Chapter 8 Safet y Assessment
83. General.
a. Basic Safety Requirements. Section 25.1309 defines the basic safety requirements for airworthiness approval of airplane systems and AC 25.1309-1A, System Design and Analysis, provides an acceptable means of demonstrating compliance with this rule. This chapter provides additional guidance and interpretive material for the application of § 25.1309 to the approval of FGS.
b. Safety Assessment. A safety assessment should be performed to identify the failure conditions, classify their hazard level according to the guidance of AC 25.1309-1A, and establish that the failure conditions occur with a probability corresponding to the hazard classification or are mitigated as intended. The safety assessment should include the rationale and coverage of the FGS protection and monitoring philosophies employed. The safety assessment should include an appropriate evaluation of each of the identified FGS failure conditions and an analysis of the exposure to common mode/cause or cascade failure in accordance with AC 25.1309-1A.
Additionally, the safety assessment should include justification and description of any functional partitioning schemes employed to reduce the effect/likelihood of failures of integrated components or functions.
c. Validation of Safety Assessment. There may be situations where the severity of the effect of a failure condition identified in the safety analysis needs to be validated.
Laboratory, simulator, or flight test may accomplish the validation of the assessment.
The test facility for each validation test should be chosen based on specific criteria, such as the difficulty of setting up the test conditions, potential hazard to an airplane and the test crew, and the validity of that validation test in that specific facility.
d. Coordination with the FAA. It is recommended that the safety analysis plan and the functional hazard assessment be coordinated with the FAA early in the certification program.
84. FGS Failure conditions. One of the initial steps in establishing compliance with § 25.1309 for a system is to identify the failure conditions that are associated with that system. The failure conditions are typically characterized by an undesired change in the intended function of the system. The failure condition should identify the functionality affected and the effect on the airplane and/or its occupants, specify any considerations relating to phase of flight, and identify any flightcrew action or other means of mitigation that are relevant.
a. Functionality. The primary functions of a FGS may include the following:
(1) Automatic control of the airplane’s flight path utilizing the airplane’s aerodynamic control surfaces.
(2) Guidance provided to the flightcrew to achieve a particular desired flight path or maneuver via information presented on a HUD or HDD system.
(3) Control of the thrust applied to the airplane.
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 120-29A, Criteria for Approval of Category I and Category II Weather Minima for Approach, and AC 120-28D, Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout.
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 sub-types:
(a) Hardover. The control or display goes to full displacement in a brief period of time. The resultant effect on the flight path 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 flight path 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 flight path, 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, below).
d. Assessment of Failure Conditions.
(1) Functional Failures. Appendix 1, 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 14 CFR Part 25, Subpart C, Structure, and § 25.1309, 77
Equipm ent, 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, paragraph 76, System Integration Issues.
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 De-Selection. The design of typical FGS allows for the de–selection of control and guidance elements. The long–term effects on occupants and any structural implication of oscillatory failures can be mitigated by de–selection 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 78
be identified. It is recom mended 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 condition 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, Compliance Demonstration Using Flight Test and Simulation. 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: ACs 120-29A and AC 120-28D identify additional considerations and criteria for operating the airplan e 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 flight path, 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, Compliance Demonstration Using Flight Test and Simulation). The minimum engage height – takeoff based upon this assessment should be provided in the AFM.
(a) Vertical Axis Assessment.
1 The operational objective during th e 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, Characteristics of Specific Modes).
2 The FGS failure conditions s hould be assessed for the potential for a significant reduction in the net takeoff flight path 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 lik ely 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 th e initial climb is to maintain an appropriate heading or track to provide separation from potential adjacent runway operations.
2 The FGS failure conditions shoul d 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 lik ely 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 flight path, 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 for details regarding how these failure conditions on approach should be assessed.
(b) Approach Operations.
1 A number of approach oper ations may be conducted using automatic flight control. These can include, but are not limited to, the following:
(aa) ILS, MLS, and GLS
(bb) RNAV (e.g., LNAV and VNAV)
(cc) NAV (for example, VOR, LOC, and Backcourse)
(dd) Open loop flight path management (for example, vertical speed, flight path angle, track or heading select).
2 Some operations may be conducte d 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/decision 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 flight path deviation from any significant failure condition would impact the autopilot minimum operational use height. The minimum use 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 flight path 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 lik ely 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 flight path to support the landing.
2 The FGS failure conditions s hould 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 lik ely 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, Compliance Demonstration Using Flight Test and Simulation, 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 82
the flightcrew to quickly disengage those functions. The im plication of failures that preclude the quick disengagement from functioning should be assessed consistent with the guidelines of AC 25.1309-1A. [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 27b(4), Alternative Means of Autopilot Disengagement, 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.
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Chapter 9 Compliance Demonstration Using Flgiht 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 human-machine interface 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 document.
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 human–machine interface 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.
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e. Other Applicable Guidance Material for FGS Ta keoff 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 120-29A, Criteria for Approval of Category I and II Weather Minima for Approach.
(b) AC 120-28D, Criteria for A pproval of Category III Weather Minima for Takeoff, Landing, and Rollout need to be included in the requirements to be tested.
(2) Procedures to Show Compliance. Procedures that may be used to show compliance are provided in AC 25-7C, Flight Test Guide For Certification of Transport Category Airplanes (See paragraph 181, Automatic Pilot 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.