AC 25.1329-1C CHG 2 Table 1-2. Related Advisory Circulars AC 20-115D Airborne Software Development Assurance Using July 21, 2017 EUROCAE ED-12( ) and RTCA DO-178( ) AC 20-152A RTCA, Inc., Document RTCA/DO-254, Design October 7, 2022 Assurance Guidance for Airborne Electronic Hardware AC 20-191 Airworthiness Approval of Airborne Systems used TBD for Takeoff, Precision Approach, Landing, and Rollout in Low-Visibility Conditions AC 25-7D Flight Test Guide for Certification of Transport May 4, 2018 Category Airplanes AC 25-12 Airworthiness Criteria for the Approval of Airborne November 2, 1987 Windshear Warning Systems in Transport Category Airplanes AC 25.1322-1 Flightcrew Alerting December 13, 2010 AC 120-118 Criteria for Approval/Authorization of All Weather July 2, 2018 Operations (AWO) for Takeoff, Landing, and Rollout Number Title Date AC 20-138D Airworthiness Approval of Positioning and April 7, 2016 Change 2 Navigation Systems AC 20-174 Development of Civil Aircraft and Systems September 30, 2011 AC 21-16G RTCA Document DO-160 versions D, E, F, and G, June 22, 2011 “Environmental Conditions and Test Procedures for Airborne Equipment” AC 25-11B Electronic Flight Displays October 7, 2014 AC 25.1309-1B System Design and Analysis August 30, 2024 AC 25.1581-1 Airplane Flight Manual July 14, 1997 Change 1 AC 120-41 Criteria for Operational Approval of Airborne Wind November 11, 1983 Shear Alerting and Flight Guidance Systems (cancelled) 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.
AC 25.1329-1C CHG 2
c. Related Industry Standards.
(1) The following documents are available from the Society of Automotive Engineers (SAE). You can pur chase 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 1-3. Related SAE Documents SAE ARP4761A Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment SAE ARP5366 Autopilot, Flight Director, and Autothrust Systems Number Title SAE ARP5288 Transport Category Airplane Head Up Display (HUD) Systems SAE AS8055A 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 from the RTCA website.
Table 1-4. Related RTCA Documents RTCA DO-178C/ Software Considerations in Airborne Systems and EUROCAE ED-12C Equipment Certification RTCA DO-254/ Design Assurance Guidance for Airborne Electronic EUROCAE ED-80 Hardware Number Title RTCA DO-160G/ Environmental Conditions and Test Procedures for EUROCAE ED-14G Airborne Equipment
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.
AC 25.1329-1C CHG 2
(2) FAA Order 8260.3D, United States Standard for Terminal Instrument Procedures (TERPS), dated February 16, 2018, is available at the FAA website.
5—10 [RESERVED] 5
AC 25.1329-1C CHG 2 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, the FAA added a reference to § 25.1420, Supercooled large drop icing conditions , in table 1-1; 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 t he 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.
13—16 [RESERVED] 6
CHG 1 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) Ground-based augmentation system (GBA S) landing system (GLS).
c. Flight management system (FMS). The FGS also includes those functions necessary to provide guidance and control in conjunction with an FMS. The FGS does not include the flight planning and the generation of flight path and speed profiles tied to 7
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 p art 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-Ref erences for § 25.1329 Paragraphs to AC Chapters 14 CFR Section § 25.1329(a) Chapter 3 Quick disengagement controls for the autopilot FGS Engagement, Disengagement, and autothrust functions must be provided for Indications, and Override each pilot. The autopilot quick disengagement Paragraph 27, controls must be located on both control wheels Autopilot Engagement, (or equivalent). The autothrust quick Disengagement, and Indications disengagement controls must be located on the thrust control levers. Quick disengagement Paragraph 29, controls must be readily accessible to each pilot Autothrust Engagement, while operating the control wheel (or Disengagement, and Indications equivalent) and thrust control levers.
AC Chapter/Paragraph Reference 8
§ 25.1329(b) Chapter 3 The effects of a failure of the system to FGS Engagement, Disengagement, disengage the autopilot or autothrust functions Indications, and Override when manually commanded by the pilot must Paragraph 27, be assessed in accordance with the requirements Autopilot Engagement, of § 25.1309. 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) Chapter 3 Engagement or switching of the flight guidance FGS Engagement, Disengagement, system, a mode, or a sensor may not cause a Indications, and Override transient response of the airplane’s flight path Chapter 8 any greater than a minor transient, as defined in paragraph (n)(1) of this section. Safety Assessment § 25.1329(d) Chapter 3 Under normal conditions, the disengagement of FGS Engagement, Disengagement, any automatic control function of a flight Indications, and Override guidance system may not cause a transient Chapter 4 response of the airplane’s flight path any greater than a minor transient. Controls, Indications, and Alerts Paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status Chapter 8 Safety Assessment 14 CFR Section AC Chapter/Paragraph Reference 9
§ 25.1329(e) Chapter 3 Under rare normal and non-normal conditions, FGS Engagement, Disengagement, disengagement of any automatic control Indications, and Override function of a flight guidance system may not Chapter 4 result in a transient any greater than a significant transient, as defined in Controls, Indications, and Alerts paragraph (n)(2) of this section. Paragraph 45, FGS Alerting, Warning, Caution, Advisory, and Status Chapter 8 Safety Assessment § 25.1329(f) Chapter 4 The function and direction of motion of each Controls, Indications, and Alerts command reference control, such as heading Paragraph 43, select or vertical speed, must be plainly FGS Controls indicated on, or adjacent to, each control if necessary to prevent inappropriate use or confusion.
§ 25.1329(g) Chapter 4 Under any condition of flight appropriate to its Controls, Indications, and Alerts use, the flight guidance system may not produce Paragraph 45, hazardous loads on the airplane, nor create FGS Alerting, Warning, Caution, hazardous deviations in the flight path . This Advisory, and Status applies to both fault-free operation and in the event of a malfunction, and assumes that the Chapter 5 pilot begins corrective action within a Performance of Function reasonable period of time.
Chapter 8 Safety Assessment Chapter 9 Compliance Demonstration Using Flight Test and Simulation 14 CFR Section AC Chapter/Paragraph Reference 10
§ 25.1329(h) Chapter 5 When the flight guidance system is in use, a Performance of Function means must be provided to avoid excursions Paragraph 57, beyond an acceptable margin from the speed Speed Protection 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.
§ 25.1329(i) Chapter 4 The flight guidance system functions, controls, Controls, Indications, and Alerts 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.
§ 25.1329(j) Chapter 3 Following disengagement of the autopilot, a FGS Engagement, Disengagement, warning (visual and auditory) must be provided Indications, and Override to each pilot and be timely and distinct from all Paragraph 27, other cockpit warnings. Autopilot Engagement, Disengagement, and Indications Chapter 8 Safety Assessment 14 CFR Section AC Chapter/Paragraph Reference 11
14 CFR Section § 25.1329(k) Chapter 3 Following disengagement of the autothrust FGS Engagement, Disengagement, function, a caution must be provided to each Indications, and Override pilot. Paragraph 27, Autopilot Engagement, Disengagement, and Indications Chapter 8 Safety Assessment § 25.1329(l) Chapter 3 The autopilot may not create a potential hazard FGS Engagement, Disengagement, when the flightcrew applies an override force to Indications, and Override the flight controls. 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) Chapter 3 During autothrust operation, it must be possible FGS Engagement, Disengagement, for the flightcrew to move the thrust levers Indications, and Override without requiring excessive force . The Paragraph 30, autothrust may not create a potential hazard Override of the FGS when the flightcrew applies an override force to the thrust levers. Chapter 8 Safety Assessment AC Chapter/Paragraph Reference 12
AC 25.1329-1C CHG 2 § 25.1329(n) Appendix B—Definitions 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.
14 CFR Section AC Chapter/Paragraph Reference Note 1: The text for each paragraph in § 25.1329 is included in this AC for reference only.
Please refer to the a ctual text in part 25.
Note 2: § 25.143(d) is the appropriate paragraph of the regulation for a certification basis later than amendment 121.
21—26 [RESERVED] 13
14 CHAPTER 3. F GS 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 und ue 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 norm al 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 targ et 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: