U.S. Department

of Transportation

Federal Aviation

Administration

Advisory

Circular

 

Subject: APPROVAL OF FLIGHT 

GUIDANCE SYSTEMS

Date: DRAFT

Initiated by: ANM-110

AC No: 25.1329-1B

Change: 1

1. PURPOSE. 

a. This change updates Advisory Circular (AC) 25.1329-1B, Approval of Flight Guidance 

Systems, dated July 16, 2006. 

b. The change number and the date of the changed material are shown at the top of each 

page with the change number and date. 

2. PRINCIPAL CHANGES.

a. Minor revisions were made to reflect guidance changes associated with the issuance of 

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

Mixed Phase, and Ice Crystal Icing Conditions, published XXXXXXX, 200X (7X FR XXXXX). 

These changes include a reference to the icing considerations of § 25.1420. Grammatical and 

typographical errors were also corrected. 

b. Paragraph 4a is revised to include § 25.1420 in the table.

c. Paragraph 4b is revised to include AC 25-7A, Flight Test Guide for Certification of 

Transport Category Airplanes in the table.

d. Paragraph 4d is revised to provide an Internet address and change a reference to an FAA 

document. 

e. Paragraph 18b is revised to change “part of the FGS” to “equipment or component.”

f. Paragraph 19b(3) is revised to add the words “landing system (GLS).” 

g. Paragraph 19b(4) was deleted.

h. Paragraph 28b is revised to change “may” to “should.”
Draft AC 25.1329-1B

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i. Paragraph 29a(1) is revised to change “should” to “must” and add “(See § 25.1329(i)).”

j. Paragraph 42b is revised to change “should” to “must.”

k. Table 5-1 is revised to add “and applicable icing conditions covered by 14 CFR 

part 25, Appendix X”

l. Paragraph 99d(5)(a) is revised to change “2.77” to “2.75” and “3.75” to “3.77.”

m. Paragraph 99f(2) is revised to change “must” to “may.”

n. Paragraph 100f(3) is revised to change “3.75” to “3.77.”

3. WEBSITE AVAILABILITY. To access AC 25.1329-1B electronically, log on to the 

Regulatory Guidance Library (RGL) website: http://rgl.faa.gov.

Page Control Chart

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(Signature Block)
Draft AC 25.1329-1B

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4. RELATED REGULATIONS, ACs, AND INDUSTRY DOCUMENTS.

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

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

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

from the Government Printing Office (GPO), Superintendent of Documents, Attn: New Orders, 

PO Box 371954, Pittsburgh, PA 15250-7954. 

Section Title

§ 25.143 Controllability and Maneuverability, General

§ 25.301 Loads

§ 25.671 Control systems, General

§ 25.672 Stability augmentation and automatic and power-operated systems

§ 25.677 Trim systems

§ 25.777 Cockpit controls

§ 25.779 Motion and effect of cockpit controls

§ 25.781 Cockpit control knob shape

§ 25.901 Installation 

§ 25.903 Engines

§ 25.1301 Function and installation

§ 25.1303 Flight and navigations instruments

§ 25.1309 Equipment, systems, and installations

§ 25.1322 Warning, caution, and advisory lights

§ 25.1420 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 
Draft AC 25.1329-1B

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

http://rgl.faa.gov. A paper copy may be ordered from the U.S. Department of Transportation, 

Subsequent Distribution Office, M-30, Ardmore East Business Center, 3341 Q 75th Avenue, 

Landover, MD 20795.

Number Title

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

Commission for Aeronautics] 

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

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

and Alaska

AC 20-152 Design Assurance Guidance for Airborne Electronic Hardware 

RTCA/DO 254

AC 25-7A Flight Test Guide for Certification of Transport Category 

Airplanes 

AC 25-11 Transport Category Airplane Electronic Display Systems

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

Warning Systems in Transport Category Airplanes

AC 25.672-1 Active Flight Controls

AC 25.1309-1A System Design and Analysis

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

Conditions

AC 25.1581-1 Airplane Flight Manual

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

Takeoff, Landing, and Rollout

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

Minima for FAR 121 Operators

AC 120-41 Criteria for Operational Approval of Airborne Windshear 

Alerting and Flight Guidance Systems

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

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

table may reference a specific version of an AC that has been superseded. The 

latest version of each AC should always be used, even though this document may 

reference a previous version of an AC.
Draft AC 25.1329-1B

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

downloaded from the Internet at 

http://www.faa.gov/aircraft/air_cert/design_approvals/csta/publications/. 

Title

Federal Aviation Administration Human Factors Team Report on: The Interfaces 

Between Flight Crews and Modern Flight Deck Systems – June 18, 1996

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

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

Transportation, Subsequent Distribution Office, M-30, Ardmore East Business Center, 3341 Q 

75th Avenue, Landover, MD 20795.

Number Title

FAA Order 

8260.3

United States Standard for Terminal Instrument Procedures

Ali Bahrami

Manager, Transport Airplane Directorate

Aircraft Certification Service
Draft AC 25.1329-1B

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CHAPTER 2 OVERVIEW OF FLIGHT GUIDANCE SYSTEMS (FGS)

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

control and tactical guidance of the airplane. The system may also provide workload relief to the 

pilots and provide a means to fly a flight path more accurately to support specific operational 

requirements, such as reduced vertical separation minimum (RVSM) or required navigation 

performance (RNP). 

18. FGS COMPONENTS. 

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

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

supplies, servo–motors/actuators, and associated wiring. It includes any indications and 

controllers necessary for the pilot to manage and supervise the system.

b. Excluded from Definition. Any 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)
Draft AC 25.1329-1B

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(3) Global navigation satellite system (GNSS) landing system (GLS)

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

provide guidance and control in conjunction with a FMS. The FGS does not include the flight 

planning and the generation of flight path and speed profiles tied to waypoints and other flight 

planning aspects of the FMS. However, it does include the interface between the FMS and FGS 

necessary for the execution of flight path and speed commands. 

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

philosophy that supports the intended operational use regarding FGS behavior, modes of 

operation, the pilot interface with controls, indications, alerts, and mode functionality.

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

operation should be addressed from flightcrew and maintenance perspectives in appropriate 

documentation and training material.

20. COMPLIANCE WITH § 25.1329. 

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

compliance with § 25.1329 and the applicability of other Part 25 rules to FGS (e.g., §§ 25.1301 

and 25.1309). The demonstrated means of compliance may include a combination of analysis, 

laboratory testing, flight testing, and simulator testing. The applicant should coordinate with the 

FAA early in the certification program via a certification plan to reach agreement on the methods 

to be used to demonstrate compliance. 

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

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

complying with § 25.1329. 

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

to § 25.1329 and other specific rules, as compliance to a rule is not optional. In these cases, 

the AC text supplies a reference to the specific rule being discussed. 

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

§ 25.1329 and indicates where acceptable means of compliance with each paragraph can be 

found within this AC. 
Draft AC 25.1329-1B

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

Systems in Transport Category Airplanes; and AC 120–41, Criteria for Operational Approval of 

Airborne Windshear Alerting and Flight Guidance. 

b. FD Disengagement. There 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 deselected. 

29. AUTOTHRUST ENGAGEMENT, DISENGAGEMENT, AND INDICATIONS. 

Engagement and disengagement should be accomplished in a manner consistent with other 

flightcrew procedures and tasks and should not require undue attention. 

a. Autothrust Engagement. 

(1) Indication. The autothrust function 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 57b, Low 

Speed Protection.) If such automatic engagement occurs, it should be clear to the flightcrew that 

automatic engagement has occurred. The automatic engagement may not cause a transient larger 

than a minor transient. [See § 25.1329(c)] The transition to the engaged state should be smooth, 

and should not cause large, unexpected changes in pitch attitudes or pitching moments. The 

reason for automatic engagement should be clear and obvious to the flightcrew.

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

engage the autothrust function outside of the normal flight envelope or at 

excessive or too low engine thrust. It is not expected that the autothrust feature 

should compensate for situations outside the normal flight envelope or normal 

engine operation range, unless that is part of the intended function of the 

autothrust system. 
Draft AC 25.1329-1B

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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, Compliance Demonstration Using Flight Test and Simulation, for 

more discussion of appropriate analyses which may include, for example, cognitive task analysis 

as a basis for evaluation. 

43. FGS CONTROLS. 

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

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

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

control must be readily apparent or plainly indicated on or adjacent to each control, if needed to 

prevent inappropriate use or confusion. (See § 25.1329(f)) Sections 25.777(b) and 25.779(a) 

provide requirements regarding direction of motion of flight deck controls. These requirements 

apply for FGS command reference controls which select target values, such as heading select or 

vertical speed. Section 25.781 also provides requirements for the shapes of the knobs. 

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

considerations: 

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

confusing speed and heading knobs on the MSP. 
Draft AC 25.1329-1B

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Table 5-1

Examples of Normal Conditions

No failure conditions All airplane systems that are associated with airplane 

performance are fully operational. Failures of those systems 

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

perform its functions. 

Light to moderate 

winds

Constant wind in a specific direction that may cause a slight 

deviation in intended flight path or a small difference between 

airspeed and groundspeed.

Light to moderate 

wind gradients

Variation in wind velocity—as a function of altitude, position, 

or time—which may cause slight erratic or unpredictable 

changes in intended flight path.

Light to moderate 

gusts

Non–repetitive momentary changes in wind velocity that can 

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

remains in positive control at all times.

Light turbulence Turbulence that momentarily causes slight, erratic changes in 

altitude and/or attitude (pitch, roll, or yaw).

Moderate turbulence Similar to light turbulence but of greater intensity. Changes in 

altitude and/or attitude occur, but the aircraft remains in positive 

control at all times. 

Light chop Turbulence that causes slight, rapid, and somewhat rhythmic 

bumpiness without appreciable changes in altitude or attitude.

Moderate chop Similar to light chop but of greater intensity. It causes rapid 

bumps or jolts without appreciable changes in aircraft altitude or 

attitude. 

Icing All icing conditions covered by 14 CFR part 25, Appendix C

and applicable icing conditions covered by 14 CFR part 25, 

Appendix X, with the exception of “asymmetric icing” 

discussed under “Rare Normal Conditions” in Table 5-2.

NOTE: Representative levels of the environmental effects should be established 

consistent with the airplane’s intended operation. 
Draft AC 25.1329-1B

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(ff) An engine failure during go–around after go–around power is 

reached.

(gg) Initiation altitude (for example, in ground effect, not in ground 

effect during flare).

2 Characteristics to be evaluated: 

(aa) The pitch response of the airplane during the initial transition.

(bb) Speed performance during airplane reconfiguration and climbout.

(cc) Integrated autopilot and autothrust operation.

(dd) Transition to missed approach altitude.

(ee) Lateral performance during an engine failure.

3 Go-Around Initiation Heights. Where height loss during a go–around 

maneuver is significant or is required to support specific operational approval, demonstrated 

values for various initiation heights should be included in the AFM. 

(5) Steep Approach [special authorization].

(a) Flight Test/Simulator Demonstration. Typical approach operations include 

glidepath angles between 2.75 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.

4 Demonstration of Go–Around mode from a steep approach.

(b) Autopilot Use on Steep Approaches. For autopilot use at approach angles 

greater than 4.5 degrees, the criteria of AC 25-7A, Flight Test Guide for Certification of 

Transport Category Airplanes, Chapter 8, Airworthiness-Miscellaneous Items, should be 

satisfied. This advisory material contains airworthiness and transition criteria for steep 

approaches. In addition, the criteria of AC 120-29A, Criteria for Approval of Category I and II 

Weather Minima for Approach, paragraph 6.2.8, Approach Ban Applicability, should be assessed 

depending on the operational and low visibility requirements. 
Draft AC 25.1329-1B

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NOTE: AC 120-28D, Appendix 3, Airworthiness Approval for Airborne 

Systems Used to Land and Rollout in Low Visibility Conditions, Section 8, 

Airborne Systems, contains guidance for evaluating autopilot override for systems 

supporting low visibility operations.

(2) Autothrust Override. The capability of the flightcrew to override the autothrust 

system should be demonstrated 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. Refer to Chapter 3, paragraph 30 of this AC for additional guidance. 

If the autothrust system automatically disengages due to the override, the alerts that accompany 

the disengagement should be assessed to ensure flightcrew awareness.

(3) Pitch Trim System Evaluation During Autopilot Override.

(a) Effect of Override. The effect of flightcrew override during automatic control 

on the automatic trim systems should be conducted. The pilot should also apply an input to the 

pitch cockpit controller below that which would cause the autopilot to disengage and verify that 

the automatic pitch trim system meets the intent in Chapter 3, paragraph 30. 

(b) No Automatic Disengagement. If the system design is such that the autopilot 

does not have an automatic disengagement on override feature, the pilot should initiate an 

intentional override for an extended period of time. The autopilot should then be disengaged 

with the quick disconnect button and any transient response assessed in compliance with 

Chapter 3, paragraph 30. The effectiveness and timeliness of any alerts used to mitigate the 

effects of the override condition should be assessed during this evaluation.

100. FAILURE CONDITIONS REQUIRING VALIDATION: § 25.1309.

a. General. 

(1) Safety Assessment. The safety assessment process identified in Chapter 8 should 

identify any failure condition responses that would require pilot evaluation to assess the severity 

of the effect, and the validity of any assumptions used for pilot recognition and mitigation. The 

classification of a failure condition can vary according to flight condition and may need to be 

confirmed by simulator or flight test.

(2) Evaluation of Failure Conditions. This chapter provides guidance on the test 

criteria, including recognition considerations, for flight evaluation of these failure conditions. In 

addition, certain probable failures should be demonstrated to assess the performance of the FGS 

and the adequacy of any applicable flightcrew procedures. 
Draft AC 25.1329-1B

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(2) Approach Without Vertical Path Reference. For an approach without vertical path 

reference (for example, VOR, Non Directional Beacon (NBD), localizer only) the FGS mode of 

operation is typically vertical speed/flight path angle (that is, a cruise mode). The worst case 

failure condition for this type of mode should be demonstrated in the approach configuration. 

An appropriate height loss should then be established in accordance with the method described in 

Appendix 2, Flight Test Procedures, paragraph 5c(4)(c). 

(3) Steep Approach. In support of an approval to use the FGS on glidepath angles of 

greater than 3.77 degrees (see Chapter 9, paragraph 99d(5), Steep Approach (special 

authorization)), an assessment should be made of the effects of failure conditions for this type of 

operation. For use of autopilot, an appropriate MUH should be established in accordance with 

the deviation profile method described in this chapter. For this assessment, the obstacle plane 

associated with a nominal 3 degree glidepath angle (1:29 slope) should be adjusted, according to 

the maximum approach angle for which approval is sought.

g. Specific Conditions.

(1) Failure Conditions to be Evaluated. The following are failure conditions that should 

be considered as part of the FGS evaluation program:

(a) Engine failure during approach; continue approach to decision 

altitude/minimum descent altitude (DA/MDA). 

(b) Potential fuel imbalance.

(c) Airplane system failures that affect FGS operational capabilities. Examples 

include failures of hydraulic systems, electrical systems, flight controls, and FGS related sensors. 

When possible, these failure conditions should be evaluated with a flight test. However, some 

failure conditions, due to their potential hazards, may best be evaluated via a high fidelity flight 

simulation.

(2) Failure to Disengage. The probability of failure of an FGS element to disengage 

when the quick disengagement control is operated should be shown to be acceptable by the 

safety analysis process. If credit is to be taken for acceptable continued manual operation with 

the FGS elements remaining engaged, that is, without operating any of the other disengagement 

controls, then a flight demonstration should be conducted though approach, landing and rollout. 

101. CRITERIA SUPPORTING OPERATIONAL USE OF AUTOPILOT.

a. Typical Flight Operations. The criteria contained in this paragraph are intended to 

identify how the functional capability of the FGS, established during the certification, can be 

utilized to support typical flight operations. The criteria are based on experience