(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.
71 1 Factors to be considered. A demonstration program should be established that confirms acceptable operation when the following factors are considered:
(aa) Airplane weight and CG.
(bb) Various landing configurations.
(cc) Use of manual thrust or autothrust.
(dd) Consequences of thrust derates with selection of Go-Around mode.
(ee) An engine failure at the initiation of go-around.
(ff) An engine failure during go-around after go-around power is reached.
(gg) Initiation altitude (for example, in ground effect, not in ground effect during flare).
2 Characteristics to be evaluated:
(aa) The pitch response of the airplane during the initial transition.
(bb) Speed performance during airplane reconfiguration and climbout.
(cc) Integrated autopilot and autothrust operation.
(dd) Transition to missed approach altitude.
(ee) Lateral performance during an engine failure.
3 Go-around initiation heights. Where height loss during a go-around maneuver is significant or is required to support specific operational approval, demonstrated values for various initiation heights should be included in the AFM.
(5) Steep approach (special authorization).
(a) Flight test/simulator demonstration. Typical approach operations include glidepath angles between 2.75 and 3.77 degrees. Application for approval to conduct operations on glidepath angles of greater than 3.77 degrees requires additional evaluation. See AC 120-29, section 4.3, Landing, for further discussion. For such an approval, the FGS flight test and simulator demonstration should include the following:
1 Approach path capture, tracking, and speed control.
2 Recovery of the system from abuse cases (for example, glidepath angle and speed).
3 Assessment of autopilot disengagement transient.
72 4 Demonstration of Go-Around mode from a steep approach.
(b) Autopilot use on steep approaches. For autopilot use at approach angles greater than 4.5 degrees, the criteria of AC 25-7C, Chapter 8, Airworthiness: Miscellaneous Items, should be satisfied. This advisory material contains airworthiness and transition criteria for steep approaches. In addition, the criteria of AC 120-29, paragraph 6.2.8, “Approach Ban” Applicability, should be assessed depending on the operational and low-visibility requirements.
(6) Indication of and response to an out-of-trim condition. An assessment should be performed to determine the acceptability of the out-of-trim annunciation and subsequent response to disengagement. (See Chapter 4, paragraph 45.d of this AC.)
e. FD/HUD.
(1) General. The guidance aspect of an FGS may be provided by a head-down FD or a HUD. HDDs normally use FD guidance cues that provide steering commands—such as a cross-pointer or single “V”—for the pilot to follow. HUDs normally display a flightpath vector that represents the instantaneous flightpath. The vector is based on the aircraft energy state and directional vectors. Various new display media are evolving (for example, Enhanced Vision Systems (EVS) and simulated vision system) that may integrate guidance elements with situational elements.
(a) Flight test or simulator program. The flight test or simulator program should demonstrate that the FD or HUD guidance elements provide smooth, accurate, and damped guidance in all applicable modes to achieve satisfactory control task performance without pilot compensation or excessive workload.
(b) Performance. The FD guidance should provide adequate performance for operations under the conditions listed below. Some pilot compensation may be acceptable for these conditions:
1 Stability augmentation off.
2 Alternate fly-by-wire control modes (for example, direct law), if any.
3 An engine inoperative.
(c) Non-stationary tracking reference. FDs designed to work with a nonstationary tracking reference (such as a flightpath angle or flightpath vector, which are commonly used with HUD guidance) should be evaluated in conditions, which bring these guidance symbols to the field of view limits of the display. Crosswinds and certain combinations of airspeed, gross weight, CG, and flap/slat/gear configurations might cause such conditions. At these limits, the dynamics of the guidance response to pilot control inputs can differ, with potentially adverse effects on tracking performance, pilot compensation, and workload.
(d) Primary instrument references. It should be demonstrated that movement of the FD and its tracking reference do not interfere with primary instrument references
73 throughout their range of motion. The pilot’s ability to interpret the guidance and essential flight information should not be adversely affected by the movement dynamics or range of motion.
(2) Specific demonstrations for HUD.
(a) Compliance. These demonstrations are intended to show compliance with the following paragraphs of this AC:
1 Chapter 3, paragraph 28, FD Engagement, Disengagement, and Indications.
2 Chapter 4, paragraph 44, FGS Mode Selection, Annunciation, and Indication.
3 Chapter 4, paragraph 46, FGS Considerations for HUDs.
4 Chapter 5, paragraph 53, Normal Performance (specifically, criteria for FD guidance).
(b) Demonstrations required. When the PF is using the HUD, the HUD is where the pilot is looking for the basic flight information, and the pilot is less likely to be scanning the head-down instruments. Therefore, the following should be demonstrated:
1 The location and presentation of the HUD information (for example, guidance, flight information, and alerts/annunciations) does not distract the pilot or obscure the pilot’s outside view. For example, the pilot should be able to track the guidance to the runway without having the view of runway references or hazards along the flightpath obscured by the HUD symbology.
2 Pilot awareness of primary flight information, annunciations, and alerts is satisfactory when using any HUD display mode. Some display modes that are designed to minimize “clutter” could degrade pilot awareness of essential information. For example, a “digital-only” display mode may not provide sufficient speed and altitude awareness during high-speed descents.
3 It should be demonstrated that the pilot can positively detect cases when conformal symbology is field of view limited.
4 Approach mode guidance, if provided, should be satisfactory throughout the intended range of conditions, including at the minimum approach speed and maximum crosswind with expected gust components, for which approval is sought.
5 Visual cautions and warnings associated with the FGS can be immediately detected by the PF while using the HUD.
6 The PF can immediately respond to wind shear warnings, ground proximity warnings, TCAS warnings, and other warnings requiring immediate flight control
74 action, such as a go-around, while using the HUD without having to revert to a head down flight display.
(c) PM. In certain phases of flight, it is important that the PM and the flightcrew be aware of problems with the HUD used by the PF. Therefore, it should also be demonstrated that the PM can immediately be made aware of any visual cautions and warnings associated with the HUD for applicable phases of flight.
(d) Approach mode. If Approach mode guidance is provided, satisfactory performance should be demonstrated throughout the intended range of operating conditions for which approval is sought. For example, performance should be demonstrated at the minimum approach speed and maximum crosswind with expected gust components.
(e) Recovery guidance. If recovery guidance is provided, it should be demonstrated that the pilot can immediately detect and recover from unusual attitudes when using the HUD. Specialized unusual attitude recovery symbology, if provided, should be shown to provide unequivocal indications of the attitude condition (for example, sky/ground, pitch, roll, and horizon) and to correctly guide the pilot to the nearest horizon. The stroke presentation of flight information on a HUD may not be as inherently intuitive for recognition and recovery as the conventional HDD (for example, contrasting color, area fill, shading versus line strokes). The HUD display design needs to be able to compensate for these differences to provide adequate pilot recognition and recovery cues.
(3) Simulator demonstration for HUD. If a pilot-in-the-loop flight simulation is used for some demonstrations, then a high-fidelity, engineering quality facility is typically required. The level of simulator may vary with the functionality being provided and the types of operation being conducted. Factors for validation of the simulation for demonstration purposes include the following:
(a) Guidance and control system interfaces.
(b) Motion base suitability.
(c) Adequacy of stability derivative estimates used.
(d) Adequacy of any simplification assumptions used for the equations of motion.
(e) Fidelity of flight controls and consequent simulated aircraft response to control inputs. (A correlation of the simulator performance to flight test results should be made.)
(f) Fidelity of the simulation of aircraft performance. (A correlation of the simulator performance to flight test results should be made.)
(g) Adequacy of flight deck instruments and displays.
(h) Adequacy of simulator and display transient response to disturbances or failures (for example, engine failure, autofeather, electrical bus switching).
(i) Visual reference availability, fidelity, and delays.
(j) Suitability of visibility restriction models, such as appropriate calibration of visual references for the tests to be performed for day, night, and dusk conditions.
(k) Fidelity of any other significant factor or limitation relevant to the validity of the simulation.
f. Flightcrew override of the FGS. A flight evaluation should be conducted to demonstrate compliance with Chapter 3, paragraph 30 of this AC. The flight evaluation should consider the implication of system configuration for various flight phases and operations.
(1) Autopilot override.
(a) Assessing effect. The effect of flightcrew override should be assessed by applying an input on the cockpit controller (for example, the control column or equivalent) to each axis for which the FGS is designed to disengage. The evaluation should be repeated with progressively increasing rate of force application to assess FGS behavior. This assessment should include the initial application of force on the cockpit controller as well as when the force is removed from the controllers. The effects of speed and altitude should be considered when conducting the evaluation.
(b) No automatic disengagement. If the autopilot is designed such that it does not automatically disengage due to a pilot override, verify that no potential hazards are generated due to the override per Chapter 3, paragraph 30 of this AC.
(c) Multiple channel engagement. If the design of the autopilot provides for multiple channel engagement for some phases of flight that results in a higher override force, these conditions should be evaluated.
Note: AC 120-28, Appendix 3, Airworthiness Approval for Airborne Systems Used to Land and Rollout in Low Visibility Conditions, section 8, Airborne Systems, contains guidance for evaluating autopilot override for systems supporting low-visibility operations.
(2) Autothrust override. The capability of the flightcrew to override the autothrust system should be conducted at various flight phases. The evaluation should include an override of the autothrust system with a single hand on the thrust levers while maintaining control of the airplane using the opposite hand on the control wheel (or equivalent). This action may not result in a potential hazard per § 25.1329(m), either during the override or after the pilot releases the thrust levers. See Chapter 3, paragraph 30 of this AC for additional guidance. If the autothrust system automatically disengages due to the override, the alerts that accompany the disengagement should be assessed to ensure flightcrew awareness.
(3) Pitch trim system evaluation during autopilot override.
(a) Effect of override. The effect of flightcrew override during automatic control on the automatic trim systems should be conducted. The pilot should also apply an input