b. Effect of failure condition. Dependent upon the functionality provided in a specific FGS, the failure conditions could potentially impact the following:
(1) Control of the airplane in the pitch, roll, and directional axes.
(2) Control of thrust.
(3) Integrity and availability of guidance and system annunciations/alerts provided to the flightcrew.
(4) Structural integrity of the airplane.
(5) Ability of the flightcrew to cope with adverse operating conditions.
(6) Flightcrew performance and workload.
(7) Safety of the occupants of the airplane.
Note: The safety assessment of a FGS for use in supporting takeoff, approach, and landing operations in low-visibility conditions is further addressed in AC 120-29 and AC 120-28.
85. Type and Severity of Failure Conditions.
a. Types of failure conditions. The type of the FGS failure conditions will depend, to a large extent, upon the architecture, design philosophy, and implementation of the system. Types of failure conditions can include the following:
(1) Loss of function. A control or display element no longer provides control, guidance, or system annunciation/alerting.
(2) Malfunction. A control or display element performs in an inappropriate manner, including the following subtypes:
(a) Hardover. The control or display goes to full displacement in a brief period of time. The resultant effect on the flightpath and occupants of the airplane and possible adverse structural effects are the primary concerns.
(b) Slowover. The control or display moves away from the correct control or display value over a relatively long period of time. The potential delay in recognizing the situation and the effect on the flightpath are the primary concerns.
(c) Oscillatory. The control or display is replaced or augmented by an oscillatory element. In addition to difficulty controlling the aircraft flightpath, there may be effects on structural integrity and occupant well-being.
b. Cause. Failure conditions are a manifestation of malfunctions in:
(1) Primary FGS elements, such as autopilot, FD, or HUD.
(2) Interfacing sensors to the FGS, such as inertial or air data.
(3) Control and display elements, such as servos or PFDs.
(4) Interfacing systems, such as the FMS or primary flight controls.
(5) Basic aircraft services, such as electrical or hydraulic power.
c. Severity. The severity of the FGS failure conditions and their associated classifications will frequently depend on the phase of flight, airplane configuration, and the type of operation being conducted. The effect of any control system variability (for example, tolerances and rigging) on the failure conditions should be considered. The severity of the failure conditions can be mitigated by various design strategies. (See paragraph 86, Mitigation of Failure Conditions.)
d. Assessment of failure conditions.
(1) Functional failures. Appendix A, Safety Assessment, of this AC presents some considerations for use when assessing the type and severity of condition that results from functional failures. The classifications of failure conditions that have been identified on previous airplane certification programs are identified. The classifications of failure conditions should be agreed with the FAA during the § 25.1309 safety assessment process.
(2) Airframe loads. With the exception of the catastrophic failure condition, the classification of failure conditions leading to the imposition of airframe loads should be assessed in accordance with part 25, Subpart C, Structure, and § 25.1309, Equipment, systems, and installations. The assessment needs to account for loads occurring during the active malfunction, recovery, or continuation of the flight with the system in the failed state.
(3) Assessing total effect of failure. Complex integrated systems may require that the total effect resulting from single failure be assessed. For example, some failures may result in a number of failure conditions that, if assessed individually, may be considered major effects but when considered in combination may be hazardous. Special consideration concerning complex integration of systems can be found in Chapter 7, Flight Guidance System Integration, paragraph 76, System Integration Issues, of this AC.
86. Mitigation of Failure Conditions.
a. General. The propagation of potential failure conditions to their full effect may be nullified or mitigated by a number of methods. These methods could include, but are not limited to, the following:
(1) Failure detection and monitoring.
(2) Fault isolation and reconfiguration.
(3) Redundancy.
(4) Authority limiting.
(5) Flightcrew action to intervene.
b. Identification of methods. Means to assure continued performance of any system design mitigation methods should be identified. The mitigation methods should be described in the safety analysis/assessment document or be available by reference to another document (for example, a system description document).
c. FGS deselection. The design of typical FGS allows for the deselection of control and guidance elements. The long-term effects on occupants and any structural implication of oscillatory failures can be mitigated by deselection of the FGS.
87. Validation of Failure Conditions.
a. General. The method of validation of failure conditions will depend on the effect of the condition, any assumptions made, and any associated risk. The severity of some failure conditions may be obvious, and other conditions may be somewhat subjective. If flightcrew action is used to mitigate the propagation of the effect of a failure condition, the information available to the flightcrew to initiate appropriate action (for example, motion, alerts, and displays) and the assumed flightcrew response should be identified. It is recommended that there be early coordination with the FAA to identify any program necessary to validate any of these assumptions.
b. Validation options. The validation options for failure conditions include the following:
(1) Analysis.
(2) Laboratory testing.
(3) Simulation.
(4) Flight test.
c. Validation methodology. The validation testing and analysis should take account of architectural strategies (for example, redundant channels, high integrity components, rate limit/magnitude limiting, etc.). It is anticipated that the majority of failure conditions can be validated by analysis to support the probability aspect of the § 25.1309 assessment. It may be necessary to substantiate the severity of a failure condition effect by ground simulation or flight test. This is particularly true where pilot recognition of the failure condition requires justification, or if there is some variability in the response of the airplane. Failure conditions that are projected to be less probable than 10-7 per flight hour, independent of effect severity, need not be demonstrated with a flight test. Guidance on the assessment of traditional failure conditions is provided in Chapter 9 of this AC. New and novel functionality may require additional assessment methods to be agreed to with the authority.
88. Specific Considerations. The following paragraphs identify specific considerations that should be given to potential failure conditions for various phases of flight.
a. FGS action during ground operations. The potential hazard that may result due to inappropriate autopilot, autothrust, or other system control action during maintenance operations should be assessed. This includes while the airplane is parked at the gate and during taxi operations. System interlocks or crew or maintenance procedures and placards may mitigate these hazards.
b. FGS operations in close proximity to the ground. The response of the airplane to failures in the FGS could have implications for the safety of operations when the airplane is close to the ground. For the purpose of this AC, close to the ground can be assumed to be less than 500 feet above the liftoff point or touchdown zone or a runway. A specific safety assessment should be performed if approval for operations where the autopilot is engaged or remains engaged in close proximity to the ground.
Note: AC 120-29 and AC 120-28 identify additional considerations and criteria for operating the airplane in low-visibility conditions.
(1) Takeoff. If approval is sought for engagement of the autopilot below 500 feet after liftoff, an assessment of the effect of any significant FGS failure conditions on the net vertical flightpath, the speed control, and the bank angle of the airplane should be conducted.
An autopilot minimum engage height - takeoff will be established based, in part, on the characteristics of the airplane in response to the failures and the acceptability of flightcrew recognition of the condition. A pilot assessment of certain failure conditions may be required.
(See Chapter 9 of this AC.) The minimum engage height - takeoff based upon this assessment should be provided in the AFM.
(a) Vertical axis assessment.
1 The operational objective during the initial climb is to maintain an appropriate climb profile to assure obstacle clearance and to maintain an appropriate speed profile during climbout. (See Chapter 6 of this AC).
2 The FGS failure conditions should be assessed for the potential for a significant reduction in the net takeoff flightpath below 500 feet and a significant increase in pitch attitude that results in the airplane speed dropping to unacceptable values.
3 For failure conditions that are likely to occur more frequently than 1 x 10-7 per flight hour or would have hazardous or catastrophic effects without pilot intervention, the ability of pilots to adequately perform the intervention should be evaluated and documented in AFM limitations or procedures, if those failure conditions require operational limitations or flightcrew procedures.
(b) Lateral axis assessment.
1 The operational objective during the initial climb is to maintain an appropriate heading or track to provide separation from potential adjacent runway operations.
63 2 The FGS failure conditions should be assessed for the potential for producing a bank angle that results in significant deviation from the runway track or intended track.
3 For failure conditions that are likely to occur more frequently than 1 x 10-7 per flight hour or would have hazardous or catastrophic effects without pilot intervention, the ability of pilots to adequately perform the intervention should be evaluated and, as necessary, documented in AFM limitations or procedures.
(2) Approach.
(a) Assessment of failure conditions. If the autopilot is to remain engaged below 500 feet above the touchdown zone during approach, an assessment of the effect of any significant FGS failure conditions on the net vertical flightpath, the speed control, and the bank angle of the airplane should be conducted. The lowest point on the approach appropriate for the use of the autopilot will be established based on the characteristics of the airplane in response to the failure conditions and the acceptability of flightcrew recognition of the condition. See Chapter 9 of this AC for details regarding how these failure conditions on approach should be assessed.
(b) Approach operations.
1 A number of approach operations may be conducted using automatic flight control. These can include, but are not limited to, the following:
• ILS and GLS.
• RNAV (for example, LNAV and VNAV).
• NAV (for example, VOR, LOC, and Backcourse).
• Open loop flightpath management (for example, vertical speed, flightpath angle, track or heading select).
2 Some operations may be conducted with a single autopilot channel engaged, and some operations may be conducted with multiple autopilots engaged. The engagement of multiple autopilots may have the effect of mitigating the effect of certain failure conditions. The effectiveness of these mitigation methods should be established.
3 The type of operation and the prevailing visibility conditions will determine the decision altitude (height) (DA(H)) or minimum descent altitude (height) (MDA(H)) for a particular flight operation. The operation may continue using automatic flight control if the visual requirements are met.
4 The lowest altitude at which the autopilot should remain engaged may vary with the type of operation being conducted. The resultant flightpath deviation from any significant failure condition would impact the autopilot minimum operational use height. The minimum use altitude/height (MUH) for approach should be provided in the AFM.
(c) Vertical axis assessment.
1 The operational objective during the approach is to maintain an appropriate descent profile to assure obstacle clearance and to maintain an appropriate speed profile.
2 FGS failure conditions should be assessed for the potential for a significant reduction in the approach flightpath when below 500 feet above touchdown and for a significant increase in pitch attitude that results in the airplane speed dropping to unacceptable values.
3 For failure conditions that are likely to occur more frequently than 1 x 10-7 per flight hour or would have hazardous or catastrophic effects without pilot intervention, the ability of pilots to adequately perform the intervention should be evaluated and documented in AFM limitations or procedures, if those failure conditions require operational limitations or flightcrew procedures.
(d) Lateral axis assessment.
1 The operational objective during the approach is to maintain an appropriate track to provide alignment with the runway centerline or intended flightpath to support the landing.
2 The FGS failure conditions should be assessed for the potential for producing a bank angle that results in significant deviation from the runway track or intended track.
3 For failure conditions that are likely to occur more frequently than 1 x 10-7 per flight hour or would have hazardous or catastrophic effects without pilot intervention, the ability of pilots to adequately perform the intervention should be evaluated and, as necessary, documented in AFM limitations or procedures.
c. Cruise operations. The primary concerns during cruise operations are adverse effects that the airplane response to failure conditions may have on the structure of the airplane and to the occupants. At a minimum, the accelerations and attitude resulting from any failure condition should be assessed. The mitigation of the effect of a failure condition by the flightcrew may not be as immediate as during takeoff and landing operations. Chapter 9 of this AC provides guidance and considerations for this phase of flight.
d. Asymmetric thrust during autothrust operation. During autothrust operation, it is possible that a failure (for example, engine failure, throttle lever jam, or thrust control cable jam) could result in a significant asymmetric thrust failure condition that may be aggravated by the continued use of the autothrust system. Because the FGS could potentially compensate for the asymmetric condition with roll (and possibly yaw) control, the pilot may not immediately be aware of the developing situation. Therefore, an alert should be considered as a means of mitigation to draw the pilot’s attention to an asymmetric thrust condition during FGS operation.
89. Failure to Disengage the FGS. The requirement for quick disengagement for the autopilot and autothrust functions is intended to provide a routine and intuitive means for the flightcrew to quickly disengage those functions. The implication of failures that preclude the quick disengagement from functioning should be assessed consistent with the guidelines of AC 25.1309-1. See § 25.1329(b). This assessment should consider the effects of failure to disengage the autopilot and/or autothrust functions during the approach using the quick disengagement controls. The feasibility of the use of the alternative means of disengagement defined in Chapter 3, paragraph 27.b(4), Alternative means of autopilot disengagement, of this AC should be assessed. If the assessment indicates the aircraft can be landed manually with the autopilot and/or autothrust engaged, this should be demonstrated in flight test.
90—97 [RESERVED]
66 CHAPTER 9. COMPLIANCE DEMONSTRATION USING FLIGHT TEST AND
SIMULATION
98. General.
a. Methods of compliance demonstration. Validation of the performance and integrity of FGS operation will typically be accomplished by a combination of analysis, laboratory test, simulation, and flight test. This chapter focuses on compliance demonstration by flight test or simulation with flightcrew participation. The chapter includes the evaluation necessary to confirm acceptable performance of intended functions, including the HMI and the acceptability of failure scenarios. The specific requirements for flight or simulator evaluation will consider the specifics of the applicant’s design, the supporting engineering analysis, and the scope and depth of the applicant’s laboratory testing. The type and extent of the various validation methods may vary, depending upon the FGS functionality, certification considerations, the applicant’s facilities, and other practical and economic constraints.
b. Criteria for establishing compliance. The criteria to be used for establishing compliance with §§ 25.1301, 25.1309, and 25.1329 may be found in Chapters 3 through 8 of this AC.
c. Elements of certification flight test. The certification flight test program should investigate representative phases of flight and aircraft configurations used by the FGS. The program should evaluate all of the FGS modes through appropriate maneuvers and representative environmental conditions, including turbulence. Combinations of FGS elements (for example, autopilot engaged and autothrust disengaged) should be considered. Certain failure scenarios may require flight or simulator demonstration. The airplane should be equipped with sufficient instrumentation to record the parameters appropriate to the test. Examples of parameters that could be recorded include normal acceleration, airspeed, height, pitch and roll angles, and autopilot engagement state. The flight test instrumentation should not affect the behavior of the FGS or any other system.
d. Pilot-in-the-loop evaluation. An important part of the pilot-in-the-loop evaluation is validation of human factors. A thorough evaluation of the HMI is required to ensure safe, effective, and consistent FGS operation. Portions of this evaluation will be conducted during flight test. Representative simulators can be used to accomplish the evaluation of human factors and workload studies. The level and fidelity of the simulator used should be commensurate with the certification credit being sought, and its use should be agreed upon with the FAA.
e. Other applicable guidance material for FGS takeoff and/or approach modes.
(1) Criteria to consider. The following criteria should be considered for applicability in developing the overall and integrated flight test and simulation requirements:
(a) AC 120-28, Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout. The criteria need to be included in the requirements to be tested.
(b) AC 120-29, Criteria for Approval of Category I and II Weather Minima for Approach.
(2) Procedures to show compliance. Procedures that may be used to show compliance are provided in AC 25-7. (See paragraph 181, Flight Guidance System -§ 25.1329.)
f. Relationship of AC chapters. Figure 9-1 depicts the relationship between this chapter and the other chapters contained within this AC.
Figure 9-1. Relationship Between AC Chapters
99. Performance Demonstration (Fault Free): § 25.1301.
a. General.
(1) Certification plan. The certification plan should identify the specific functionality provided by the FGS. The flight test and/or simulator program will typically assess this functionality under representative operational conditions, including applicable airplane
68 configurations and a representative range of airplane weight, center of gravity (CG), and operational envelope.
(2) Evaluating performance. The performance of the FGS in each of its guidance and control modes should be evaluated. The acceptability of the performance of the FGS may be based on test pilot assessment, taking into account the experience acquired from similar equipment capabilities and the general behavior of the airplane. The FGS should be evaluated for its low and high maneuvering capability. AC 25-7 provides additional information on FGS test procedures.
(3) Evaluating compatibility of mode controls, indications, and alerts.
The acceptability of mode controls and annunciations, any associated alerts, and general compatibility with cockpit displays should be evaluated. The FGS should be free from unexpected disengagement, and no confusion should result from FGS mode changes. Additional considerations relating to the assessment of human factors is provided in Chapter 9, paragraph 103, Assessment of Human Factors, of this AC.
b. Normal performance.
(1) Demonstration. Normal performance is considered to be performance during operations well within the airplane’s flight envelope and with routine atmospheric and environmental conditions. Normal performance should be demonstrated over a range of conditions that represent typical conditions experienced in operational use.
(2) Evaluation. The FGS should be evaluated to determine the acceptability of the following characteristics:
(a) Stability and tracking of automatic control elements.
(b) Controllability and tracking of guidance elements.
(c) Acquisition of flightpaths for capture modes.
(d) Consistency of integration of modes (of this chapter).
(3) Expected errors. Performance should be assessed in the presence of errors that can reasonably be expected in operation (for example, the incorrect selection of approach speed).
c. Performance in “other than normal” conditions. Other than normal conditions are the combination of “rare normal” and “non-normal” conditions. Performance in other than normal conditions is considered to be performance of the system under conditions that are experienced infrequently by the airplane during operational use. Examples of rare normal and non-normal conditions are given in Chapter 5, paragraph 54 and paragraph 56 of this AC.
(1) Test program. The test program should assess the FGS performance in more challenging environmental conditions (for example, winds, wind gradients, and various levels of turbulence) as the opportunity presents itself. Rare environmental conditions may require the FGS to operate at the limits of its capabilities. The intent of the evaluation is to assess the
69 performance of the FGS under more demanding conditions that may be experienced infrequently in service. Due to the severity of some environmental conditions, such as severe and extreme turbulence or flights into severe wind shear, it is neither recommended nor required that the FGS flight evaluations include demonstration under those conditions. Those conditions are more appropriately addressed by simulator evaluation.
(2) Evaluation of FGS. The FGS should be evaluated to determine the acceptability of the following characteristics:
(a) Stability of automatic control elements and ability to resume tracking following any upset.
(b) Controllability of guidance elements and ability to resume tracking following any upset.
(c) Acceptability of mode transitions and overall cockpit system integration.
(3) Additional guidance for wind shear. If the FGS provides wind shear escape guidance, performance demonstration requirements should be conducted consistent with AC 25-12.
d. Specific performance conditions. The following paragraphs identify specific performance conditions requiring evaluation by flight test and/or simulation.
(1) Icing considerations. The FGS performance and safety in icing conditions should be demonstrated by flight test and/or simulation tests and be supported by analysis where necessary. The implications of continued use of the automatic flight control elements of the FGS in icing conditions should be assessed. Ice accumulation on the airplane wings and surfaces can progressively change the aerodynamic characteristics and stability of the airplane. Even though the FGS may perform safely under these conditions, its continued use may mask this change, which, in turn, can lead to pilot handling difficulties and potential loss of control, should the autopilot become disengaged (either automatically or manually).
(a) Test program. A test program should assess the potential vulnerability of the FGS to icing conditions by evaluating autopilot performance during artificial ice shape tests or during natural icing tests.
1 Sufficient autopilot testing and analysis should be conducted to ensure that the autopilot’s performance is acceptable. This testing and analysis should include evaluation of the low-speed protection threshold in the event that stall warning parameters are adjusted to account for icing conditions. For more information, see Chapter 5, paragraph 55, Icing Considerations in Normal and Rare Normal Conditions, of this AC.
2 In general, it is not necessary to conduct an autopilot evaluation that encompasses all weights, CG positions (including lateral asymmetry), altitudes, and deceleration device configurations. However, if the autopilot performance with ice accretion shows a significant difference from the noncontaminated airplane or testing indicates marginal performance, additional tests may be necessary.
70 3 If significant autopilot inputs are required to compensate for the icing conditions, then the acceptability of the indication of a significant out-of-trim condition should be assessed, and the subsequent response of the airplane when the autopilot disengages (manual or automatic) should be determined. For more information, see Chapter 3, paragraph 27.b and Chapter 4, paragraph 45.d of this AC.
(b) Deicing. If the airplane is configured with a deicing system, the autopilot should demonstrate satisfactory performance during the shedding of ice from the airplane.
(c) AFM limitations. Where degradation is noted that is not significant enough to require changes to the autopilot system or to deicing or anti-icing systems, appropriate limitations and procedures should be established and presented in the AFM.
(2) Low-speed protection. The FGS should be assessed for the acceptability of the low-speed protection performance under the following conditions:
(a) High-altitude cruise with a simulated engine failure.
(b) Climb to altitude capture at low altitude with a simulated engine failure during capture.
(c) Vertical speed with insufficient climb power.
(d) Approach with speed abuse.
(3) High-speed protection. The FGS should be assessed for the acceptability of the high-speed protection performance under the following conditions:
(a) High-altitude level flight with autothrust function.
(b) High-altitude level flight without autothrust function.
(c) High-altitude descending flight with autothrust function.
(4) Go-around.
(a) Mode assessment. The objective of the Go-Around mode (see Chapter 6, paragraph 65.b, Go-Around Mode, of this AC) is to quickly change the flightpath of the airplane from approach to landing to a safe climbout trajectory. The mode has specific utility in low-visibility conditions when operations are predicated on a DA(H), and a go-around is necessary if visual references are not acquired at the DA(H). Therefore, the assessment of the Go-Around mode may be conducted in conjunction with the evaluation of the FGS to support low-visibility operations, using additional criteria contained in AC 120-28 and AC 120-29.
(b) Flight evaluation. The flight evaluation should be conducted to assess the rotation characteristics of the airplane and the performance of the airplane in acquiring and maintaining a safe flightpath. The acceptability of the operation if contact is made with the runway during the missed approach or balked landing should be established.