Advisory 

Circular

Subject: APPROVAL OF 

FLIGHT GUIDANCE SYSTEMS 

Date: 7/17/06

Initiated by: ANM-110 

AC No: 25.1329-1B 

1. PURPOSE. This advisory circular (AC) describes an acceptable means for showing 

compliance with certain requirements of Title 14, Code of Federal Regulations (CFR) 

25.1329, Flight guidance systems. While Part 25 contains the airworthiness standards 

applicable to transport category airplanes, the guidance in this AC pertains to the 

functions of autopilots, flight directors (FD), and automatic thrust control as well as any 

interactions with stability augmentation and trim functions. 

2. CANCELLATION. Advisory Circular 25.1329-1A, dated July 8, 1968, is 

canceled. 

3. APPLICABILITY. 

a. The guidance provided in this document is directed to airplane manufacturers, 

modifiers, and operators of certain transport category airplanes. 

b. This material is neither mandatory nor regulatory in nature and does not 

constitute a regulation. It describes acceptable means, but not the only means, for 

demonstrating compliance with the applicable regulations. The Federal Aviation 

Administration (FAA) will consider other methods of demonstrating compliance that an 

applicant may elect to present. While these guidelines are not mandatory, they are 

derived from extensive FAA and industry experience in determining compliance with the 

relevant regulations. On the other hand, if we become aware of circumstances that 

convince us that following this AC would not result in compliance with the applicable 

regulations, we will not be bound by the terms of this AC, and we may require additional 

substantiation or design changes as a basis for finding compliance. 

c. This material does not change, create any additional, authorize changes in, or 

permit deviations from, regulatory requirements. 
7/17/06 AC 25.1329-1B 

ii

4. RELATED REGULATIONS, ACs, AND INDUSTRY DOCUMENTS. 

a. Regulations. The following related documents are provided for information 

purposes and are not necessarily directly referenced in this AC. The full text of 14 CFR 

can be downloaded from the Internet at http://www.gpoaccess.gov/nara. A paper copy 

may be ordered from the Government Printing Office (GPO), Superintendent of 

Documents, Attn: New Orders, PO Box 371954, Pittsburgh, PA 15250-7954. 

Section Title 

§ 25.143 Controllability and Maneuverability, General 

§ 25.301 Loads 

§ 25.671 Control systems, General 

§ 25.672 Stability augmentation and automatic and power-operated systems 

§ 25.677 Trim systems 

§ 25.777 Cockpit controls 

§ 25.779 Motion and effect of cockpit controls 

§ 25.781 Cockpit control knob shape 

§ 25.901 Installation 

§ 25.903 Engines 

§ 25.1301 Function and installation 

§ 25.1303 Flight and navigations instruments 

§ 25.1309 Equipment, systems, and installations 

§ 25.1322 Warning, caution, and advisory lights 

§ 25.1581 Airplane Flight Manual, General 

§ 25.1583 Airplane Flight Manual, Operating limitations 

§ 25.1585 Airplane Flight Manual, Operating procedures 
7/17/06 AC 25.1329-1B 

iii

b. ACs. An electronic copy of the following ACs can be downloaded from the 

Internet at http://www.airweb.faa.gov/rgl. A paper copy may be ordered from the U.S. 

Department of Transportation, Subsequent Distribution Office, M-30, Ardmore East 

Business Center, 3341 Q 75th Avenue, Landover, MD 20795. 

Number Title 

AC 20-115B RTCA, Inc., Document RTCA/DO 178B [Radio Technical 

Commission for Aeronautics] 

AC 20-129 Airworthiness Approval of Vertical Navigation (VNAV) 

Systems for use in the U.S. National Airspace System (NAS) 

and Alaska 

AC 20-152 Design Assurance Guidance for Airborne Electronic Hardware 

RTCA/DO 254 

AC 25-11 Transport Category Airplane Electronic Display Systems 

AC 25-12 Airworthiness Criteria for the Approval of Airborne Windshear 

Warning Systems in Transport Category Airplanes 

AC 25.672-1 Active Flight Controls 

AC 25.1309-1A System Design and Analysis 

AC 25.1419-1 Certification of Transport Category Airplanes for Flight in Icing 

Conditions 

AC 25.1581-1 Airplane Flight Manual 

AC 120-28D Criteria for Approval of Category III Weather Minima for 

Takeoff, Landing, and Rollout 

AC 120-29A Criteria for Approving Category 1 and Category II Landing 

Minima for FAR 121 Operators 

AC 120-41 Criteria for Operational Approval of Airborne Windshear 

Alerting and Flight Guidance Systems 

NOTE: This table represents the latest version of each AC at the time of 

publication. These individual ACs may be updated in the future, such that 

this table may reference a specific version of an AC that has been 

superseded. The latest version of each AC should always be used, even 

though this document may reference a previous version of an AC. 
7/17/06 AC 25.1329-1B 

iv

c. Related Industry Standards. The following documents are available from the 

Society of Automotive Engineers (SAE). These documents can be obtained from the 

Society of Automotive Engineers, Inc., 400 Commonwealth Drive, Warrendale, 

Pennsylvania, 15096. 

Number Title 

SAE ARP5366 Autopilot, Flight Director and Autothrust Systems 

SAE ARP4754 Certification Considerations for Highly Integrated or Complex 

Aircraft Systems 

SAE ARP4761 Guidelines and Methods for Conducting the Safety 

Assessment Process on Civil Airborne Systems and Equipment 

SAE ARP5288 Transport Category Airplane Head Up Display Systems 

SAE AS8055 Minimum Performance Standards for Airborne Head Up 

Display 

The following documents are available from the RTCA, Inc. These documents can be 

obtained from RTCA, Inc., 1140 Connecticut Avenue., Northwest, Suite 1020, 

Washington, D.C., 20036-4001 U.S.A. 

Number Title 

RTCA DO-178B/ 

EUROCAE 

ED-12B 

Software Considerations in Airborne Systems and Equipment 

Certification 

RTCA DO-160E/ 

EUROCAE 

ED–14D 

Environmental Conditions and Test Procedures for Airborne 

Equipment 

RTCA DO-254/ 

EUROCAE 

ED-80 

Design Assurance Guidance for Airborne Electronic Hardware 
7/17/06 AC 25.1329-1B 

v

d. FAA Documents. An electronic copy of the following FAA document can be 

downloaded from the Internet at http://ntl.bts.gov/databases.html. Click on NTL Catalog, 

then enter the document number in the search box. A paper copy may be ordered from 

the National Technical Information Service, 5285 Port Royal Road, Springfield, Virginia 

22161. 

Number Title 

DOT/FAA/ 

CT–96/1 

Human Factors Design Guide for Acquisition of CommercialOff-the-Shelf Subsystems, Non-Developmental Items, and 

Developmental Systems 

An electronic copy of the following FAA Order can be downloaded from the Internet at 

http://www.airweb.faa.gov/rgl. A paper copy may be ordered from the U.S. Department 

of Transportation, Subsequent Distribution Office, M-30, Ardmore East Business Center, 

3341 Q 75th Avenue, Landover, MD 20795. 

Number Title 

FAA Order 

8260.3 

United States Standard for Terminal Instrument Procedures 

/s/ 

Ali Bahrami 

Manager, Transport Airplane Directorate 

Aircraft Certification Service 
7/17/06 AC 25.1329-1B 

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CONTENTS 

Paragraph Page 

1 – 10. [RESERVED] 13 

CHAPTER 1. BACKGROUND 

11. General 13 

 a. Advisory Circular (AC) 25.1329-1A 13 

 b. Scope 13 

12. Evolution of Flight Guidance Systems (FGS) 13 

 a. Complexity 13 

 b. Guidance Considerations 13 

13 – 16. [RESERVED] 13 

CHAPTER 2. OVERVIEW OF FLIGHT GUIDANCE SYSTEMS (FGS) 

17. Purpose of FGS 15 

18. FGS Components 15 

 a. Included in Definition 15 

 b. Excluded from Definition 15 

19. FGS Functions 15 

 a. Elements of FGS 15 

 b. Approach and Landing System 15 

 c. Flight Management System (FMS) 16 

 d. Design Philosophy 16 

 e. Description of FGS Behavior and Operation 16 

20. Compliance with § 25.1329 16 

 a. General 16 

 b. Relevant Paragraphs 16 

 Table 2-1 Cross-References § 25.1329 Paragraphs to AC 

Chapters 

17 

21 – 26 [RESERVED] 22 

CHAPTER 3. FGS ENGAGEMENT, DISENGAGEMENT, 

INDICATIONS, AND OVERRIDE 

27. Autopilot Engagement, Disengagement, and Indications 23 

 a. Autopilot Engagement 23 

 (1) Single Switch Action 23 

 (2) Engagement Status of Autopilot 23 

 (3) Multiple Autopilots 23 

 (4) Acceptable Transients 23 
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Paragraph Page 

 (5) Flight Director (FD) Not Engaged 23 

 (6) FD Engaged 24 

 b. Autopilot Disengagement 24 

 (1) General 24 

 (2) Autopilot Disengagement Alerts 25 

 (3) Quick Disengagement Control 26 

 (4) Alternative Means of Autopilot Disengagement 26 

 (5) Flightcrew Pitch Trim Input 27 

28. FD Engagement, Disengagement, and Indications 27 

 a. FD Engagement 27 

 (1) General 27 

 (2) Guidance Cue(s) 28 

 (3) Reactive Windshear Guidance System 28 

 b. FD Disengagement 29 

29. Autothrust Engagement, Disengagement, and Indications 29 

 a. Autothrust Engagement 29 

 (1) Indication 29 

 (2) Accessibility of Controls 29 

 (3) Inadvertent Engagement/Disengagement 29 

 (4) Automatic Engagement 29 

 b. Autothrust Disengagement 30 

 (1) Indication of Disengagement 30 

 (2) Inadvertent Disengagement 30 

 (3) Consequence of Disengagement 30 

 (4) Autothrust Quick Disengagement Control 30 

30. Override of the FGS 31 

 a. General 31 

 b. Autopilot Override 31 

 (1) With Automatic Disengagement 31 

 (2) Without Automatic Disengagement 31 

 c. Autothrust Override 32 

 (1) Force Required 32 

 (2) Response to Override 32 

 (3) Engagement Status with Override 32 

31. FGS Engagement Mode Compatibility 33 

 a. FGS Mode Engagement Philosophy 33 

 b. Engagement Mode Compatibility 33 

32 — 41. [RESERVED] 33 
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CHAPTER 4. CONTROLS, INDICATIONS, AND ALERTS 

Paragraph Page 

42. General 35 

 a. Human-Machine Interface 35 

 b. Design of Controls, Indications, and Alerts 35 

 c. Evaluation of Information Provided by FGS 35 

43. FGS Controls 35 

 a. General 35 

 b. Design Considerations 35 

 (1) Differentiation of Knob Shape and Position 35 

 (2) Design to Support Correct Selection of Target Values 36 

 (3) Commonality Across Aircraft Types 36 

 (4) Positioning of Controls and Related Indications 36 

 (5) Inadvertent Operation 36 

44. FGS Mode Selection, Annunciation, and Indication 36 

 a. Annunciation of Engagement of the FGS 36 

 b. Description of FGS Modes 36 

 c. FGS Mode Annunciations 36 

 (1) Active and Armed Modes 36 

 (2) Location 37 

 (3) Discriminators 37 

 (4) Color 37 

 d. Mode Changes 37 

 (1) Operationally Relevant Mode Changes 37 

 (2) Attention-Getting Features 37 

 (3) Use of Alerts 38 

 e. Failure to Engage or Arm 38 

 f. FGS Mode Display and Indications 38 

45. FGS Alerting, Warning, Caution, Advisory, and Status 38 

 a. General 38 

 (1) Alerting Provisions 38 

 (2) Determination of FGS Capability 38 

 b. Speed Protection Alerts 38 

 (1) Alerts to Crew 38 

 (2) Alert Specifications 39 

 (3) Consistency 39 

 (4) Nuisance Alerts 39 

 c. Loss of Autopilot Approach Mode 39 

 d. Awareness of Potential Significant Transient Condition 

(“Bark Before Bite”) 

39 

 (1) General 39 

 (2) Flightcrew Alert(s) 39 

 (3) Situations to Consider for an Alert 40 
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Paragraph Page 

 (4) Automatic Disengagement 40 

46. FGS Considerations for Head Up Displays (HUD) 41 

 a. General 41 

 b. Characteristics of HUD Guidance 41 

 (1) Supplemental Use Display System 41 

 (2) External Visual References 41 

 (3) Interference and Occlusion 41 

 (4) Display Criteria 42 

 (5) Conformal Symbology 42 

 (6) Low Visibility Approach 42 

 (7) Clutter 42 

 c. HUD/Head Down Display (HDD) Compatibility 42 

 (1) General 42 

 (2) Consistency and Compatibility 42 

 d. Alerting Issues 43 

 (1) Alerts Displayed on HUDs 43 

 (2) Monochrome HUDs 44 

 (3) Mulit-Color HUDs 44 

 (4) Single HUD Installation 44 

 (5) Dual HUD Installations 44 

 (6) Ground Proximity Warning System (GPWS)/Traffic Alert 

and Collision Avoidance System (TCAS) Alerts On HUD 

44 

 e. Upset/Unusual Attitude Recovery Guidance 44 

 (1) Guidance Cues 44 

 (2) Orientation Cues 45 

47 – 51. [RESERVED] 45 

CHAPTER 5. PERFORMANCE OF FUNCTION 

52. General 47 

 a. Intended Function 47 

 b. Effect of System Tolerances 47 

53. Normal Performance 47 

 a. General 47 

 b. Conditions to Consider 47 

 Table 5.1 Examples of Normal Conditions 48 

 c. Level of Performance 49 

 (1) Significant Characteristics 49 

 (2) Configuration Changes 49 

 (3) Compliance Evaluation 49 

 (4) Prescribed Levels of Performance 49 

 (5) Equivalence of Performance 49 

 (6) Autopilot 49 
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Paragraph Page 

 (7) Flight Director (FD) 49 

 (8) Autothrust 49 

 (9) Automatic Trim 49 

54. Performance in Rare Normal Conditions 50 

 a. General 50 

 (1) Range of Conditions 50 

 (2) Effect on Performance 50 

 b. Conditions to Consider 50 

 Table 5-2 Examples of Rare Normal Conditions 51 

 c. Level of Performance 52 

 (1) Performance Standard 52 

 (2) Autopilot Authority Limit 52 

 (3) Masking of Potential Hazard 52 

55. Icing Considerations in Normal and Rare Normal Conditions 52 

 a. Effect on Performance 52 

 b. Alerts 52 

 c. Effect on Speed Protection 53 

56. Performance in Non-Normal Conditions 53 

 a. General 53 

 b. Considerations to Consider 53 

 Table 5-3 Examples of Non-Normal Conditions 54 

 c. Level of Performance 55 

 (1) Performance Standard 55 

 (2) Autopilot Authority Limit 55 

 (3) Masking of Potential Hazard 55 

57. Speed Protection 55 

 a. General 55 

 (1) Speed Excursions 55 

 (2) Compliance with § 25.1329(h) 55 

 (3) Design Standard 56 

 b. Low Speed Protection 57 

 (1) General 57 

 (2) Factors to Consider 57 

 (3) Low Speed Protection During Approach Operations 57 

 (4) Windshear Recovery Guidance 59 

 c. High Speed Protection 59 

 (1) General 59 

 (2) Factors to Consider 59 

58 – 61. [RESERVED] 60 
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CHAPTER 6. CHARACTERISTICS OF SPECIFIC MODES 

Paragraph Page 

62. General 61 

63. Lateral Modes 61 

 a. Heading or Track Hold 61 

 b. Heading or Track Select 61 

 c. Lateral Navigation (LNAV) Mode 61 

 (1) Lateral Flight Path 61 

 (2) Automatic Mode Transitions 61 

 (3) Takeoff or Go-Around (TOGA) 62 

64. Vertical Modes 62 

 a. Target Altitude Selection 62 

 b. Vertical Speed 62 

 c. Flight Path Angle 62 

 d. Indicated Airspeed (IAS)/Mach Hold [speed on elevator] 62 

 e. Indicated Airspeed (IAS)/Mach Select [speed on elevator] 62 

 f. Flight Level (FL) Change [speed on elevator] 63 

 g. Altitude Capture 63 

 (1) Mode Transition 63 

 (2) Guidelines for Altitude Capture Mode 63 

 h. Altitude Hold 64 

 (1) Entering Mode 64 

 (2) Mode Transition Annunciation 64 

 (3) Barometric Pressure Adjustment 64 

 i. VNAV 65 

 (1) FMS Path 65 

 (2) Climb or Descent 65 

65. Multi-Axis Modes 66 

 a. Takeoff Mode 66 

 (1) Vertical Guidance 66 

 (2) Lateral Guidance 66 

 (3) Rotation Guidance 66 

 (4) Autothrust 66 

 (5) Operation and Performance 66 

 b. Go-Around Mode 67 

 (1) Vertical Elements 67 

 (2) Speed 67 

 (3) Autothrust 67 

 (4) Engagement 68 

 (5) All Engine and Engine Out Capability 68 

 c. Approach Mode 68 

 (1) Final Approach Path 68 

 (2) Mode Annunciations 68 
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Paragraph Page 

 (3) Sub-Modes 68 

 (4) Mode Engagement Sequence 68 

66. Autothrust Modes 68 

 a. Thrust Mode 68 

 b. Speed Mode 68 

 c. Retard Mode 69 

67. Special Considerations for VNAV Approach Operations 

Related to Selecting a Target Altitude 

69 

 a. Approach Operations 69 

 b. VNAV Path Operations 69 

68. Control Wheel Steering (CWS) [control steering through the 

autopilot] 

69 

 a. General 69 

 b. Design 69 

 (1) Control Forces 69 

 (2) Bank and Pitch Attitudes 70 

 (3) Control Behavior 70 

 (4) Stall and Stall Recovery 70 

 (5) Adjustments to Trim 70 

 (6) Takeoff and Landing 70 

 (7) Automatic Reversion to CWS Mode 70 

 c. Terminology 71 

 (1) Touch Control Steering (TCS) 71 

 (2) Supervisory Override 71 

69. Special Considerations for the Integration of Fly-by-Wire Flight 

Control Systems and FGS 

71 

 a. Speed Protection 71 

 b. System Degradation 71 

70 – 74 [RESERVED] 71 

CHAPTER 7. FGS INTEGRATION 

75. General 73 

76. System Integration Issues 73 

 a. Common and Cascading Failure Modes 73 

 b. Risk of Error 73 

 c. System Safety Analysis 73 

77. Functional Interfaces 74 

 a. Interfacing Sensors 74 

 b. Potential Inconsistencies Between Systems 74 

 c. Function and Performance 74 

78 – 82 [RESERVED] 74 
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CHAPTER 8. SAFETY ASSESSMENT 

Paragraph Page 

83. General 75 

 a. Basic Safety Requirements 75 

 b. Safety Assessment 75 

 c. Validation of Safety Assessment 75 

 d. Coordination with the FAA 75 

84. FGS Failure Conditions 75 

 a. Functionality 76 

 b. Effect of Failure Condition 76 

85. Type and Severity of Failure Conditions 76 

 a. Types of Failure Conditions 76 

 (1) Loss of Function 76 

 (2) Malfunction 77 

 b. Cause 77 

 c. Severity 77 

 d. Assessment of Failure Conditions 77 

 (1) Functional Failures 77 

 (2) Airframe Loads 77 

 (3) Assessing Total Effect of Failure 78 

86. Mitigation of Failure Conditions 78 

 a. General 78 

 b. Identification of Methods 78 

 c. FGS De-Selection 78 

87. Validation of Failure Conditions 78 

 a. General 78 

 b. Validation Options 79 

 c. Validation Methodology 79 

88. Specific Considerations 79 

 a. FGS Action During Ground Operations 79 

 b. FGS Operations in Close Proximity to the Ground 79 

 (1) Takeoff 80 

 (2) Approach 81 

 c. Cruise Operations 82 

 d. Asymmetric Thrust During Autothrust Operation 82 

89. Failure to Disengage the FGS 83 

90 – 97 [RESERVED] 83 
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CHAPTER 9. COMPLIANCE DEMONSTRATION USING 

FLIGHT TEST AND SIMULATION 

Paragraph Page 

98. General 85 

 a. Methods of Compliance Demonstration 85 

 b. Criteria for Establishing Compliance 85 

 c. Elements of Certification Flight Test 85 

 d. Pilot–in–the–Loop Evaluation 85 

 e. Other Applicable Guidance Material for FGS Takeoff and/or 

Approach Modes 

86 

 (1) Criteria to Consider 86 

 (2) Procedures to Show Compliance 86 

 f. Relation of AC Chapters 86 

 Figure 9-1, Relationship Between AC Chapters 87 

99. Performance Demonstration (Fault Free): § 25.1301 88 

 a. General 88 

 (1) Certification Plan 88 

 (2) Evaluating Performance 88 

 (3) Evaluating Compatibility of Mode Controls, Indications, 

and Alerts 

88 

 b. Normal Performance 88 

 (1) Demonstration 88 

 (2) Evaluation 88 

 (3) Expected Errors 89 

 c. Performance in “Other Than Normal” Conditions 89 

 (1) Test Program 89 

 (2) Evaluation of FGS 89 

 (3) Additional Guidance for Windshear 89 

 d. Specific Performance Conditions 89 

 (1) Icing Considerations 89 

 (2) Low-Speed Protection 90 

 (3) High-Speed Protection 91 

 (4) Go-Around 91 

 (5) Steep Approach [special authorization] 92 

 (6) Indication Of and Response To an Out Of Trim Condition 93 

 e. Flight Director (FD) /Head Up Display (HUD 93 

 (1) General 93 

 (2) Specific Demonstrations for HUD 94 

 (3) Simulator Demonstration for HUD 95 

 f. Flightcrew Override of the FGS 96 

 (1) Autopilot Override 96 

 (2) Autothrust Override 97 

 (3) Pitch Trim System Evaluation During Autopilot Override 97 
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Paragraph Page 

100. Failure Conditions Requiring Validation: § 25.1309 97 

 a. General 97 

 (1) Safety Assessment 97 

 (2) Evaluation of Failure Conditions 97 

 b. Validation Elements 98 

 (1) Assessment 98 

 (2) Failure Condition 98 

 (3) Pilot Recognition 98 

 (4) Pilot Reaction Time 100 

 (5) Pilot Recovery 100 

 c. Takeoff 101 

 (1) Worst Case Failure Condition 101 

 (2) Failures to be Evaluated 101 

 (3) On Runway Guidance 101 

 d. Climb, Cruise, Descent, and Holding 101 

 e. Maneuvering 101 

 f. Approach 101 

 (1) Approach With Vertical Path Reference 101 

 (2) Approach Without Vertical Path Reference 103 

 (3) Steep Approach 103 

 g. Specific Conditions 103 

 (1) Failure Conditions to be Evaluated 103 

 (2) Failure to Disengage 103 

101. Criteria Supporting Operational Use of Autopilot 103 

 a. Typical Flight Operations 103 

 b. Operations in Close Proximity to the Ground 104 

 (1) General 104 

 (2) Considerations 104 

 (3) Autopilot Engagement Altitude/Height After Takeoff 104 

 (4) Autopilot Engagement During Approach 105 

 (5) Circling Approach 107 

 c. Climb, Cruise, Descent, and Holding 107 

 d. Maneuvering 107 

102. Automatic Disengagement of the Autopilot 108 

103. Assessment of Human Factors 108 

 a. General 108 

 b. Flight Evaluations 108 

 (1) During Certification 108 

 (2) Prior to Certification 108 

 (3) Credit to Establish FGS Compliance 109 

 (4) Simulation Versus Flight Tests 109 

 c. Structured Subjective Evaluations 109 

 d. Pilot Training and Experience 110 
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Paragraph Page 

 e. Evaluating New or Unique Design Features 110 

 f. Certification Planning Documentation 110 

104 – 108 [RESERVED] 110 

CHAPTER 10. AIRPLANE FLIGHT MANUAL 

109. General 111 

 a. AFM Terminology 111 

 b. Low-Visibility Operations 111 

110. Information Supporting Operational Use of Autopilot 111 

 a. General 111 

 b. Specifying a Minimum Use Height (MUH) 111 

 (1) Takeoff 111 

 (2) Cruise 111 

 (3) Approach 111 

 c. Maximum Displacement Deviation 112 

111. Limitations 112 

112. Non-Normal/Emergency Procedures 112 

113. Normal Procedures 112 

 a. Documentation of Normal Procedures 112 

 b. Maximum Displacement Deviation 112 

 c. Procedures for Use of FGS in Icing Conditions 112 

 d. Aircraft With Published Flightcrew Operation Manuals 

(FCOM)

112 

 e. Aircraft Without Published FCOMs 113 

APPENDICES 

1 Safety Assessment A1-1 

 1. General A1-1 

 2. Identification of Failure Conditions A1-1 

 3. Considerations When Assessing The Severity of Failure 

Condition Effects 

A1-2 

 4. Failure Condition Classification A1-3 

2 Flight Test Procedures A2-1 

 1. General A2-1 

 2. Purpose and Scope of Flight Demonstration Program A2-1 

 3. Protection Features of FGS A2-1 

 4. Environmental Conditions A2-5 

 5. Failure Conditions A2-6 

3 Definitions A3-1 

4 Acronyms A4-1 
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 12 

TABLES 

Number Page 

2-1 Cross-References § 25.1329 Paragraphs to AC Chapters 17 

5-1 Examples of Normal Conditions 48 

5-2 Examples of Rare Normal Conditions 51 

5-3 Examples of Non-Normal Conditions 54 

FIGURES 

Number Page 

9-1 Relationship Between AC Chapters (Chapter 9) 87 

2-1 Deviation Profile Method (Appendix 2) A2-10 

2-2 Height Loss Method (Appendix 2) A2-11 
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 13 

1 — 10 [RESERVED] 

CHAPTER 1 BACKGROUND 

11. GENERAL. 

a. Advisory Circular (AC) 25.1329-1A. This advisory material replaces material 

previously provided in AC 25.1329-1A for autopilots. The automatic control and 

guidance systems in current aircraft have evolved to a level that dictates a revision to 

current advisory material. 

b. Scope. This advisory material provides information for flight guidance systems 

(FGS), which include any autopilot functions, flight director (FD) functions, and 

automatic thrust control functions as well as any interactions with stability augmentation 

and trim functions. 

12. EVOLUTION OF FGSs. 

a. Complexity. There have been dramatic changes in technology and system 

design, which have resulted in much higher levels of integration, automation, and 

complexity. These changes have also redefined the allocation of functions and interfaces 

between systems. Relatively simple, dedicated systems have been replaced with digital 

multi–function systems with more modes and automatic changes in modes of operation. 

The introduction of fly–by–wire flight control systems has created new interface 

considerations for the FGS. 

b. Guidance Considerations. These new systems are capable of providing better 

performance, increased safety, and decreased workload. But, if designed without 

consideration for the criteria in this AC, these systems could also be confusing and not 

immediately intuitive for the flightcrew. Significant operational experience has been 

gained on new generation systems, and this guidance material is provided based on that 

experience. 

13 — 16 [RESERVED] 
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 15 

CHAPTER 2 OVERVIEW OF FLIGHT GUIDANCE SYSTEMS (FGS) 

17. PURPOSE OF FGS. The FGS is primarily intended to assist the flightcrew in the 

basic control and tactical guidance of the airplane. The system may also provide 

workload relief to the pilots and provide a means to fly a flight path more accurately to 

support specific operational requirements, such as reduced vertical separation minimum 

(RVSM) or required navigation performance (RNP). 

18. FGS COMPONENTS. 

a. Included in Definition. For the purpose of this AC, the term “FGS” includes all 

the equipment necessary to accomplish the FGS function, including the sensors, 

computers, power supplies, servo–motors/actuators, and associated wiring. It includes 

any indications and controllers necessary for the pilot to manage and supervise the 

system. 

b. Excluded from Definition. Any part of the FGS that remains mechanically 

connected to the primary flight controls or propulsion controls when the FGS is not in use 

is regarded as a part of the primary flight controls and propulsion system, and the 

provisions for such systems are applicable. 

19. FGS FUNCTIONS. 

a. Elements of FGS. 

 (1) Flight guidance and control (for example, autopilot, FD displayed head 

down or head up). 

 (2) Autothrottle/autothrust systems. The term “autothrust” is generic in nature 

and includes power control systems for propeller driven airplanes. 

 (3) Interactions with stability augmentation and trim systems. 

 (4) Alerting, status, mode annunciation, and situation information associated 

with flight guidance and control functions. 

b. Approach and Landing System. The FGS includes those functions necessary to 

provide guidance and control in conjunction with an approach and landing system, such 

as the following: 

 (1) Instrument landing system (ILS) 

 (2) Microwave landing system (MLS) 
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 16 

 (3) Global navigation satellite system (GNSS) 

 (4) GNSS landing system (GLS) 

c. Flight Management System (FMS). The FGS also includes those functions 

necessary to provide guidance and control in conjunction with a FMS. The FGS does not 

include the flight planning and the generation of flight path and speed profiles tied to 

waypoints and other flight planning aspects of the FMS. However, it does include the 

interface between the FMS and FGS necessary for the execution of flight path and speed 

commands. 

d. Design Philosophy. The applicant should establish, document, and follow a 

design philosophy that supports the intended operational use regarding FGS behavior, 

modes of operation, the pilot interface with controls, indications, alerts, and mode 

functionality. 

e. Description of FGS Behavior and Operation. A description of the FGS behavior 

and operation should be addressed from flightcrew and maintenance perspectives in 

appropriate documentation and training material. 

20. COMPLIANCE WITH § 25.1329. 

a. General. Subsequent chapters of this advisory material provide acceptable means 

of compliance with § 25.1329 and the applicability of other Part 25 rules to FGS 

(e.g., §§ 25.1301 and 25.1309). The demonstrated means of compliance may include a 

combination of analysis, laboratory testing, flight testing, and simulator testing. The 

applicant should coordinate with the FAA early in the certification program via a 

certification plan to reach agreement on the methods to be used to demonstrate 

compliance. 

NOTE: This AC uses the terminology “should” and “should not” when discussing 

compliance to the AC itself, as the AC represents one, but not the only, method of 

complying with § 25.1329. 

NOTE: This AC uses the terminology “must” and “may not” when discussing 

compliance to § 25.1329 and other specific rules, as compliance to a rule is not 

optional. In these cases, the AC text supplies a reference to the specific rule being 

discussed. 

b. Relevant Paragraphs. The following table (Table 2-1) lists the relevant 

paragraphs of § 25.1329 and indicates where acceptable means of compliance with each 

paragraph can be found within this AC. 
7/17/06 AC 25.1329-1B 

 

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Table 2-1 Cross-References 

§ 25.1329 Paragraphs to AC Chapters 

Section/Paragraph AC Chapter 

Reference/Paragraph 

§ 25.1329(a) 

Quick disengagement controls for the autopilot and 

autothrust functions must be provided for each pilot. 

The autopilot quick disengagement controls must be 

located on both control wheels (or equivalent). The 

autothrust quick disengagement controls must be 

located on the thrust control levers. Quick 

disengagement controls must be readily accessible to 

each pilot while operating the control wheel (or 

equivalent) and thrust control levers. 

Chapter 3 

FGS Engagement, Disengagement, 

Indications, and Override 

Paragraph 27, 

Autopilot Engagement, 

Disengagement, and 

Indications 

Paragraph 29, Autothrust 

Engagement, Disengagement, 

Indications 

§ 25.1329(b) 

The effects of a failure of the system to disengage the 

autopilot or autothrust functions when manually 

commanded by the pilot must be assessed in 

accordance with the requirements of § 25.1309.

Chapter 3 

FGS Engagement, Disengagement, 

Indications, and Override 

Paragraph 27, 

Autopilot Engagement, 

Disengagement, Indications 

Paragraph 29, 

Autothrust Engagement, 

Disengagement, and 

Indications 

Paragraph 30, 

Override of the FGS 

Chapter 8 

Safety Assessment 

Paragraph 89, 

Failure to Disengage the FGS 
7/17/06 AC 25.1329-1B 

 

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Section/Paragraph AC Chapter 

Reference/Paragraph 

§ 25.1329(c) 

Engagement or switching of the flight guidance 

system, a mode, or a sensor may not cause a transient 

response of the airplane’s flight path any greater than 

a minor transient, as defined in paragraph (n)(1) of 

this section. 

Chapter 3 

FGS Engagement, Disengagement, 

Indications, and Override 

Chapter 8

Safety Assessment 

§ 25.1329(d) 

Under normal conditions, the disengagement of any 

automatic control function of a flight guidance 

system may not cause a transient response of the 

airplane’s flight path any greater than a minor 

transient. 

Chapter 3 

FGS Engagement, Disengagement, 

Indications, and Override 

Chapter 4 

Controls, Indications, and Alerts 

Paragraph 45, 

FGS Alerting, Warning, 

Caution, Advisory, and Status 

Chapter 8 

Safety Assessment 

§ 25.1329(e) 

Under rare normal and non-normal conditions, 

disengagement of any automatic control function of a 

flight guidance system may not result in a transient 

any greater than a significant transient, as defined in 

paragraph (n)(2) of this section. 

Chapter 3 

FGS Engagement, Disengagement, 

Indications, and Override 

Chapter 4 

Controls, Indications, and Alerts 

Paragraph 45, 

FGS Alerting, Warning, 

Caution, Advisory, and Status 

Chapter 8 

Safety Assessment 

§ 25.1329(f) 

The function and direction of motion of each 

command reference control, such as heading select or 

vertical speed, must be plainly indicated on, or 

adjacent to, each control if necessary to prevent 

inappropriate use or confusion. 

Chapter 4 

Controls, Indications and Alerts 

Paragraph 43, 

FGS Controls
7/17/06 AC 25.1329-1B 

 

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Section/Paragraph AC Chapter 

Reference/Paragraph 

§ 25.1329(g) 

Under any condition of flight appropriate to its use, 

the flight guidance system may not produce 

hazardous loads on the airplane, nor create hazardous 

deviations in the flight path. This applies to both 

fault-free operation and in the event of a malfunction, 

and assumes that the pilot begins corrective action 

within a reasonable period of time. 

Chapter 4 

Controls, Indications and Alerts 

Paragraph 45, 

FGS Alerting, Warning, 

Caution, Advisory, and Status 

Chapter 5 

Performance of Function 

Chapter 8 

Safety Assessment 

Chapter 9 

Compliance Demonstration Using 

Flight Test and Simulation 

§ 25.1329(h)

When the flight guidance system is in use, a means 

must be provided to avoid excursions beyond an 

acceptable margin from the speed range of the 

normal flight envelope. If the airplane experiences 

an excursion outside this range, a means must be 

provided to prevent the flight guidance system from 

providing guidance or control to an unsafe speed. 

Chapter 5 

Performance of Function 

Paragraph 57, 

Speed Protection 

§ 25.1329(i) 

The flight guidance system functions, controls, 

indications, and alerts must be designed to minimize 

flightcrew errors and confusion concerning the 

behavior and operation of the flight guidance system. 

Means must be provided to indicate the current mode 

of operation, including any armed modes, transitions, 

and reversions. Selector switch position is not an 

acceptable means of indication. The controls and 

indications must be grouped and presented in a 

logical and consistent manner. The indications must 

be visible to each pilot under all expected lighting 

conditions. 

Chapter 4 

Controls, Indications, and Alerts 

 
7/17/06 AC 25.1329-1B 

 

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Section/Paragraph AC Chapter 

Reference/Paragraph 

§ 25.1329(j) 

Following disengagement of the autopilot, a warning 

(visual and auditory) must be provided to each pilot 

and be timely and distinct from all other cockpit 

warnings. 

Chapter 3 

FGS Engagement, Disengagement, 

Indications, and Override 

Paragraph 27, 

Autopilot Engagement, 

Disengagement, and 

Indications, 

Chapter 8

Safety Assessment 

§ 25.1329(k) 

Following disengagement of the autothrust function, 

a caution must be provided to each pilot. 

Chapter 3 

FGS Engagement, Disengagement, 

Indications, and Override 

Paragraph 27, 

Autopilot Engagement, 

Disengagement, and 

Indications 

Chapter 8 

Safety Assessment 

§ 25.1329(l)

The autopilot may not create a potential hazard when 

the flightcrew applies an override force to the flight 

controls. 

Chapter 3 

FGS Engagement, Disengagement, 

Indications, and Override 

Paragraph 30, 

Override of the FGS 

Chapter 4 

Controls, Indications, and Alerts 

Paragraph 45, 

FGS Alerting, Warning, 

Caution, Advisory, and Status 

Chapter 8 

Safety Assessment 
7/17/06 AC 25.1329-1B 

 

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Section/Paragraph AC Chapter 

Reference/Paragraph 

§ 25.1329(m) 

During autothrust operation, it must be possible for 

the flightcrew to move the thrust levers without 

requiring excessive force. The autothrust may not 

create a potential hazard when the flightcrew applies 

an override force to the thrust levers. 

Chapter 3 

FGS Engagement, Disengagement, 

Indications, and Override 

Paragraph 30, Override of 

the FGS 

Chapter 8 

Safety Assessment 

§ 25.1329(n) 

For purposes of this section, a transient is a 

disturbance in the control or flight path of the 

airplane that is not consistent with response to 

flightcrew inputs or environmental conditions. 

(1) A minor transient would not significantly reduce 

safety margins and would involve flightcrew actions 

that are well within their capabilities. A minor 

transient may involve a slight increase in flightcrew 

workload or some physical discomfort to passengers 

or cabin crew. 

(2) A significant transient may lead to a significant 

reduction in safety margins, an increase in flightcrew 

workload, discomfort to the flightcrew, or physical 

distress to the passengers or cabin crew, possibly 

including non-fatal injuries. Significant transients do 

not require, in order to remain within or recover to 

the normal flight envelope, any of the following: 

(i) Exceptional piloting skill, alertness, or strength. 

(ii) Forces applied by the pilot which are greater than 

those specified in § 25.143(c). 

(iii) Accelerations or attitudes in the airplane that 

might result in further hazard to secured or 

non-secured occupants. 

Appendix 3 - Definitions
7/17/06 AC 25.1329-1B 

 

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NOTE: The text for each paragraph in § 25.1329 is included in this AC 

for reference only. Please refer to the actual text in 14 CFR part 25. 

21 — 26 [RESERVED] 
7/17/06 AC 25.1329-1B 

 

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CHAPTER 3 FGS ENGAGEMENT, DISENGAGEMENT, 

INDICATIONS, AND OVERRIDE 

27. AUTOPILOT ENGAGEMENT, DISENGAGEMENT, AND INDICATIONS. 

Autopilot engagement and disengagement should be accomplished in a manner consistent 

with other flightcrew procedures and tasks and should not require undue attention. 

a. Autopilot Engagement. 

 (1) Single Switch Action. Each pilot should be able to select the autopilot 

function of the FGS with a single switch action. The single switch action should engage 

both the pitch and roll axes. 

 (2) Engagement Status of Autopilot. The autopilot system should provide 

positive indication to the flightcrew that the system has been engaged. The selector 

switch position is not acceptable as a means of indication. [See § 25.1329(i)] Regardless 

of the method used, the engagement status (and changes in status) of the autopilot(s) 

should be clearly indicated and should not require undue attention or recall. 

NOTE: If an operational need is identified for split–axis engagement, 

then annunciation or indication should be provided for each axis. 

 (3) Multiple Autopilots. If a single autopilot within a multiple autopilot 

installation can be individually selected by the flightcrew, the engagement annunciation 

should reflect the flightcrew selection. It should not be possible for multiple autopilots to 

be engaged in different modes. For modes that use multiple autopilots, the additional 

autopilots may engage automatically at selection of the mode or after arming the mode. 

A means should be provided to determine that adequate autopilot capability exists to 

support the intended operation (for example, "Land 2" and "Land 3" are used in some 

aircraft). 

 (4) Acceptable Transients. The engagement of the autopilot should be free of 

perceptible transients. Under dynamic conditions, including maneuvering flight, minor 

transients are acceptable. However, with normal operating conditions, significant 

transients may not occur. [See § 25.1329(d)] 

 (5) Flight Director (FD) Not Engaged. Without a FD engaged, the initial lateral 

and vertical modes should be consistent with minimal disturbance from the flight path. 

For example, the lateral mode at engagement may roll the airplane to wings level and 

then hold the airplane heading/track or maintain the existing bank angle (if in a normal 

range). A heading/track pre–select at engagement function may be provided, if 

precautions are taken to ensure that selection reflects the current intent of the flightcrew. 

The modes at engagement should be annunciated and any associated selected target 

values should be displayed. 
7/17/06 AC 25.1329-1B 

 

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 (6) FD Engaged. With a FD engaged, the autopilot should engage in a mode 

consistent with (that is, the same as, or if that is not possible, then compatible with) the 

active FD mode of operation. Consideration should be given to the mode in which the 

autopilot will engage when large commands are present on either or both FDs. The 

following examples list some of these considerations: 

 (a) Should the autopilot retain the active FD mode or should it engage in 

the basic default mode? 

 (b) Are there any implications for current flight path references and targets? 

 (c) Is there any potential for flightcrew confusion and unintended changes 

in flight path or modes? 

NOTE: The design should consider the possibility that the pilot may 

attempt to engage the autopilot outside of the normal flight envelope. It is 

not required that the autopilot compensate for unusual attitudes or other 

situations outside the normal flight envelope, unless that is part of the 

autopilot’s intended function. 

b. Autopilot Disengagement. 

 (1) General. 

 (a) Normal Conditions. Under normal conditions, automatic or manual 

disengagement of the autopilot should be free of perceptible transients or out–of–trim 

forces that are not consistent with the maneuvers being conducted by the airplane at the 

time of disengagement. A disengagement in normal conditions may not result in a 

transient any greater than a minor transient. If multiple autopilots are engaged, any 

disengagement of an individual autopilot may not result in a transient any greater than a 

minor transient. [See § 25.1329(d)] The disengagement of an individual autopilot should 

not adversely affect the operation of the remaining engaged autopilot(s). 

 (b) Other Than Normal Conditions. Under other than normal conditions 

(that is, non-normal or rare normal conditions), disengagement of the autopilot may not 

result in a transient any greater than a significant transient. [See § 25.1329(e)] (See 

Chapter 5, Performance of Function, paragraphs 54 and 56 of this AC for a discussion of 

non-normal and rare normal conditions). The flightcrew should be able to respond to a 

significant transient without using any of the following: 

 1 exceptional piloting skill, alertness, or strength, 

 2 forces greater than those given in § 25.143(c), or 
7/17/06 AC 25.1329-1B 

 

 25 

 3 accelerations or attitudes in the airplane that might result in a hazard 

to secured or non–secured occupants. 

 (c) Potential for Significant Transient. The flightcrew should be made 

aware (via a suitable alerting or other indication) of conditions or situations (for example, 

continued out–of–trim) that could result in a significant transient at disengagement. [See 

Chapter 4, Controls, Indications, and Alerts, paragraph 45d, Awareness of Potential 

Significant Transient Condition (“Bark before Bite”).] 

NOTE: See Appendix 3 for definitions of significant transient and minor 

transient. See Chapter 5, Performance of Function, paragraphs 53, 54, 

and 56 for a discussion of normal conditions, rare normal conditions, and 

non-normal conditions. 

 (2) Autopilot Disengagement Alerts. 

 (a) Alert Type. Since it is necessary for a pilot to immediately assume 

manual control following disengagement (manual or automatic) of the autopilot, a 

warning (both visual and aural) must be given. [See § 25.1329(j)] 

 (b) Aural Warning Specifications. A timely aural warning must be 

provided and must be distinct from all other cockpit warnings. [See § 25.1329(j)] It 

should sound long enough to ensure that it is heard and recognized by the pilot and other 

flight crewmembers, but not so long that it adversely affects communication between 

crewmembers or is a distraction. The aural warning should continue until silenced by 

one of the following means: 

 1 Activation of the autopilot quick disengagement control 

 2 Re–engagement of the autopilot 

 3 Another acceptable means. 

 (c) Multiple Autopilot System. 

 1 Disengagement of an autopilot channel within a multiple-channel 

autopilot system, downgraded system capability, or a reduction in the level of system 

redundancy which requires immediate flightcrew awareness and possible timely action 

should cause a caution level alert to be issued to the flightcrew. 

 2 Disengagement of an autopilot channel within a multiple-channel 

autopilot system which requires only flightcrew awareness should cause a suitable 

advisory level alert to be issued to the flightcrew. 
7/17/06 AC 25.1329-1B 

 

 26 

 (3) Quick Disengagement Control. 

 (a) Purpose. The purpose of the quick disengagement control is to ensure 

the capability for each pilot to manually disengage the autopilot quickly with a minimum 

of pilot hand/limb movement. 

 (b) Location. The quick disengagement control must be located on each 

control wheel or equivalent. [See § 25.1329(a)] It should be within easy reach of one or 

more fingers/thumb of the pilot’s hand when the hand is in a position for normal use on 

the control wheel or equivalent. 

 (c) Criteria. The quick disengagement control: 

 1 Must be operable with one hand on the control wheel or equivalent 

and the other hand on the thrust levers. [See § 25.1329(a)] 

 2 Should be accessible and operable from a normal hands–on position 

without requiring a shift in hand position or grip on the control wheel or equivalent. 

 3 Should be easily locatable by the pilot without having to first locate 

the control visually. 

 4 Should be designed so that any action to operate the quick 

disengagement control should not cause an unintended input to the control wheel (or 

equivalent). 

 5 Should be designed to minimize inadvertent operation and 

interference from other nearby control wheel (or equivalent) switches/ devices, such as 

radio control or trim. 

NOTE: When establishing location of the quick disengagement control, 

consideration should be given to its accessibility with large displacements 

of or forces on the control wheel (or equivalent) and the possible need to 

operate the quick disengagement control with the other hand. 

 (4) Alternative Means of Autopilot Disengagement. 

 (a) Factors to Consider. When a § 25.1309 assessment shows a need for an 

alternative means of disengagement, the following factors should be addressed: 

 1 Independence from primary quick disengagement control. 

 2 Whether alternative means are readily accessible to each pilot. 

 3 Reliability and the possibility of latent failure of those alternative 

means. 
7/17/06 AC 25.1329-1B 

 

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 (b) Acceptable Means. The following means of providing an alternative 

disengagement have been found to be acceptable: 

 1 Selecting the engagement control to the “off” position. 

 2 Disengaging the bar on mode select panel (MSP). 

 3 Activating the trim switch on the yoke. 

NOTE: Use of circuit breakers as a means of disengagement is not 

considered acceptable. 

 (5) Flightcrew Pitch Trim Input. If the autopilot is engaged and the pilot applies 

manual pitch trim input, and the autopilot is designed to disengage because of that flight 

crew action, the autopilot must disengage with no more than a minor transient. [See 

§ 25.1329(c)] Alternatively, pitch trim changes may be inhibited, such that the potential 

for a transient is removed. 

28. FD ENGAGEMENT, DISENGAGEMENT AND INDICATIONS. Engagement and 

disengagement of the FD should be accomplished consistent with other flightcrew 

procedures and tasks and should not require undue attention. 

a. FD Engagement. 

 (1) General.

 (a) Selection. A means may be provided for each pilot to select (that is, 

turn on) and de-select (that is, turn off) the FD for display on his or her primary flight 

display (PFD). 

 (b) Engagement. A FD is considered “engaged” if it is selected and 

displaying guidance cues. 

NOTE: The distinction is made between “engaged” and “selected,” 

because the FD might be selected but not displaying guidance cue(s) (for 

example, the cue(s) are biased out of view). 

 (c) Engagement Status of FD. The selection status of the FD and the 

source of FD guidance should be clear and unambiguous. Failure of a selected FD should 

be clearly annunciated. 

 (d) Multiple FDs. If there are multiple FDs and if necessary for crew 

awareness, indications should be provided to denote which FD is engaged (for example, 

FD1, FD2, HUD (head up display)). For airplanes with multiple FDs installed, all 
7/17/06 AC 25.1329-1B 

 

 28 

engaged FDs should always be in the same armed and active FGS modes. The selection 

status of each FD should be clear and unambiguous for each pilot. In addition, 

indications should be provided to denote loss of FD independence (for example, first 

officer selection of captain’s FD). 

 (e) Autopilot Engaged. A FD should engage into the current modes and 

targets of an already engaged autopilot or FD, if any. With no autopilot engaged, the 

basic modes at engagement of the FD functions should be established consistent with 

typical flight operations. 

NOTE: The engagement of the pitch axis in Vertical Speed or Flight Path 

Angle and engagement of the lateral axis in Heading Hold, Heading 

Select, or Bank Angle Hold have been found to be acceptable. 

 (f) HUD. Since the HUD can display flight guidance, the HUD guidance 

mode should be indicated to both pilots and should be compatible with the active head 

down FD mode. 

 (g) Maneuvering Flight. Engagement during maneuvering flight should be 

considered. 

NOTE: The design should consider the safety consequences if it is 

possible for the FD to engage outside of the normal flight envelope. It is 

not required that the FD should compensate for unusual attitudes or other 

situations outside the normal flight envelope, unless that is part of the 

FD’s intended function. 

 (2) Guidance Cue(s). 

 (a) Display. The FD command guidance cue(s) will typically be displayed 

under the following conditions: 

 1 When the FD is selected and valid command guidance is available, 

or 

 2 When the FD is automatically providing guidance (See 

paragraph (3), below). 

 (b) Engagement Indication. The display of guidance cue(s) (for example, 

FD bars) is sufficient indication that the FD is engaged. 

 (c) Invalid Guidance. The FD guidance cue(s) should be removed when 

guidance is determined to be invalid. 

 (3) Reactive Windshear Guidance System. For airplanes equipped with a FD 

windshear guidance system, FD engagement should be provided consistent with the 
7/17/06 AC 25.1329-1B 

 

 29 

criteria in AC 25–12, Airworthiness Criteria for the Approval of Airborne Windshear 

Warning Systems in Transport Category Airplanes; and AC 120–41, Criteria for 

Operational Approval of Airborne Windshear Alerting and Flight Guidance. 

b. FD Disengagement. There may be a means for each pilot to readily de-select his 

or her on–side FD function. Flightcrew awareness of disengagement and de-selection is 

important. Removal of guidance cue(s) alone is not sufficient indication of de-selection, 

because the guidance cue(s) may be removed from view for a number of reasons, 

including invalid guidance or autopilot engagement. Therefore, the FD function should 

provide clear and unambiguous indication to the flightcrew that the function has been 

deselected. 

29. AUTOTHRUST ENGAGEMENT, DISENGAGEMENT, AND INDICATIONS. 

Engagement and disengagement should be accomplished in a manner consistent with 

other flightcrew procedures and tasks and should not require undue attention. 

a. Autothrust Engagement. 

 (1) Indication. The autothrust function should provide the flightcrew positive 

indication that the system has been engaged. 

 (2) Accessibility of Controls. The autothrust engagement controls should be 

accessible to each pilot. 

 (3) Inadvertent Engagement/Disengagement. The autothrust function should 

normally be designed to prevent inadvertent engagement and inadvertent application of 

thrust for both on–ground and in–air operations. For example, separate arm and engage 

functions may be provided. 

 (4) Automatic Engagement. Intended automatic engagement, such as a “wake 

up” mode to protect for unsafe speeds, may be acceptable. (See Chapter 5, 

paragraph 57b, Low Speed Protection.) If such automatic engagement occurs, it should 

be clear to the flightcrew that automatic engagement has occurred. The automatic 

engagement may not cause a transient larger than a minor transient. [See § 25.1329(c)] 

The transition to the engaged state should be smooth, and should not cause large, 

unexpected changes in pitch attitudes or pitching moments. The reason for automatic 

engagement should be clear and obvious to the flightcrew. 

NOTE: The design should consider the possibility that the pilot may 

attempt to engage the autothrust function outside of the normal flight 

envelope or at excessive or too low engine thrust. It is not expected that 

the autothrust feature should compensate for situations outside the normal 

flight envelope or normal engine operation range, unless that is part of the 

intended function of the autothrust system. 
7/17/06 AC 25.1329-1B 

 

 30 

b. Autothrust Disengagement. 

 (1) Indication of Disengagement. Positive indication of disengagement of the 

autothrust function must result in a caution level alert to the flightcrew. [See 

§ 25.1329(k)] The autothrust “engage” status annunciations should be deleted. 

 (a) Visual Indication.

 1 Following automatic disengagement: Visual indication of 

disengagement should persist until canceled by flightcrew action. 

 2 Following manual disengagement: If an aural alert is provided, 

visual indication of disengagement should persist for some minimum period. If an aural 

alert is not provided, visual indication of disengagement should persist until canceled by 

flightcrew action. 

 (b) Aural alert. If provided, the aural disengagement alert should be of 

sufficient duration and volume to assure that the flightcrew has been alerted that 

disengagement has occurred. However, an extended cycle of an aural alert is not 

acceptable following disengagement, if such an alert can significantly interfere with 

flightcrew coordination or radio communication. 

 (2) Inadvertent Disengagement. The autothrust normally should be designed to 

preclude inadvertent disengagement during activation of autothrust modes of operation. 

 (3) Consequence of Disengagement. Under normal conditions, autothrust 

disengagement may not cause a transient any greater than a minor transient. [See 

§ 25.1329(d)] The transition to the disengaged state should be smooth and not cause 

unexpected changes in pitch attitude or pitching moment or a significant thrust transient. 

The disengagement should not preclude, inhibit, or interfere with timely thrust changes 

for go-around, landing, or other maneuvers requiring manual thrust changes. 

 (4) Autothrust Quick Disengagement Control. Autothrust quick disengagement 

controls must be provided for each pilot on the respective thrust control (thrust lever or 

equivalent). A single–action, quick disengagement switch must be incorporated on the 

thrust control, so that switch activation can be executed when the pilot’s other hand is on 

the control wheel (or equivalent). [See § 25.1329(a)] The disengagement control should 

be positioned such that inadvertent disengagement of the autothrust function is unlikely. 

Positioning the control on the outboard side has been shown to be acceptable for multi–

engine aircraft. Thrust lever knob, end-mounted disengagement controls available on 

both sides to facilitate use by either pilot have been shown to be preferable to those 

positioned to be accessible by the pilot’s palm. 
7/17/06 AC 25.1329-1B 

 

 31 

30. OVERRIDE OF THE FGS.

 a. General. An override of an engaged FGS function is defined as an action taken 

by the flightcrew intended to prevent, oppose, or alter an operation being conducted by 

the FGS function without first disengaging that function. 

b. Autopilot Override. 

 (1) With Automatic Disengagement. 

 (a) Override Force. The autopilot should disengage when the flightcrew 

applies a significant override force to the controls. The applicant should interpret 

“significant” as a force that is consistent with an intention to overpower the autopilot by 

either or both pilots. The autopilot should not disengage by minor application of force to 

the controls, such as a pilot gently bumping the control column while entering or exiting 

a pilot seat during cruise. 

NOTE: Twenty-five pounds (25 lbs) of force at the control column or 

wheel has been determined to be a significant override force level for 

other than approach operations on some aircraft types. To reduce nuisance 

disengagement, higher forces have been found acceptable for certain 

approach, landing, and go-around operations on some aircraft. The force 

to disengage an autopilot is not necessarily the force required at the 

column to oppose autopilot control, that is, to cause elevator movement. 

The corresponding forces for a sidestick controller may be different. 

 (b) Transients Resulting from Override. In normal operating conditions, a 

transient larger than a minor transient may not result from autopilot disengagement when 

the flightcrew applies an override force to the controls. [See § 25.1329(d)] Mitigation 

may be accomplished through provision of an appropriate alert and flightcrew procedure. 

 (c) Sustained Override Force Below Level Required for Automatic 

Disconnect. Sustained application of force below the disengagement threshold may not 

result in a potential hazard. [See § 25.1329(l)] For example, the automatic trim should 

not run to oppose the override of the autopilot by the flightcrew that would result in 

unacceptable airplane motion, if the autopilot were to automatically disengage or be 

manually disengaged. 

 (2) Without Automatic Disengagement.

 (a) Potential Hazard. If the FGS is not designed to disengage in response 

to any override force, then the response to an override may not result in a potential 

hazard. Sustained application of an override force may not result in a potential hazard, 

such as when the flightcrew abruptly releases the force on the controls. [See 

§ 25.1329(l)] Mitigation may be accomplished through provision of an appropriate alert 

and flightcrew procedure. 
7/17/06 AC 25.1329-1B 

 

 32 

NOTE: The term “sustained application of override force” is intended to 

describe a force that is applied to the controls which may be small, slow, 

and sustained for some period of time. This may be due to an inadvertent 

crew action or may be an intentional crew action meant to “assist” the 

autopilot in a particular maneuver. (See Chapter 9, Compliance 

Demonstration Using Flight Test and Simulation, paragraph 99f, 

Flightcrew Override of the FGS, for more information.) 

 (b) Transients Resulting from Override. In normal operating conditions, a 

transient larger than a minor transient may not result from manual autopilot 

disengagement after the flightcrew has applied an override force to the controls. [See 

§ 25.1329(d)] Mitigation may be accomplished through provision of an appropriate alert 

and flightcrew procedure. 

NOTE: The term “override force” is intended to describe a pilot action 

that is intended to prevent, oppose or alter an operation being conducted 

by a flight guidance function without first disengaging that function. One 

possible reason for this action could be an avoidance maneuver, such as 

responding to a traffic alert and collision avoidance system (TCAS) 

resolution advisory that requires immediate action by the flightcrew and 

would typically involve a rapid and forceful input from the flightcrew. 

NOTE: For control wheel steering considerations, see Chapter 6, 

Characteristics of Specific Modes, paragraph 68, Control Wheel Steering 

(CWS) [control steering through the autopilot]. 

c. Autothrust Override. 

 (1) Force Required. It should be possible for the pilot to readily override the 

autothrust function and set thrust by moving the thrust levers (or equivalent) with one 

hand. 

 (2) Response To Override. The autothrust response to a flightcrew override 

may not create a potential hazard. [See § 25.1329(m)] The autothrust response to the 

flightcrew override should not result in an abrupt change of pitch attitude, an abrupt 

pitching moment, or an uncontrolled change of thrust. 

 (3) Engagement Status With Override. Autothrust functions may be designed to 

safely remain engaged during pilot override. Alternatively, autothrust functions may 

disengage as a result of pilot override, provided that the design prevents unintentional 

autothrust disengagement and adequately alerts the flightcrew to ensure pilot awareness. 
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31. FGS ENGAGEMENT MODE COMPATIBILITY. 

a. FGS Mode Engagement Philosophy. A description of the philosophy used for the 

mode at engagement of the autopilot, FD, and autothrust functions should be provided in 

flightcrew training material. 

b. Engagement Mode Compatibility. It should not be possible to select 

incompatible FGS command or guidance functions, such as commanding speed through 

elevator and autothrust at the same time. 

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CHAPTER 4 CONTROLS, INDICATIONS, AND ALERTS 

42. GENERAL. 

a. Human-Machine Interface. The human-machine interface with the FGS is crucial 

to ensuring safe, effective, and consistent FGS operation. The manner in which FGS 

information is depicted to the flightcrew is vital to flightcrew awareness and, therefore, to 

safe operation of the FGS. 

b. Design of Controls, Indications, and Alerts. These features must be designed to 

minimize flightcrew errors and confusion. [See § 25.1329(i)] Indications and alerts 

should be presented in a manner compatible with the procedures and assigned tasks of the 

flightcrew and provide the necessary information to perform those tasks. The indications 

must be grouped and presented in a logical and consistent manner and should be visible 

from each pilot’s station under all expected lighting conditions. [See § 25.1329(i)] The 

choice of colors, fonts, font size, location, orientation, movement, graphical layout, and 

other characteristics—such as steady or flashing—should all contribute to the 

effectiveness of the system. Controls, indications, and alerts should be implemented in a 

consistent manner. 

c. Evaluation of Information Provided by FGS. It is recommended that the 

applicant evaluate the adequacy and effectiveness of the information provided by the 

FGS interface, that is the controls, indications, alerts, and displays, to ensure flightcrew 

awareness of FGS behavior and operation. See Chapter 9, Compliance Demonstration 

Using Flight Test and Simulation, for more discussion of appropriate analyses which may 

include, for example, cognitive task analysis as a basis for evaluation. 

43. FGS CONTROLS. 

a. General. The FGS controls must be designed and located to prevent crew errors, 

confusion, and inadvertent operation. [See § 25.1329(i)] They should be designed to 

provide convenience of operation to each crewmember. The function and direction of 

motion of each control must be readily apparent or plainly indicated on or adjacent to 

each control, if needed to prevent inappropriate use or confusion. [See § 25.1329(f)] 

Sections 25.777(b) and 25.779(a) provide requirements regarding direction of motion of 

flight deck controls. These requirements apply for FGS command reference controls 

which select target values, such as heading select or vertical speed. Section 25.781 also 

provides requirements for the shapes of the knobs. 

b. Design Considerations. The design of the FGS should address the following 

specific considerations: 

 (1) Differentiation of Knob Shape and Position. Errors by the flightcrew have 

included confusing speed and heading knobs on the MSP. 
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 (2) Design to Support Correct Selection of Target Values. Use of a single 

control, such as concentric controls, for selecting multiple command reference targets has 

resulted in erroneous target value selection. 

 (3) Commonality Across Aircraft Types. Use of uniform control design in 

different aircraft prevents negative transfer of learning with respect to operation of the 

controls. Using knowledge about the operation of the controls on one type of airplane 

has resulted in activation of the incorrect control when pilots move from that type of 

airplane to another type of airplane. An example of this is confusion between the 

takeoff/go-around button and the autothrust disengagement button when transitioning 

between aircraft types with differing designs and locations for these controls. 

 (4) Positioning of Controls and Related Indications. Individual FGS controls, 

flight mode annunciator (FMA), and related PFD information should be positioned so 

that, as much as reasonably practical, items of related function have similarly related 

positions. Misinterpretation and confusion have occurred due to the inconsistent 

arrangement of FGS controls with the annunciations on the FMA. 

 (5) Inadvertent Operation. The FGS controls must be located to discourage or 

avoid inadvertent operation, such as inadvertent engagement or disengagement. [See 

§ 25.777(a)] Further design aspects of the FGS controls, other than location of the 

controls, should also be used discourage or avoid inadvertent operation. Examples 

include the shape of the control and the force required for operation. 

44. FGS MODE SELECTION, ANNUNCIATION, AND INDICATION. 

a. Annunciation of Engagement of the FGS. Engagement of the FGS functions 

must be suitably annunciated to each pilot. [See § 25.1329(i)] Guidance is provided in 

Chapter 3, FGS Engagement, Disengagement, Indications, and Override. 

b. Description of FGS Modes. The operator should be provided with appropriate 

descriptions of the FGS modes and their behaviors. 

c. FGS Mode Annunciations. Mode annunciation must indicate the state of the 

system. [See § 25.1329(i)] Mode annunciation should be presented in a manner 

compatible with flightcrew procedures/tasks and be consistent with the mode 

annunciation design for the specific aircraft type. That is, the mode annunciation should 

be compatible with other flight deck systems mode annunciations. Mode selector switch 

position or status is not acceptable as the sole means of mode annunciation. [See 

§ 25.1329(i)] Modes and mode changes should be depicted in a manner that achieves 

flightcrew attention and awareness. 

 (1) Active and Armed Modes. The FGS mode annunciations must effectively 

and unambiguously indicate the active and armed modes of operation. [See § 25.1329(i)] 
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The mode annunciation should convey explicitly, and as simply as possible, what the 

FGS is doing (for active modes), what it will be doing (for armed modes), and target 

information (such as selected speed, heading, and altitude) for satisfactory flightcrew 

awareness. 

 (2) Location. Mode annunciations should be located in the forward field of 

view. For example, a suitable location is on the PFD. Engaged modes should be 

annunciated at different locations than armed modes to assist in mode recognition. 

 (3) Discriminators. Color, font type, font size, location, highlighting, and 

symbol flashing have historical precedent as good discriminators, when implemented 

appropriately. The fonts and font size should be chosen so that annunciation of FGS 

mode and status information is readable and understandable without eyestrain when 

viewed by the pilot seated at the design eye position. The use of graphical or symbolic 

(that is, non-textual) indications is acceptable. Implementation of such discriminators 

should follow accepted guidelines, as described in applicable international standards 

(such as AC 25-11, Transport Category Airplane Electronic Display Systems), and 

should be evaluated for their consistency with and integration with the flight deck design. 

 (4) Color. Color should be used in a consistent manner and assure compatibility 

with the overall use of color on the flight deck. Specific colors should be used such that 

the FGS displays are consistent with other flight deck systems, such as a flight 

management system (FMS). The use of monochrome displays is not precluded, provided 

that the aspects of flightcrew attention and awareness are satisfied. Engaged modes 

should be annunciated with different colors than armed modes to assist in mode 

recognition. 

d. Mode Changes.

 (1) Operationally Relevant Mode Changes. Mode changes that are 

operationally relevant—especially mode reversions and sustained speed protection—

should be clearly and positively annunciated to ensure flightcrew awareness. Altitude 

Capture is an example of an operationally relevant mode that should be annunciated, 

because pilot actions during that brief time the mode is operational may have different 

effects on the airplane. Annunciation of sustained speed protection should be clear and 

distinct to ensure flightcrew awareness. The FGS sub–modes that are not operationally 

relevant (for example, sub-modes as the FGS transitions from instrument landing system 

Localizer (LOC) Capture to Localizer Track) need not be annunciated. 

 (2) Attention-Getting Features. The transition from an armed mode to an 

engaged mode should provide an additional attention–getting feature, such as boxing and 

flashing on an electronic display (see AC 25-11) for a suitable, but brief, period (for 

example, ten seconds) to assist in flightcrew awareness. Aural notification of mode 

changes should be limited to special considerations. 
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 (3) Use Of Alerts. In–service experience has shown that mode annunciation 

alone may be insufficient—unclear or not compelling enough—to communicate mode 

changes to the flightcrew, especially in high workload situations. Therefore, the safety 

consequences of the flightcrew not recognizing mode changes should be considered. If 

necessary, an appropriate alert should be used. 

e. Failure to Engage or Arm. The failure of a mode to engage or arm when selected 

by the pilot should be apparent. 

f. FGS Mode Display and Indications. Mode information provided to the flightcrew 

should be sufficiently detailed, so that the consequences of the interaction between the 

FGS and the flightcrew can be determined unambiguously. The FGS interface should 

provide timely and positive indication when the FGS deviates from the pilot's direct 

commands (for example, a target altitude or speed setting) or from the pilot's 

pre-programmed set of commands (for example, waypoint crossing). The interface 

should also provide clear indication when there is a difference or conflict between 

pilot-initiated commands. An example would be when a pilot engages positive vertical 

speed and then selects an altitude that is lower than the aircraft altitude. The default 

action taken by the FGS should be made apparent. 

45. FGS ALERTING, WARNING, CAUTION, ADVISORY, and STATUS. 

a. General. 

 (1) Alerting Provisions. Alerting information must follow the provisions of 

§ 25.1322. The advisory material associated with § 25.1322 should be consulted. Alerts 

for FGS engagement and disengagement are described in Chapter 3, FGS Engagement, 

Disengagement, Indications, and Override. 

 (2) Determination of FGS Capability. There should be some method for the 

flightcrew to determine and monitor the availability or capability of the FGS (for 

example, prior to dispatch), where the intended operation is predicated on the use of the 

FGS. The method of monitoring provided should take account of the hazard resulting 

from the loss of the autopilot function for the intended operation. 

b. Speed Protection Alerts. 

 (1) Alerts to Crew. To assure crew awareness, an alert should be provided when 

a sustained speed protection condition is detected. This is in addition to any 

annunciations associated with mode reversions that occur as a consequence of invoking 

speed protection (See Chapter 5, Performance of Function, paragraph 57, Speed 

Protection). 
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 (2) Alert Specifications.

 (a) Low Speed Protection. Low speed protection alerts should include both 

an aural and a visual component. 

 (b) High Speed Protection. High-speed protection alerts need include only 

a visual alert component because of existing high–speed aural alert requirements. 

However, giving an alert prior to that required aural alert is not precluded. [Refer to 

§ 25.1303(c)(1) for overspeed alerting regulations.] 

 (3) Consistency. Alerts for speed protection should be consistent with the 

protection provided and with the other alerts in the flight deck. 

 (4) Nuisance Alerts. Care should be taken to set appropriate values for 

indicating speed protection that would not be considered a nuisance for the flightcrew. 

c. Loss of Autopilot Approach Mode. The loss of the Approach mode requires 

immediate flightcrew awareness. This may be accomplished through autopilot 

disengagement described in AC 120-28D, Criteria for Approval of Category III Weather 

Minima for Takeoff, Landing, and Rollout. If the autopilot remains engaged and reverts 

to a non-approach mode, an appropriate aural warning and/or visual alert should be 

provided. 

d. Awareness of Potential Significant Transient Condition (“Bark Before Bite”). 

 (1) General. There have been situations where an autopilot was engaged, 

operating normally and controlling up to the limit of its authority for an extended period 

of time, and the flightcrew was unaware of the potential hazard. This service experience 

has shown that, without timely flightcrew awareness and action, such a situation can 

progress to a loss of control after autopilot disengagement, particularly in rare normal or 

non-normal conditions. However, with adequate flightcrew awareness and pilot action, 

loss of control may be prevented. 

 (2) Flightcrew Alert(s). To help ensure flightcrew awareness and timely action, 

appropriate alert(s)—normally a caution or warning—should be provided to the 

flightcrew for conditions that could require exceptional piloting skill or alertness for 

manual control following autopilot disengagement (for example, significantly out of trim 

conditions). The number and type of alerts required should be determined by the unique 

situations that are being detected and by the crew procedures required to address those 

situations. Any alert should be clear and unambiguous and should be consistent and 

compatible with other flight deck alerts. Care should be taken to set appropriate 

thresholds for these alerts so that they are not considered a nuisance for the flightcrew. 
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 (3) Situations to Consider for an Alert. 

 (a) Sustained Lateral Control Command. If the autopilot is holding a 

sustained lateral control command, it could be indicative of an unusual operating 

condition for which the autopilot is compensating. Examples of such unusual operating 

conditions are asymmetric lift and/or drag due to asymmetric icing, fuel imbalance, or 

asymmetric thrust. In the worst case, the autopilot may be operating at or near its full 

authority in one direction. If the autopilot were to disengage while holding this lateral 

trim, the result would be that the airplane could undergo a rolling moment that could 

possibly take the pilot by surprise. Therefore, a timely alert should be considered to 

permit the crew to manually disengage the autopilot and take control prior to any 

automatic disengagement that might result from the condition. 

 (b) Sustained Pitch Command. If the autopilot is holding sustained pitch 

command, it could be indicative of an unusual operating condition (for example, 

inoperative automatic horizontal trim) for which the autopilot is compensating. If the 

autopilot were to disengage while holding this pitch command, the result would be that 

the airplane could undergo an abrupt change in pitch that could possibly take the pilot by 

surprise. Therefore, a timely alert should be considered to permit the crew to manually 

disengage the autopilot and take control prior to any automatic disengagement that might 

result from the condition. 

 (c) Bank and Pitch Angles. Most autopilots are designed with operational 

limits in both the pitch and roll axes, such that those predetermined limits will not be 

purposely exceeded. If the airplane exceeds those limits, it could be indicative of a 

situation that requires the pilot to intervene. Such a situation may not be covered by 

paragraphs (a) or (b) above. Therefore, a timely alert should bring this condition to the 

attention of the flightcrew and permit the crew to manually disengage the autopilot and 

take control prior to any automatic disengagement which might result. 

 (4) Automatic Disengagement. It is preferable that the autopilot remains 

engaged during out–of–trim conditions. However, if there is an automatic disengagement 

feature due to excessive out–of–trim, an alert should be generated and should precede any 

automatic disengagement with sufficient margin to permit timely flightcrew recognition 

and manual disengagement. See also Chapter 3, FGS Engagement, Disengagement, 

Indications, and Override, paragraph 30, Override of the FGS, for related material. 

NOTE: This paragraph is not intended to require alerting for all instances 

of automatic autopilot disengagement. It is intended only for conditions 

that would, if not addressed, lead to disengagement that could result in a 

significant transient (or greater) for which the pilot may be unprepared. 

The intent is to provide crew awareness that would allow the flightcrew to 

be prepared with hands on controls and take appropriate corrective action 

before the condition results in a potentially hazardous airplane 

configuration or state. 
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NOTE: This paragraph describes alerting requirements for conditions 

resulting in unintended out–of–trim operation. There are FGS functions 

that can intentionally produce out-of-trim conditions. Examples would be 

parallel rudder operation in align or engine failure compensation modes, 

pitch trim operation during the approach/landing to provide trim up/flare 

spring bias, and pitch trim operation for certain types of speed/Mach trim 

systems. It is not the intent of this paragraph to require alerts for functions 

producing intentional out–of–trim conditions. Other system indications 

(for example, mode and status annunciations) should be provided to make 

the crew aware of the operation of these functions, where appropriate. 

46. FGS CONSIDERATIONS FOR HEAD UP DISPLAYS (HUD). 

a. General. HUDs have unique characteristics compared to flight displays installed 

on the instrument panel. Most of these HUD differences are addressed during HUD 

certification whether or not the HUD provides flight guidance functions. The intent of 

this chapter is to address how such HUD differences may affect FGS functions. (See 

SAE ARP5288, Transport Category Airplane Head Up Display Systems, for further 

information on this subject. An address for obtaining SAE ARP documents is provided 

in paragraph 4 of this AC.) 

b. Characteristics of HUD Guidance.

 (1) Supplemental Use Display System. If the HUD is designed as a 

supplemental use display system, it does not replace the requirement for standard head 

down display (HDD) of flight instrument data. The HUD is intended for use during 

takeoff, climb, cruise, descent, approach, and landing under day, night, visual 

meteorological conditions (VMC), and instrument meteorological conditions (IMC) 

conditions. When it can be reasonably expected that the pilot will operate primarily by 

reference to the HUD, it should be shown that the HUD is satisfactory for manually 

controlling the airplane and for monitoring the performance of the FGS. 

 (2) External Visual References. During takeoff and landing in certain light and 

visibility conditions, HUD symbology can be extremely dominant in comparison to 

external visual references. When visual references are relatively dim, extremely active 

symbology dynamics and guidance cue gains can lead the pilot to make excessively 

strong corrections. It should be shown that—if HUD guidance cues are followed 

regardless of the appearance of external visual references—they do not cause the pilot to 

take unsafe actions. 

 (3) Interference and Occlusion. It should be shown that there is no interference 

between the indications of primary flight information and the flight guidance cues. In 

takeoff, approach, and landing FGS modes, the flight guidance symbology should have 

occlusion priority. That is, the flight guidance symbology should not be obscured or 

covered by the primary flight information. The HUD guidance symbology should not 
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excessively interfere with pilots’ forward view, ability to visually maneuver the airplane, 

acquire opposing traffic, and see the runway environment. 

 (4) Display Criteria. Generally, the criteria for the mechanization of guidance 

displayed on the HUD should be no different than for guidance displayed on the HDD. 

See Chapter 5, Performance of Function, for FD performance criteria. 

 (5) Conformal Symbology. Unlike HDDs, HUDs are capable of displaying 

certain symbology conformal to the outside scene, including guidance cues. 

Consequently, the range of motion of this conformal symbology, including the guidance 

cues, can present certain challenges in rapidly changing and high crosswind conditions. 

In certain cases, the motion of the guidance and the primary reference cue may be limited 

by the field of view. It should be shown that, in such cases, the guidance remains usable 

and that there is a positive indication that it is no longer conformal with the outside scene. 

 (6) Low Visibility Approach. The HUD guidance is often used in cases, like the 

low visibility approach, where the pilot will need to reference both the information 

displayed on the HUD and outside references. Consequently, it should be shown that the 

location and presentation of the HUD information does not distract the pilot or obscure 

the pilot’s outside view. For example, it would be necessary for the pilot to track the 

guidance to the runway without having the view of runway references or hazards along 

the flight path obscured by the HUD symbology. 

 (7) Clutter. The HUD display should present flight guidance information in a 

clear and unambiguous manner. Display clutter should be minimized. Some flight 

guidance data elements are essential or critical and should not be removed by any declutter function. 

c. HUD/Head Down Display (HDD) Compatibility. 

 (1) General. The HUD FGS symbology should be compatible and consistent 

with symbology on other FGS displays such as head down electronic flight instruments. 

The FGS-related display parameters should be consistent to avoid misinterpretation of 

similar information, but the display presentations need not be identical. The HUD and 

head down PFD formats and data sources need to be compatible to ensure that the same 

FGS-related information presented on both displays have the same intended meaning. 

 (2) Consistency and Compatibility. While not all information displayed on the 

HUD is directly related to the FGS, the pilot is likely to use most of the displayed 

information while using the HUD-displayed guidance and FGS annunciations. 

Therefore, when applicable, the guidelines below for the presentation of FGS-related 

display information should be followed as much as possible. Certain deviations from 

these guidelines may be appropriate due to conflict with other information display 

characteristics or requirements unique to HUDs. These may include minimization of 

display clutter, minimization of excessive symbol flashing, and the presentation of certain 

information conformal to the outside scene. 
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 (a) Symbology. Symbols should be the same format. For example, a 

triangle-shaped pointer head down should appear as a triangle pointer head up. However, 

some differences in HUD symbology, such as the FD “circle” versus head down FD 

“bars” or “wedge,” have been found acceptable. 

 (b) Location. Information (symbols) should appear in the same general 

location relative to other information. 

 (c) Alphanumeric Data. Alphanumeric readouts should have the same 

resolution, units, and labeling. For example, the command reference indication for 

“vertical speed” should be displayed in the same foot-per-minute increments and be 

labeled with the same characters as the HDDs. 

 (d) Analog Scales. Analog scales or dials should have the same range and 

dynamic operation. For example, a glideslope deviation scale displayed head up should 

have the same displayed range as the glideslope deviation scale displayed head down, and 

the direction of movement should be consistent. 

 (e) Display of FGS Modes. The FGS modes and status state transitions 

should be displayed on the HUD using consistent methods, except for the use of color. 

That is, the method used head down to indicate a FD mode transitioning from armed to 

captured should also be used head up. 

 (f) Information Sources. Information sources should be consistent between 

the HUDs and the HDDs used by the same pilot. 

 (g) Display of FGS Command Information. When FGS command 

information (for example, a FD command) is displayed on the HUD in addition to the 

HDDs, the HUD depiction and guidance cue deviation “scaling” needs to be consistent 

with that used on the HDDs. This is intended to provide comparable pilot performance 

and workload when using either HUDs or HDDs. 

 (h) Display for Pilot Not Flying. The same information concerning current 

HUD system mode, reference data, status state transitions, and alert information that is 

displayed to the pilot flying on the HUD should also be displayed to the pilot not flying 

on the HDDs. Consistent nomenclature should be used to ensure unambiguous 

awareness of the HUD operation. 

d. Alerting Issues.

 (1) Alerts Displayed on HUDs. Although HUDs are typically not intended to be 

classified as integrated caution and warning systems, they may display cautions, 

warnings, and advisories as part of their FGS function. In this regard, HUDs should 

provide the equivalent alerting functionality as the head down PFD(s). Warnings that 

require continued flightcrew attention on the PFD also should be presented on the HUD 
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(for example, TCAS, windshear, and ground proximity warning (GPWS) annunciations). 

If master alerting indications are not provided within the peripheral field of view of the 

pilot while using the HUD, the HUD should provide annunciations that inform the pilot 

of caution and/or warning conditions. 

 (2) Monochrome HUDs. For monochrome HUDs, appropriate use of attentiongetting properties, such as flashing, outline boxes, brightness, size, and/or location are 

necessary to adequately compensate for the lack of color normally assigned to distinguish 

and call attention to cautions and warnings. 

 (3) Multi–Color HUDs. For multi-color HUDs, the use of red, amber, or yellow 

for symbols not related to caution and warning functions should be avoided, so that the 

effectiveness of distinguishing characteristics of true cautions and warnings is not 

reduced. 

 (4) Single HUD Installation. Single HUD installations rely on the fact that the 

pilot not flying (PNF) will monitor the head down instruments and alerting systems for 

failures of systems, modes, and functions not associated with PFDs. 

 (5) Dual HUD Installations. Dual HUD installations require special 

consideration for alerting systems. It should be assumed that both pilots will be head up 

simultaneously (full or part-time), especially when the HUD is being used as the primary 

flight reference or when the HUD is required equipment for the operation being 

conducted. If master alerting indications are not provided within the peripheral field of 

view of each pilot while using the HUD, then each HUD should provide annunciations 

that direct the pilot’s attention to head down alerting displays. The types of information 

that should trigger the HUD master alerting display are any cautions or warnings not 

already duplicated on the HUD from head down primary display as well as any caution 

level or warning level engine indications or system alerts. 

NOTE: The objective is to avoid redirecting attention of the pilot flying 

(PF) to other display when an immediate maneuver is required (for 

example, TCAS resolution advisory, windshear). 

 (6) Ground Proximity Warning System (GPWS)/Traffic Alert and Collision 

Avoidance System (TCAS) on HUD. If a GPWS, wind shear detection system, wind 

shear escape guidance system, or a TCAS is installed, then the guidance, warnings and 

annunciations required to be a part of these systems and normally required to be in the 

pilot’s primary field of view should be displayed on the HUD. 

e. Upset/Unusual Attitude Recovery Guidance. 

 (1) Guidance Cues. Upsets due to wake turbulence or other environmental 

conditions may result in near instantaneous excursions in pitch and bank angles and a 

subsequent unusual attitude. If the HUD is designed to provide guidance for recovery 

from upsets or unusual attitudes, recovery steering guidance commands should be distinct 
7/17/06 AC 25.1329-1B 

 

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from and not confused with orientation symbology such as horizon “pointers.” For 

example, a cue for left control wheel/stick input should not be confused with a cue 

indicating direction to the nearest horizon. Guidance should be removed if cues become 

invalid at extreme attitudes, such as zenith, nadir, overbanked (for example, greater than 

90 degrees bank angle), or inverted. For extreme attitudes, it is acceptable for the pilot to 

transition to the HDD, provided that the cues to transition from the HUD are clear and 

unambiguous. 

 (2) Orientation Cues. If the HUD is designed to provide only orientation during 

upsets or unusual attitudes (that is, no guidance cues are provided), those orientation cues 

should be designed to prevent them from being mistaken as flight control input 

commands. 

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CHAPTER 5 PERFORMANCE OF FUNCTION 

52. GENERAL. 

a. Intended Function. The FGS should provide guidance or control, as appropriate, 

for the intended function of the active mode(s) in a safe and predictable manner within 

the airplane’s normal flight envelope. There are special considerations when the FGS is 

operating at the limits of its performance capabilities or under significant environmental 

conditions. The following paragraphs provide criteria for acceptable means of 

compliance and interpretive material pertaining to these considerations. 

b. Effect of System Tolerances. Where system tolerances have a significant effect 

on autopilot authority limits, consideration should be given to the effect on autopilot 

performance. Factors to be considered include, but are not limited to, tolerances of servo 

authority, servo clutch setting, “cam-out” settings, control friction, and sensor tolerances. 

53. NORMAL PERFORMANCE. 

a. General. The FGS should be designed to provide guidance or control, as 

appropriate, for the intended function of the active mode(s) in a safe and predictable 

manner within the airplane’s normal flight envelope. 

b. Conditions to Consider. The design of the FGS should consider the normal 

conditions listed in the following table. This table does not fully define every condition 

that may be encountered during an airplane’s life and unequivocally categorize what can 

be considered to be a “normal condition.” Rather, the table is intended to give examples 

to be used during system development and testing. By the very nature of the phenomena 

involved, there will always be some subjectivity to these categorizations. Also, the same 

conditions may affect different airplane models in very different ways. These differences 

should be considered in determining how to characterize the severity of the conditions 

discussed in the table. 
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Table 5-1 

Examples of Normal Conditions 

No failure conditions All airplane systems that are associated with airplane 

performance are fully operational. Failures of those systems 

could impair the flight guidance system's (FGS) ability to 

perform its functions. 

Light to moderate 

winds 

Constant wind in a specific direction that may cause a slight 

deviation in intended flight path 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—which may cause slight erratic or unpredictable 

changes in intended flight path. 

Light to moderate 

gusts 

Non–repetitive momentary changes in wind velocity that can 

cause changes in altitude and/or attitude to occur, but the aircraft 

remains in positive control at all times. 

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 14 CFR Part 25, Appendix C, 

with the exception of “asymmetric icing” discussed under “Rare 

Normal Conditions” in Table 5-2. 

NOTE: Representative levels of the environmental effects should be 

established consistent with the airplane’s intended operation. 
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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, reduced vertical separation minimums (RVSM), required navigation 

performance (RNP)). 

 (5) Equivalence of Performance. The FGS performance of intended functions 

should at least be equivalent to that expected of a pilot for a similar task. AC 25-7A, 

Flight Test Guide for Certification of Transport Category Airplanes, and the SAE 

ARP5366, Autopilot, Flight Director and Autothrust Systems, may prove useful for 

establishing the general behavior of the FGS. When integrated with navigation (NAV) 

sensors or the FMS, the FGS should satisfy the flight technical error tolerances expected 

for the use of those systems in performing their intended functions. 

 (6) Autopilot. The autopilot should provide smooth and accurate control 

without divergent or perceptible sustained nuisance oscillation. 

 (7) Flight Director (FD). The FD, in each available display presentation (for 

example, single cue, cross–pointer, flight path director) should provide smooth and 

accurate guidance and be appropriately damped, to achieve satisfactory control task 

performance without pilot compensation or excessive workload. 

 (8) Autothrust. The autothrust function should provide smooth and accurate 

control of thrust without significant or sustained oscillatory power changes or excessive 

overshoot of the required power setting. 

 (9) Automatic Trim. The automatic pitch trim function should operate at a rate 

sufficient to mitigate excessive control surface deflections or limitations of control 
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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 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 following table. 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. 
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Table 5-2 

Examples of Rare Normal Conditions 

Significant winds Constant wind in a specific direction that may cause a 

large change in intended flight path or groundspeed or 

cause a large difference between airspeed and 

groundspeed. 

Significant wind 

gradients 

Variation in wind velocity—as a function of altitude, 

position, or time—which may cause large changes in 

intended flight path. 

Windshear/microburst A wind gradient of such magnitude that it may cause 

damage to the aircraft. 

Large gusts Non-repetitive momentary changes in wind velocity that 

can cause large changes in altitude and/or attitude to occur. 

Aircraft may be momentarily out of control. 

Severe turbulence Turbulence that causes large, abrupt changes in altitude or 

attitude. It usually causes large variations in indicated 

airspeed. Aircraft may be momentarily out of control. 

Asymmetric icing Icing conditions that result in ice accumulations that 

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 § 121.358 for windshear warning and 

guidance need FD windshear guidance. The FGS may also provide 

suitable autopilot control during windshear. Refer to AC 25-12 and 

AC 120-41 for wind shear guidance system requirements. 
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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, FGS Engagement, 

Disengagement, Indications, and Override, paragraph 27b, Autopilot Disengagement, and 

the criteria for flight guidance alerting in Chapter 4, Controls, Indications, and Alerts, 

paragraph 45d, Awareness of Potential Significant Transient Condition (“Bark Before 

Bite”). 

 (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. See Chapter 4, 

paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status, for discussion of 

alerting under such conditions. 

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. 
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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 57b, Low Speed Protection, of this 

chapter. 

56. PERFORMANCE IN NON-NORMAL CONDITIONS. 

a. General. The FGS will occasionally be operating when the airplane transitions 

outside of its normal flight envelope, due to failures or non-standard ferry configurations. 

Under such circumstances, the FGS characteristics and flightcrew interaction with the 

FGS should be shown to be safe. 

b. Conditions to Consider. The following table 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. 
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Table 5-3 

Examples of Non-Normal Conditions 

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.

Non-standard ferry flight 

configurations 

Possible aerodynamic drag (both symmetrical and 

unsymmetrical) caused by non-standard 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 which 

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. 
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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, FGS Engagement, 

Disengagement, Indications, and Override, paragraph 27b, Autopilot Disengagement, and 

the criteria for flight guidance alerting in Chapter 4, Controls, Indications, and Alerts, 

paragraph 45d, Awareness of Potential Significant Transient Condition (“Bark Before 

Bite”). 

 (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. See Chapter 4, 

paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status, for discussion of 

alerting under such conditions. 

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: 
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 (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. 

See Chapter 4, Controls, Indications, and Alerts, paragraph 45b, Speed 

Protection Alerts, for additional discussion of speed protection alerting. 

 (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. 
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b. Low Speed Protection. 

 (1) General. When the FGS is engaged in any modes (with the possible 

exception of Approach, as discussed in paragraph (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 drift–down speed 

schedule, if the FGS (or other integrated sensors/systems) can determine 

that the thrust deficiency is due to an engine failure. 

 (c) Approach. 

 (d) Transition from approach to go–around and go–around climb. 

NOTE: A low speed alert and a transition to the Speed Protection mode 

at approximately 1.13 VSR (reference stall speed) for the landing flap 

configuration has been found to be acceptable. 

 (3) Low Speed Protection During Approach Operations. 

 (a) Non–Interference. Speed protection should not interfere with the 

approach and landing phases of flight. 

 (b) Autothrust Operation. It is assumed that with autothrust operating 

normally, the combination of thrust control and pitch control during the approach will be 

sufficient to maintain speed and desired vertical flight path. In cases where it is not 

sufficient, an alert should be provided in time for the flightcrew to take appropriate 

corrective action. 

 (c) Defined Vertical Path. For approach operations with a defined vertical 

path (for example, ILS, microwave landing system (MLS), GNSS landing system (GLS), 
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Lateral Navigation (LNAV) mode, VNAV mode), if the thrust is insufficient to maintain 

both the desired flight path and the desired approach speed, there are several ways to 

meet the intent of low speed protection: 

 1 The FGS may maintain the defined vertical path as the airplane 

decelerates below the desired approach speed until the airspeed reaches the low speed 

protection value. At that time, the FGS would provide guidance to maintain the low 

speed protection value as the airplane departs the defined vertical path. The FGS mode 

reversion and low speed alert should be activated to ensure pilot awareness. 

NOTE: The pilot is expected to take corrective action to add thrust and 

return the airplane to the defined vertical path or go–around, as necessary. 

 2 The FGS may maintain the defined vertical path as the airplane 

decelerates below the desired approach speed to the low speed protection value. The 

FGS will then provide a low speed alert while remaining in the existing FGS approach 

mode. 

NOTE: The pilot is expected to take corrective action to add thrust to 

cause the airplane to accelerate back to the desired approach speed while 

maintaining the defined vertical path or go–around, as necessary. 

 3 The FGS may maintain the defined vertical path as the airplane 

decelerates below the desired approach speed until the airspeed reaches the low speed 

protection value. The FGS will then provide a low speed alert and disengage. 

NOTE: The pilot is expected to take corrective action when alerted to the 

low speed condition and the disengagement of the autopilot, to add thrust, 

and to manually return the airplane to the desired vertical path or go–

around. 

 (d) Vertical Flight Path Not Protected. If the speed protection is invoked 

during approach such that vertical flight path is not protected, the subsequent behavior of 

the FGS after speed protection should be carefully considered. Activating low speed 

protection during the approach, resuming the Approach mode, and re–acquiring 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 120-28D, 

Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout, 

appendix 3, section 8.1, Automatic Flight Control Systems, which states that it should not 

be possible to change the flight path of the airplane with the autopilot(s) engaged, except 

by initiating an automatic go–around. 

 (4) Windshear Recovery Guidance. The interaction between low speed 

protection and windshear recovery guidance is a special case. Windshear recovery 

guidance that meets the criteria found in AC 25-12 and AC 120-41 provides the 
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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 windshear 

recovery guidance activates, unless autopilot operation has been shown to be safe in these 

conditions and provides effective automatic windshear 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 highspeed 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, six 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). 

 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, six knots), 
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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, six 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] 
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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, 

Controls, Indications, and Alerts, paragraph 44, FGS Mode Selection, Annunciation, and 

Indication. 

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 five 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. Lateral Navigation (LNAV) Mode . 

 (1) Lateral Flight Path. In the LNAV mode, the FGS should acquire and 

maintain the lateral flight path 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 
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engaged state at a point where the lateral flight path can be smoothly acquired and 

tracked. 

 (3) Takeoff or Go–Around (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. 

64. VERTICAL MODES. This paragraph discusses modes that are implemented in 

many FGSs that are used primarily for pitch control of the airplane. The criteria 

identified reflect operational experience gained from the use of these modes. 

a. Target Altitude Selection. To avoid unconstrained climbs or descents for any 

altitude transitions when using applicable vertical modes, the altitude select controller 

should be set to a new target altitude before the vertical mode can be selected. If the 

design allows the vertical mode to be selected before setting the target altitude, then 

consideration should be given to the potential vulnerability of unconstrained climb or 

descent leading to an altitude violation or controlled flight into terrain. Consideration 

should also be given to appropriate annunciation of the deviation from previously 

selected altitude and/or subsequent required pilot action to reset the selected altitude. 

b. Vertical Speed. In the Vertical Speed mode, the FGS should smoothly acquire 

and maintain a selected vertical speed. Consideration should be given to the situation 

where the selected value is outside the performance capability of the airplane. Another 

situation to consider is use of Vertical Speed mode without autothrust. These situations 

could potentially lead to a low-speed or high-speed condition and corresponding pilot 

awareness vulnerabilities. See Chapter 5, paragraph 57, Speed Protection, for discussion 

of acceptable means of compliance for these situations. 

c. Flight Path Angle. In the Flight Path Angle mode, the FGS should smoothly 

acquire and maintain the selected flight path angle. Consideration should be given to the 

situation where the selected value is outside the performance capability of the airplane. 

The use of Flight Path Angle mode without autothrust—potentially leading to a lowspeed or high-speed condition and corresponding pilot awareness vulnerabilities—should 

also be considered. Acceptable means of compliance have included a reversion to an 

envelope protection mode or a timely annunciation of the situation. 

d. Indicated Airspeed (IAS)/Mach Hold [speed on elevator]. In the Airspeed/Mach 

Hold mode, the FGS should maintain the airspeed or Mach at the time of engagement. 

e. Indicated Airspeed (IAS)/Mach Select [speed on elevator]. In the Airspeed/Mach 

Select mode, the FGS should acquire and maintain a selected airspeed or Mach. The 
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selected airspeed or Mach may be either pre–selected or synchronized to the airspeed or 

Mach at the time of engagement. 

f. Flight Level (FL) Change [speed on elevator]. In the FL Change mode, the FGS 

should change altitude in a coordinated way with thrust control on the airplane. The 

autopilot/FD will typically maintain speed control through the elevator. The autothrust 

function, if engaged, should control the thrust to the appropriate value for climb or 

descent.

g. Altitude Capture.

 (1) Mode Transition. The Altitude Capture mode should command the FGS to 

transition from a vertical mode to smoothly capture and maintain the selected target 

altitude with consideration of the rates of climb and descent experienced in service. 

 (2) Guidelines for Altitude Capture Mode. In–service experience has shown 

that certain implementations have the potential to cause pilot confusion that may lead to 

altitude violations. Accordingly, the following are guidelines for the Altitude Capture 

mode. 

 (a) Automatic Arming of Mode. The Altitude Capture mode should be 

automatically armed to ensure capture of the selected altitude. Annunciation of the 

armed status is not required if the Altitude Capture mode is armed at all times. If the 

FGS is in the Altitude Capture mode, it should be annunciated. 

 (b) Engagement From Any Vertical Mode. The Altitude Capture mode 

should engage from any vertical mode if the computed flight path will intercept the 

selected altitude and the altitude capture criteria are satisfied, except as specified during 

an approach (for example, when the glidepath for Approach mode is active). 

 (c) Changing Climb/Descent Command References. Changes in the 

climb/descent command references (for example, the vertical speed command reference), 

with the exception of those made by the flightcrew using the altitude select controller, 

should not prevent capture of the target altitude. 

 (d) Capturing Selected Altitude. The Altitude Capture mode should 

smoothly capture the selected altitude, using an acceptable acceleration limit and pitch 

attitude with consideration for occupant comfort. 

 (e) Minimizing Acceleration Overshoot. The acceleration limit may, under 

certain conditions, result in an overshoot. To minimize the altitude overshoot, the normal 

acceleration limit may be increased, consistent with occupant safety. 

 (f) Selecting Other Vertical Modes. Pilot selection of other vertical modes 

at the time of altitude capture should not prevent or adversely affect the level off at the 

target altitude. One means of compliance is to inhibit transition to other pilot–selectable 
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vertical modes (except Altitude Hold, Go–Around, and Approach modes) during altitude 

capture, unless the target altitude is changed. If glidepath capture criteria are satisfied 

during altitude capture, then the FGS should transition to glidepath capture. 

 (g) Changing Target Altitude. The FGS must be designed to minimize 

flightcrew confusion concerning the FGS operation when the target altitude is changed 

during altitude capture. [See § 25.1329(i)] It should be suitably annunciated and 

appropriate for the phase of flight. 

 (h) Barometric Pressure Adjustment. Adjusting the datum pressure at any 

time during altitude capture should not result in loss of the capture mode. The transition 

to the pressure altitude should be accomplished smoothly. 

 (i) Maintaining Reference Airspeed. If the autothrust function is active 

during altitude capture, the autopilot and autothrust functions should be designed such 

that the FGS maintains the reference airspeed during the level-off maneuver. For 

example, if the autopilot changes from a speed mode to an altitude capture or control 

mode, then the autothrust should transition to a speed mode to maintain the reference 

airspeed. 

h. Altitude Hold.

 (1) Entering Mode. The Altitude Hold mode may be entered either by 

flightcrew selection or by transition from another vertical mode. 

 (a) Pilot Selection. 

 1 Level flight. When initiated by pilot action in level flight, the 

Altitude Hold mode should provide guidance or control to maintain altitude at the time 

the mode is selected. 

 2 Climbing or descending. When initiated by pilot action when the 

airplane is either climbing or descending, the FGS should immediately initiate a pitch 

change to arrest the climb or descent and maintain the altitude when level flight (for 

example, less than 200 feet per minute) is reached. The intensity of the leveling 

maneuver should be consistent with occupant comfort and safety. 

 (b) Automatic Transition. When initiated by an automatic transition from 

Altitude Capture, the Altitude Hold mode should provide guidance or control to the 

selected altitude. 

 (2) Mode Transition Annunciation. Automatic transition into the Altitude Hold 

mode from another vertical mode should be clearly annunciated for flightcrew awareness. 

 (3) Barometric Pressure Adjustment. Any airplane response due to an 

adjustment of the datum pressure should be smooth. 
7/17/06 AC 25.1329-1B 

 

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i. VNAV.

 (1) FMS Path. 

 (a) Acquire Vertical Path. In the VNAV mode, the FGS should acquire 

and maintain the vertical flight path commanded by a flight management function (that is, 

FMS or equivalent). If the airplane is not on the desired FMS path when the VNAV 

mode is selected, the FGS VNAV mode should go into an armed state or provide 

guidance to smoothly acquire the FMS path. The flightcrew should establish the airplane 

on a flight profile to intercept the desired FMS path. The FGS should transition from the 

armed state to an engaged state at a point where the FGS can smoothly acquire and track 

the FMS path. 

 (b) Deviation from Vertical Path. If the aircraft is flying a vertical path (for 

example, VNAV path), then the deviation from that path should be displayed in the 

primary field of view, such as the PFD, navigation display (ND), or other acceptable 

display. 

 (2) Climb or Descent. 

 (a) Autothrust Function. When VNAV is selected for climb or descent, the 

autothrust function (if installed and engaged) should maintain the appropriate thrust 

setting. When leveling after a VNAV climb or descent, the autothrust function should 

maintain the target speed. 

 (b) Preclude VNAV Climb. The FGS should preclude a VNAV climb, 

unless the mode select panel (MSP) altitude window is set to an altitude above the current 

altitude. 

 (c) Preclude VNAV Descent. The FGS should preclude a VNAV descent, 

unless the MSP altitude window is set to an altitude below the current altitude, except 

when on a final approach to a runway. 

 (d) Preclude MSP Altitude Fly–Through. The FGS should not allow the 

VNAV climb or descent to pass through a MSP altitude, except when on a final approach 

to a runway. 

NOTE: See paragraph 67, Special Considerations for VNAV Approach 

Operations Related to Selecting a Target Altitude. 
7/17/06 AC 25.1329-1B 

 

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65. MULTI-AXIS MODES. This paragraph discusses modes that are implemented in 

many FGSs that are used in an integrated manner for pitch, lateral/directional control, and 

thrust management of the airplane. The criteria identified reflect operational experience 

gained from the use of these modes. 

a. Takeoff Mode.

 (1) Vertical Guidance. In the Takeoff mode, the vertical element of the FGS 

should provide vertical guidance to acquire and maintain a safe climb out speed after 

initial rotation for takeoff. 

 (2) Lateral Guidance. In the Takeoff mode, the lateral element of the FGS, if 

implemented, should maintain runway heading/track or wings level after liftoff. A 

separate lateral mode annunciation should be provided. 

 (3) Rotation Guidance. If rotation guidance is provided, the use of the guidance 

should not result in a tail strike. It should be consistent with takeoff methods necessary 

to meet takeoff performance requirements up to 35 feet above ground level (AGL). If no 

rotation guidance is provided, the pitch command bars may be displayed during takeoff 

roll but should not be considered as providing rotation guidance, unless it is part of the 

intended function. 

 (4) Autothrust. The autothrust function should increase and maintain engine 

thrust to the selected thrust limits (for example, full takeoff thrust, de-rate). 

 (5) Operation and Performance. The FGS design should address all engine and 

engine-inoperative conditions consistent with the following takeoff system performance 

characteristics after liftoff. 

 (a) Transitions Between Flight Phases. Takeoff system operation should be 

continuous and smooth through transition from the runway portion of the takeoff to the 

airborne portion and reconfiguration for en route climb. The pilot should be able to 

continue the use of the same primary display(s) for the airborne portion as for the runway 

portion. Changes in guidance modes and display formats should be automatic. 

 (b) Pitch Attitude and Climb Speed/Normal Operation. The vertical axis 

guidance of the takeoff system during normal operation should result in the appropriate 

pitch attitude and climb speed for the airplane considering the following factors: 

 1 Normal rate rotation of the airplane to the commanded pitch attitude 

at VR (takeoff rotation speed) - 10 knots for all engines operative and VR - 5 knots for 

engine out should not result in a tail-strike. 

 2 The system should provide commands that lead the airplane to 

smoothly acquire a pitch attitude that results in capture and tracking of the all–engine 

takeoff climb speed, V2 (takeoff safety speed) + X. X is the all-engine speed additive 
7/17/06 AC 25.1329-1B 

 

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from the AFM (normally 10 knots or higher). If pitch limited conditions are encountered, 

a higher climb airspeed may be used to achieve the required takeoff path without 

exceeding the pitch limit. 

 (c) Engine–Out Operation. For engine-out operation, the system should 

provide commands that lead the airplane to smoothly acquire a pitch attitude that results 

in capture and tracking of the following reference speeds: 

 1 V2, for engine failure at or below V2. This speed should be attained 

by the time the airplane has reached 35 feet altitude. 

 2 Airspeed at engine failure for failures between V2 and V2 + X. 

 3 V2 + X, for failures at or above V2 + X. Alternatively, the airspeed 

at engine failure may be used, provided it has been shown that the minimum takeoff 

climb gradient can still be achieved at that speed. 

 (d) Lateral Commands During Takeoff Mode. If implemented, the lateral 

element of the Takeoff mode should maintain runway heading/track or wings level after 

liftoff and a separate lateral mode annunciation should be provided. 

b. Go-Around Mode. Characteristics of the Go–Around mode and resulting flight 

path should be consistent with a manually flown go-around. 

 (1) Vertical Elements. The vertical element of the FGS Go–Around mode 

should initially rotate the airplane or provide guidance to rotate the airplane to arrest the 

rate of descent. 

 (2) Speed. The FGS should acquire and maintain a safe speed during climbout 

and airplane configuration changes. Typically, a safe speed for go-around climb is V2, 

but a different speed may be found safe for windshear recoveries (See AC 25-12). 

 (3) Autothrust. 

 (a) Thrust During Climb. The autothrust function, if installed, should 

increase thrust and either maintain thrust to specific thrust limits or maintain thrust for an 

adequate, safe climb. The autothrust function should not exceed thrust limits (for 

example, full go–around thrust or de–rated go–around thrust limits). Furthermore, the 

autothrust function should not reduce thrust for wind below the minimum value required 

for an adequate, safe climb or reduce the thrust lever position below a point that would 

cause a warning system to activate. 

 (b) Pitch Attitude During Go–Around. The initial go–around maneuver 

may require a significant change in pitch attitude. It is acceptable to reduce thrust to 

lower the pitch attitude for comfort of the occupants when a safe climb gradient has been 

established. It should be possible for the pilot to re-select the full thrust value, if needed. 
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 (4) Engagement. The Go-Around mode should engage when go-around is 

selected by the pilot, even if the MSP selected altitude is at or below the go–around 

initiation point. The airplane should climb until another vertical mode is selected or the 

MSP altitude is adjusted to an altitude above the present aircraft altitude. 

 (5) All Engine and Engine Out Capability. The FGS design of the Go-Around 

mode should address all engine and engine–out operation. The design should consider an 

engine failure resulting in a go-around and the engine failure occurring during an all 

engine go-around. 

c. Approach Mode.

 (1) Final Approach Path. In the Approach mode, the FGS should capture and 

track a final approach lateral and vertical path, if applicable, from a navigation (NAV) or 

landing system, for example, ILS, MLS, GLS, RNP, area navigation (RNAV) (see 

AC 120-28D and AC 120-29A). 

 (2) Mode Annunciations. The FGS should annunciate all operationally relevant 

approach modes. Modes that are armed and waiting for capture criteria to be satisfied 

should be indicated in addition to the active pre-capture mode. A positive indication of 

the capture of the previously armed mode should be provided. 

 (3) Sub–Modes. The FGS may have sub–modes that become active without 

additional crew selection. An assessment of the significance of these sub-mode 

transitions to the flightcrew should be made. If assessed to be significant (for example, 

flare), positive annunciation of the transition should be provided. 

 (4) Mode Engagement Sequence. Glideslope capture mode engagement may 

occur prior to localizer capture. However, it is the flightcrew’s responsibility to ensure 

proper safe obstacle/terrain clearance when following vertical guidance when the airplane 

is not established on the final lateral path. 

NOTE: Additional guidance and criteria are contained in AC 120-29A 

and AC 120-28D. 

66. AUTOTHRUST MODES. This paragraph discusses modes that are implemented in 

many FGSs that are used primarily for controlling the engines on the airplane. The 

criteria identified reflect operational experience gained from the use of these modes. 

a. Thrust Mode. In the Thrust mode, the FGS should command the autothrust 

function to achieve a selected target thrust value. 

b. Speed Mode. In the Speed mode, the FGS should command the autothrust 

function to acquire and maintain the selected target speed value, assuming that the 
7/17/06 AC 25.1329-1B 

 

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selected speed is within the speed range of the normal flight envelope. The autothrust 

system may fly a higher airspeed than the selected target speed during an approach when 

operating in winds or turbulent conditions. 

c. Retard Mode. If a Retard mode is implemented in the FGS, it should work in the 

same manner for both automatic and manual landings when the autothrust function is 

engaged. 

67. SPECIAL CONSIDERATIONS FOR VNAV APPROACH OPERATIONS 

RELATED TO SELECTING A TARGET ALTITUDE. 

a. Approach Operations. The FGS vertical modes should allow the pilot to set the 

target altitude to a missed approach value prior to capturing the final approach segment. 

This should be possible for capturing from both above and below the final approach 

segment. 

b. VNAV Path Operations. It should be possible to define a descent path to the 

final approach fix and another path from the final approach fix to the runway with the 

target altitude set for the missed approach altitude. Appropriate targets and descent 

points should be identified by the FMS. 

68. CONTROL WHEEL STEERING(CWS) [CONTROL STEERING THROUGH THE 

AUTOPILOT]. 

a. General. In the CWS mode, the FGS allows the flightcrew to maneuver the 

airplane manually through the autopilot control path. CWS is considered an FGS mode, 

as it is a specific function of the FGS. However, during CWS operation, the pilot is in 

control of the aircraft, rather than the FGS. Operationally, CWS is identical to the pilot 

flying the airplane during manual flight. In both cases, the pilot is in control of the flight 

path and speed of the airplane. The only difference is the mechanization of how the 

actual flight control surfaces are moved. No automatic FGS commands are involved 

during CWS operation, with the possible exception of automatic aircraft trim function(s). 

Therefore, paragraphs in this AC, such as those which discuss speed protection and 

performance objectives, should be applied only to those autopilot modes with which the 

FGS is in control of the flight path of the airplane. Such paragraphs in this AC should 

not be applied to CWS, unless such features are specifically provided while the FGS is in 

the CWS mode. 

b. Design. If provided, a CWS mode should meet the following: 

 (1) Control Forces. It should be possible for the pilot to maneuver the airplane 

using the normal flight controls with the CWS mode engaged and to achieve the 

maximum available control surface deflection without using forces so high that the 
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controllability requirements of § 25.143(c) are not met. Excessive discontinuities in 

control force that might adversely affect the flight path should not be encountered. 

 (2) Bank and Pitch Attitudes. The maximum bank and pitch attitudes that can 

be achieved, without physically overpowering the autopilot, should be limited to those 

necessary for the normal operation of the airplane. 

NOTE: Normal operational limits are typically 35 degrees in roll and 

+20 degrees to –10 degrees in pitch. 

NOTE: Limits that can be physically overcome with forceful control 

commands by the pilot are sometimes referred to as “soft limits.” 

 (3) Control Behavior. It should be possible to perform all normal maneuvers 

smoothly and accurately without nuisance oscillation. It should also be possible to 

counter all normal changes of trim due to change of configuration or power, within the 

range of flight conditions in which CWS may be used. 

 (4) Stall and Stall Recovery. The stall and stall recovery characteristics of the 

airplane should remain acceptable. It should be assumed that recovery from such a stall 

is made with CWS in use, unless automatic disengagement of the autopilot is provided. 

 (5) Adjustments to Trim. In showing compliance with § 25.143(f), 

consideration should be given to adjustments to trim that may be made by the autopilot in 

the course of maneuvers that can reasonably be expected. Some alleviation may be 

acceptable in the case of unusually prolonged maneuvers, provided that the reduced 

control forces would not be potentially hazardous. 

 (6) Takeoff and Landing. If the use of CWS for takeoff and landing is to be 

permitted, it should be shown that: 

 (a) Sufficient control, both in amplitude and rate, is available without 

encountering force discontinuities. 

 (b) Reasonable mishandling is not hazardous (for example, engaging the 

autopilot while the elevators or ailerons are held in an out-of-trim position). 

 (c) Runaway rates and control forces are such that the pilot can readily 

overpower the autopilot with no significant deviation in flight path. 

 (d) Any lag in aircraft response induced by the CWS mode is acceptable 

for the intended maneuver. 

 (7) Automatic Reversion to CWS Mode. The autopilot, when engaged, should 

not automatically revert to the CWS mode by applying an input to the control column or 

wheel, unless the autopilot is in a capture mode (for example, Altitude Capture or 
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Localizer Capture). When the force is released, the autopilot should return to the 

previously engaged capture mode or to the Track/Heading Hold mode. 

c. Terminology and Application of Advisory Material. The term “CWS” is 

currently used by industry to refer to several different types of systems. Several other 

types of systems that are similar in nature to CWS, but functionally different from it, are 

described below. Paragraph 68b, Design, applies to systems as described in paragraph 

68a, General. It does not apply to those systems described in paragraphs 68c(1) and 

68c(2), below. These descriptions are intended to be generic. Implementation may vary 

from airplane to airplane. 

 (1) Touch Control Steering (TCS). A TCS system is available on many 

business and commuter aircraft. With a TCS system, a pilot is able to physically 

disengage the autopilot servos from the flight control system, usually by pushing and 

holding a button on the control wheel, without causing the autopilot system itself to 

disengage or lose its currently selected modes. The pilot may then maneuver the airplane 

as desired using the aircraft’s flight control system (that is, the autopilot servos are not 

part of the control loop). The pilot is then able to reconnect the autopilot servos to the 

flight control system by releasing the TCS button. Using the new orientation of the 

aircraft as a basis, the autopilot will then reassume control of the airplane using the same 

mode selections as were present before the selection of TCS. This type of system on 

some aircraft is also sometimes referred to as CWS. 

 (2) Supervisory Override. A function available on some aircraft is referred to as 

a “supervisory override” of an engaged autopilot. With this function, a pilot is able to 

physically overpower an engaged autopilot servo by applying force to the flight deck 

controls. With a supervisory override, the autopilot does not automatically disengage due 

to the pilot input. This allows the pilot to position the airplane as desired using the flight 

deck controls without first disengaging the autopilot. When the pilot releases the 

controls, the autopilot reassumes control of the airplane using the same mode selections 

as were present before the supervisory override. 

69. SPECIAL CONSIDERATIONS FOR THE INTEGRATION OF FLY-BY-WIRE 

FLIGHT CONTROL SYSTEMS AND FGS. 

a. Speed Protection. Speed protection features may be implemented in the fly-bywire flight control system. However, if speed protection is also implemented within the 

FGS, it should be compatible with the envelope protection features of the fly–by–wire 

flight control system. The FGS speed protection (normal flight envelope) should operate 

to or within the limits of the flight control system (limit flight envelope). 

b. System Degradation. Information should be provided to the flightcrew about the 

impact on the FGS following degradation of the fly-by-wire flight control systems. 

70 — 74 [RESERVED] 
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CHAPTER 7 FGS INTEGRATION 

75. GENERAL. Throughout the preceding chapters of the document, FGSs and 

functions have been considered as being separate and distinct from other systems and 

functions on the aircraft. It is recognized that in complex aircraft designs, the flight 

guidance functions are closely integrated with other avionics functions and that the 

physical integration of these systems may have a bearing on how airplane level safety is 

assessed. The following paragraphs provide guidance on the likely FGS system 

integration issues found in more complex aircraft system designs and the interfaces that 

should be considered within the bounds of demonstrating the intended function, 

performance, and safety of the FGS. 

76. SYSTEM INTEGRATION ISSUES. 

 a. Common and Cascading Failure Modes. Integration of other aircraft systems 

with the FGS has the potential of reducing the independence of failure effects and 

partitioning between functions. This is particularly the case where hardware and 

software resources are shared by different systems and functions (for example, aircraft 

data network and integrated modular avionics (IMA) architectures). In addition to 

considering the reliability and integrity aspects of the FGS as a separate system, it may be 

necessary to address the effects of FGS failures with respect to fault propagation, 

detection, and isolation within other systems. The overall effect on the aircraft of a 

combination of individual system failure conditions occurring as a result of a common or 

cascade failure, may be more severe than the individual system effect. For example, 

failure conditions classified under § 25.1309 as ”minor” or ”major” by themselves may 

have hazardous effects at the aircraft level when considered in combination. With regard 

to isolation of failures and particularly combination of failures, the ability of the alerting 

system to provide clear and unambiguous information to the flightcrew becomes of 

significant importance. (See also Chapter 8, Safety Assessment.) 

 b. Risk of Error. Complex and highly integrated avionics systems present greater 

risk for development error. With non-traditional human-machine interfaces, there is also 

the potential for operational flightcrew errors. Moreover, integration of systems may 

result in a greater likelihood of undesirable and unintended effects. 

 c. System Safety Analysis. When credit is taken for shared resources or partitioning 

schemes, these should be justified and documented within the system safety analysis. 

When considering the functional failures of the system, where such partitioning schemes 

cannot be shown to provide the necessary isolation, possible combination failure modes 

should be taken into account. An example of this type of failure would be multi–axis 

active failures, where the control algorithms for more than one axis are hosted on a single 

processing element. Further, the functional integration of control functions such as 
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control surface trimming, yaw channel, and stability augmentation, while not strictly 

FGS, should be considered. 

77. FUNCTIONAL INTERFACES. 

 a. Interfacing Sensors. In its simplest form, the FGS may be considered as 

interfacing with sensors that provide the necessary inputs to enable computation of its 

various functions. Typically, these sensors will include air and inertial data, engine 

control, and NAV sensors such as ILS, very high frequency omni range (VOR), and 

distance measuring equipment (DME). In the case of engine control, a feedback loop 

may also be provided. The FGS may also be considered as providing inner loop closure 

to outer loop commands. The most common interface is with the FMS, which provides 

targets for LNAV and VNAV in the form of steering orders. 

 b. Potential Inconsistencies Between Systems. In demonstrating the intended 

function and performance of both the FGS and systems providing outer loop commands, 

the applicant needs to address potential inconsistencies between limits provided by the 

two different systems. One example of this is possible conflicts between FMS steering 

commands with basic FGS pitch and bank angle limits. Failure to address these points 

can result in discontinuities, mode switching, and reversions, leading to erroneous NAV 

and other possible safety issues (for example, buffet margin at high altitude). Similar 

issues arise in the inner loop, across the functional interface between FGS and flight 

controls. In fly–by–wire aircraft, the loss of synchronization between the two can result 

in mode anomalies and autopilot disengagement. 

 c. Function and Performance. The applicant should demonstrate the intended 

function and performance of the FGS across all possible functional interfaces. The 

alerting system should also be assessed to ensure that accurate and adequate information 

is provided to the flightcrew when dealing with failures across functional interfaces. 

78 — 82 [RESERVED] 
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CHAPTER 8 SAFETY 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. 
7/17/06 AC 25.1329-1B 

 

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 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. 
7/17/06 AC 25.1329-1B 

 

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 (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, 
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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, 

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 be identified. It 
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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 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. 
7/17/06 AC 25.1329-1B 

 

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NOTE: ACs 120-29A and AC 120-28D 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 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 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, Characteristics of Specific Modes). 

 2 The FGS failure conditions should 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 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. 

 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. 
7/17/06 AC 25.1329-1B 

 

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 (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 operations 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 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/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. 
7/17/06 AC 25.1329-1B 

 

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 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 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 flight path 

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, 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. 
7/17/06 AC 25.1329-1B 

 

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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 25.1309-1A. [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. 

90 — 97 [RESERVED] 
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7/17/06 AC 25.1329-1B 

 

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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 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. 
7/17/06 AC 25.1329-1B 

 

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 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 120-29A, Criteria for Approval of Category I and II Weather 

Minima for Approach. 

 (b) AC 120-28D, Criteria for Approval 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 25-7A, 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. 
7/17/06 AC 25.1329-1B 

 

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Figure 9-1 

Relationship Between AC Chapters 

Analysis

Safety 

Assessment

CHAPTER 

8 

 CHAPTER 

 10 

 AFM

Flight Test

Simulator

Failure 

Conditions

Analysis 

Only

Flight 

Validation

FGS Function

 CHAPTERS 3 - 7 

CHAPTER 

9

Flight Test 

 & 

Simulation 

FGS

AC 25-7A 

 & 

Appendix 2

Test 

Methods

Test 

Criteria
7/17/06 AC 25.1329-1B 

 

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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 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. Advisory 

Circular 25-7A, Flight Test Guide for Certification of Transport Category Airplanes, may 

provide 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. 

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 flight paths for capture modes. 

 (d) Consistency of integration of modes (of this chapter). 
7/17/06 AC 25.1329-1B 

 

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 (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, 

paragraphs 54, Performance in Rare Normal Conditions, and 56, Performance in 

Non-Normal Conditions. 

 (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 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 windshear, 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 Windshear. If the FGS provides windshear escape 

guidance, performance demonstration requirements should be conducted consistent with 

AC 25-12, Airworthiness Criteria for the Approval of Airborne Windshear Warning 

Systems in Transport Category Airplanes. 

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 
7/17/06 AC 25.1329-1B 

 

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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. See Chapter 5, paragraph 54e, 

Icing Considerations in Normal and Rare Normal Conditions, for more information. 

 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 non–contaminated airplane or testing 

indicates marginal performance, additional tests may be necessary. 

 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. See Chapter 3, 

paragraph 27b, Autopilot Disengagement, and Chapter 4, paragraph 45d, Awareness of 

Potential Significant Transient Condition (“Bark Before Bite”), for more information. 

 (b) De-icing. If the airplane is configured with a de-icing system, the 

autopilot should demonstrate satisfactory performance during the shedding of ice from 

the airplane. 

 (c) Airplane Flight Manual (AFM) Limitations. Where degradation is 

noted which is not significant enough to require changes to the autopilot system or to 

de-icing 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. 
7/17/06 AC 25.1329-1B 

 

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 (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 65b, Go-Around Mode) is to quickly change the flight path 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 120-28D and AC 120-29A. 

 (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 flight path. The acceptability of the operation if contact 

is made with the runway during the missed approach or balked landing should be 

established. 

 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 de–rates with selection of 

Go-Around mode. 

 (ee) An engine failure at the initiation of go-around. 
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 (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 2.77 and 3.75 degrees. Application for approval to 

conduct operations on glidepath angles of greater than 3.75 degrees requires additional 

evaluation. See AC 120-29A, 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. 

 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 25-7A, Flight Test Guide for 

Certification of Transport Category Airplanes, 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 120-29A, Criteria for 
7/17/06 AC 25.1329-1B 

 

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Approval of Category I and II Weather Minima for Approach, 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 45d, Awareness of 

Potential Significant Transient Condition (“Bark Before Bite”)). 

e. Flight Director (FD)/Head Up Display (HUD). 

 (1) General. The guidance aspect of an FGS may be provided by a head down 

FD or a HUD. Head down displays (HDD) normally utilize FD guidance cues that 

provide steering commands—such as a cross pointer or single “V”—for the pilot to 

follow. HUDs normally display a flight path vector that represents the instantaneous 

flight path. The vector is based on the aircraft energy state and directional vectors. 

Various new display media are evolving (for example, enhanced vision system (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. Flight directors designed to work 

with a non-stationary tracking reference (such as a flight path angle or flight path 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 affects on tracking performance, 

pilot compensation, and workload. 
7/17/06 AC 25.1329-1B 

 

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 (d) Primary Instrument References. It should be demonstrated that 

movement of the FD and its tracking reference do not interfere with primary instrument 

references 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 Head Up Displays 

(HUD) 

 4 Chapter 5, paragraph 53, Normal Performance (specifically, criteria 

for FD guidance) 

 (b) Demonstrations Required. When the pilot flying (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 flight path 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. 
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 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 windshear warnings, ground 

proximity warnings, TCAS warnings, and other warnings requiring immediate flight 

control action, such as a go–around, while using the HUD without having to revert to a 

head down flight display. 

 (c) Pilot Not Flying (PNF). In certain phases of flight, it is important that 

the PNF and the flightcrew be aware of problems with the HUD used by the PF. 

Therefore, it should also be demonstrated that the PNF 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 vs. 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. 
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 (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, auto-feather, 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, Override of the FGS. 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. 

 (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. 
7/17/06 AC 25.1329-1B 

 

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NOTE: AC 120-28D, 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 must not result in a potential hazard per § 25.1329(m) , either 

during the override or after the pilot releases the thrust levers. Refer to 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 to the pitch cockpit controller below that which would cause the autopilot to 

disengage and verify that the automatic pitch trim system meets the intent in Chapter 3, 

paragraph 30. 

 (b) No Automatic Disengagement. If the system design is such that the 

autopilot does not have an automatic disengagement on override feature, the pilot should 

initiate an intentional override for an extended period of time. The autopilot should then 

be disengaged with the quick disconnect button and any transient response assessed in 

compliance with Chapter 3, paragraph 30. The effectiveness and timeliness of any alerts 

used to mitigate the effects of the override condition should be assessed during this 

evaluation. 

100. FAILURE CONDITIONS REQUIRING VALIDATION: § 25.1309. 

a. General. 

 (1) Safety Assessment. The safety assessment process identified in Chapter 8 

should identify any failure condition responses that would require pilot evaluation to 

assess the severity of the effect, and the validity of any assumptions used for pilot 

recognition and mitigation. The classification of a failure condition can vary according to 

flight condition and may need to be confirmed by simulator or flight test. 

 (2) Evaluation of Failure Conditions. This chapter provides guidance on the test 

criteria, including recognition considerations, for flight evaluation of these failure 

conditions. In addition, certain probable failures should be demonstrated to assess the 

performance of the FGS and the adequacy of any applicable flightcrew procedures. 
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Appendix 2, Flight Test Procedures, provides guidance on test methods for particular 

types of failure condition that have been identified by the safety assessment. 

b. Validation Elements. 

 (1) Assessment. The safety assessment described in Chapter 8 establishes the 

FGS failure condition for which appropriate testing should be undertaken. Assessment of 

failure conditions has the following elements: 

 (a) Failure condition insertion. 

 (b) Pilot recognition of the effects of the failure condition. 

 (c) Pilot reaction time. That is, the time between pilot recognition of the 

failure condition and initiation of the recovery. 

 (d) Pilot recovery. 

 (2) Failure Condition.

 (a) Autopilot. Failure conditions of the autopilot should be simulated such 

that overall response is representative of how that failure condition would affect the 

airplane and its systems. Failure conditions should include multi–axis failures and 

automatic trim failures, if those failure conditions exist (given the architecture of a 

specific FGS) and are relevant to the safety assessment. 

 (b) Flight Director (FD). FD failure conditions should be validated, if 

those failure conditions are relevant to the safety assessment. 

 (c) Most Critical Conditions. The flight conditions under which the failure 

condition is inserted should be the most critical. Examples of these flight conditions are 

CG, weight, flap setting, altitude, speed, power, and thrust. If an autothrust system is 

installed, the tests should be performed with the autothrust system engaged or 

disengaged, whichever is the more adverse case. 

 (3) Pilot Recognition.

 (a) General. The pilot may detect a failure condition through airplane 

motion cues or by cockpit flight instruments and alerts. The specific recognition cues 

will vary with flight condition, phase of flight, and crew duties. 

 (b) Hardover. The recognition point should be that at which a pilot 

operating in non-visual conditions may be expected to recognize the need to take action. 

Recognition of the effect of the failure may be through the behavior of the airplane (for 

example, in the pitch axis by aircraft motion and associated normal acceleration cues and 

in the roll axis by excessive bank angle) or an appropriate alerting system. Control 
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column or wheel movements alone should not be used for recognition. The recognition 

time should not normally be less than 1 second. If a recognition time of less than 

1 second is asserted, specific justification will be required. For example, additional tests 

may be required to ensure that the recognition time is representative in light of the cues 

available to the pilot. 

 (c) Slowover. This type of failure condition is typically recognized by a 

path deviation indicated on primary flight instruments (for example, course deviation 

indicator (CDI), altimeter, or vertical speed indicator). It is important that the recognition 

criteria are agreed upon with the FAA. The following identify examples of recognition 

criteria as a function of flight phase: 

 1 En–route cruise. Recognition through the altitude alerting system 

can be assumed for vertical path deviation. The lateral motion of the airplane may go 

unrecognized for a significant period of time, unless a bank angle alerting system is 

installed and the slowover results in a bank angle in excess of the limit allowed by the 

alerting system. 

 2 Climb and descent. Recognition through increasing/decreasing 

vertical speed, pitch attitude, roll attitude, or heading can be assumed. 

 3 On an approach with vertical path reference. A displacement 

recognition threshold should be identified and selected for testing that is appropriate for 

the display(s) and failure condition(s) to be assessed. 

NOTE: For an ILS or GLS approach in a significant wind gradient, a 

value of 1 dot on the displayed ILS/GLS deviation scale is considered a 

reasonable value for crew recognition. In smooth atmospheric conditions 

with steady state tracking with the vertical flight path typically maintained 

at less than a fraction of a needle width, a detection and recognition 

threshold even below 1/2 dot may be suitable. 

NOTE: For RNAV systems that do not use dots, some multiple of needle 

width related to an established crew monitoring tolerance of normal 

performance may be appropriate (for example, three needle widths of 

deviation on the VNAV scale). 

NOTE: Credit may be taken for excessive deviation alerts, if available. 

 4 On an approach without vertical path reference, criteria similar to 

the climb/descent condition can be assumed. 

 (d) Oscillatory. It is assumed that oscillatory failures that have structural 

implications are addressed under 14 CFR Part 25, Subpart C, and § 25.1309. It can be 

assumed that the flightcrew will disengage the automatic control elements of the FGS 

that have any adverse oscillatory effect and will not follow any adverse oscillatory 
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guidance. However, if there are any elements of the FGS that can not be disconnected in 

the presence of an oscillatory failure condition, the long term effects on crew workload 

and the occupants will need to be evaluated. 

 (4) Pilot Reaction Time. The pilot reaction time is considered dependent upon 

the pilot attentiveness, based upon the phase of flight and associated duties. The 

following assumptions are considered acceptable: 

 (a) Climb, Cruise, Descent and Holding. Recovery action should not be 

initiated until three seconds after the recognition point. 

 (b) Maneuvering Flight. Recovery action should not be initiated until one 

second after the recognition point. 

 (c) Approach. The demonstration of malfunctions should be consistent 

with operation in non–visual conditions. The pilot can be assumed to be carefully 

monitoring the airplane performance and will respond rapidly once the malfunction has 

been recognized. A reaction time of one second between recognition point and initiation 

of recovery is appropriate for this phase of flight. 

NOTE: For the final phase of landing (for example, below 80 feet), the 

pilot can be assumed to react upon recognition without delay. 

NOTE: For phases of flight where the pilot is exercising manual control 

using CWS, if implemented, the pilot can be assumed to commence 

recovery action at the recognition point. 

 (5) Pilot Recovery.

 (a) Pilot Recovery Action. The recovery action should be commenced after 

the reaction time. Following such delay, the pilot should be able to return the airplane to 

its normal flight attitude under full manual control without engaging in any dangerous 

maneuvers during recovery and without control forces exceeding the values given in 

§ 25.143 (c). During the recovery, the pilot may overpower the autopilot or disengage it. 

 (b) Minimum Autopilot Use Height. For the purpose of determining the 

minimum height at which the autopilot may be used during an approach or for height loss 

assessments, a representative pilot recovery maneuver appropriate to the airplane type 

and flight condition should be performed. This maneuver should not lead to an unsafe 

speed excursion to resume a normal flight path. An incremental normal acceleration in 

the order of 0.5 g is considered the maximum for this type of maneuver. 
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c. Takeoff.

 (1) Worst Case Failure Condition. The primary concern for the takeoff phase of 

flight is the effect of the worst case failure condition identified by the safety assessment 

on the following: 

 (a) The net effect on the flight path of the airplane after takeoff, and 

 (b) The airplane’s attitude and speed during climbout. 

 (2) Failures to be Evaluated. Failures that would cause the airplane to pitch up, 

pitch down, and bank during the takeoff should be evaluated, if those failure conditions 

exist (given the architecture of a specific FGS) and are relevant to the safety assessment. 

 (3) On Runway Guidance. If the FGS provides on runway guidance for takeoff, 

the effect of any failures of that takeoff guidance should be assessed. See AC 120-28D 

for additional information. 

d. Climb, Cruise, Descent, and Holding. Where the safety analysis identifies a 

failure condition requiring flight/simulator evaluation with pilot assessment, the height 

loss should be established in accordance with the method described in the flight test 

procedures Appendix 2, Flight Test Procedures, paragraph 5c(4)(c), Assessment of 

Approach Without Vertical Path Reference. 

e. Maneuvering. Where the safety analysis identifies a failure condition that has a 

dynamic effect on the roll control of the airplane, the failure condition should be 

introduced at the bank angle for normal operation. The bank angle should not exceed 

60 degrees when the pilot recognition and the recovery times identified above are 

applied. 

f. Approach. A discussion of the operational considerations for approach operations 

is contained in paragraph 101, Criteria Supporting Operational Use of the Autopilot, 

below. This chapter identifies test criteria to support those considerations. The safety 

assessment process should identify the demonstration of specific failure conditions during 

the approach. The fault demonstration process during approach should include the four 

phases identified in paragraph 100b, Validation Elements, above. The failure condition 

should be inserted at a safe but representative height. The deviation profile should be 

identified and applied as indicated in Appendix 2, Flight Test Procedures, 

paragraph 5c(4), Approach. 

 (1) Approach With Vertical Path Reference.

 (a) xLS (that is, ILS, MLS, or GLS).

 1 ILS and MLS operations are typically conducted on instrument 

approach procedures designed in accordance with United States Standard for Terminal 
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Instrument Procedures (TERPS). See FAA Order 8260.3, United States Standard for 

Terminal Instrument Procedures, or the International Civil Aviation Organization (ICAO) 

Procedures for Air Navigation Services – Operations (PANS-OPS) criteria, or equivalent. 

These criteria together with ICAO Annex 14 are generally intended to take into account 

obstacles beneath a reference obstacle identification surface. 

NOTE: The FAA may establish operational criteria for GLS at a later 

time. 

 2 In assessing the implication of the effect of failures during autopilot 

operations, a reference 1:29 slope penetration boundary has been applied against the 

deviation profile to identify an appropriate altitude for continued autopilot operation. 

The 1:29 slope has been found to provide an acceptable margin above obstacles on an 

approach. 

 3 The worst case failure condition identified by the safety assessment 

(See Chapter 8, paragraph 87, Validation of Failure Conditions.) should be demonstrated 

against the deviation profile criteria and a minimum use height (MUH) established (See 

Appendix 2, Flight Test Procedures, Section 5c(4)(b), Assessment of Approach With 

Vertical Path Reference). 

 (b) Area Navigation (RNAV).

 1 For RNAV coupled approach operations, a vertical flight path 

similar to an xLS flight path will be used (for example, 3 degree path starting 50 feet 

above the threshold). However, due to sensor characteristics it is assumed that RNAV 

operations will be conducted with a DA(H) or MDA(H) that is higher than an equivalent 

MUH on an xLS approach to the same runway. Further, for this type of operation, it 

should be noted that the MUH is always in the visual segment of the approach, as the 

failure recognition and recovery maneuver are assumed to be conducted with the pilot 

having established outside visual reference. 

 2 In order to derive only one MUH value for simplicity of use, it is 

assumed that the effects of failure on the autopilot in RNAV operation are no worse than 

for the xLS operation. No further determination or demonstration is required. However, 

the applicant should show that due account has been taken in the safety assessment of the 

differences between the RNAV and xLS inputs to the autopilot (for example, barometric 

altitude input, FMS position and guidance commands, and their failure effects). If these 

effects can be bounded or otherwise reconciled, then the xLS demonstrated MUH may 

also be considered applicable to RNAV operations. 

 3 If these effects cannot be bounded or accounted for within those for 

the xLS operation, the MUH should be determined in accordance with an Approach 

Without Vertical Path Reference, below. 
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 (2) Approach Without Vertical Path Reference. For an approach without 

vertical path reference (for example, VOR, Non Directional Beacon (NBD), localizer 

only) the FGS mode of operation is typically vertical speed/flight path angle (that is, a 

cruise mode). The worst case failure condition for this type of mode should be 

demonstrated in the approach configuration. An appropriate height loss should then be 

established in accordance with the method described in Appendix 2, Flight Test 

Procedures, paragraph 5c(4)(c). 

 (3) Steep Approach. In support of an approval to use the FGS on glidepath 

angles of greater than 3.75 degrees (see Chapter 9, paragraph 99d(5), Steep Approach 

[special authorization]), an assessment should be made of the effects of failure conditions 

for this type of operation. For use of autopilot, an appropriate MUH should be 

established in accordance with the deviation profile method described in this chapter. For 

this assessment, the obstacle plane associated with a nominal 3 degree glidepath angle 

(1:29 slope) should be adjusted, according to the maximum approach angle for which 

approval is sought. 

g. Specific Conditions.

 (1) Failure Conditions to be Evaluated. The following are failure conditions 

that should be considered as part of the FGS evaluation program: 

 (a) Engine failure during approach; continue approach to decision 

altitude/minimum descent altitude (DA/MDA). 

 (b) Potential fuel imbalance. 

 (c) Airplane system failures that affect FGS operational capabilities. 

Examples include failures of hydraulic systems, electrical systems, flight controls, and 

FGS related sensors. When possible, these failure conditions should be evaluated with a 

flight test. However, some failure conditions, due to their potential hazards, may best be 

evaluated via a high fidelity flight simulation. 

 (2) Failure to Disengage. The probability of failure of an FGS element to 

disengage when the quick disengagement control is operated should be shown to be 

acceptable by the safety analysis process. If credit is to be taken for acceptable continued 

manual operation with the FGS elements remaining engaged, that is, without operating 

any of the other disengagement controls, then a flight demonstration should be conducted 

though approach, landing and rollout. 

101. CRITERIA SUPPORTING OPERATIONAL USE Of AUTOPILOT. 

a. Typical Flight Operations. The criteria contained in this paragraph are intended 

to identify how the functional capability of the FGS, established during the certification, 

can be utilized to support typical flight operations. The criteria are based on experience 
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gained from certification programs and functionality provided by traditional systems. A 

FGS providing non-traditional functionality—using new or novel technology and/or 

implementation techniques—may require additional criteria to be established. 

b. Operations in Close Proximity to the Ground.

 (1) General. During low visibility operations, multiple redundant autopilot 

channels may be used, and the effect of any autopilot failures on the flight path may be 

eliminated or substantially minimized by the protection provided by that redundancy. 

The considerations in the following paragraphs apply primarily to single channel 

operations where performance or integrity aspects may require further consideration. See 

Chapter 8, paragraph 88b, FGS Operations in Close Proximity to the Ground, for specific 

considerations relating to autopilot operations close to the ground in the presence of 

failures. 

 (2) Considerations. The minimum engagement point for the autopilot after 

takeoff and the minimum use of the autopilot during approach should take into 

consideration the effect of the following: 

 (a) Failures and their effects (that is, failure conditions). 

 (b) Fault–free performance. 

 (c) Any specific operational considerations and/or mitigation. 

 (3) Autopilot Engagement Altitude/Height After Takeoff.

 (a) Deviation from Flight Path. The potential deviation of the airplane 

from the desired flight path due to the effect of a failure condition may necessitate 

delaying the engagement of an autopilot to an acceptable height above the departure 

runway. 

 (b) Worst–Case Deviation Profile. To support the determination of the 

minimum engagement height, if autopilot failure condition(s) are identified that will 

cause a significant deviation below the intended vertical flight path, the worst–case 

deviation profile should be identified. This profile and the recovery of the airplane 

should not result in penetration of the net flight path as defined in § 25.115. 

 (c) Other Effects. If the failure condition(s) have a neutral effect on the 

flight path but has implications for speed control during takeoff, the acceptability of cues 

for the flightcrew detection of the condition should be made. The effect of any failure 

condition relating to the bank angle of the airplane should also be assessed. In all of the 

above, account should be taken of operating the airplane at the 

weight/altitude/temperature (WAT) limit. 
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 (d) Minimum Engagement Height. The minimum engagement height will 

typically be established, based on the greatest of the following factors: 

 1 The lowest altitude or height where the flightcrew could reasonably 

be assumed to engage the autopilot. Consideration should be given to normal flightcrew 

tasks during rotation and liftoff (typically 100 feet or greater). 

 2 Any allowance for the acceptability of the performance of the 

autopilot during the basic engagement/mode transition. 

 3 The lowest altitude or height consistent with the response of the 

airplane to any identified autopilot failure condition(s). 

 4 Activation of an armed stall identification system, such as a stick 

pusher (if installed). 

 (e) Deviation Information. If the response to the worst-case failure 

condition causes a significant transition below the intended vertical flight path, the 

deviation information should be provided in the AFM. 

 (4) Autopilot Engagement During Approach.

 (a) Deviation from Flight Path. The potential deviation of the airplane 

from the desired flight path due to the effect of a failure condition may necessitate the 

disengagement of an autopilot at an appropriate height on the approach to landing. 

 (b) MUH - Approach. The operational MUH for approach will be 

established, based on the following considerations: 

 1 The altitude or height at which the performance of the automatic 

control is no longer acceptable. 

 2 The lowest altitude or height consistent with the response of the 

airplane to a subsequent autopilot failure. 

 3 Any specific operational consideration. 

 (c) Approach with Vertical Path Reference. 

 1 Approaches with vertical path reference can include xLS (that is, 

ILS, MLS or GLS) or RNAV. Operations using xLS can be assumed to be conducted 

with respect to a flight path prescribed or established as an integral part of navigation 

service provided by the State. RNAV approach operations will be conducted using an 

onboard database that provides a navigation flight path to the runway. 
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 2 The operational consideration for this type of operation relates to an 

assessment of the adequacy of continued use of the autopilot in maintaining the desired 

vertical flight path. Considerations include the lowest altitude consistent with the 

response of the airplane to an autopilot failure. 

 3 To support this determination, if one or more autopilot failure 

condition(s) is identified that causes a significant transition below the intended vertical 

flight path, the worst-case deviation profile should be identified using the method 

identified in Chapter 9, paragraph 100f(1), Approach With Vertical Path Reference. If 

the failure condition(s) has a neutral effect on the flight path, the acceptability of cues for 

the flightcrew detection of the condition should be made. The effect of any failure 

condition relating to the bank angle of the airplane should be assessed. 

 4 For the purpose of the airworthiness assessment, the vertical flight 

path an xLS and RNAV approach can be assumed to be a flight path of 3 degrees that 

passes through the runway threshold at an altitude of 50 feet. Considerations for steep 

approaches are provided in paragraph Chapter 9, paragraph 99d(5), Steep Approach 

[special authorization]. 

 5 The vertical flight path control for an xLS approach will be made 

with reference to the path defined by the navigation service. The RNAV vertical flight 

path will typically be conducted with reference to barometric altitude. An appropriate 

adjustment to the MUH may be necessary to take into account the vertical accuracy of 

RNAV operations. 

NOTE: Temperature effect compensation should be considered as part of 

the operational authorization. 

 6 The MUH - Approach is the value identified using method 

identified in Appendix 2, Flight Test Procedures, paragraph 5c(4), Approach. 

 (d) Approach Without Vertical Path Reference.

 1 Flight operations with no vertical path reference are conducted with 

an appropriate visual segment for final approach path. In the interest of providing 

appropriate automatic control to assist in a stabilized approach, the minimum use of the 

autopilot should be consistent with the performance needed for the descent (for example, 

vertical speed/flight path angle) and the pilot detection and recovery from an autopilot 

failure. 

 2 To support this determination, if one or more autopilot failure 

conditions is identified that causes a significant transition below the intended vertical 

flight path, the worst-case deviation profile should be identified. If the failure condition 

has a neutral effect on the flight path but has implications for speed control during 

takeoff, the acceptability of cues for the flightcrew detection of the condition should be 
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made. The effect of any failure condition relating to the bank angle of the airplane 

should be assessed. 

 3 For FGS that are failure protected (that is, fail–passive), the MUH 

will typically be no lower than 50 feet above runway elevation. However, when 

determining this limitation, account should be taken of the handling task presented to the 

pilot when regaining manual control, especially in limiting crosswind conditions. 

 4 For FGS that are not failure protected (that is, not fail-passive), the 

demonstrated MUH-Approach will typically be established based on the greater of the 

following considerations: 

 (aa) 50 feet above runway elevation. 

 (bb) Two times the height loss for the airplane as a result of any 

identified autopilot failure condition, using the method identified in Appendix 2, Flight 

Test Procedures, paragraph 5c(4), Approach. 

 (5) Circling Approach. For the purpose of this AC, circling approaches may be 

considered to have the following visual segments associated with the approach: 

 (a) A segment at or above the minimums prescribed by the procedure that 

parallel the runway in the opposite direction of the landing runway, 

 (b) A turning segment to align with the runway that can be level or partially 

descending, and 

 (c) A final descending segment to landing. 

Operationally, the autopilot may remain engaged even after leaving the minimum altitude 

(MDA(H)) for reasons related to safety and flightcrew workload relief. This operational 

procedure should be balanced against unacceptable performance or failure characteristics. 

As this procedure is in the visual segment, no specific constraints for the use of the 

autopilot are considered necessary for this phase of flight, unless specific unacceptable 

performance or failure characteristics related to circling approach are identified during 

the certification program. 

c. Climb, Cruise, Descent, and Holding. The value of the use of the autopilot in 

providing flightcrew workload relief in climb, cruise, descent, and holding phases of 

flight should be balanced against the failure characteristics of the autopilot. No specific 

constraints for the use of the autopilot are considered necessary for these phases of flight, 

unless specific unacceptable performance or failure characteristics related to climb, 

cruise, or descent are identified during the certification program. 

d. Maneuvering. No specific constraints for the use of the autopilot are considered 

necessary for maneuvering flight, unless unacceptable performance or failure 
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characteristics are identified during the certification program. Chapter 9, paragraph 98e, 

Maneuvering, provides assessment criteria for maneuvering flight for autopilot failures. 

102. AUTOMATIC DISENGAGEMENT OF THE AUTOPILOT. The automatic 

disengagement of the FGS will occur for several reasons, such as system failures, sensor 

failures, or unusual accelerations. The automatic disengagement characteristics of the 

FGS should be investigated throughout the flight envelope. These disengagement cases 

should be analyzed to determine the ones that can be demonstrated during the test 

program. For each disengagement case, the transients, warnings, and pilot workload for 

recovery should be evaluated, and compliance with § 25.1329(d) and (e) must be verified. 

The use of simulation is recommended for all conditions that are expected to result in 

significant transients. 

103. ASSESSMENT OF HUMAN FACTORS. 

a. General. The evaluation, demonstration and testing should assess the 

acceptability of the human–machine interface with the FGS and the potential for 

flightcrew errors and confusion concerning the behavior and operation of the FGS, when 

used by a representative range of pilots. The evaluation of normal and non–normal FGS 

operations should include the representative range of conditions in terms of crew mental 

or physical workload, required crew response timeliness, and potential for confusion or 

indecision. The set of test cases should represent operationally relevant scenarios and the 

assumptions about pilot training and skill level should be documented. 

b. Flight Evaluations. 

 (1) During Certification. Flight evaluation during certification is a final 

assessment and is intended to validate the design. 

 (2) Prior to Certification. Evaluations prior to certification are typically 

conducted in a variety of ways and at different levels of fidelity in order to finalize the 

design. These may include the following: 

 (a) Engineering evaluations and task analyses, including cognitive and 

physical tasks. 

 (b) Mock–up evaluations and demonstrations. 

 (c) Part–task evaluations and demonstrations. 

 (d) Simulator evaluations, demonstrations, and tests. 

 (e) Engineering flight evaluations, demonstrations, and tests. 
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 (3) Credit to Establish FGS Compliance. The data and experiences from these 

evaluations may be useful for credit to establish FGS compliance with regulations having 

human factors considerations. In addition, applicants have successfully used 

comparisons to previously certificated designs to obtain such credit (although such credit 

is not assured). Additional testing may be necessary for new FGS flightcrew interface 

designs or functions. 

 (4) Simulation Versus Flight Tests. In some cases, the FAA may consider that 

less flight testing may be required to show compliance, if the simulation evaluations have 

added confidence with respect to the reduced potential for crew error and confusion and 

other human factors attributes of the pilot/FGS interface. 

 (a) Selecting Methodology. In many cases the evaluation, demonstration, 

and test scenarios, including failures and environmental events, will determine whether 

the data should be obtained in simulation or in flight because of safety considerations or 

unavailability of the necessary environmental conditions. 

 (b) High Fidelity Simulation. In some test scenarios, a very high fidelity 

simulation will be needed. In addition to the simulation validation considerations 

identified in Chapter 9, paragraph 99e(3)(b), Motion Base Suitability, the simulation used 

may need to include the following features, depending on the functionality of the FGS: 

 1 Physical implementation of flight deck controls, displays, indicators, 

and annunciators for all flightcrew positions that are relevant to the objectives of the 

evaluation. 

 2 Adequate emulations of relevant equipment (hardware and software 

function), including capability to introduce failures. 

 3 Weather simulation, including gusts, turbulence, windshear, and 

visibility. 

 4 Representation of the operational environments, including 

interaction with air traffic services, day/night operations, etc., as relevant to the functions 

and pilot tasks being evaluated. 

 5 Data collection capabilities. 

 (c) Simulation Conformity. In some cases, certification credit or 

demonstration of compliance using simulations may not be granted due to inability to 

find simulation conformity. Conformity should be accounted for when seeking 

certification testing credit using a simulator. 

 c. Structured Subjective Evaluations. Simulator evaluations and tests are intended 

to generate objective and/or subjective data. It may not always be possible or necessary 

to obtain quantifiable measurements of flightcrew performance, even with high fidelity 
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flight or simulation evaluation, demonstration, or test scenarios. In these cases, 

evaluation procedures should be based on the use of structured subjective methods, such 

as rating scales, questionnaires, and/or interviews. When there is dependence on this type 

of data, evaluations should consider multiple data collection techniques with an 

appropriate number of pilot evaluators. 

 d. Pilot Training And Experience. In order to provide sound evaluations, pilots 

should be trained appropriately on the FGS operation and procedures. They should have 

experience in the kinds of operation and aircraft types for which the FGS is intended. 

Finally, they should be familiar with the intended function of the FGS, its operational and 

design philosophy, and the way that this philosophy fits with the overall flight deck and 

its operational and design philosophy. 

 e. Evaluating New or Unique Design Features. Rationale should be provided for 

decisions regarding new or unique features in a design. It should be confirmed that the 

data resulting from the evaluations support acceptability of any new or unique features. 

 f. Certification Planning Documentation. The certification planning documentation 

should describe the means to show compliance with the human factors related 

considerations of the FGS with this AC. 

104 — 108 [RESERVED] 
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CHAPTER 10 AIRPLANE FLIGHT MANUAL 

109. GENERAL. The following paragraphs provide guidance on material to be provided 

in the AFM to ensure that the appropriate information related to FGS operation is 

translated into air carrier operations. For additional guidance, note that AC 25.1581-1, 

Airplane Flight Manual, addresses requirements of the AFM for transport category 

aircraft and distinguishes between those aircraft that are used in air carrier operations and 

those not in air carrier service. 

a. AFM Terminology. The terminology used in the AFM should be consistent with 

the intended operational use. 

b. Low-Visibility Operations. Appropriate AFM information related to lowvisibility operations are addressed in AC 120–28D and AC 120–29A. 

110. INFORMATION SUPPORTING OPERATIONAL USE OF AUTOPILOT. 

a. General. The airworthiness certification process will assess the effect of autopilot 

Failure Conditions as identified in Chapters 8, Safety Assessment, and 9, Compliance 

Demonstration Using Flight Test Simulation. If a specific MUH is necessary, then the 

height should be provided in the AFM limitations section. If the design is such that the 

effects of one or more failure conditions do not require establishment of a MUH, then the 

pertinent deviation profile or height loss information should be provided in the normal or 

non-normal section of the AFM, as applicable. 

 b. Specifying a Minimum Use Height (MUH). If a MUH or a height loss value is 

applicable, it should be specified as follows: 

 (1) Takeoff — Autopilot engagement altitude or height. 

 (2) Cruise — height loss. 

 (3) Approach — MUH or height loss. 

 (a) Approach With Vertical Path Reference. The MUH should be 

determined based on clearance above a 1:29 plane using the deviation profile method, as 

discussed in Appendix 2, paragraph 5, Failure Conditions, and shown in Figure 2-1, 

Deviation Profile Method, of this document. 

 (b) Approach Without Vertical Path Reference. The height loss should be 

determined using the height loss method, as discussed in Appendix 2, paragraph 5, and 

shown in Figure 2-2, Height Loss Method, of this document. 
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c. Maximum Displacement Deviation. If minimum engagement altitude(s) or 

height(s) are not specified, then “maximum displacement deviation” information from a 

pertinent takeoff flight path and approach profile should be provided in the AFM normal 

procedures section, or in the associated flightcrew operation manual (FCOM). 

111. LIMITATIONS. The AFM limitations section presents those FGS operating 

limitations appropriate to the airplane model as established in the course of the type 

certification process. The FGS operational limitations listed in the AFM (should any 

exist) must specify any configuration/envelope restrictions. [See §§ 25.1581(a)(1), 

and 25.1583] 

112. NON-NORMAL/EMERGENCY PROCEDURES. The AFM must include 

non-normal or emergency procedures appropriate to the FGS identified during the 

certification program. [See §§ 25.1581(a)(1), and 25.1585(a)(2), (a)(3)] 

113. NORMAL PROCEDURES. 

a. Documentation of Normal Procedures. The normal procedures for use of the 

FGS must be documented in the AFM. [See §§25.1581(a)(1), and 25.1585(a)(1)] These 

procedures should be demonstrated during the type certification process. 

b. Maximum Displacement Deviation. In lieu of specification of minimum 

engagement altitude(s) or height(s) (see paragraph 110, above), the AFM may alternately 

specify “maximum displacement deviations” from a specified takeoff flight path or from 

a specified approach profile. This information may be based on typical departure or 

approach flight paths suited for the aircraft type and for failure conditions that are 

determined applicable to the type of FGS and modes suitable for use. 

c. Procedures for Use of FGS in Icing Conditions. The flight manual should include 

any necessary procedures for the use of the FGS in icing conditions (including moderate 

and heavy icing conditions). In particular, the procedures should include any necessary 

changes in operating speeds, required either operationally or as a result of relevant design 

features of the speed protection function of the FGS. For example, variations in 

minimum speeds as a function of de/anti-icing system selection, speed increments during 

approach, and landing in turbulence should be part of the documented procedures. 

d. Aircraft With Published Flightcrew Operations Manuals (FCOM). The AFMs 

for aircraft for which the manufacturer has published a FCOM must contain essential 

information on normal operating procedures that are considered unique to the operation 

of the FGS for the aircraft type or are otherwise necessary for safe operation. [See 

§ 25.1581(a)(2)] FGS description and integration with the overall flight deck design 

philosophy, specification, and operational procedures that are normally associated with 

FGSs should be included in the FCOM. If applicable, a FCOM may contain the 
7/17/06 AC 25.1329-1B 

 

 113 

“maximum displacement deviation” information described in paragraph 110, above, in 

either numeric or graphic form. 

e. Aircraft Without Published FCOMs. For aircraft that rely on the AFM as the sole 

operating manual, the AFM must contain operating information sufficient for flightcrew 

reference. [See § 25.1581(a)(2)] The FGS description and integration with the overall 

flight deck design philosophy, specification and operational procedures normally 

associated with FGSs, should be made available so that an appropriately trained 

flightcrew may operate the FGS under normal conditions. 
7/17/06 AC 25.1329-1B 

 

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7/17/06 AC 25.1329-1B 

 Appendix 1 

 1-1 

APPENDIX 1. SAFETY ASSESSSMENT 

1. GENERAL. This appendix provides material that may be useful in supporting the 

safety assessment activities identified in Chapter 8, Safety Assessment. 

2. IDENTIFICATION OF FAILURE CONDITIONS. The following cases should be 

considered for applicability when establishing failure conditions, as indicated in 

Chapter 8. 

a. Loss of autopilot in single or multiple axes. 

b. Loss of guidance in single or multiple axes. 

c. Loss of thrust control. 

d. Partial loss or degradation of autopilot function. 

e. Unintended autopilot commands in a single axis or multiple axes simultaneously 

(for example, hardover, slowover, and oscillatory failure modes). 

f. Unintended guidance commands in a single axis or multiple axes. 

g. Unintended thrust control. 

h. A sustained out–of–trim condition with the autopilot engaged without an alert. 

i. Autopilot disengagement in an out–of–trim condition. 

j. Autopilot disengagement without a warning. 

k. Inability to disengage the autopilot or autothrust function. 

l. Un–commanded engagement of an autopilot or autothrust. 

m. Jamming or loading of primary flight controls. 

n. Un–intended thrust asymmetry. 

A typical failure condition statement may be of the form: 

‘[Failure]’ during ‘[Phase of Flight]’ that ‘[Effect]’ when ‘[Mitigation Consideration]’ 
7/17/06 AC 25.1329-1B 

 Appendix 1 

 1-2 

Failure conditions may result from failures within the FGS or from failure associated 

with aircraft interfacing systems or components (for example, NAV receivers, attitude 

heading reference systems, flight management systems (FMS), hydraulics, electrical 

systems, etc.). 

3. CONSIDERATIONS WHEN ASSESSING THE SEVERITY OF FAILURE 

CONDITION EFFECTS.

 a. Complete Definition of Failure Condition. The “failure condition” definition is 

complete (as defined in AC 25.1309-1A, System Design and Analysis) when the effects 

resulting from the failure are identified. The complete definition of the “failure 

condition” and its effect will then support the subsequent failure condition hazard 

classification. 

 b. Factors To Consider. When assessing the effect that results from a failure, the 

following factors should be considered for various phases of flight: 

 (1) The impact of the loss of control or unintended control on the structural 

integrity of the airplane as a result of simple loading or as a result of excitation of 

aerodynamic or structural modes—both at the time of occurrence and while the flight 

continues. 

 (2) Implications of the airplane response in terms of attitude, speed, 

accelerations, flight path, and the impact on the occupants and on flightcrew 

performance. 

 (3) Degradation in the stability or other flying qualities of the airplane. 

 (4) The duration of the condition. 

 (5) The aircraft configuration. 

 (6) The aircraft motion cues that will be used by the flightcrew for recognition. 

 (7) Availability, level, and type of alerting provided to the flightcrew. 

 (8) Expected flightcrew corrective action on detection of the failure. 

 c. Possible Characteristics of Failure Conditions. Failure conditions may include 

the following characteristics: 

 (1) “Hardover” effects—typically are considered significant and are readily 

detectable by the flightcrew, based on the resulting aircraft motion or guidance cues. 
7/17/06 AC 25.1329-1B 

 Appendix 1 

 1-3 

 (2) “Slowover” effects—typically are not readily detected by the flightcrew. 

The effect may involve departures from intended flight path that are not initially 

detectable by aircraft motion alone and may be detectable only by motion cues when a 

significant flight path deviation has occurred or by an appropriate flightcrew alert. 

 (3) “Oscillatory” effects—typically are repetitive motions or guidance 

conditions not related to intended guidance or control. The magnitude, period, and 

duration of the condition and any mitigation considerations will determine the final 

effect. 

 (4) “Loss of” effects—typically is the removal of control, guidance or 

functionality that may have an immediate effect or may not be immediately apparent to 

the flightcrew. 

NOTE: Chapter 9, Compliance Demonstration Using Flight Test and 

Simulation, provides guidance on crew recognition considerations. 

4. FAILURE CONDITION CLASSIFICATION.

The following are examples of the type of failure condition effects that have been 

identified in previous airplane certification programs. The specific number and type of 

failure condition may vary with airplane type, airplane system architecture, and FGS 

design philosophy (for example, failure detection, redundancy management, failure 

annunciation, etc.). 

 a. Catastrophic Failure Conditions. The following effects have been assessed 

catastrophic in previous airplane certification programs: 

 (1) A load on any part of the primary structure sufficient to cause a structural 

failure preventing safe flight and landing. 

 (2) Unrecoverable loss of flight path control. 

 (3) Exceedance of VDF/ MDF (demonstrated flight dive speed/demonstrated 

flight dive Mach). 

 (4) Flutter or vibration that causes a structural failure preventing safe flight and 

landing. 

 (5) A temporary loss of control (for example, stall) where the flightcrew is 

unable to prevent contact with obstacles or terrain. 

 (6) Deviations in flight path from which the flightcrew are unable to prevent 

contact with obstacles, terrain, or other aircraft. 
7/17/06 AC 25.1329-1B 

 Appendix 1 

 1-4 

 b. Hazardous Failure Conditions. The following effects have been assessed 

hazardous in previous airplane certification programs: 

 (1) Exceedance of an airspeed halfway between VMO (maximum operating limit 

speed) and VDF or a Mach number halfway between MMO (maximum operating limit 

Mach) and MDF. 

 (2) A stall, even if the flightcrew is able to recover safe flight path control. 

 (3) A load factor less than zero. 

 (4) Bank angles of more than 60 degrees en route or more than 30 degrees 

below a height of 1000 ft. (304.8 m above an applicable airport elevation). 

 (5) Degradation of the flying qualities of the airplane that excessively increases 

flightcrew workload. 

 (6) Failure that could result in a rejected takeoff (RTO) and high speed overrun 

(for example, 60 knots). 

 (7) A flight path deviation that requires a severe maneuver to prevent contact 

with obstacle, terrain or other aircraft. 

NOTE: Severe maneuver includes risk of serious injury or death of a 

small number of occupants. 

 c. Major Failure Conditions. The following effects have been assessed “major” in 

previous airplane certification programs: 

 (1) A flight path deviation, including the required recovery maneuver, which 

may result in passenger injuries. Consideration should be given to phases of flight where 

the occupants may reasonably be moving about the airplane or be serving or consuming 

hot drinks. See Appendix 3, for the definition of significant transient. 

 (2) Degradation of the flying qualities of the airplane that significantly increase 

flightcrew workload. 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-1 

APPENDIX 2. FLIGHT TEST PROCEDURES 

1. GENERAL. This Appendix contains flight test procedures that can be used to 

validate the operation of an FGS consistent with the acceptable means of compliance 

documented in this AC. This information is in addition to that contained in AC 25-7A, 

Flight Test Guide for Certification of Transport Category Airplanes. 

2. PURPOSE AND SCOPE OF FLIGHT DEMONSTRATION PROGRAM.

A flight test program should be established that confirms the performance of the FGS for 

the modes of operation and the operational capabilities supported by its design. The 

operational implications of certain failures and failure conditions may require flight 

evaluation. The pilot interface with FGS controls and displays in the cockpit will need to 

be assessed. 

Some aspects of the design may be validated by laboratory test and/or simulator 

evaluation. It is recommended that an applicant provide a certification flight 

demonstration plan to the FAA at a timely point in the development program. 

The scope of the flight demonstration program will be dependent on the operational 

capability being provided, including any new and novel features. Early coordination with 

the FAA is recommended to reduce certification risks associated with the flight 

demonstration program. 

The intent of the flight demonstration program is to confirm that the operation of the FGS 

is consistent with its use for the intended flight operations of the airplane type and 

configuration. 

The modes of the FGS should be demonstrated in representative airplane configurations 

and under a representative range of flight conditions. 

The following paragraphs describe specific test procedure that can assist in that 

demonstration program. 

3. PROTECTION FEATURES OF THE FGS. 

 a. General. Protection features are included in the design of an FGS to assist the 

flightcrew in ensuring that boundaries of the flight envelope or operational limits are not 

exceeded, leading to an unsafe condition. The means to alert the flightcrew to a 

condition or for the system to intervene to preclude the condition may vary, but certain 

operational scenarios can be used to assess the performance of the system in providing 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-2 

the protection function. The procedures in the following paragraphs can be used to 

evaluate the protection functions of an FGS. 

 b. Low Speed Protection. 

 (1) Purpose. The low speed protection feature in an FGS is intended to prevent 

loss of speed to an unsafe condition (See Chapter 5, paragraph 57b, Low Speed 

Protection). This may be accomplished by a number of means but should be evaluated 

under a number of scenarios. 

 (2) Cases To Consider. There are four cases that should be considered when 

evaluating the low speed protection function of a FGS: 

 (a) High Altitude Cruise Evaluation.

 1 At high altitude at normal cruise speed, engage the FGS into an 

Altitude Hold mode and a heading or LNAV mode. 

 2 Engage the autothrust into a speed mode. 

 3 Manually reduce one engine to idle thrust. 

 4 As the airspeed decreases, observe the FGS behavior in maintaining 

altitude and heading/course. 

 5 When the low speed protection condition becomes active, note the 

airspeed and the associated aural and visual alerts including possible mode change 

annunciations for acceptable operation. 

 (b) Altitude Capture Evaluation at Low Altitude.

 1 At approximately 3000 feet MSL and 250 knots, engage the FGS 

into Altitude Hold and a heading or LNAV mode. 

 2 Engage the autothrust into a speed mode. 

 3 Set the altitude pre-selector to 8000 feet MSL. 

 4 Make a flight level change to 8000 feet with a 250 knots climb at 

maximum climb power. 

 5 When the FGS first enters the Altitude Capture mode, retard an 

engine to idle power. 

 6 As the airspeed decreases, observe the airplane trajectory and 

behavior. 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-3 

 7 When the low speed protection condition becomes active, note the 

airspeed and the associated aural and visual alerts including possible mode change 

annunciations for acceptable operations. 

 (c) High Vertical Speed Evaluation.

 1 Engage the FGS in the Vertical Speed mode with a very high rate of 

climb. 

 2 Set the thrust to a value that will cause the airplane to decelerate at 

approximately 1 knot per second. 

 3 As the airspeed decreases, observe the airplane trajectory and behavior. 

 4 When the low speed protection condition becomes active, note the 

airspeed and the associated aural and visual alerts including possible mode change 

annunciations for acceptable operation. 

 (d) Approach Evaluation.

 1 Conduct an instrument approach with vertical path reference. 

 2 Couple the FGS to the localizer and glideslope (or LNAV/VNAV, etc.). 

 3 Cross the final approach fix/outer marker at a high speed (approximately 

Vref + 40 knots) with the thrust at idle power until low speed protection activates. 

 4 As the airspeed decreases, observe the airplane trajectory and behavior. 

 5 When the low speed protection becomes active, note the airspeed and the 

associated aural and visual alerts including possible mode change annunciation for 

acceptable operation. 

 6 Note the pilot response to the alert and the recovery actions taken to 

recover to the desired vertical path and the re-capture to that path and the acceleration 

back to the desired approach speed. 

NOTE: If the FGS remains in the existing mode with reversion to low 

speed protection, the FGS should provide a suitable alert to annunciate the 

low speed condition. In this case, note the pilot response to the alert and 

the recovery actions taken to maintain the desired vertical path and to 

accelerate back to the desired approach speed. 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-4 

 c. High-Speed Protection.

 (1) Purpose. The high-speed protection feature in an FGS is intended to prevent 

a gain in airspeed to an unsafe condition (See Chapter 5, paragraph 57c, High Speed 

Protection). This may be accomplished by a number of means but should be evaluated 

under a number of scenarios. 

 (2) Cases to Consider. Three cases that should be considered when evaluating 

the high-speed protection function of a FGS are: 

 (a) High Altitude Level Flight Evaluation with Autothrust Function.

 1 Select autothrust off, if an automatic wake-up function is provided; 

otherwise, select autothrust on. 

 2 Engage the FGS in Altitude Hold. 

 3 Select a thrust level that will result in an acceleration that would 

cause, without intervention (either automatic or manual), a speed/Mach beyond 

VMO/MMO.

 4 As the airspeed increases, observe the behavior of the high-speed 

protection condition and any autothrust reactivation and thrust reduction, as applicable. 

 5 Assess the performance of the FGS to control the airspeed to 

VMO/MMO, or other appropriate speed. 

 (b) High Altitude Level Flight Evaluation Without Autothrust Function.

 1 Select a thrust value that will result in acceleration that would cause, 

without intervention (either automatic or manual), a speed/Mach beyond VMO/MMO. 

 2 As the airspeed increases, observe the basic airplane overspeed 

warning activate between VMO + 1 and VMO + 6 knots. 

 3 Observe the high-speed protection condition become active as 

evidenced by the unique visual alert and note possible FGS mode change. 

 4 Maintain the existing thrust level and observe the airplane depart the 

selected altitude. 

 5 After sufficient time has elapsed to verify and record FGS behavior, 

reduce the thrust as necessary to cause the airplane to begin a descent. 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-5 

 6 Observe the FGS behavior during the descent and subsequent 

altitude capture at the original selected altitude. 

 (c) High Altitude Descending Flight Evaluation With Autothrust Function.

 1 Select autothrust off (with automatic wake-up function) with thrust 

set to maintain airspeed 10% below VMO/MMO with the FGS engaged in Altitude Hold. 

 2 Select Vertical Speed mode that will result in acceleration that 

would cause, without intervention (either automatic or manual), a speed/Mach beyond 

VMO/MMO. 

 3 As the airspeed increases observe the autothrust function reactivate 

and reduce thrust towards idle. 

 4 Observe the activation of FGS high-speed protection condition. 

 5 Observe the reduction in pitch. 

NOTE: If the FGS remains in the existing mode with reversion to high 

speed protection, the FGS should provide a suitable alert to annunciate the 

high-speed condition. In this case, note the pilot response to the alert and 

the recovery actions taken to maintain the desired vertical path and to 

decelerate back to the desired speed. 

4. ENVIRONMENTAL CONDITIONS.

 a. General. Some environmental conditions have created operational problems 

during FGS operations. It should be the objective of the flight demonstration program to 

expose the FGS to a range of environmental conditions as the opportunity presents itself. 

These include winds, mountain-wave, turbulence, icing, etc. However, some specific test 

conditions may have to be created to emulate operational conditions that are not readily 

achieved during normal flight test. 

 b. Icing.

 (1) General. The accumulation of ice on the airplane wings and airframe can 

have an effect on airplane characteristics and FGS performance. FGS operations may 

mask the onset of an airplane configuration that would present the pilot with handling 

difficulties when resuming manual control, particularly following any automatic 

disengagement of the FGS. 

 (2) Evaluation of FGS During Icing. During the flight test program the 

opportunity should be taken to evaluate the FGS during natural icing conditions including 

the shedding of the ice, as applicable. It is also recommended that the operation of the 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-6 

FGS be evaluated during basic airplane evaluation with artificial “ice shapes” installed. 

The following conditions should be considered for evaluating FGS performance under 

icing conditions: 

 (a) Icing in Holding Conditions. See AC 25.1419-1, Certification of 

Transport Category Airplanes for Flight in Icing Conditions, for information. 

 (b) Medium to Light Weight, Symmetric Fuel Loading.

 1 High lift devices retracted configuration: 

 (aa) Slow at 1 knot/sec to automatic autopilot disengage, stall 

warning, or entry into the speed protection function. 

 (bb) Recovery should be initiated a reasonable period after the 

onset of stall warning or other appropriate warning. 

 (cc) The airplane should exhibit no hazardous characteristics. 

 2 Full instrument approach. If the autopilot has the ability to fly a 

coupled instrument approach and go-around, it should demonstrate the following 

conditions: 

 (aa) Instrument approach using all normal flap selections. 

 (bb) Go-around using all normal flap selections. 

 (cc) Glideslope capture from above the glidepath. 

 3 If the airplane accretes or sheds ice asymmetrically, it should be 

possible to disengage the autopilot at any time without unacceptable out of trim forces. 

 4 General maneuverability should be assessed, including normal turns, 

maximum angle of bank commanded by the FGS in one direction and then rapid reversal 

of command reference to the maximum FGS angle of bank in the other direction. 

5. FAILURE CONDITIONS. 

 a. General. This paragraph contains criteria relating to airplane system failure 

conditions identified for validation by a system safety assessment.

 b. Test Methods.

 (1) Fault Simulation. The test method for most failure conditions will require 

some type of fault simulation technique with controls that provide for controlled insertion 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-7 

and removal of the type of fault. The insertion point will typically be at a major control 

or guidance point on the airplane (for example, control surface command, guidance 

command, thrust command). 

 (2) Assessment of Failure Conditions. The implication of the effect of the 

failure condition on various flight phases should be assessed and the demonstration 

condition established. This assessment should identify the parameters that need to be 

measured and the instrumentation required. 

 The role of any monitoring and alerting in the evaluation should be identified. 

 The alertness of the crew to certain airplane response cues may vary with phase 

of flight and other considerations. Guidance on this is provided in the following 

paragraphs. 

 The “success criteria” or operational implications should be identified and 

agreed with the FAA prior to the conduct of the test. Guidance on this is provided in the 

following paragraphs. 

 c. Fault Recognition and Pilot Action.

 (1) Safety Assessment. The safety assessment process may identify a 

vulnerability to these types of failure condition: hardover, slowover, and oscillatory. The 

various types of effect will cause differing response in the airplane and resultant motion 

and other cues to the flightcrew to alert them to the condition. The flightcrew attention 

may be gained by additional alerting provided by systems on the airplane. The 

recognition is then followed by appropriate action including recovery. 

 The assessment of the acceptability of the failure condition and the validation of 

the safety assessment assumptions are complete when a stable state is reached as 

determined by the test pilot. 

 The following paragraphs provide guidance for specific phases of flight. 

 (2) Takeoff. This material addresses the use of a FGS after rotation for takeoff. 

Chapter 8, Safety Assessment, identified the key considerations for this phase of flight to 

be the effect on the net flight path and the speed control after liftoff. Automatic control is 

not typically provided for the takeoff roll. It may however be selected soon after liftoff. 

Failure conditions may be introduced with this engagement. 

 For the initial liftoff through flap retraction, it can be assumed that the flightcrew is 

closely monitoring the airplane movements and a maximum crew response time after 

recognition would be one second. 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-8 

 (3) Climb, Cruise, Descent and Holding and Maneuvering. The demonstration 

of applicable failure conditions during these phases of flight would include the potential 

for occupants to be out of their seats and moving about the cabin. 

 (4) Approach. There are two types of approach operations to consider—an 

approach with vertical path reference and one without vertical path reference. The 

approach with vertical path reference will be assessed against ground-based criteria using 

a deviation profile assessment. A height loss assessment is used for approaches without 

vertical path reference. 

 (a) Fault Demonstration Process. The worst-case malfunction has first to 

be determined, based on factors such as the following: 

 1 Failure conditions identified by the system safety assessment. 

 2 System characteristics such as variations in authority or monitor 

operation. 

 3 Mitigation provided by any system alerts. 

 4 Aircraft flight characteristics relevant to failure recognition. 

 Once the worst-case malfunction has been determined, flight tests of the 

worst-case malfunction should be flown in representative conditions (for example, 

coupled to an ILS), with the malfunction being initiated at a safe height. The pilot should 

not initiate recovery from the malfunction until one second after the recognition point. 

The delay is intended to simulate the variability in response to effectively a “hands off” 

condition. It is expected that the pilot will follow through on the controls until the 

recovery is initiated. 

 (b) Assessment of Approach With Vertical Path Reference. Figure 2-1, 

“Deviation of Profile Method,” of this Appendix provides a depiction of the deviation 

profile method. The first step is to identify the deviation profile from the worst-case 

malfunction. The next step is to “slide” the deviation profile down the glidepath, until it 

is tangential to the 1:29 line or the runway. The failure condition contribution to the 

MUH-approach may be determined from the geometry of the aircraft wheel height 

determined by the deviation profile, relative to the 1:29 line intersecting a point 15 feet 

above the threshold. The method of determination may be made by graphical or by 

calculation. 

NOTE: The MUH-approach is based on the recovery point for the 

following reasons: 

1. It is assumed that in service the pilot will be “hands off” until the 

autopilot is disengaged at the MUH in normal operation. 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-9 

2. The test technique assumes a worst-case based on the pilot being 

“hands off” from the point of malfunction initiation to the point of 

recovery. 

3. A failure occurring later in the approach than the point of initiation of 

the worst-case malfunction described above is therefore assumed to be 

recovered earlier and in consequence to be less severe. 

 (c) Assessment of Approach Without Vertical Path Reference. Figure 2-1 of 

this Appendix provides a depiction of the height loss method. A descent path of 

3 degrees, with nominal approach speed, should be used unless the autopilot is to be 

approved for significantly steeper descents. The vertical height loss is determined by the 

deviation of the aircraft wheel height relative to the nominal wheel flight path. 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-10 

Figure 2-1 Deviation Profile Method 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-11 

 

Figure 2-2 Height Loss Method 
7/17/06 AC 25.1329-1B 

 Appendix 2 

 2-12 

d. Autopilot Override.

 (1) Initial Tests. The initial tests to demonstrate compliance should be 

accomplished at an intermediate altitude and airspeed, e.g., 15000 feet MSL and 

250 knots. With the autopilot engaged in Altitude Hold, the pilot should apply a low 

force to the control wheel (or equivalent) and verify that the automatic trim system does 

not produce motion resulting in a hazardous condition. The pilot should then gradually 

increase the applied force to the control wheel (or equivalent) until the autopilot 

disengages. When the autopilot disengagement occurs, observe the transient response of 

the airplane. Verify that the transient response is in compliance with Chapter 3, 

paragraph 30, Override of the FGS. 

 (a) Automatic Disengagement. Disengagement caused by flightcrew 

override should be verified by applying an input on the control wheel (or equivalent) to 

each axis for which the FGS is designed to disengage, that is, the pitch and roll yoke, or 

the rudder pedals (if applicable). The inputs by the pilot should increase for each test 

case to a point where the input is sharp and forceful, so that the FGS can immediately be 

disengaged for the flightcrew to assume manual control of the airplane. 

 (b) Non-Automatic Disengagement. If the autopilot is designed such that it 

does not automatically disengage during an autopilot override—and instead provides a 

flight deck alert to mitigate any potentially hazardous conditions—the timeliness and 

effectiveness of this alert should be evaluated. The pilot should follow the evaluation 

procedure identified above until such time as an alert is provided. At that time, the pilot 

should respond to the alert in a responsive manner consistent with the level of the alert 

(that is, a caution, a warning) and with the appropriate flightcrew procedure defined for 

that alert. When the autopilot is manually disengaged, observe the transient response of 

the airplane and verify that the transient response is in compliance with Chapter 3, 

paragraph 30. 

NOTE: During hardover testing as described in AC 25-7A, Flight Test 

Guide for Certification of Transport Category Airplanes, Chapter 6, 

Equipment, there will be several opportunities throughout the flight 

envelope to conduct these tests. The evaluation of the manual disconnects 

would include the forces required for an autopilot disengage, (not too 

light, but not too high), the transients characteristics associated with each 

one (that is, what type of motion and “g’s” that are produced), and the 

warnings that are generated. 

 (2) Repeated Test Conditions. After the initial tests have been successfully 

completed, the above tests should be repeated at higher altitudes and airspeeds until 

reaching MMO at high cruise altitudes. 
7/17/06 AC 25.1329-1B 

 Appendix 3 

 3-1 

APPENDIX 3. DEFINITIONS 

The following definitions apply to the requirements of § 25.1329 and the guidance 

material provided in this AC. They should not be assumed to apply to the same or similar 

terms used in other regulations or ACs. Terms for which standard dictionary definitions 

apply are not defined in this AC. 

TERM DEFINITION 

Abnormal Condition See non-normal condition. 

Advisory Crew awareness is required and subsequent crew action 

may be required. 

Alert A generic term used to describe a flight deck indication 

meant to attract the attention of the flightcrew to a nonnormal operational or airplane system condition without 

implying the degree or level of urgency for recognition and 

corrective action by the crew. Warnings, cautions, and 

advisories are considered alerts. 

Analysis The terms “analysis” and “assessment” are used 

throughout. Each has a broad definition and the two terms 

are to some extent interchangeable. However, the term 

analysis generally implies a more specific, more detailed 

evaluation, while the term assessment may be a more 

general or broader evaluation but may include one or more 

types of analysis. 

Arm A condition where the intent to transition to a new mode or 

state has been established but the criteria necessary to 

make that transition has not been satisfied. 

Assessment See the definition of analysis above. 
7/17/06 AC 25.1329-1B 

 Appendix 3 

 3-2 

TERM DEFINITION 

Autopilot The autopilot function provides automatic control of the 

airplane, typically in pitch, roll, and yaw. The term 

includes the sensors, computers, power supplies, servomotors/actuators and associated wiring, necessary for its 

function. It includes any indications and controllers 

necessary for the pilot to manage and supervise the system. 

Any part of the autopilot system that remains connected to 

the primary flight controls when the autopilot is not in use 

is regarded as a part of the primary flight controls. 

Autothrust The autothrust function provides automatic control of the 

thrust of the airplane. The term includes the sensors, 

computers, power supplies, servo-motors/actuators and 

associated wiring, necessary for its function. It includes 

any indications and controllers necessary for the pilot to 

manage and supervise the system. Any part of the 

autothrust that remains connected to the engine controls 

when the autothrust is not in use is regarded as a part of the 

engine control system. 

Caution A flight deck indication that alerts the flightcrew to a nonnormal operational or airplane system condition that 

requires immediate crew awareness. Subsequent pilot 

corrective compensatory action will be required. 

Cognitive Task Analysis An analysis that focuses on the mental processes, skills, 

strategies, and use of information required for task 

performance. 

Complex A system is complex when its operation, failure modes, or 

failure effects are difficult to comprehend without the aid 

of analytical methods. 

Conformal Positioned and scaled with respect to the outside view. 
7/17/06 AC 25.1329-1B 

 Appendix 3 

 3-3 

TERM DEFINITION 

Control Wheel Steering 

(CWS) 

A flight guidance system (FGS) function which, when 

engaged, enables the pilot/first officer to manually fly the 

airplane by positioning the flight control surfaces using the 

autopilot servos. The positions of the flight deck controls 

(for example, control column, control wheel) are 

determined by the FGS, which converts them into autopilot 

servo commands. The autopilot servos, in turn, drive the 

appropriate flight control surfaces. 

Conventional A system is considered to be conventional if its 

functionality, the technological means used to implement 

its functionality, and its intended usage are all the same as, 

or closely similar to, that of previously approved systems 

that are commonly used. 

Engage A steady state that exists when a flightcrew request for 

mode or system functionality has been satisfied. 

Error An omission or incorrect action by a crewmember or 

maintenance personnel, or a mistake in requirements, 

design, or implementation. 

Failure 

NOTE: 

An occurrence that affects the operation of a component, 

part, or element such that it can no longer function as 

intended (this includes both loss of function and 

malfunction). 

Errors may cause failures, but are not considered to be 

failures. 

Failure Condition A condition having an effect on the airplane and/or its 

occupants, either direct or consequential, which is caused 

or contributed to by one or more failures or errors, 

considering flight phase and relevant adverse operational 

or environmental conditions, or external events. 

Fail Operational System A system capable of completing an operation, following 

the failure of any single element or component of that 

system, without pilot action. 
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TERM DEFINITION 

Fail Passive System A system which, in the event of a failure, results in: 

(a) no significant deviation in the aircraft flight path 

or attitude, and 

(b) no out-of-trim condition at disengagement that is 

not easily controlled by the pilot. 

Flight Director (FD) A visual cue or set of cues that are used during manual 

control of the airplane as command information to direct 

the pilot how to maneuver the airplane, usually in pitch, 

roll and/or yaw, to track a desired flight path. The FD, 

displayed on the pilot's primary head down attitude 

indicator (ADI) or head up display (HUD), is a component 

of the flight guidance system (FGS) and is integrated with 

airborne attitude, air data, and navigation (NAV) systems. 

Flight Guidance System 

(FGS) 

A system consisting of one or more of the following 

elements: 

• autopilot 

• flight director (FD), and 

• automatic thrust control. 

A flight guidance system (FGS) also includes any 

interactions with stability augmentation and trim systems. 

Flight Management 

System (FMS) 

An aircraft area navigation (RNAV) system and associated 

displays and input/output devices(s) having complex multiwaypoint lateral (LNAV) and vertical (VNAV) capability 

(or equivalent), data entry capability, data base memory to 

store route and instrument flight procedure information, 

and display readout of NAV parameters. The flight 

management system (FMS) provides guidance commands 

to the FGS for the purpose of automatic navigation and 

speed control when the FGS is engaged in an appropriate 

mode or modes (for example, VNAV, LNAV, RNAV). 
7/17/06 AC 25.1329-1B 

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TERM DEFINITION 

Head Up Display (HUD) A transparent optical display system located level with and 

between the pilot and the forward windscreen. The HUD 

displays a combination of control, performance, navigation 

(NAV), and command information superimposed on the 

external field of view. It includes the display element, 

sensors, computers and power supplies, indications and 

controls. It is integrated with airborne attitude, air data and 

navigation NAV systems, and as a display of command 

information is considered a component of the flight 

guidance system (FGS). 

Inadvertent A condition or action that was not planned or intended. 

Latent Failure A failure is latent until it is made known to the flightcrew 

or maintenance personnel. A significant latent failure is 

one which would, in combination with one or more 

specific failures or events, result in a hazardous or 

catastrophic failure condition. 

Limit Flight Envelope The outermost flight envelope of the airplane, normally 

associated with the limit loads of the airplane structure. 

Minimum Engage 

Height 

The minimum height after takeoff at which the flightcrew 

is permitted to engage the autopilot. Also referred to as 

minimum use height takeoff. See “minimum use height.” 

Minimum Use Height 

(MUH) 

A height specified during airworthiness demonstration or 

review above which, under standard or specified 

conditions, a probable failure of a system is not likely to 

cause a significant path displacement unacceptably 

reducing flight path clearance from specified reference 

surfaces (for example, airport elevation) or specified 

obstacle clearance surfaces. 

Minor Transient See “Transient.” 

Mode A mode is system configuration that corresponds to a 

single (or set of) FGS behavior(s). 
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TERM DEFINITION 

Mode Reversion An automatic mode change that returns the FGS to a 

previously engaged mode or to a pre-determined “default” 

mode for that condition. This may occur due to several 

reasons, such as specific criteria becoming satisfied or 

because the FGS cannot perform the currently selected 

operation. This type of mode change is not requested by 

the flightcrew and therefore may be unexpected. 

Non-Normal Condition A condition or configuration of the airplane that would not 

normally be experienced during routine flight operations—

usually due to failures. Also includes unusual airplane 

ferry configurations, such as transporting a spare engine or 

landing gear locked down ferry flight. 

Normal Condition Any fault free condition typically experienced in normal 

flight operations. Operations typically well within the 

aircraft flight envelope, and with routine atmospheric and 

environmental conditions. 

Normal Flight Envelope The range of altitude and operating speeds that are defined 

by the airplane manufacturer as consistent with conducting 

flight operations for which the airplane is designed. This 

envelope is generally associated with practical, routine 

operation and/or prescribed conditions, whether all-engine 

or engine inoperative. 

Other Than Normal 

Condition 

The summation of rare normal and non-normal conditions. 

Override (of engaged 

FGS functions) 

An action taken by the flightcrew intended to prevent, 

oppose, or alter an operation being conducted by a flight 

guidance function, without first disengaging that function. 

Rare Normal Condition A fault-free condition that is experienced infrequently by 

the airplane due to severe environmental conditions (for 

example, significant wind, turbulence, or asymmetric 

icing). 

Redundancy The presence of more than one independent means for 

accomplishing a given function or flight operation. 
7/17/06 AC 25.1329-1B 

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TERM DEFINITION 

Select The flightcrew action of requesting functionality or an end 

state condition. 

Significant Transient See “Transient.” 

Stability Augmentation 

System 

Automatic systems that provide or enhance stability for 

specific aerodynamic characteristics of an airplane (for 

example, yaw damper, longitudinal stability augmentation 

system, Mach trim). 

System A combination of components, parts, and elements that are 

inter-connected to perform one or more specific functions. 

Transient A disturbance in the control or flight path of the airplane 

that is not consistent with response to flightcrew inputs or 

environmental conditions. 

 

Minor Transient: A transient that would not significantly reduce safety 

margins and which involves flightcrew actions that are 

well within their capabilities involving a slight increase in 

flightcrew workload or some physical discomfort to 

passengers or cabin crew 

 Significant Transient: A transient that would lead to a significant reduction in 

safety margins, an increase in flightcrew workload, 

discomfort to the flightcrew, or physical distress to 

passengers or cabin crew, possibly including non-fatal 

injuries 

 

 NOTE: The flightcrew should be able to respond to any significant 

transient without: 

• Exceptional piloting skill, alertness, or strength, 

• Forces greater than those given in § 25.143(c), and 

• Accelerations or attitudes in the airplane that might 

result in further hazard to secured or non-secured 

occupants. 

Warning A flight deck indication that alerts the flightcrew to a nonnormal operational or airplane system requiring immediate 

recognition. Immediate corrective or compensatory action 

by the flightcrew is required. 

 
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7/17/06 AC 25.1329-1B 

 Appendix 4 

 4-1 

APPENDIX 4. ACRONYMS 

AC Advisory Circular 

ADI Attitude Director Indicator 

AFM Airplane Flight Manual 

AGL Above Ground Level 

AIM Airman’s Information Manual 

ARP Aerospace Recommended Practice 

ATC Air Traffic Control 

AWO All Weather Operations 

CDI Course Deviation Indicator 

CG Center of Gravity 

CFR Code of Federal Regulations 

CWS Control Wheel Steering 

DA Decision Altitude 

DA(H) Decision Altitude (Height) 

DME Distance Measuring Equipment 

EFIS Electronic Flight Instrument System 

EVS Enhanced Vision System 

FAA Federal Aviation Administration 

FCOM Flightcrew Operations Manual 

FD Flight Director 

FGS Flight Guidance System 

FL Flight Level 

FMA Flight Mode Annunciator 

FMS Flight Management System 

GA Go-Around 

GLS GNSS Landing System 

GNSS Global Navigation Satellite System 

GPWS Ground Proximity Warning System 

HDD Head Down Display 

HUD Head Up Display 

IAS Indicated Airspeed 

ICAO International Civil Aviation Organization 

ILS Instrument Landing System 

IMA Integrated Modular Avionics 

IMC Instrument Meteorological Conditions 
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 4-2 

LNAV Lateral Navigation 

LOC Instrument Landing System Localizer 

MDA(H) Minimum Descent Altitude (Height) 

MLS Microwave Landing System 

MSL Mean Sea Level 

MSP Mode Select Panel 

MUH Minimum Use Height [autopilot] 

NAV Navigation 

ND Navigation Display 

NDB Non Directional Beacon 

NPRM Notice of Proposed Rulemaking 

PAN-OPS Procedures for Air Navigation Services–Operations 

PF Pilot Flying 

PFD Primary Flight Display 

PNF Pilot Not Flying 

RNAV Area Navigation 

RNP Required Navigation Performance 

RTO Rejected Takeoff 

RVSM Reduced Vertical Separation Minimum 

SAE Society of Automotive Engineers 

TCAS Traffic Alert and Collision Avoidance System 

TCS Touch Control Steering 

TERPS Terminal Instrument Procedures 

TO Takeoff 

TOGA Takeoff or Go–Around 

VDF/ MDF Demonstrated Flight Dive Speed/Demonstrated 

Flight Dive Mach 

VMO/ MMO Maximum Operating Limit Speed/Maximum 

Operating Limit Mach 

VR Rotation Speed 

VSR Reference Stall Speed (See § 25.103) 

V2 Takeoff Safety Speed 

VMC Visual Meteorological Conditions 

VNAV Vertical Navigation 

VOR Very High Frequency Omnirange Station 

WAT Weight Altitude Temperature 

xLS ILS, MLS, or GLS