0 

U.S. Department 

of Transportation 

Federal Aviation 

Administration 

Advisory

Circular 

Subject: Approval of Flight Guidance Systems Date: I 0/27/14 AC No: 25 .1329-IC 

Initiated By: ANM-11 0 

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, and automatic thrust 

control as well as any interactions with stability augmentation and trim functions. This revision 

incorporates minor changes associated with issuance of the final rule for Airplane and Engine 

Certification Requirements in Supercooled Large Drop, Mixed Phase, and Ice Crystal Icing 

Conditions. 

If you have suggestions for improving this AC, you may use the feedback form at the end ofthis 

AC. 

Michael Kaszycki 

Acting Manager, Transport Airplane Directorate 

Aircraft Certification Service 
10/27/14 AC 25.1329-1C

CONTENTS

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ii

INTRODUCTION ....................................................................................................................... 1

1. Purpose............................................................................................................................. 1

2. Applicability .................................................................................................................... 1

3. Cancellation ..................................................................................................................... 1

4. Related Regulations, ACs, and Industry Documents....................................................... 1

a. Regulations ................................................................................................................ 1

b. Advisory Circulars..................................................................................................... 2

c. Related Industry Standards........................................................................................ 4

d. Other FAA documents............................................................................................... 4

5—10 [RESERVED] ............................................................................................................ 5

CHAPTER 1. BACKGROUND................................................................................................... 6

11. General............................................................................................................................. 6

a. AC 25.1329-1C.......................................................................................................... 6

b. Scope.......................................................................................................................... 6

12. Evolution of FGS ............................................................................................................. 6

a. Complexity................................................................................................................. 6

b. Guidance considerations............................................................................................ 6

13—16 [RESERVED] ............................................................................................................ 6

CHAPTER 2. OVERVIEW OF FLIGHT GUIDANCE SYSTEMS ................................................ 7

17. Purpose of FGS................................................................................................................ 7

18. Components..................................................................................................................... 7

a. Included in definition................................................................................................. 7

b. Excluded from definition ........................................................................................... 7

19. FGS Functions ................................................................................................................. 7

a. Elements of FGS........................................................................................................ 7

b. Approach and landing system.................................................................................... 7

c. Flight management system ........................................................................................ 7

d. Design philosophy ..................................................................................................... 8

e. Description of FGS behavior and operation .............................................................. 8
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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20. Compliance with § 25.1329 ............................................................................................. 8

a. General....................................................................................................................... 8

b. Relevant paragraphs................................................................................................... 8

21—26 [RESERVED] .......................................................................................................... 13

CHAPTER 3. FGS ENGAGEMENT, DISENGAGEMENT, INDICATIONS, AND OVERRIDE.. 14

27. Autopilot Engagement, Disengagement, and Indications.............................................. 14

a. Autopilot engagement.............................................................................................. 14

(1) Single switch action ........................................................................................ 14

(2) Engagement status of autopilot....................................................................... 14

(3) Multiple autopilots.......................................................................................... 14

(4) Acceptable transients...................................................................................... 14

(5) FD not engaged............................................................................................... 14

(6) FD engaged ..................................................................................................... 14

b. Autopilot disengagement ......................................................................................... 15

(1) General............................................................................................................ 15

(2) Autopilot disengagement alerts....................................................................... 16

(3) Quick disengagement control ......................................................................... 16

(4) Alternative means of autopilot disengagement............................................... 17

(5) Flightcrew pitch trim input ............................................................................. 17

28. FD Engagement, Disengagement, and Indications........................................................ 18

a. FD engagement ........................................................................................................ 18

(1) General............................................................................................................ 18

(2) Guidance cue(s) .............................................................................................. 19

(3) Reactive windshear guidance system.............................................................. 19

b. FD disengagement ................................................................................................... 19

29. Autothrust Engagement, Disengagement, and Indications............................................ 19

a. Autothrust engagement ............................................................................................ 19

(1) Indication ........................................................................................................ 19

(2) Accessibility of controls ................................................................................. 19
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CONTENTS (CONTINUED)

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(3) Inadvertent engagement/disengagement......................................................... 19

(4) Automatic engagement ................................................................................... 19

b. Autothrust disengagement ....................................................................................... 20

(1) Indication of disengagement........................................................................... 20

(2) Inadvertent disengagement ............................................................................. 20

(3) Consequence of disengagement...................................................................... 20

(4) Autothrust quick disengagement control ........................................................ 20

30. Override of the FGS....................................................................................................... 21

a. General..................................................................................................................... 21

b. Autopilot override.................................................................................................... 21

(1) With automatic disengagement....................................................................... 21

(2) Without automatic disengagement.................................................................. 21

c. Autothrust override .................................................................................................. 22

(1) Force required ................................................................................................. 22

(2) Response to override....................................................................................... 22

(3) Engagement status with override .................................................................... 22

31. FGS Engagement Mode Compatibility.......................................................................... 22

a. FGS mode engagement philosophy ......................................................................... 22

b. Engagement mode compatibility ............................................................................. 23

32—41 [RESERVED] .......................................................................................................... 23

CHAPTER 4. CONTROLS, INDICATIONS, AND ALERTS ..................................................... 24

42. General........................................................................................................................... 24

a. Human-machine interface........................................................................................ 24

b. Design of controls, indications, and alerts............................................................... 24

c. Evaluation of information provided by FGS ........................................................... 24

43. FGS Controls ................................................................................................................. 24

a. General..................................................................................................................... 24

b. Design considerations.............................................................................................. 24

(1) Differentiation of knob shape and position..................................................... 24
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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(2) Design to support correct selection of target values....................................... 24

(3) Commonality across aircraft types.................................................................. 25

(4) Positioning of controls and related indications............................................... 25

(5) Inadvertent operation ...................................................................................... 25

44. FGS Mode Selection, Annunciation, and Indication ..................................................... 25

a. Annunciation of engagement of the FGS................................................................. 25

b. Description of FGS modes....................................................................................... 25

c. FGS mode annunciations......................................................................................... 25

(1) Active and armed modes................................................................................. 25

(2) Location .......................................................................................................... 25

(3) Discriminators................................................................................................. 26

(4) Color ............................................................................................................... 26

d. Mode changes .......................................................................................................... 26

(1) Operationally relevant mode changes............................................................. 26

(2) Attention-getting features ............................................................................... 26

(3) Use of alerts .................................................................................................... 26

e. Failure to engage or arm .......................................................................................... 26

f. FGS mode display and indications .......................................................................... 26

45. FGS Alerting, Warning, Caution, Advisory, and Status................................................ 27

a. General..................................................................................................................... 27

(1) Alerting provisions.......................................................................................... 27

(2) Determination of FGS capability.................................................................... 27

b. Speed protection alerts............................................................................................. 27

(1) Alerts to crew.................................................................................................. 27

(2) Alert specifications ......................................................................................... 27

(3) Consistency..................................................................................................... 27

(4) Nuisance alerts................................................................................................ 27

c. Loss of autopilot approach mode............................................................................. 27

d. Awareness of potential significant transient condition (“bark before bite”) ........... 28

(1) General............................................................................................................ 28
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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(2) Flightcrew alert(s)........................................................................................... 28

(3) Situations to consider for an alert ................................................................... 28

46. FGS Considerations for HUDs ...................................................................................... 29

a. General..................................................................................................................... 29

b. Characteristics of HUD guidance ............................................................................ 29

(1) Supplemental use display system.................................................................... 29

(2) External visual references............................................................................... 29

(3) Interference and occlusion .............................................................................. 30

(4) Display criteria................................................................................................ 30

(5) Conformal symbology .................................................................................... 30

(6) Low visibility approach .................................................................................. 30

(7) Clutter ............................................................................................................. 30

c. HUD/HDD compatibility......................................................................................... 30

(1) General............................................................................................................ 30

(2) Consistency and compatibility........................................................................ 30

d. Alerting issues.......................................................................................................... 31

(1) Alerts displayed on HUDs.............................................................................. 31

(2) Monochrome HUDs........................................................................................ 32

(3) Multi-color HUDs........................................................................................... 32

(4) Single HUD installation.................................................................................. 32

(5) Dual HUD installations................................................................................... 32

(6) GPWS/TCAS on HUD ................................................................................... 32

e. Upset/unusual attitude recovery guidance ............................................................... 32

(1) Guidance cues................................................................................................. 32

(2) Orientation cues.............................................................................................. 33

47—51 [RESERVED] .......................................................................................................... 33

CHAPTER 5. PERFORMANCE OF FUNCTION...................................................................... 34

52. General........................................................................................................................... 34

a. Intended function ..................................................................................................... 34
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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b. Effect of system tolerances...................................................................................... 34

53. Normal Performance...................................................................................................... 34

a. General..................................................................................................................... 34

b. Conditions to consider ............................................................................................. 34

c. Level of performance............................................................................................... 36

(1) Significant characteristics............................................................................... 36

(2) Configuration changes.................................................................................... 36

(3) Compliance evaluation.................................................................................... 36

(4) Prescribed levels of performance.................................................................... 36

(5) Equivalence of performance ........................................................................... 36

(6) Autopilot ......................................................................................................... 36

(7) Flight director.................................................................................................. 36

(8) Autothrust ....................................................................................................... 36

(9) Automatic trim................................................................................................ 36

54. Performance in Rare Normal Conditions....................................................................... 37

a. General..................................................................................................................... 37

(1) Range of conditions ........................................................................................ 37

(2) Effect on performance..................................................................................... 37

b. Conditions to consider ............................................................................................. 37

c. Level of performance............................................................................................... 38

(1) Performance standard...................................................................................... 38

(2) Autopilot authority limit ................................................................................. 39

(3) Masking of potential hazard............................................................................ 39

55. Icing Considerations in Normal and Rare Normal Conditions...................................... 39

a. Effect on performance.............................................................................................. 39

b. Alerts........................................................................................................................ 39

c. Effect on speed protection ....................................................................................... 39

56. Performance in Non-Normal Conditions....................................................................... 39

a. General..................................................................................................................... 39

b. Conditions to consider ............................................................................................. 39
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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c. Level of performance............................................................................................... 40

(1) Performance standard...................................................................................... 40

(2) Autopilot authority limit ................................................................................. 40

(3) Masking of potential hazard............................................................................ 41

57. Speed Protection ............................................................................................................ 41

a. General..................................................................................................................... 41

(1) Speed excursions............................................................................................. 41

(2) Compliance with § 25.1329(h) ....................................................................... 41

(3) Design standard............................................................................................... 42

b. Low-speed protection............................................................................................... 42

(1) General............................................................................................................ 42

(2) Factors to consider.......................................................................................... 42

(3) Low-speed protection during approach operations......................................... 43

(4) Windshear recovery guidance......................................................................... 44

c. High-speed protection.............................................................................................. 44

(1) General............................................................................................................ 44

(2) Factors to consider.......................................................................................... 44

58—61 [RESERVED] .......................................................................................................... 45

CHAPTER 6. CHARACTERISTICS OF SPECIFIC MODES.................................................... 46

62. General........................................................................................................................... 46

63. Lateral Modes ................................................................................................................ 46

a. Heading or track hold............................................................................................... 46

b. Heading or track select............................................................................................. 46

c. LNAV mode............................................................................................................. 46

(1) Lateral flight path............................................................................................ 46

(2) Automatic mode transitions............................................................................ 46

(3) Takeoff or go-around ...................................................................................... 46

64. Vertical Modes............................................................................................................... 47

a. Target altitude selection........................................................................................... 47
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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b. Vertical speed........................................................................................................... 47

c. Flight path angle ...................................................................................................... 47

d. Indicated airspeed/Mach hold (speed on elevator) .................................................. 47

e. Indicated airspeed/Mach select (speed on elevator) ................................................ 47

f. Flight level change (speed on elevator) ................................................................... 47

g. Altitude capture........................................................................................................ 47

(1) Mode transition ............................................................................................... 47

(2) Guidelines for altitude capture mode.............................................................. 48

h. Altitude hold ............................................................................................................ 49

(1) Entering mode................................................................................................. 49

(2) Mode transition annunciation ......................................................................... 49

(3) Barometric pressure adjustment...................................................................... 49

i. VNAV...................................................................................................................... 49

(1) FMS path......................................................................................................... 49

(2) Climb or descent ............................................................................................. 49

65. Multi-Axis Modes.......................................................................................................... 50

a. Takeoff mode ........................................................................................................... 50

(1) Vertical guidance ............................................................................................ 50

(2) Lateral guidance.............................................................................................. 50

(3) Rotation guidance ........................................................................................... 50

(4) Autothrust ....................................................................................................... 50

(5) Operation and performance............................................................................. 50

b. Go-around mode ...................................................................................................... 51

(1) Vertical elements ............................................................................................ 51

(2) Speed............................................................................................................... 51

(3) Autothrust ....................................................................................................... 51

(4) Engagement..................................................................................................... 52

(5) All engine and engine out capability .............................................................. 52

c. Approach mode........................................................................................................ 52

(1) Final approach path......................................................................................... 52
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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(2) Mode annunciations........................................................................................ 52

(3) Sub-modes....................................................................................................... 52

(4) Mode engagement sequence ........................................................................... 52

66. Autothrust Modes........................................................................................................... 52

a. Thrust mode ............................................................................................................. 52

b. Speed mode.............................................................................................................. 53

c. Retard mode ............................................................................................................. 53

67. Special Considerations for VNAV Approach Operations Related to Selecting a Target 

Altitude .......................................................................................................................... 53

a. Approach operations................................................................................................ 53

b. VNAV path operations ............................................................................................ 53

68. Control Wheel Steering (Control Steering through the Autopilot)................................ 53

a. General..................................................................................................................... 53

b. Design ...................................................................................................................... 53

(1) Control forces.................................................................................................. 53

(2) Bank and pitch attitudes.................................................................................. 53

(3) Control behavior ............................................................................................. 54

(4) Stall and stall recovery.................................................................................... 54

(5) Adjustments to trim......................................................................................... 54

(6) Takeoff and landing ........................................................................................ 54

(7) Automatic reversion to CWS mode ................................................................ 54

c. Terminology and application of advisory material.................................................. 54

(1) Touch control steering .................................................................................... 55

(2) Supervisory override....................................................................................... 55

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

and FGS ......................................................................................................................... 55

a. Speed protection....................................................................................................... 55

b. System degradation.................................................................................................. 55

70—74 [RESERVED] .......................................................................................................... 55
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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CHAPTER 7. FGS INTEGRATION.......................................................................................... 56

75. General........................................................................................................................... 56

76. System Integration Issues .............................................................................................. 56

a. Common and cascading failure modes.................................................................... 56

b. Risk of error............................................................................................................. 56

c. System safety analysis ............................................................................................. 56

77. Functional Interfaces...................................................................................................... 56

a. Interfacing sensors................................................................................................... 56

b. Potential inconsistencies between systems.............................................................. 57

c. Function and performance ....................................................................................... 57

78—82 [RESERVED] .......................................................................................................... 57

CHAPTER 8. SAFETY ASSESSMENT ................................................................................... 58

83. General........................................................................................................................... 58

a. Basic safety requirements........................................................................................ 58

b. Safety assessment..................................................................................................... 58

c. Validation of safety assessment............................................................................... 58

d. Coordination with the FAA ..................................................................................... 58

84. FGS Failure Conditions................................................................................................. 58

a. Functionality ............................................................................................................ 58

b. Effect of failure condition........................................................................................ 59

85. Type and Severity of Failure Conditions....................................................................... 59

a. Types of failure conditions ...................................................................................... 59

(1) Loss of function .............................................................................................. 59

(2) Malfunction..................................................................................................... 59

b. Cause........................................................................................................................ 59

c. Severity .................................................................................................................... 60

d. Assessment of failure conditions............................................................................. 60

(1) Functional failures .......................................................................................... 60

(2) Airframe loads ................................................................................................ 60

(3) Assessing total effect of failure....................................................................... 60
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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86. Mitigation of Failure Conditions ................................................................................... 60

a. General..................................................................................................................... 60

b. Identification of methods......................................................................................... 61

c. FGS de-selection...................................................................................................... 61

87. Validation of Failure Conditions ................................................................................... 61

a. General..................................................................................................................... 61

b. Validation options.................................................................................................... 61

c. Validation methodology........................................................................................... 61

88. Specific Considerations ................................................................................................. 62

a. FGS action during ground operations...................................................................... 62

b. FGS operations in close proximity to the ground .................................................... 62

(1) Takeoff............................................................................................................ 62

(2) Approach......................................................................................................... 63

c. Cruise operations ..................................................................................................... 64

d. Asymmetric thrust during autothrust operation ....................................................... 64

89. Failure to Disengage the FGS........................................................................................ 65

90—97 [RESERVED] .......................................................................................................... 65

CHAPTER 9. COMPLIANCE DEMONSTRATION USING FLIGHT TEST 

AND SIMULATION.................................................................................................................. 66

98. General........................................................................................................................... 66

a. Methods of compliance demonstration.................................................................... 66

b. Criteria for establishing compliance ........................................................................ 66

c. Elements of certification flight test.......................................................................... 66

d. Pilot-in-the-loop evaluation ..................................................................................... 66

e. Other applicable guidance material for FGS takeoff and/or approach modes......... 66

(1) Criteria to consider.......................................................................................... 66

(2) Procedures to show compliance...................................................................... 67

f. Relationship of AC chapters.................................................................................... 67

99. Performance Demonstration (Fault Free): § 25.1301 ................................................... 67

a. General..................................................................................................................... 67
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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(1) Certification plan ............................................................................................ 67

(2) Evaluating performance .................................................................................. 68

(3) Evaluating compatibility of mode controls, indications, and alerts................ 68

b. Normal performance ................................................................................................ 68

(1) Demonstration................................................................................................. 68

(2) Evaluation ....................................................................................................... 68

(3) Expected errors ............................................................................................... 68

c. Performance in “other than normal” conditions ...................................................... 68

(1) Test program................................................................................................... 68

(2) Evaluation of FGS........................................................................................... 69

(3) Additional guidance for windshear................................................................. 69

d. Specific performance conditions.............................................................................. 69

(1) Icing considerations........................................................................................ 69

(2) Low-speed protection...................................................................................... 70

(3) High-speed protection..................................................................................... 70

(4) Go-around ....................................................................................................... 70

(5) Steep approach (special authorization)........................................................... 71

(6) Indication of and response to an out-of-trim condition .................................. 72

e. Flight director/head up display ................................................................................ 72

(1) General............................................................................................................ 72

(2) Specific demonstrations for HUD................................................................... 73

(3) Simulator demonstration for HUD.................................................................. 74

f. Flightcrew override of the FGS ............................................................................... 75

(1) Autopilot override........................................................................................... 75

(2) Autothrust override ......................................................................................... 75

(3) Pitch trim system evaluation during autopilot override.................................. 75

100. Failure Conditions Requiring Validation: § 25.1309.................................................... 76

a. General..................................................................................................................... 76

(1) Safety assessment............................................................................................ 76

(2) Evaluation of failure conditions...................................................................... 76
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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b. Validation elements ................................................................................................. 76

(1) Assessment...................................................................................................... 76

(2) Failure condition ............................................................................................. 76

(3) Pilot recognition.............................................................................................. 77

(4) Pilot reaction time........................................................................................... 78

(5) Pilot recovery.................................................................................................. 79

c. Takeoff..................................................................................................................... 79

(1) Worst-case failure condition........................................................................... 79

(2) Failures to be evaluated .................................................................................. 79

(3) On runway guidance ....................................................................................... 79

d. Climb, cruise, descent, and holding......................................................................... 79

e. Maneuvering ............................................................................................................ 79

f. Approach.................................................................................................................. 79

(1) Approach with vertical path reference............................................................ 80

(2) Approach without vertical path reference....................................................... 81

(3) Steep approach................................................................................................ 81

g. Specific conditions................................................................................................... 81

(1) Failure conditions to be evaluated .................................................................. 81

(2) Failure to disengage ........................................................................................ 81

101. Criteria Supporting Operational Use of Autopilot......................................................... 81

a. Typical flight operations.......................................................................................... 81

b. Operations in close proximity to the ground............................................................ 82

(1) General............................................................................................................ 82

(2) Considerations................................................................................................. 82

(3) Autopilot engagement altitude/height after takeoff........................................ 82

(4) Autopilot engagement during approach.......................................................... 83

(5) Circling approach............................................................................................ 85

c. Climb, cruise, descent, and holding......................................................................... 85

d. Maneuvering ............................................................................................................ 85

102. Automatic Disengagement of the Autopilot .................................................................. 85
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

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103. Assessment of Human Factors....................................................................................... 85

a. General..................................................................................................................... 85

b. Flight evaluations..................................................................................................... 86

(1) During certification......................................................................................... 86

(2) Prior to certification ........................................................................................ 86

(3) Credit to establish FGS compliance................................................................ 86

(4) Simulation versus flight tests.......................................................................... 86

c. Structured subjective evaluations ............................................................................ 87

d. Pilot training and experience ................................................................................... 87

e. Evaluating new or unique design features............................................................... 87

f. Certification planning documentation...................................................................... 87

104—108 [RESERVED] ................................................................................................. 87

CHAPTER 10. AIRPLANE FLIGHT MANUAL......................................................................... 88

109. General........................................................................................................................... 88

a. AFM terminology .................................................................................................... 88

b. Low-visibility operations......................................................................................... 88

110. Information Supporting Operational Use of Autopilot.................................................. 88

a. General..................................................................................................................... 88

b. Specifying a MUH ................................................................................................... 88

c. Maximum displacement deviation........................................................................... 88

111. Limitations..................................................................................................................... 89

112. Non-Normal/Emergency Procedures............................................................................. 89

113. Normal Procedures......................................................................................................... 89

a. Documentation of normal procedures...................................................................... 89

b. Maximum displacement deviation........................................................................... 89

c. Procedures for use of FGS in icing conditions ........................................................ 89

d. Aircraft with published FCOMs.............................................................................. 89

e. Aircraft without published FCOMs......................................................................... 89
10/27/14 AC 25.1329-1C

CONTENTS (CONTINUED)

APPENDICES

Number Page

A Safety Assessment ..............................................................................................A-1

1. General............................................................................................................A-1

2. Identification of Failure Conditions................................................................A-1

3. Considerations when Assessing the Severity of Failure Condition Effects....A-2

4. Failure Condition Classification .....................................................................A-3

B Definitions........................................................................................................... B-1

C Acronyms............................................................................................................ C-1

TABLES

Number Page

1 Related 14 CFR Sections........................................................................................ 2

2 Related Advisory Circulars..................................................................................... 3

3 Related SAE Documents......................................................................................... 4

4 Related RTCA Documents ..................................................................................... 4

2-1 Cross-References for § 25.1329 Paragraphs to AC Chapters................................. 8

5-1 Examples of Normal Conditions........................................................................... 35

5-2 Examples of Rare Normal Conditions.................................................................. 38

5-3 Examples of Non-Normal Conditions .................................................................. 40

B-1 Terms and Definitions......................................................................................... B-1

C-1 Acronyms and Definitions.................................................................................. C-1

FIGURES

Number Page

9-1 Relationship between AC Chapters...................................................................... 67
10/27/14 AC 25.1329-1C

1

INTRODUCTION

1. Purpose. This AC describes an acceptable means for showing compliance with certain 

requirements of 14 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 (FDs), and automatic thrust control as well 

as any interactions with stability augmentation and trim functions.

2. Applicability.

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

modifiers, operators of certain transport category airplanes, foreign regulatory authorities, and 

Federal Aviation Administration (FAA) transport airplane type certificate engineers and their 

designees.

b. The guidance in this AC 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. We will consider other methods of showing 

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. If, however, 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. The material in this AC does not change or create any additional regulatory 

requirements, nor does it authorize changes in, or permit deviations from, regulatory 

requirements.

3. Cancellation. This AC cancels AC 25.1329-1B Change 1, dated October 16, 2012.

4. Related Regulations, ACs, and Industry Documents. The following related documents 

are provided for information purposes and are not necessarily directly referenced in this AC.

a. Regulations. Table 1 identifies the related 14 CFR regulations that are referenced in this 

AC. The full text of these regulations can be downloaded from the U.S. Government Printing 

Office e-CFR. A paper copy may be ordered from the Government Printing Office, 

Superintendent of Documents, Attn: New Orders, PO Box 371954, Pittsburgh, PA, 15250-7954.
10/27/14 AC 25.1329-1C

2

Table 1. Related 14 CFR Sections

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 Flightcrew Alerting

§ 25.1420 Supercooled large drop icing conditions

§ 25.1581 Airplane Flight Manual—General

§ 25.1583 Airplane Flight Manual—Operating limitations

§ 25.1585 Airplane Flight Manual—Operating procedures

b. Advisory Circulars. Table 2 identifies the ACs that are related to the guidance in this 

AC. The table lists the latest version of each AC at the time of publication. If any AC is revised 

after publication of this AC, you should refer to the latest version for guidance. You can find the 

latest version at the FAA website.
10/27/14 AC 25.1329-1C

3

Table 2. Related Advisory Circulars

Number Title Date

AC 20-115C Airborne Software Assurance July 19, 2013

AC 20-138D

Change 1

Airworthiness Approval of Positioning and 

Navigation Systems

October 10, 2014

AC 20-152 RTCA, Inc., Document RTCA/DO-254,

Design Assurance Guidance for Airborne 

Electronic Hardware

July 5, 2005

AC 20-174 Development of Civil Aircraft and Systems September 30, 2011

AC 25-7C Flight Test Guide for Certification of 

Transport Category Airplanes

October 16, 2012

AC 25-11B Electronic Flight Displays October 7, 2014

AC 25-12 Airworthiness Criteria for the Approval of 

Airborne Windshear Warning Systems in 

Transport Category Airplanes

November 2, 1987

AC 25.672-1 Active Flight Controls November 15, 1983

AC 25.1309-1A System Design and Analysis October 3, 2011

AC 25.1419-2 Compliance with the Ice Protection 

Requirements of §§ 25.1419(e), (f), (g), and 

(h)

October 27, 2009

AC 25.1581-1

Change 1

Airplane Flight Manual July 14, 1997

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

Minima for Takeoff, Landing, and Rollout

July 13, 1999

AC 120-29A Criteria for Approving Category I and 

Category II Weather Minima for Approach

August 12, 2002

AC 120-41 Criteria for Operational Approval of Airborne 

Wind Shear Alerting and Flight Guidance 

Systems

November 11, 1983

Note: This table identifies the latest version of each AC at the time of publication. If any 

AC is revised after publication of this AC, you should refer to the latest version for 

guidance.
10/27/14 AC 25.1329-1C

4

c. Related Industry Standards.

(1) The following documents are available from the Society of Automotive Engineers 

(SAE). You can purchase these documents by mail from SAE Customer Service, 

400 Commonwealth Drive, Warrendale, PA, 15096; by faxing your order information to 

(724) 776-0790; or from the SAE website.

Table 3. Related SAE Documents

Number Title

SAE ARP4754A Guidelines for Development of Civil Aircraft and 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 (HUD) Systems

SAE ARP5366 Autopilot, Flight Director, and Autothrust Systems

SAE AS8055 Minimum Performance Standard for Airborne Head Up Display

(HUD)

(2) The following documents are available from RTCA, Inc. (formerly the Radio 

Technical Commission for Aeronautics). You can purchase these documents by mail from 

RTCA Inc., 1150 18th Street NW., Suite 910, Washington, DC 20036; by faxing the Document 

Order Form to (202) 833-9434; or from the RTCA website.

Table 4. Related RTCA Documents

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

d. Other FAA documents.

(1) Federal Aviation Administration Human Factors Team Report on: The Interfaces 

Between Flightcrews and Modern Flight Deck Systems, dated June 18, 1996, is available at the

FAA website.
10/27/14 AC 25.1329-1C

5

(2) FAA Order 8260.3B, United States Standard for Terminal Instrument Procedures 

(TERPS), Changes 1 – 19, is available at FAA website.

5—10 [RESERVED]
10/27/14 AC 25.1329-1C

6

CHAPTER 1. BACKGROUND

11. General.

a. AC 25.1329-1C. This revision incorporates minor changes associated with issuance of 

the final rule for Airplane and Engine Certification Requirements in Supercooled Large Drop, 

Mixed Phase, and Ice Crystal Icing Conditions (79 FR 65508, November 4, 2014). Specifically, 

in this AC, we added a reference to § 25.1420, Supercooled large drop icing conditions, in

table 1; excepted “large drop icing” as an icing normal condition in table 5-1; and added “large 

drop icing” as a rare normal condition in table 5-2.

b. Scope. This advisory material provides information for flight guidance systems (FGS), 

which include any autopilot functions, FD functions, and automatic thrust control functions as 

well as any interactions with stability augmentation and trim functions.

12. Evolution of FGS.

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]
10/27/14 AC 25.1329-1C

7

CHAPTER 2. OVERVIEW OF FLIGHT GUIDANCE SYSTEMS

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. 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 equipment or component 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).

(3) Ground-based augmentation system (GBAS) landing system (GLS).

c. Flight management system. The FGS also includes those functions necessary to 

provide guidance and control in conjunction with a flight management system (FMS). The FGS 

does not include the flight planning and the generation of flight path and speed profiles tied to 
10/27/14 AC 25.1329-1C

8

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, and 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 (for 

example, §§ 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 1: 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 2: 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. 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.

Table 2-1. Cross-References for § 25.1329 Paragraphs to AC Chapters

14 CFR Section AC Chapter/Paragraph Reference

§ 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, and Indications
10/27/14 AC 25.1329-1C

9

14 CFR Section AC Chapter/Paragraph Reference

§ 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, and 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

§ 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
10/27/14 AC 25.1329-1C

10

14 CFR Section AC Chapter/Paragraph Reference

§ 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

§ 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
10/27/14 AC 25.1329-1C

11

14 CFR Section AC Chapter/Paragraph Reference

§ 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

§ 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
10/27/14 AC 25.1329-1C

12

14 CFR Section AC Chapter/Paragraph Reference

§ 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

§ 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
10/27/14 AC 25.1329-1C

13

14 CFR Section AC Chapter/Paragraph Reference

§ 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 B—Definitions

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]
10/27/14 AC 25.1329-1C

14

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) 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.

(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:
10/27/14 AC 25.1329-1C

15

(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, rare 

normal or non-normal conditions), disengagement of the autopilot may not result in a transient 

any greater than a significant transient. See § 25.1329(e). (For a discussion of rare normal and 

non-normal conditions, see Chapter 5, Performance of Function, paragraph 54, Performance in 

Rare Normal Conditions, and paragraph 56, Performance in Non-Normal Conditions, of this 

AC.) 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(d), or

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 45.d, Awareness of Potential Significant Transient 

Condition (“Bark before Bite”).)
10/27/14 AC 25.1329-1C

16

Note: See appendix B for definitions of significant transient and minor 

transient. For a discussion of normal conditions, rare normal conditions, and 

non-normal conditions, see Chapter 5, paragraphs 53, 54, and 56, respectively, 

of this AC.

(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 flightcrew members, 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 that 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 that requires only flightcrew awareness should cause a suitable advisory level 

alert to be issued to the flightcrew.

(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:
10/27/14 AC 25.1329-1C

17

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.

(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 flightcrew 

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.
10/27/14 AC 25.1329-1C

18

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). 

For airplanes with multiple FDs installed, all 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.
10/27/14 AC 25.1329-1C

19

(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 28.a(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 criteria in 

AC 25-12 and AC 120-41.

b. FD disengagement. There should 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 de-selected.

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 must provide the flightcrew positive indication 

that the system has been engaged. See § 25.1329(i).

(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 57.b, 

Low-Speed Protection, of this AC.) If such automatic engagement occurs, it should be clear to 

the flightcrew that automatic engagement has occurred. The automatic engagement may not 
10/27/14 AC 25.1329-1C

20

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.

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 
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21

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.

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.
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22

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 99.f, Flightcrew Override of the FGS, of this 

AC 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 1: 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 2: For control wheel steering considerations, see Chapter 6, 

Characteristics of Specific Modes, paragraph 68, Control Wheel Steering 

(Control Steering through the Autopilot), of this AC.

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 

remain safely 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.

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.
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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.

32—41 [RESERVED]
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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 must 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 of this AC 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 that 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.

(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.
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25

(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 to 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, of this AC.

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). 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.
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26

(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 25-11A, Flight Test Guide for Certification of 

Transport Category Airplanes), and should be evaluated for their consistency 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 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 25-11A) 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.

(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 
10/27/14 AC 25.1329-1C

27

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 of this AC.

(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, paragraph 57, Speed Protection, of this AC).

(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. See § 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. If the autopilot remains engaged and reverts to a non-approach mode, an 

appropriate aural warning and/or visual alert should be provided.
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28

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.

(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 45.d(3)(a) or 45.d(3)(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 that might result.
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29

(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, paragraph 30, Override of the FGS, of this AC for related 

material.

Note 1: 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.

Note 2: 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 HUDs.

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 for further 

information on this subject. See paragraph 4.c of this AC for information on how to obtain SAE 

documents.)

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 
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30

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 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 of this AC.

(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 de-clutter function.

c. HUD/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 
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31

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.

(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 (PF) on the HUD should also be displayed to the pilot not flying (PNF) 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 
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32

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 (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 attention-getting 

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 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 to another

display when an immediate maneuver is required (for example, TCAS resolution 

advisory, windshear).

(6) GPWS/TCAS on HUD. If a GPWS, windshear detection system, windshear 

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 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 
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33

(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.

47—51 [RESERVED]
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34

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 table 5-1 of this AC. This table does not fully define every condition that may be 

encountered during an airplane’s life and unequivocally categorize what can be considered to be 

a “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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35

Table 5-1. Examples of Normal Conditions

Condition Example

No failure 

conditions

All airplane systems that are associated with airplane 

performance are fully operational. Failures of those 

systems could impair the FGS’s ability to perform its 

functions.

Light to 

moderate winds

Constant wind in a specific direction that may cause a 

slight deviation in intended 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—that 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” and “large drop icing” discussed under “Rare 

Normal Conditions” in table 5-2 of this AC.

Note: Representative levels of the environmental effects should be established 

consistent with the airplane’s intended operation.
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36

c. Level of performance.

(1) Significant characteristics. Any performance characteristics that are operationally 

significant or operationally limiting must be identified with an appropriate statement or 

limitation in the airplane flight manual (AFM). See § 25.1581.

(2) Configuration changes. The FGS should perform its intended function during 

routine airplane configuration or power changes, including the operation of secondary flight 

controls and landing gear.

(3) Compliance evaluation. Evaluation of FGS performance for compliance should be 

based on the minimum level of performance needed for its intended functions. Subjective 

judgment may be applied to account for experience acquired from similar equipment and levels 

that have been established as operationally acceptable by the end-user.

(4) Prescribed levels of performance. There are certain operations that dictate a 

prescribed level of performance. When the FGS is intended for operations that require specific 

levels of performance, the use of FGS should be shown to meet those specific levels of 

performance (for example, low visibility operations—Category II and III operations, RVSM, 

RNP).

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

at least be equivalent to that expected of a pilot for a similar task. AC 25-7C and SAE ARP5366,

Autopilot, Flight Director, and Autothrust Systems, may prove useful for establishing the general 

behavior of the FGS. When integrated with navigation (NAV) sensors or the FMS, the FGS 

should satisfy the flight technical error tolerances expected for the use of those systems in 

performing their intended functions.

(6) Autopilot. The autopilot should provide smooth and accurate control without 

divergent or perceptible sustained nuisance oscillation.

(7) Flight director. 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 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.
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37

54. Performance in Rare Normal Conditions.

a. General.

(1) Range of conditions. The FGS will encounter a wide range of conditions in normal 

operations, some of which may be infrequent but levy a greater than average demand on the FGS 

capabilities. Certain environmental conditions, as discussed in paragraph 54.b below, are prime 

examples. The FGS performance during such rare normal conditions should be assessed.

(2) Effect on performance. Rare normal conditions may degrade the performance of 

the FGS but should be safe for operation of the FGS. The relative infrequency of such conditions 

may also be a factor in the flightcrew’s ability to detect and mitigate in a timely manner any 

limited capability of the FGS to cope with them.

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

conditions listed in the table 5-2 of this AC. This table does not fully define every condition that 

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

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

used during system development and testing. By the very nature of the phenomena involved, 

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

affect different airplane models in very different ways. These differences should be considered in 

determining how to characterize the severity of the conditions discussed in the table.
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Table 5-2. Examples of Rare Normal Conditions

Condition Example

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

large change in intended 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—that 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.

Large drop icing Applicable icing conditions covered by 14 CFR 

part 25, Appendix O.

Asymmetric icing Icing conditions that result in ice accumulations that 

causes the FGS, if engaged, to counter the 

aerodynamic effect of the icing conditions with a 

sustained pitch, roll, or yaw command that 

approaches its maximum authority.

Note: Airplanes intended to meet § 121.358 for windshear warning and 

guidance need FD windshear guidance. The FGS may also provide suitable 

autopilot control during windshear. See AC 25-12 and AC 120-41 for 

windshear guidance system requirements.

c. Level of performance.

(1) Performance standard. For rare normal conditions, the FGS should be designed to 

provide guidance or control, as appropriate, for the intended function of the active mode(s) in a 

safe and predictable manner, both within the normal flight envelope and for momentary 

excursions outside the normal flight envelope. If a determination is made that there are 

environmental conditions in which the FGS cannot be safely operated, appropriate operational 

limitations should be placed in the AFM.
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39

(2) Autopilot authority limit. Operations in rare normal conditions may result in 

automatic or pilot-initiated autopilot disengagement close to the limit of autopilot authority. 

Autopilot disengagement in rare normal conditions should meet the safety criteria for autopilot

disengagement found in Chapter 3, paragraph 27.b, Autopilot Disengagement, and the criteria for 

flight guidance alerting in Chapter 4, paragraph 45.d of this AC.

(3) Masking of potential hazard. It is not necessary that the FGS always be 

disengaged when rare normal conditions that may degrade its performance or capability are 

encountered. The FGS may significantly help the flightcrew during such conditions. However, 

the design should address the potential for the FGS to mask a condition from the flightcrew or 

otherwise delay appropriate flightcrew action. For discussion of alerting under such conditions,

see Chapter 4, paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status, of this AC.

55. Icing Considerations in Normal and Rare Normal Conditions.

a. Effect on performance. The FGS typically will be designed to provide acceptable 

performance in all standard airplane configurations. Operating an airplane in icing conditions can 

have significant implications on the aerodynamic characteristics of the airplane (for example, ice 

accretion on wings, tail, and engines) and, consequently, on FGS performance. Ice accretion may 

be slow, rapid, symmetric, or asymmetric. During autopilot operation, the flightcrew may not be 

aware of the gradual onset of icing conditions or the effect that the accumulation of ice is having 

on the handling qualities of the airplane.

b. Alerts. Means should be provided to alert the flightcrew when icing conditions begin to 

have an effect upon FGS performance, as described in Chapter 4, paragraph 45 of this AC.

c. Effect on speed protection. The implication of icing conditions on speed protection 

should be assessed. If the threshold of the stall warning system is adjusted due to icing 

conditions, appropriate adjustments should also be made to the FGS low-speed protection 

threshold, which is discussed in paragraph 57.b of this chapter.

56. Performance in Non-Normal Conditions.

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

of its normal flight envelope, due to failures or 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. Table 5-3 of this AC shows examples of non-normal 

conditions. The table does not fully define every condition that may be encountered during an 

airplane’s life and unequivocally categorize what can be considered to be a “non-normal 

condition.” Rather, the table is intended to give examples to be used during system development 

and testing. By the very nature of the phenomena involved, there will always be some 

subjectivity to these categorizations. Also, the same conditions may affect different airplane 

models in very different ways. These differences should be considered in determining how to 

characterize the severity of the conditions discussed in the following table.
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Table 5-3. Examples of Non-Normal Conditions

c. Level of performance.

(1) Performance standard. For non-normal conditions, the FGS should be designed 

to provide guidance or control, as appropriate, for the intended function of the active mode(s) in 

a safe and predictable manner, both within the normal flight envelope and for momentary 

excursions outside the normal flight envelope. If a determination is made that there are 

non-normal conditions in which the FGS cannot be safely operated, appropriate operational 

limitations should be placed in the AFM.

(2) Autopilot authority limit. Operations in non-normal conditions may result in 

automatic or pilot-initiated autopilot disengagement close to the limit of autopilot authority. 

Autopilot disengagement in non-normal conditions should meet the safety criteria for autopilot 

disengagement found in Chapter 3, paragraph 27.b and the criteria for flight guidance alerting in 

Chapter 4, paragraph 45.d of this AC.

Condition Term

Significant fuel

imbalance

Large variation of the amount of fuel between the two 

wing tanks (and center and tail tanks, if so equipped) 

that causes the FGS, if engaged, to counter the 

aerodynamic effect of the fuel imbalance with a pitch, 

roll, or yaw command that is approaching maximum 

system authority.

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 

that the FGS is certified to operate.

Inoperative Ice 

Detection/Protection 

System

Loss of ice detection/protection system on an airplane 

so equipped where the FGS is certified for operation 

in icing conditions with that failure present.
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41

(3) Masking of potential hazard. It is not necessary that the FGS always be 

disengaged when non-normal conditions that may degrade its performance or capability are 

encountered. The FGS may significantly help the flightcrew during such conditions. However, 

the design should address the potential for the FGS to mask a condition from the flightcrew or 

otherwise delay appropriate flightcrew action. For discussion of alerting under such conditions, 

see Chapter 4, paragraph 45 of this AC.

57. Speed Protection.

a. General.

(1) Speed excursions. The requirement for speed protection is based on the premise 

that reliance on flightcrew attentiveness to airspeed indications alone during FGS operation is 

not adequate to avoid unacceptable speed excursions outside the speed range of the normal flight 

envelope. Many existing FGS systems have no provisions to avoid speed excursions outside the 

normal flight envelope. Some FGSs will remain engaged until the aircraft slows to stall 

conditions and also to speeds well above maximum operating limit speed/maximum operating 

limit Mach (VMO/MMO).

(2) Compliance with § 25.1329(h). Standard stall warning and high-speed alerts are 

not always timely enough for the flightcrew to intervene to prevent unacceptable speed 

excursions during FGS operation. The intent of § 25.1329(h) is for the FGS to provide a speed 

protection function for all operating modes, so that the airspeed can be safely maintained within 

an acceptable margin of the speed range of the normal flight envelope. The FGS design may use 

any of the following ways or a combination of ways to provide acceptable speed protection:

(a) The FGS provides speed protection. In this case, the following are acceptable 

means to comply with this rule:

1 The FGS may detect the speed protection condition, alert the flightcrew, 

and provide speed protection control or guidance.

2 The FGS may detect the speed protection condition, alert the flightcrew, 

and then disengage the FGS.

3 The FGS may detect the speed protection condition, alert the flightcrew, 

and remain engaged in the active mode without providing speed protection control or guidance.

(b) Other systems provide speed protection. Other systems, such as the primary 

flight control system or the FMS (when in a vertical navigation (VNAV) mode) may be used to 

provide equivalent speed protection.

Note: If compliance with this requirement is based on use of alerting alone, 

the alerts should be shown to be appropriate and timely to ensure flightcrew 

awareness and enable the pilot to keep the airplane within an acceptable 

margin from the speed range of the normal flight envelope. For additional 

discussion of speed protection alerting, see Chapter 4, paragraph 45.b, Speed 

Protection Alerts.
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42

(3) Design standard.

(a) Interaction of FGS elements. The design should consider how and when the 

speed protection is provided for combinations of autopilot, FDs, and autothrust operation. Care 

should be taken to set appropriate values for transitioning into and out of speed protection such 

that the flightcrew does not consider the transitions a nuisance.

(b) Integration of pitch and thrust control. The speed protection function should 

integrate pitch and thrust control. Consideration should be given to automatically activating the 

autothrust function when speed protection is invoked. If an autothrust function is either not 

provided or is unavailable, speed protection should be provided through pitch control alone.

(c) Interaction of systems. The role and interaction of autothrust with elements of 

the FMS, the primary flight control system, and the propulsion system, as applicable, should be 

accounted for in the design for speed protection.

(d) Engine inoperative. Consideration should be given to the effects of an engine 

inoperative condition on the performance of speed protection.

b. Low-speed protection.

(1) General. When the FGS is engaged in any mode (with the possible exception of 

Approach, as discussed in paragraph 57.b(3) below) for which the available thrust is insufficient 

to maintain a safe operating speed, the low-speed protection function should be invoked to avoid 

unsafe speed excursions.

(2) Factors to consider. Activation of speed protection should take into account such 

factors as the phase of flight, turbulence and gusty wind conditions, and compatibility with the 

speed schedules. The low-speed protection function should activate at a suitable margin to stall 

warning that will not result in nuisance alerts. Consider the operational speeds, as specified in the 

AFM, for all-engine and engine-inoperative cases during the following phases of flight:

(a) Takeoff.

(b) Departure, climb, cruise, descent, and terminal area operations. During these 

flight phases, airplanes are normally operated at or above the minimum maneuvering speed for 

the given flap configuration.

Note: For high altitude operations, it may be desirable to incorporate 

low-speed protection at the appropriate engine out 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.
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43

Note: A low-speed alert and a transition to the Speed Protection mode at 

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

configuration has been found to be acceptable.

(3) Low-speed protection during approach operations.

(a) Non-interference. Speed protection should not interfere with the approach and 

landing phases of flight.

(b) Autothrust operation. It is assumed that with autothrust operating normally, the 

combination of thrust control and pitch control during the approach will be sufficient to maintain 

speed and desired vertical 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, MLS, GLS, 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 
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44

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, 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 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 high-speed 

protection through FGS control of airspeed.

(b) High-speed protection while in altitude hold mode:

1 Climbing to control airspeed is not desirable because departing an 

assigned altitude can be disruptive to air traffic control (ATC) and potentially hazardous (for 

example, in RVSM airspace). As long as the speed does not exceed a certain margin beyond 

VMO/MMO (for example, six knots), it is better that the FGS remain in Altitude Hold mode.

2 The autothrust function, if operating normally, should effect high-speed 

protection by limiting its speed reference to the normal speed envelope (that is, at or below 

VMO/MMO).
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45

3 The basic airplane high-speed alert should be sufficient for the pilot to 

recognize the overspeed condition and take corrective action to reduce thrust. However, if the 

airspeed exceeds a margin beyond VMO/MMO (for example, six knots), the FGS may transition 

from Altitude Hold to the Overspeed Protection mode and depart (that is, climb above) the 

selected altitude.

(c) High-speed protection during climbs and descents:

1 When the elevator channel of the FGS is not controlling airspeed, the 

autothrust function, if engaged, should reduce thrust, as needed, to prevent sustained airspeed 

excursions beyond VMO/MMO (for example, 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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46

CHAPTER 6. CHARACTERISTICS OF SPECIFIC MODES

62. General. There are certain operational modes of the FGS that have been implemented in

different ways in different airplanes and systems. The following paragraphs provide guidance 

and interpretative material that clarifies the operational intent of these modes, and criteria shown 

to be acceptable in current operations. The guidance in this chapter does not preclude other mode 

implementations. Pilot understanding of the mode behavior is especially important to avoid 

confusion. Therefore, mode behavior should be clearly annunciated, as described in Chapter 4,

paragraph 44, FGS Mode Selection, Annunciation, and Indication, of this AC.

63. Lateral Modes. This paragraph discusses modes that are implemented in many FGSs that 

are used primarily for lateral/directional control of the airplane. The criteria below identify 

acceptable mode operation, based on past operational experience gained from the use of these 

modes.

a. Heading or track hold. In the Heading or Track Hold mode, the FGS should maintain 

the airplane heading or track. When the airplane is in a bank when the Heading or Track Hold 

mode is engaged, the FGS should roll the airplane to a wings-level condition and maintain the 

heading or track when wings-level is achieved (typically less than 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. 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 engaged state at a 

point where the lateral flight path can be smoothly acquired and tracked.

(3) Takeoff or go-around. For an FGS incorporating the LNAV mode during the 

takeoff or go-around (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.
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47

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. For 

discussion of acceptable means of compliance for these situations, see Chapter 5, paragraph 57

of this AC.

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 low-speed 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/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/Mach select (speed on elevator). In the Airspeed/Mach Select 

mode, the FGS should acquire and maintain a selected airspeed or Mach. The selected airspeed 

or Mach may be either pre-selected or synchronized to the airspeed or Mach at the time of 

engagement.

f. Flight level change (speed on elevator). In the flight level (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.
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48

(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 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.
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49

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.

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.
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(b) Preclude VNAV climb. The FGS should preclude a VNAV climb, unless the 

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, of this AC.

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.
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(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 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 
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52

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.

(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 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.
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53

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 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 (Control Steering through the Autopilot).

a. General. In the control wheel steering (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 

that 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 controllability requirements of 

§ 25.143(d) 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 during CWS operations should be limited to those necessary for the normal operation 

of the airplane.
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54

Note 1: Normal operational limits are typically 35 degrees in roll and +20 degrees 

to –10 degrees in pitch.

Note 2: 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, in both 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 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 68.b, Design, of this AC applies to systems as described in paragraph 68.a, General, 

of this AC. It does not apply to those systems described in paragraphs 68.c(1) and 68.c(2) below. 
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55

These descriptions are intended to be generic. Implementation may vary from airplane to 

airplane.

(1) Touch control steering. A touch control steering (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-by-wire 

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, of this AC.)

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 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 
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57

as ILS, very high frequency omnirange (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.

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.
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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 

and AC 120-28D.

85. Type and Severity of Failure Conditions.

a. Types of failure conditions. The type of the FGS failure conditions will depend, to a 

large extent, upon the architecture, design philosophy, and implementation of the system. Types 

of failure conditions can include the following:

(1) Loss of function. A control or display element no longer provides control, 

guidance, or system annunciation/alerting.

(2) Malfunction. A control or display element performs in an inappropriate manner, 

including the following sub-types:

(a) Hardover. The control or display goes to full displacement in a brief period of 

time. The resultant effect on the flight path and occupants of the airplane and possible adverse 

structural effects are the primary concerns.

(b) Slowover. The control or display moves away from the correct control or 

display value over a relatively long period of time. The potential delay in recognizing the 

situation and the effect on the flight path are the primary concerns.

(c) Oscillatory. The control or display is replaced or augmented by an oscillatory 

element. In addition to difficulty controlling the aircraft flight path, there may be effects on 

structural integrity and occupant well-being.

b. Cause. Failure conditions are a manifestation of malfunctions in—

(1) Primary FGS elements, such as autopilot, FD, or HUD.
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(2) Interfacing sensors to the FGS, such as inertial or air data.

(3) Control and display elements, such as servos or PFDs.

(4) Interfacing systems, such as the FMS or primary flight controls.

(5) Basic aircraft services, such as electrical or hydraulic power.

c. Severity. The severity of the FGS failure conditions and their associated classifications 

will frequently depend on the phase of flight, airplane configuration, and the type of operation 

being conducted. The effect of any control system variability (for example, tolerances and 

rigging) on the failure conditions should be considered. The severity of the failure conditions can 

be mitigated by various design strategies. (See paragraph 86, Mitigation of Failure Conditions.)

d. Assessment of failure conditions.

(1) Functional failures. Appendix A, Safety Assessment, of this AC presents some 

considerations for use when assessing the type and severity of condition that results from 

functional failures. The classifications of failure conditions that have been identified on previous 

airplane certification programs are identified. The classifications of failure conditions should be 

agreed with the FAA during the § 25.1309 safety assessment process.

(2) Airframe loads. With the exception of the catastrophic failure condition, the 

classification of failure conditions leading to the imposition of airframe loads should be assessed 

in accordance with 14 CFR part 25, Subpart C, Structure, and § 25.1309, Equipment, systems, 

and installations. The assessment needs to account for loads occurring during the active 

malfunction, recovery, or continuation of the flight with the system in the failed state.

(3) Assessing total effect of failure. Complex integrated systems may require that the 

total effect resulting from single failure be assessed. For example, some failures may result in a 

number of failure conditions that, if assessed individually, may be considered major effects but 

when considered in combination may be hazardous. Special consideration concerning complex 

integration of systems can be found in Chapter 7, FGS Integration, paragraph 76, System 

Integration Issues, of this AC.

86. Mitigation of Failure Conditions.

a. General. The propagation of potential failure conditions to their full effect may be 

nullified or mitigated by a number of methods. These methods could include, but are not limited 

to, the following:

(1) Failure detection and monitoring.

(2) Fault isolation and reconfiguration.

(3) Redundancy.

(4) Authority limiting.
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(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 is recommended that there 

be early coordination with the FAA to identify any program necessary to validate any of these 

assumptions.

b. Validation options. The validation options for failure conditions include the following:

(1) Analysis.

(2) Laboratory Testing.

(3) Simulation.

(4) Flight Test.

c. Validation methodology. The validation testing and analysis should take account of 

architectural strategies (for example, redundant channels, high integrity components, rate 

limit/magnitude limiting, etc.). It is anticipated that the majority of failure conditions can be 

validated by analysis to support the probability aspect of the § 25.1309 assessment. It may be 

necessary to substantiate the severity of a failure condition effect by ground simulation or flight 

test. This is particularly true where pilot recognition of the failure condition requires 

justification, or if there is some variability in the response of the airplane. Failure conditions that 

are projected to be less probable than 10-7 per flight hour, independent of effect severity, need 

not be demonstrated with a flight test. Guidance on the assessment of traditional failure 

conditions is provided in Chapter 9 of this AC. New and novel functionality may require 

additional assessment methods to be agreed to with the authority.
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88. Specific Considerations. The following paragraphs identify specific considerations that 

should be given to potential failure conditions for various phases of flight.

a. FGS action during ground operations. The potential hazard that may result due to 

inappropriate autopilot, autothrust, or other system control action during maintenance operations 

should be assessed. This includes while the airplane is parked at the gate and during taxi 

operations. System interlocks or crew or maintenance procedures and placards may mitigate 

these hazards.

b. FGS operations in close proximity to the ground. The response of the airplane to

failures in the FGS could have implications for the safety of operations when the airplane is close 

to the ground. For the purpose of this AC, close to the ground can be assumed to be less than 

500 feet above the liftoff point or touchdown zone or a runway. A specific safety assessment 

should be performed if approval for operations where the autopilot is engaged or remains 

engaged in close proximity to the ground.

Note: AC 120-29A 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 of this AC.) The minimum engage height - takeoff based upon this assessment 

should be provided in the AFM.

(a) Vertical axis assessment.

1 The operational objective during the initial climb is to maintain an 

appropriate climb profile to assure obstacle clearance and to maintain an appropriate speed 

profile during climbout. (See Chapter 6 of this AC).

2 The FGS failure conditions should be assessed for the potential for a 

significant reduction in the net takeoff 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.
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2 The FGS failure conditions should be assessed for the potential for 

producing a bank angle that results in significant deviation from the runway track or intended 

track.

3 For failure conditions that are likely to occur more frequently than 

1 x 10-7 per flight hour or would have hazardous or catastrophic effects without pilot 

intervention, the ability of pilots to adequately perform the intervention should be evaluated and, 

as necessary, documented in AFM limitations or procedures.

(2) Approach.

(a) Assessment of failure conditions. If the autopilot is to remain engaged below 

500 feet above the touchdown zone during approach, an assessment of the effect of any 

significant FGS failure conditions on the net vertical 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 of this AC for details regarding how these failure conditions on approach should be 

assessed.

(b) Approach Operations.

1 A number of approach operations may be conducted using automatic 

flight control. These can include, but are not limited to, the following:

• ILS, MLS, and GLS.

• RNAV (for example, LNAV and VNAV).

• NAV (for example, VOR, LOC, and Backcourse).

• 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 (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.
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(c) Vertical axis assessment.

1 The operational objective during the approach is to maintain an 

appropriate descent profile to assure obstacle clearance and to maintain an appropriate speed 

profile.

2 FGS failure conditions should be assessed for the potential for a 

significant reduction in the approach flight path when below 500 feet above touchdown and for a 

significant increase in pitch attitude that results in the airplane speed dropping to unacceptable 

values.

3 For failure conditions that are 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 of this AC provides 

guidance and considerations for this phase of flight.

d. Asymmetric thrust during autothrust operation. During autothrust operation, it is 

possible that a failure (for example, engine failure, throttle lever jam, or thrust control cable jam) 

could result in a significant asymmetric thrust failure condition that may be aggravated by the 

continued use of the autothrust system. Because the FGS could potentially compensate for the 

asymmetric condition with roll (and possibly yaw) control, the pilot may not immediately be 

aware of the developing situation. Therefore, an alert should be considered as a means of 

mitigation to draw the pilot’s attention to an asymmetric thrust condition during FGS operation.
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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 27.b(4), Alternative Means of Autopilot Disengagement, of this 

AC should be assessed. If the assessment indicates the aircraft can be landed manually with the 

autopilot and/or autothrust engaged, this should be demonstrated in flight test.

90—97 [RESERVED]
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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 

AC.

c. Elements of certification flight test. The certification flight test program should 

investigate representative phases of flight and aircraft configurations used by the FGS. The 

program should evaluate all of the FGS modes through appropriate maneuvers and representative 

environmental conditions, including turbulence. Combinations of FGS elements (for example, 

autopilot engaged and autothrust disengaged) should be considered. Certain failure scenarios 

may require flight or simulator demonstration. The airplane should be equipped with sufficient 

instrumentation to record the parameters appropriate to the test. Examples of parameters that 

could be recorded include normal acceleration, airspeed, height, pitch and roll angles, and 

autopilot engagement state. The flight test instrumentation should not affect the behavior of the 

FGS or any other system.

d. Pilot-in-the-loop evaluation. An important part of the pilot-in-the-loop evaluation is 

validation of human factors. A thorough evaluation of the 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.

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 Approving Category I and II Weather Minima for 

Approach.
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(b) AC 120-28D, Criteria for Approval of Category III Weather Minima for 

Takeoff, Landing, and Rollout. The criteria need to be included in the requirements to be tested.

(2) Procedures to show compliance. Procedures that may be used to show 

compliance are provided in AC 25-7C. (See paragraph 181, Flight Guidance System -

§ 25.1329.)

f. Relationship of AC chapters. Figure 9-1 depicts the relationship between this chapter 

and the other chapters contained within this AC.

Figure 9-1. Relationship between AC Chapters

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

a. General.

(1) Certification plan. The certification plan should identify the specific functionality 

provided by the FGS. The flight test and/or simulator program will typically assess this 

functionality under representative operational conditions, including applicable airplane 
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configurations and a representative range of airplane weight, center of gravity (CG), and 

operational envelope.

(2) Evaluating performance. The performance of the FGS in each of its guidance and 

control modes should be evaluated. The acceptability of the performance of the FGS may be 

based on test pilot assessment, taking into account the experience acquired from similar 

equipment capabilities and the general behavior of the airplane. The FGS should be evaluated for 

its low and high maneuvering capability. AC 25-7C provides additional information on FGS test 

procedures.

(3) Evaluating compatibility of mode controls, indications, and alerts. The

acceptability of mode controls and annunciations, any associated alerts, and general 

compatibility with cockpit displays should be evaluated. The FGS should be free from 

unexpected disengagement, and no confusion should result from FGS mode changes. Additional 

considerations relating to the assessment of human factors is provided in Chapter 9, 

paragraph 103, Assessment of Human Factors, of this AC.

b. Normal performance.

(1) Demonstration. Normal performance is considered to be performance during 

operations well within the airplane’s flight envelope and with routine atmospheric and 

environmental conditions. Normal performance should be demonstrated over a range of 

conditions that represent typical conditions experienced in operational use.

(2) Evaluation. The FGS should be evaluated to determine the acceptability of the 

following characteristics:

(a) Stability and tracking of automatic control elements.

(b) Controllability and tracking of guidance elements.

(c) Acquisition of flight paths for capture modes.

(d) Consistency of integration of modes (of this chapter).

(3) Expected errors. Performance should be assessed in the presence of errors that can 

reasonably be expected in operation (for example, the incorrect selection of approach speed).

c. Performance in “other than normal” conditions. Other than normal conditions are 

the combination of “rare normal” and “non-normal” conditions. Performance in “other than 

normal conditions” is considered to be performance of the system under conditions that are 

experienced infrequently by the airplane during operational use. Examples of rare normal and 

non-normal conditions are given in Chapter 5, paragraph 54 and paragraph 56 of this AC.

(1) Test program. The test program should assess the FGS performance in more 

challenging environmental conditions (for example, winds, wind gradients, and various levels of 

turbulence) as the opportunity presents itself. Rare environmental conditions may require the 

FGS to operate at the limits of its capabilities. The intent of the evaluation is to assess the 
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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.

d. Specific performance conditions. The following paragraphs identify specific 

performance conditions requiring evaluation by flight test and/or simulation.

(1) Icing considerations. The FGS performance and safety in icing conditions should 

be demonstrated by flight test and/or simulation tests and be supported by analysis where 

necessary. The implications of continued use of the automatic flight control elements of the FGS 

in icing conditions should be assessed. Ice accumulation on the airplane wings and surfaces can 

progressively change the aerodynamic characteristics and stability of the airplane. Even though 

the FGS may perform safely under these conditions, its continued use may mask this change, 

which, in turn, can lead to pilot handling difficulties and potential loss of control, should the 

autopilot become disengaged (either automatically or manually).

(a) Test program. A test program should assess the potential vulnerability of the 

FGS to icing conditions by evaluating autopilot performance during artificial ice shape tests or 

during natural icing tests.

1 Sufficient autopilot testing and analysis should be conducted to ensure 

that the autopilot’s performance is acceptable. This testing and analysis should include 

evaluation of the low-speed protection threshold in the event that stall warning parameters are 

adjusted to account for icing conditions. For more information, see Chapter 5, paragraph 55, 

Icing Considerations in Normal and Rare Normal Conditions, of this AC.

2 In general, it is not necessary to conduct an autopilot evaluation that 

encompasses all weights, CG positions (including lateral asymmetry), altitudes, and deceleration 

device configurations. However, if the autopilot performance with ice accretion shows a 

significant difference from the non-contaminated airplane or testing indicates marginal 

performance, additional tests may be necessary.
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3 If significant autopilot inputs are required to compensate for the icing 

conditions, then the acceptability of the indication of a significant out-of-trim condition should 

be assessed, and the subsequent response of the airplane when the autopilot disengages (manual 

or automatic) should be determined. For more information, see Chapter 3, paragraph 27.b and 

Chapter 4, paragraph 45.d of this AC.

(b) 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) AFM limitations. Where degradation is noted that 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.

(c) Vertical speed with insufficient climb power.

(d) Approach with speed abuse.

(3) High-speed protection. The FGS should be assessed for the acceptability of the 

high-speed protection performance under the following conditions:

(a) High altitude level flight with autothrust function.

(b) High altitude level flight without autothrust function.

(c) High altitude descending flight with autothrust function.

(4) Go-around.

(a) Mode assessment. The objective of the Go-Around mode (see Chapter 6, 

paragraph 65.b, Go-Around Mode, of this AC) is to quickly change the 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.
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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.

(ff) An engine failure during go-around after go-around power is 

reached.

(gg) Initiation altitude (for example, in ground effect, not in ground effect 

during flare).

2 Characteristics to be evaluated:

(aa) The pitch response of the airplane during the initial transition.

(bb) Speed performance during airplane reconfiguration and climbout.

(cc) Integrated autopilot and autothrust operation.

(dd) Transition to missed approach altitude.

(ee) Lateral performance during an engine failure.

3 Go-around initiation heights. Where height loss during a go-around 

maneuver is significant or is required to support specific operational approval, demonstrated 

values for various initiation heights should be included in the AFM.

(5) Steep approach (special authorization).

(a) Flight test/simulator demonstration. Typical approach operations include 

glidepath angles between 2.75 and 3.77 degrees. Application for approval to conduct operations 

on glidepath angles of greater than 3.77 degrees requires additional evaluation. See AC 120-29A, 

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.
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4 Demonstration of Go-Around mode from a steep approach.

(b) Autopilot use on steep approaches. For autopilot use at approach angles 

greater than 4.5 degrees, the criteria of AC 25-7C, Chapter 8, Airworthiness: Miscellaneous 

Items, should be satisfied. This advisory material contains airworthiness and transition criteria 

for steep approaches. In addition, the criteria of AC 120-29A, paragraph 6.2.8, Approach Ban 

Applicability, should be assessed depending on the operational and low visibility requirements.

(6) Indication of and response to an out-of-trim condition. An assessment should be 

performed to determine the acceptability of the out-of-trim annunciation and subsequent 

response to disengagement. (See Chapter 4, paragraph 45.d of this AC.)

e. Flight director/head up display.

(1) General. The guidance aspect of an FGS may be provided by a head down FD or a 

HUD. HDDs normally use FD guidance cues that provide steering commands—such as a cross 

pointer or single “V”—for the pilot to follow. HUDs normally display a 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. FDs 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 

effects on tracking performance, pilot compensation, and workload.

(d) Primary instrument references. It should be demonstrated that movement of 

the FD and its tracking reference do not interfere with primary instrument references throughout 
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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 2, paragraph 28, FD Engagement, Disengagement, and 

Indications.

2 Chapter 3, paragraph 44, FGS Mode Selection, Annunciation, and 

Indication.

3 Chapter 4, paragraph 46, FGS Considerations for HUDs.

4 Chapter 4, paragraph 53, Normal Performance (specifically, criteria for 

FD guidance).

(b) Demonstrations required. When the PF is using the HUD, the HUD is where 

the pilot is looking for the basic flight information, and the pilot is less likely to be scanning the 

head down instruments. Therefore, the following should be demonstrated:

1 The location and presentation of the HUD information (for example, 

guidance, flight information, and alerts/annunciations) does not distract the pilot or obscure the 

pilot’s outside view. For example, the pilot should be able to track the guidance to the runway 

without having the view of runway references or hazards along the 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.

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 
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action, such as a go-around, while using the HUD without having to revert to a head down flight 

display.

(c) Pilot not flying. 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.

(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).
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(i) Visual reference availability, fidelity, and delays.

(j) Suitability of visibility restriction models, such as appropriate calibration of 

visual references for the tests to be performed for day, night, and dusk conditions.

(k) Fidelity of any other significant factor or limitation relevant to the validity of 

the simulation.

f. Flightcrew override of the FGS. A flight evaluation should be conducted to 

demonstrate compliance with Chapter 3, paragraph 30 of this AC. The flight evaluation should 

consider the implication of system configuration for various flight phases and operations.

(1) Autopilot override.

(a) Assessing effect. The effect of flightcrew override should be assessed by 

applying an input on the cockpit controller (for example, the control column or equivalent) to 

each axis for which the FGS is designed to disengage. The evaluation should be repeated with 

progressively increasing rate of force application to assess FGS behavior. This assessment 

should include the initial application of force on the cockpit controller as well as when the force 

is removed from the controllers. The effects of speed and altitude should be considered when 

conducting the evaluation.

(b) No automatic disengagement. If the autopilot is designed such that it does not 

automatically disengage due to a pilot override, verify that no potential hazards are generated 

due to the override per Chapter 3, paragraph 30 of this AC.

(c) Multiple channel engagement. If the design of the autopilot provides for 

multiple channel engagement for some phases of flight that results in a higher override force, 

these conditions should be evaluated.

Note: AC 120-28D, 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 may not result 

in a potential hazard per § 25.1329(m), either during the override or after the pilot releases the 

thrust levers. See Chapter 3, paragraph 30 of this AC for additional guidance. If the autothrust 

system automatically disengages due to the override, the alerts that accompany the 

disengagement should be assessed to ensure flightcrew awareness.

(3) Pitch trim system evaluation during autopilot override.

(a) Effect of override. The effect of flightcrew override during automatic control 

on the automatic trim systems should be conducted. The pilot should also apply an input to the 
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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 of this AC.

(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 of this AC. 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 of this 

AC 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. AC 25-7C, paragraph 181b(9)

provides guidance on test methods for particular types of failure conditions that have been 

identified by the safety assessment.

b. Validation elements.

(1) Assessment. The safety assessment described in Chapter 8 of this AC 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 
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those failure conditions exist (given the architecture of a specific FGS) and are relevant to the 

safety assessment.

(b) Flight director. 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 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 1: 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 
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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 2: 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 3: 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 guidance. However, if there 

are any elements of the FGS that cannot 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 1: For the final phase of landing (for example, below 80 feet), the pilot 

can be assumed to react upon recognition without delay.

Note 2: 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.
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(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(d). 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.

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 AC 25-7C, paragraph 181b(9)(e)4,

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 Autopilot. 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 
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paragraph 100.b, Validation Elements, of this AC. The failure condition should be inserted at a 

safe but representative height. The deviation profile should be identified and applied as indicated 

in AC 25-7C, paragraph 181b(9)(e), 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 Instrument 

Procedures (TERPS). See FAA Order 8260.3B, or the International Civil Aviation Organization 

(ICAO) Procedures for Air Navigation Services – Aircraft 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 MUH established. (See AC 25-7C, paragraph 181b(9)(e)3,

Assessment of Approach with Vertical Path Reference.)

(b) Area navigation.

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.
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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.

(2) Approach without vertical path reference. For an approach without vertical path 

reference (for example, VOR, Non Directional Beacon (NDB), 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 

AC 25-7C, paragraph 181b(9)(e)4.

(3) Steep approach. In support of an approval to use the FGS on glidepath angles of 

greater than 3.77 degrees (see Chapter 9, paragraph 99.d(5), Steep Approach (Special 

Authorization) of this AC), 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 

used to support typical flight operations. The criteria are based on experience gained from 

certification programs and functionality provided by traditional systems. A FGS providing 
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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. For specific considerations relating to 

autopilot operations close to the ground in the presence of failures, see Chapter 8, 

paragraph 88.b, FGS Operations in Close Proximity to the Ground, of this AC.

(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.

(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).
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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 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.

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 100.f(1), Approach with Vertical Path Reference, of this AC. If the failure 

condition(s) has a neutral effect on the flight path, the acceptability of cues for the flightcrew 
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84

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

Chapter 9, paragraph 99.d(5) of this AC.

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 the method identified in 

AC 25-7C, paragraph 181b(9)(e).

(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 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 AC 25-7C, paragraph 181b(9)(e).
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(5) Circling approach.

(a) For the purpose of this AC, circling approaches may be considered to have the 

following visual segments associated with the approach:

1 A segment at or above the minimums prescribed by the procedure that 

parallel the runway in the opposite direction of the landing runway,

2 A turning segment to align with the runway that can be level or partially 

descending, and

3 A final descending segment to landing.

(b) Operationally, the autopilot may remain engaged even after leaving the 

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 characteristics are 

identified during the certification program. Chapter 9, paragraph 100.e, Maneuvering, of this AC 

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 
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86

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.

(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 99.e(3)(b), Motion Base Suitability, of this AC, 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.
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87

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 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 Change 1, 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 low-visibility 

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 and 9 of this AC. 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 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 AC 25-7C, 

paragraph 181b(9)(e), and shown in figure 181-2, Deviation Profile Method, of that AC.

(b) Approach without vertical path reference. The height loss should be 

determined using the height loss method, as discussed in AC 25-7C, paragraph 181b(9)(e), and 

shown in figure 181-3, Height Loss Method, of that AC.

(c) Assessment of approach with vertical path reference. As shown in AC 25-7C, 

figure 181-2, Deviation Profile Method.

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).
10/27/14 AC 25.1329-1C

89

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) and (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 of this AC), 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 FCOMs. 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 “maximum 

displacement deviation” information described in paragraph 110 of this AC 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.
10/27/14 AC 25.1329-1C

Appendix A

A-1

APPENDIX A. SAFETY ASSESSMENT

1. General. This appendix provides material that may be useful in supporting the safety 

assessment activities identified in Chapter 8 of this AC.

2. Identification of Failure Conditions.

a. The following cases should be considered for applicability when establishing failure 

conditions, as indicated in Chapter 8 of this AC.

(1) Loss of autopilot in single or multiple axes.

(2) Loss of guidance in single or multiple axes.

(3) Loss of thrust control.

(4) Partial loss or degradation of autopilot function.

(5) Unintended autopilot commands in a single axis or multiple axes simultaneously 

(for example, hardover, slowover, and oscillatory failure modes).

(6) Unintended guidance commands in a single axis or multiple axes.

(7) Unintended thrust control.

(8) A sustained out-of-trim condition with the autopilot engaged without an alert.

(9) Autopilot disengagement in an out-of-trim condition.

(10) Autopilot disengagement without a warning.

(11) Inability to disengage the autopilot or autothrust function.

(12) Uncommanded engagement of an autopilot or autothrust.

(13) Jamming or loading of primary flight controls.

(14) Unintended thrust asymmetry.

b. A typical failure condition statement may be of the form:

‘[Failure]’ during ‘[Phase of Flight]’ that ‘[Effect]’ when ‘[Mitigation Consideration]’

c. 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, FMS, hydraulics, electrical systems, etc.).
10/27/14 AC 25.1329-1C

Appendix A

A-2

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) 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, these are considered significant and are readily 

detectable by the flightcrew, based on the resulting aircraft motion or guidance cues.

(2) “Slowover” effects. Typically, these 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, these 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, this is the removal of control, guidance, or 

functionality that may have an immediate effect or may not be immediately apparent to the 

flightcrew.
10/27/14 AC 25.1329-1C

Appendix A

A-3

Note: Chapter 9 of this AC 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.

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).
10/27/14 AC 25.1329-1C

Appendix A

A-4

(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 B for the definition of significant transient.

(2) Degradation of the flying qualities of the airplane that significantly increase 

flightcrew workload.
10/27/14 AC 25.1329-1C

Appendix B

B-1

APPENDIX B. DEFINITIONS

The following definitions apply to the requirements of § 25.1329 and the guidance material 

provided in this AC. You should not assume that these definitions 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.

Table B-1. Terms and Definitions

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 non-normal 

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.

Autopilot The autopilot function provides automatic control of the 

airplane, typically in pitch, roll, and yaw. 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 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.
10/27/14 AC 25.1329-1C

Appendix B

B-2

Term Definition

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 

non-normal 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.

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.
10/27/14 AC 25.1329-1C

Appendix B

B-3

Term Definition

Failure 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).

Note: 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.

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 FGS and is integrated with 

airborne attitude, air data, and NAV systems.

Flight Guidance System 

(FGS)

A system consisting of one or more of the following elements:

• Autopilot

• Flight director, and

• Automatic thrust control.

A flight guidance system (FGS) also includes any interactions 

with stability augmentation and trim systems.
10/27/14 AC 25.1329-1C

Appendix B

B-4

Term Definition

Flight Management System 

(FMS)

An aircraft area navigation (RNAV) system and associated 

displays and input/output devices(s) having complex 

multi-waypoint lateral (LNAV) and vertical (VNAV) 

capability (or equivalent), data entry capability, database 

memory to store route and instrument flight procedure 

information, and display readout of NAV parameters. The 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).

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, 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 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 that

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.”
10/27/14 AC 25.1329-1C

Appendix B

B-5

Term Definition

Mode A mode is system configuration that corresponds to a single (or 

set of) FGS behavior(s).

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.

Select The flightcrew action of requesting functionality or an end state 

condition.
10/27/14 AC 25.1329-1C

Appendix B

B-6

Term Definition

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 

non-normal operational or airplane system requiring immediate 

recognition. Immediate corrective or compensatory action by 

the flightcrew is required.
10/27/14 AC 25.1329-1C

Appendix C

C-1

APPENDIX C. ACRONYMS

Table C-1. Acronyms and Definitions

Acronym Definition

AC Advisory Circular

ADI Attitude Director Indicator

AFM Airplane Flight Manual

AGL Above Ground Level

ARP Aerospace Recommended Practice

ATC Air Traffic Control

CDI Course Deviation Indicator

CFR Code of Federal Regulations

CG Center of Gravity

CWS Control Wheel Steering

DA Decision Altitude

DA(H) Decision Altitude (Height)

DME Distance Measuring Equipment

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
10/27/14 AC 25.1329-1C

Appendix C

C-2

Acronym Definition

FMS Flight Management System

GBAS Ground-Based Augmentation System

GLS GBAS Landing System

GPWS Ground Proximity Warning System

HDD Head Down Display

HUD Head Up Display

ICAO International Civil Aviation Organization

ILS Instrument Landing System

IMA Integrated Modular Avionics

IMC Instrument Meteorological Conditions

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

PAN-OPS Procedures for Air Navigation – Aircraft Operations

PF Pilot Flying

PFD Primary Flight Display

PNF Pilot Not Flying
10/27/14 AC 25.1329-1C

Appendix C

C-3

Acronym Definition

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

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
Advisory Circular Feedback 

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Directives Management Officer at (202) 267-3983. 

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