AC 25.1329-1C CHG 2 advisories as part of their FGS function. In this regard, HUDs should provide the equivalent alerting functionality as the head down PFD(s) . Warnings that require continued flightcrew attention on the PFD also should be presented on the HUD (for example, 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 attention- getting properties, such as flashing, outline boxes, brightness, size, and/or location, is 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 pilotnot-flying (PNF) will monitor the head down instruments and alerting systems for failures of systems, modes, and functions not associ ated 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 to an other display when an immediate maneuver is required (for example, TCAS resolution advisory, windshear).
(6) GPWS/TCAS on HUD. If a GPWS, windshear detection system, windshear 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.
Note: See AC 25-11B, appendix F, for guidance on time-critical alerts displayed on the HUD. If it is not practical to provide time -critical alerts on the HUD, the applicant should provide compensating design features and pilot procedures to ensure that the pilot has equivalent and effective information for immediate awareness and response to the respective alerts.
AC 25.1329-1C CHG 2
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 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.
47—51 [RESERVED] 33
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52. General.
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. Normal 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 table 5-1 of this AC. 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 failure All airplane systems that are associated with airplane conditions performance are fully operational . Failures of those systems could impair the FGS’s ability to perform its functions.
Light to Variation in wind velocity—as a function of altitude, moderate wind position, or time—that may cause slight erratic or gradients unpredictable changes in intended flight path.
Light Turbulence that momentarily causes slight, erratic turbulence changes in altitude and/or attitude (pitch, roll, or yaw).
Light chop Turbulence that causes slight, rapid, and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude.
Icing All icing conditions covered by part 25, a ppendix C, with the exception of “asymmetric icing” and “large drop icing” discussed under “Rare Normal Conditions” in table 5-2 of this AC.
Condition Example Light to moderate winds Constant wind in a specific direction that may cause a slight deviation in the intended flight path or a small difference between airspeed and groundspeed.
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.
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 .
Moderate chop Similar to light chop but of greater intensity . It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude.
Note: Representative levels of the environmental effects should be established consistent with the airplane’s intended operation.
AC 25.1329-1C CHG 2
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, RVSM, 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 25-7D and 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. 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 authority without introducing adverse interactions with automatic control of the aircraft . Automatic roll and yaw trim functions, if installed, should operate without introducing adverse interactions with automatic control of the aircraft.
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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 54.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 table 5-2 of this AC. 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.
Windshear/ A wind gradient of such magnitude that it may cause microburst damage to the aircraft.
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 cause 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.
Condition Example Significant wind gradients Variation in wind velocity—as a function of altitude, position, or time—that may cause large changes in intended flight path.
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.
Large drop icing Applicable icing conditions covered by 14 CFR part 25, appendix O.
Note: Airplanes intended to meet 14 CFR 121.358 for windshea r warning and guidance need FD windshear guidance. The FGS may also provide suitable autopilot control during windshear . See AC 25-12 and AC 120-41 for windshear 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.
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(2) Autopilot authority limit. Operations in rare normal conditions may result i n a utomatic 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, paragraph 27.b, Autopilot Disengagement, and the criteria for flight guidance alerting in Chapter 4 , paragraph 45.d of this AC.
(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 . For discussion of alerting under such conditions , see Chapter 4, paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status , of this AC.
55. Icing Considerations in Normal and Rare Normal Conditions .
a. Effect on performance. The FGS will typically 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 t o h ave an effect upon FGS performance, as described in Chapter 4 , paragraph 45 of this AC.
c. Effect on speed protection. The implication of icing conditions on speed protecti on s hould 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 57.b 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. Table 5-3 of this AC 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 fuel Large variation of the amount of fuel between the two imbalance 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.
Inoperative Loss of one or more engines that causes the FGS, if engine(s) engaged, to counter the aerodynamic effect of the difference in thrust with a pitch, roll, or yaw command that is approaching maximum system authority.
Inoperative ice Loss of ice detection/protection system on an airplane detection/protection so equipped where the FGS is certified for operation system in icing conditions with that failure present.
Condition Non-standard ferry Possible aerodynamic drag (both symmetrical and flight configurations unsymmetrical) caused by non -standard airplane ferry Term 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.
Loss of one or more hydraulic systems Loss of one or more hydraulic systems down to the minimum amount of remaining operational systems that the FGS is certified to operate.
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, paragraph 27.b, and the criteria for flight guidance alerting in Chapter 4, paragraph 45.d of this AC.
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(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 . For discussion of alerting under such conditions , see Chapter 4, paragraph 45 of this AC.
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 the 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:
(a) The FGS provides speed protection . In this case, the following are acceptabl e m eans to comply with this rule:
1 The FGS may detect the speed protection condition, alert the flightcrew, and provide speed protection control or guidance.
2 The FGS may detect the speed protection condition, alert the flightcrew, and then disengage the FGS.
3 The FGS may detect the speed protection condition, alert the flightcrew, and remain engaged in the active mode without providing speed protection control or guidance.
(b) Other systems provide speed protection . Other systems, such as the primar y f light control system or the FMS (when in a vertical navigation (VNAV) mode) , may be used to provide equivalent speed protection.
Note: If compliance with this requirement is based on use of alerting alone, the alerts should be shown to be appropriate and timely to ensure flightcrew awareness and enable the pilot to keep the airplane within an acceptable margin from the speed range of the normal flight envelope. For additional discussion of speed protection alerting, s ee Chapter 4, paragraph 45.b, Speed Protection Alerts.
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(3) Design standard.
(a) Interaction of FGS elements. The design should consider how and when the speed protection is provided for combinations of autopilot, FDs, and autothrust operation. Care should be taken to set appropriate values for transitioning into and out of speed protection such that the flightcrew does not consider the transitions a nuisance.
(b) Integration of pitch and thrust control. The speed protection function shoul d i ntegrate pitch and thrust control . Consideration should be given to automatically activating the autothrust function when speed protection is invoked. If an autothrust function is either not provided or is unavailable, speed protection should be provided through pitch control alone.
(c) Interaction of systems. The role and interaction of autothrust with elements of the FMS, the primary flight control system, and the propulsion system, as applicable, should be accounted for in the design for speed protection.
(d) Engine inoperative. Consideration should be given to the effects of an engine inoperative condition on the performance of speed protection.
b. Low-speed protection.
(1) General. When the FGS is engaged in any mode (with the possible exception of Approach, as discussed in paragraph 57.b(3) below) for which the available thrust is insufficient to maintain a safe operating speed, the low -speed protection function should be invoked to avoid unsafe speed excursions.
(2) Factors to consider. Activation of speed protection should take into account suc h f actors as the phase of flight, turbulence and gusty wind conditions, and compatibility with the speed schedules. The low-speed protection function should activate at a suitable margin to stall warning that will not result in nuisance alerts . Consider the operational speeds, as specified in the AFM, for all-engine and engine-inoperative cases during the following phases of flight:
(a) Takeoff.
(b) Departure, climb, cruise, descent, and terminal area operations . During these flight phases, airplanes are normally operated at or above the minimum maneuvering speed for the given flap configuration.
Note: For high altitude operations, it may be desirable to incorporate low-speed protection at the appropriate engine out drift -down speed schedule, if the FGS (or other integrated sensors/systems) can determine that the thrust deficiency is due to an engine failure.
(c) Approach.
(d) Transition from approach t o go-around and go-around climb.
AC 25.1329-1C CHG 2 43 Note: A low-speed alert and a transition to the Speed Protection mode at approximately 1.13 VSR (reference stall speed) for the landing flap configuration has been found to be acceptable.
(3) Low-speed protection during approach operations.
(a) Non-interference. Speed protection should not interfere with the approach and landing phases of flight.
(b) Autothrust operation. It is assumed that with autothrust operating normally, the combination of thrust control and pitch control during the approach will be sufficient to maintain speed and desired vertical flight path . In cases where it is not sufficient, an alert should be provided in time for the flightcrew to take appropriate corrective action.
(c) Defined vertical path. For approach operations with a defined vertical path (for example, ILS, GLS, and RNAV using LNAV/VNAV or LPV lines of minima ), if the thrust is insufficient to maintain both the desired flight path and the desired approach speed, there are several ways to meet the intent of low -speed protection:
1 The FGS may maintain the defined vertical path as the airplane decelerates below the desired approach speed until the airspeed reaches the low -speed protection value. At that time, the FGS would provide guidance to maintain the low -speed protection value as the airplane departs the defined vertical path . The FGS mode reversion and low -speed alert should be activated to ensure pilot awareness.
Note: The pilot is expected to take corrective action to add thrust and return the airplane to the defined vertical path or go- around, as necessary.
2 The FGS may maintain the defined vertical path as the airplane decelerates below the desired approach speed to the low -speed protection value. The FGS will then provide a low-speed alert while remaining in the existing FGS approach mode.
Note: The pilot is expected to take corrective action to add thrust to cause the airplane to accelerate back to the desired approach speed while maintaining the defined vertical path or go-around, as necessary.
3 The FGS may maintain the defined vertical path as the airplane decelerates below the desired approach speed until the airspeed reaches the low -speed protection value. The FGS will then provide a low -speed alert and disengage.
Note: The pilot is expected to take corrective act ion when alerted to the low-speed condition and the disengagement of the autopilot, to add thrust, and to manually return the airplane to the desired vertical path or go-around.
(d) Vertical flight path not protected. If the speed protection is invoked during approach such that vertical flight path is not protected, the subsequent behavior of the FGS after speed protection should be carefully considered. Activating low-speed protection during the