35 Table 5-1. Examples of Normal Conditions Condition Example No failure conditions All airplane systems that are associated with airplane performance are fully operational. Failures of those systems could impair the FGS’s ability to perform its functions.
Light to moderate winds Constant wind in a specific direction that may cause a slight deviation in intended flightpath 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—that may cause slight erratic or unpredictable changes in intended flightpath.
Light to moderate gusts Nonrepetitive 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 part 25, appendix C, with the exception of “asymmetric icing” and “large drop icing” discussed under “Rare Normal Conditions” in Table 5-2 of this AC.
Note: Representative levels of the environmental effects should be established consistent with the airplane’s intended operation.
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. The current editions of AC 25-7 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) FD. The FD, in each available display presentation (for example, single cue, cross-pointer, flightpath 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.
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.
38 Table 5-2. Examples of Rare Normal Conditions Condition Example Significant winds Constant wind in a specific direction that may cause a large change in intended flightpath or groundspeed or cause a large difference between airspeed and groundspeed.
Significant wind Variation in wind velocity—as a function of altitude, gradients position, or time—that may cause large changes in intended flightpath.
Wind shear/ A wind gradient of such magnitude that it may cause microburst damage to the aircraft.
Large gusts Nonrepetitive 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.
Large drop icing Applicable icing conditions covered by 14 CFR part 25, appendix O.
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 14 CFR part 121, § 121.358 for wind shear warning and guidance need FD wind shear guidance. The FGS may also provide suitable autopilot control during wind shear. See 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, 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 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 of this AC.
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 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 nonstandard 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.
40 Table 5-3. Examples of Non-Normal Conditions Condition Term Significant fuel 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.
Nonstandard ferry flight configurations Possible aerodynamic drag (both symmetrical and unsymmetrical) caused by nonstandard 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 that 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, paragraph 27.b 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 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 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 acceptable means 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 primary flight 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, see Chapter 4, paragraph 45.b, Speed Protection Alerts.
(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 should integrate 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 such factors 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 driftdown 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 to go-around and go-around climb.
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 flightpath. 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, lateral navigation (LNAV) mode, VNAV mode), if the thrust is insufficient to maintain both the desired flightpath 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 action when alerted to the lowspeed 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 flightpath not protected. If the speed protection is invoked during approach such that vertical flightpath is not protected, the subsequent behavior of the FGS after speed protection should be carefully considered. Activating low-speed protection during the
44 approach, resuming the Approach mode, and reacquiring the defined vertical path may be an acceptable response, if the activation is sufficiently brief and not accompanied by large speed or path deviations. This response is considered consistent with criteria for Category III automatic landing systems in AC 120-28, Appendix 3, section 8.1, Automatic Flight Control Systems, which states that it should not be possible to change the flightpath of the airplane with the autopilot(s) engaged, except by initiating an automatic go-around.
(4) Wind shear recovery guidance. The interaction between low-speed protection and wind shear recovery guidance is a special case. Wind shear recovery guidance that meets the criteria found in AC 25-12 and AC 120-41 provides the necessary low-speed protection when it is activated and is considered acceptable for compliance with § 25.1329(h). The autopilot should be disengaged when the wind shear recovery guidance activates, unless autopilot operation has been shown to be safe in these conditions and provides effective automatic wind shear recovery that meets the criteria found in the ACs referenced above.
c. High-speed protection.
(1) General. Section 25.1329(h) states that means must be provided to avoid excursions beyond an acceptable margin from the speed range of the normal flight envelope.
VMO and MMO mark the upper speed/Mach limit of the normal flight envelope. This is not intended to require or preclude high-speed protection based on airplane configurations (for example, flaps extended).
(2) Factors to consider.
(a) Duration of airspeed excursions, rate of airspeed change, turbulence, and gust characteristics:
1 Operations at or near VMO/MMO in routine atmospheric conditions (for example, light turbulence) are safe. Small, brief excursions above VMO/MMO by themselves are not unsafe.
2 The FGS design should strive to strike a balance between providing adequate speed protection margin and avoiding nuisance activation of high-speed protection.
Note: The following factors apply only to designs that provide high-speed protection through FGS control of airspeed.
(b) High-speed protection while in altitude hold mode:
1 Climbing to control airspeed is not desirable because departing an assigned altitude can be disruptive to air traffic control (ATC) and potentially hazardous (for example, in RVSM airspace). As long as the speed does not exceed a certain margin beyond VMO/MMO (for example, 6 knots), it is better that the FGS remain in Altitude Hold mode.
2 The autothrust function, if operating normally, should effect highspeed protection by limiting its speed reference to the normal speed envelope (that is, at or below VMO/MMO).
45 3 The basic airplane high-speed alert should be sufficient for the pilot to recognize the overspeed condition and take corrective action to reduce thrust. However, if the airspeed exceeds a margin beyond VMO/MMO (for example, 6 knots), the FGS may transition from Altitude Hold to the Overspeed Protection mode and depart (that is, climb above) the selected altitude.
(c) High-speed protection during climbs and descents:
1 When the elevator channel of the FGS is not controlling airspeed, the autothrust function, if engaged, should reduce thrust, as needed, to prevent sustained airspeed excursions beyond VMO/MMO (for example, 6 knots) down to the minimum appropriate value.
2 When thrust is already the minimum appropriate value or the autothrust function is not operating, the FGS should begin using pitch control, as needed, for high-speed protection.
3 If conditions are encountered that result in airspeed excursions above VMO/MMO, it is preferable for the FGS to smoothly and positively guide or control the airplane back to within the speed range of the normal flight envelope.
58—61 [RESERVED]
46 CHAPTER 6. CHARACTERISTICS OF SPECIFIC MODES
62. General. There are certain operational modes of the FGS that have been implemented in different ways in different airplanes and systems. The following paragraphs provide guidance and interpretative material that clarifies the operational intent of these modes, and criteria shown to be acceptable in current operations. The guidance in this chapter does not preclude other mode implementations. Pilot understanding of the mode behavior is especially important to avoid confusion. Therefore, mode behavior should be clearly annunciated, as described in Chapter 4, paragraph 44, FGS Mode Selection, Annunciation, and Indication, of this AC.
63. Lateral Modes. This paragraph discusses modes that are implemented in many FGSs that are used primarily for lateral/directional control of the airplane. The criteria below identify acceptable mode operation, based on past operational experience gained from the use of these modes.
a. Heading or track hold. In the Heading or Track Hold mode, the FGS should maintain the airplane heading or track. When the airplane is in a bank when the Heading or Track Hold mode is engaged, the FGS should roll the airplane to a wings-level condition and maintain the heading or track when wings-level is achieved (typically less than 5 degrees of bank angle).
b. Heading or track select. In the Heading or Track Select mode, the FGS should expeditiously acquire and maintain a “selected” heading or track value consistent with occupant comfort. When the mode is initially engaged, the FGS should turn the airplane in a direction that is the shortest heading (or track) change to acquire the new heading (or track). Once the Heading/Track Select mode is active, changes in the selected value should result in changes in heading/track. The FGS must turn the airplane in the same direction as the sense of the selected heading change. See § 25.779(a)(1). That is, if the pilot turns the heading select knob clockwise, the airplane must turn to the right, even if the shortest heading (or track) change is in the opposite direction. The target heading or track value should be presented to the flightcrew.
c. LNAV mode.
(1) Lateral flightpath. In the LNAV mode, the FGS should acquire and maintain the lateral flightpath commanded by a flight management function (that is, FMS or equivalent).
(2) Automatic mode transitions. If the airplane is not on the desired lateral path or within the designed path capture criteria when LNAV is selected, the FGS LNAV mode should enter an armed state. The FGS should transition from the armed state to an engaged state at a point where the lateral flightpath can be smoothly acquired and tracked.
(3) TOGA. For an FGS incorporating the LNAV mode during the TOGA phase, the design should specify maneuvering capability immediately after takeoff and any limits that may exist. After TOGA, maneuvering should be based upon aircraft performance with the objective to prevent excessive roll attitudes where wingtip impact with the runway becomes probable, yet satisfy operational requirements where terrain and/or thrust limitations exist.