(1) Range of Conditions. The FGS will encounter a wide range of conditions in normal operations, some of which may be infrequent but levy a greater than average demand on the FGS capabilities. Certain environmental conditions, as discussed in paragraph b, below, are prime examples. The FGS performance during such rare normal conditions should be assessed.
(2) Effect on Performance. Rare normal conditions may degrade the performance of the FGS but should be safe for operation of the FGS. The relative infrequency of such conditions may also be a factor in the flightcrew’s ability to detect and mitigate in a timely manner any limited capability of the FGS to cope with them.
b. Conditions to Consider. The design of the FGS should consider the rare normal conditions listed in the following table. This table does not fully define every condition that may be encountered during an airplane’s life and unequivocally categorize what can be considered to be a “rare normal condition.” Rather, the table is intended to give examples to be used during system development and testing. By the very nature of the phenomena involved, there will always be some subjectivity to these categorizations.
Also, the same conditions may affect different airplane models in very different ways.
These differences should be considered in determining how to characterize the severity of the conditions discussed in the table.
Table 5-2 Examples of Rare Normal Conditions
Significant winds Constant wind in a specific direction that may cause a large change in intended flight path or groundspeed or cause a large difference between airspeed and groundspeed.
Significant wind gradients Variation in wind velocity—as a function of altitude, position, or time—which may cause large changes in intended flight path.
Windshear/microburst A wind gradient of such magnitude that it may cause damage to the aircraft.
Large gusts Non-repetitive momentary changes in wind velocity that can cause large changes in altitude and/or attitude to occur. Aircraft may be momentarily out of control.
Severe turbulence Turbulence that causes large, abrupt changes in altitude or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control.
Asymmetric icing Icing conditions that result in ice accumulations that causes the FGS, if engaged, to counter the aerodynamic effect of the icing conditions with a sustained pitch, roll, or yaw command that approaches its maximum authority.
Note: Airplanes intended to meet § 121.358 for windshear warning and guidance need FD windshear guidance. The FGS may also provide suitable autopilot control during windshear. Refer to AC 25-12 and AC 120-41 for wind shear guidance system requirements.
c. Level of Performance.
(1) Performance Standard. For rare normal conditions, the FGS should be designed to provide guidance or control, as appropriate, for the intended function of the active mode(s) in a safe and predictable manner, both within the normal flight envelope and for momentary excursions outside the normal flight envelope. If a determination is made that there are environmental conditions in which the FGS cannot be safely operated, appropriate operational limitations should be placed in the AFM.
(2) Autopilot Authority Limit. Operations in rare normal conditions may result in automatic or pilot-initiated autopilot disengagement close to the limit of autopilot authority. Autopilot disengagement in rare normal conditions should meet the safety criteria for autopilot disengagement found in Chapter 3, FGS Engagement, Disengagement, Indications, and Override, paragraph 27b, Autopilot Disengagement, and the criteria for flight guidance alerting in Chapter 4, Controls, Indications, and Alerts, paragraph 45d, Awareness of Potential Significant Transient Condition (“Bark Before Bite”).
(3) Masking of Potential Hazard. It is not necessary that the FGS always be disengaged when rare normal conditions that may degrade its performance or capability are encountered. The FGS may significantly help the flightcrew during such conditions.
However, the design should address the potential for the FGS to mask a condition from the flightcrew or otherwise delay appropriate flightcrew action. See Chapter 4, paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status, for discussion of alerting under such conditions.
55. Icing coniseration in normal and rare normal conditions.
a. Effect on Performance. The FGS typically will be designed to provide acceptable performance in all standard airplane configurations. Operating an airplane in icing conditions can have significant implications on the aerodynamic characteristics of the airplane (for example, ice accretion on wings, tail, and engines) and, consequently, on FGS performance. Ice accretion may be slow, rapid, symmetric, or asymmetric. During autopilot operation, the flightcrew may not be aware of the gradual onset of icing conditions or the effect that the accumulation of ice is having on the handling qualities of the airplane.
b. Alerts. Means should be provided to alert the flightcrew when icing conditions begin to have an effect upon FGS performance, as described in Chapter 4, paragraph 45.
c. Effect On Speed Protection. The implication of icing conditions on speed protection should be assessed. If the threshold of the stall warning system is adjusted due to icing conditions, appropriate adjustments should also be made to the FGS low speed protection threshold, which is discussed in paragraph 57b, Low Speed Protection, of this chapter.
56. Performance in non-normal conditions.
a. General. The FGS will occasionally be operating when the airplane transitions outside of its normal flight envelope, due to failures or non-standard ferry configurations.
Under such circumstances, the FGS characteristics and flightcrew interaction with the FGS should be shown to be safe.
b. Conditions to Consider. The following table shows examples of non–normal conditions. The table does not fully define every condition that may be encountered during an airplane’s life and unequivocally categorize what can be considered to be a “non–normal condition.” Rather, the table is intended to give examples to be used during system development and testing. By the very nature of the phenomena involved, there will always be some subjectivity to these categorizations. Also, the same conditions may affect different airplane models in very different ways. These differences should be considered in determining how to characterize the severity of the conditions discussed in the following table.
Table 5-3 Examples of Non-Normal Conditions
Significant f uel imbalance Large variation of the amount of fuel between the two wing tanks (and center and tail tanks, if so equipped) that causes the FGS, if engaged, to counter the aerodynamic effect of the fuel imbalance with a pitch, roll, or yaw command that is approaching maximum system authority.
Non-standard ferry flight configurations Possible aerodynamic drag (both symmetrical and unsymmetrical) caused by non-standard airplane ferry flight conditions such as locked high lift devices, landing gear locked in the deployed position, or an extra engine carried underneath one wing in an inoperative position.
Inoperative engine(s) Loss of one or more engines that causes the FGS, if engaged, to counter the aerodynamic effect of the difference in thrust with a pitch, roll, or yaw command that is approaching maximum system authority.
Loss of one or more hydraulic systems Loss of one or more hydraulic systems down to the minimum amount of remaining operational systems which the FGS is certified to operate.
Inoperative Ice Detection/ Protection System Loss of ice detection/protection system on an airplane so equipped where the FGS is certified for operation in icing conditions with that failure present.
c. Level of Performance.
(1) Performance Standard. For non- normal conditions, the FGS should be designed to provide guidance or control, as appropriate, for the intended function of the active mode(s) in a safe and predictable manner, both within the normal flight envelope and for momentary excursions outside the normal flight envelope. If a determination is made that there are non-normal conditions in which the FGS cannot be safely operated, appropriate operational limitations should be placed in the AFM.
(2) Autopilot Authority Limit. Operations in non-normal conditions may result in automatic or pilot-initiated autopilot disengagement close to the limit of autopilot authority. Autopilot disengagement in non-normal conditions should meet the safety criteria for autopilot disengagement found in Chapter 3, FGS Engagement, Disengagement, Indications, and Override, paragraph 27b, Autopilot Disengagement, and the criteria for flight guidance alerting in Chapter 4, Controls, Indications, and Alerts, paragraph 45d, Awareness of Potential Significant Transient Condition (“Bark Before Bite”).
(3) Masking of Potential Hazard. It is not necessary that the FGS always be disengaged when non-normal conditions that may degrade its performance or capability are encountered. The FGS may significantly help the flightcrew during such conditions.
However, the design should address the potential for the FGS to mask a condition from the flightcrew or otherwise delay appropriate flightcrew action. See Chapter 4, paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status, for discussion of alerting under such conditions.
57. Speed protection.
a. General.
(1) Speed Excursions. The requirement for speed protection is based on the premise that reliance on flightcrew attentiveness to airspeed indications alone during FGS operation is not adequate to avoid unacceptable speed excursions outside the speed range of the normal flight envelope. Many existing FGS systems have no provisions to avoid speed excursions outside the normal flight envelope. Some FGSs will remain engaged until the aircraft slows to stall conditions and also to speeds well above maximum operating limit speed/maximum operating limit Mach (VMO/MMO).
(2) Compliance with § 25.1329(h). Standard stall warning and high-speed alerts are not always timely enough for the flightcrew to intervene to prevent unacceptable speed excursions during FGS operation. The intent of § 25.1329(h) is for the FGS to provide a speed protection function for all operating modes, so that the airspeed can be safely maintained within an acceptable margin of the speed range of the normal flight envelope. The FGS design may use any of the following ways or a combination of ways to provide acceptable speed protection: