Advisory Circular U.S. Department of Transportation Federal Aviation Administration Subject: Unwanted In-Flight Thrust Reversal of Turbojet Thrust Reversers Date: 08/30/2024 AC No: 25.933-1 Initiated By: AIR-600 This advisory circular (AC) describes various acceptable means for showing compliance with the requirements of paragraphs (a)(1) and (2) of Title 14, Code of Federal Regulations (14 CFR) 25.933, Reversing systems, as they apply to unwanted in-flight turbojet thrust reversals on transport category airplanes.
If you have suggestions for improving this AC, you may use the Advisory Circular Feedback Form at the end of this AC.
DANIEL J.
Digitally signed by DANIEL J. ELGAS ELGAS Date: 2024.08.30 14:48:50 -04'00' Daniel Elgas Director, Policy and Standards Division Aircraft Certification Service
AC 25.933-1
CONTENTS
PARAGRAPH PAGE
1 Purpose1
2 Applicability1
2.1 Applicability of this AC1
2.2 Applicability of § 25.933(a)1
2.3 Applicability to Reversers Certified Under 14 CFR Part 331
3 Related Documents2
3.1 Regulations2
3.2 Advisory Circulars3
3.3 Industry Documents3
4 Background3
4.1 General3
4.2 Minimizing Adverse Effects4
4.3 Coordination With Engine Manufacturer7
5 Definitions8
6 Demonstrating Compliance with § 25.933(a)(1)8
7 Controllability Option8
7.1 General8
7.2 Structural Integrity9
7.3 Performance9
7.4 Handling Qualities12
7.5 Flightcrew Training15
8 Reliability Option: Provide Continued Safe Flight and Landing by Preventing any
Unwanted In-flight Thrust Reversal15
8.1 General15
8.2 System Safety Assessment (SSA)16
8.3 Structural Aspects18
8.4 Uncontained Rotor Failure18
8.5 Protection of Thrust Reverser Restraint Devices19
9 Mixed Controllability/Reliability Option22
10 Deactivated Reverser22
11 Demonstrating Compliance with § 25.933(a)(2)22
ii
AC 25.933-1 Contents (continued) Paragraph Page
Appendix A. DefinitionsA-1
A.1 AnalysisA-1
A.2 AssessmentA-1
A.3 Continued Safe Flight and LandingA-1
A.4 Controllable Flight Envelope and ProcedureA-1
A.5 Deactivated ReverserA-1
A.6 Exceptional Piloting Skill or StrengthA-1
A.7 In-FlightA-1
A.8 Light CrosswindA-1
A.9 Light TurbulenceA-2
A.10 Normal Flight EnvelopeA-2
A.11 Qualitative Analytical processA-2
A.12 Quantitative analystical processA-2
A.13 SystemA-2
A.14 Thrust ReversalA-2
A.15 Thrust Reverser SystemA-2
A.16 Turbojet Thrust Reversing SystemA-3
Appendix B. Best PracticesB-1
B.1 Manufacturing/QualityB-1
B.2 Reliability MonitoringB-1
B.3 Maintenance and AlterationsB-1
iii
AC 25.933-1 Contents (continued) Fi gures Number Page
Figure 1. Cross-Section of Ring Disk Drum Rotor19
Figure 2. Cross-Section of Deep Bore Disk Drum20
Figure 3. Cross-Section of Single-Stage Deep Bore Disk20
Figure 4. Cross-Section of Typical High-Bypass Turbofan Low and High Compressor with
Fan Thrust Reverser21
iv
AC 25.933-1 1 PURPOSE.
This AC describes various acceptable means for showing compliance with the requirements of § 25.933(a)(1) and (a)(2), as they apply to unwanted in-flight turbojet thrust reversals on transport category airplanes. These means are intended to provide guidance to supplement the engineering and operational judgment that forms the basis of compliance findings relative to in-flight thrust reversal of turbojet thrust reversers.
The contents of this document do not have the force and effect of law and are not meant to bind the public in any way. This document is intended only to provide clarity to the public regarding existing requirements under the law or agency policies.
2 APPLICABILITY.
2.1 Applicability of this AC.
2.1.1 The guidance in this AC is for airplane manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation Administration (FAA) type certification engineers and the Administrator’s designees.
2.1.2 Conformity with the guidance is voluntary only and nonconformity will not affect rights and obligations under existing statutes and regulations. The FAA will consider other methods of showing compliance that an applicant may elect to present. Terms such as “should,” “may,” and “must” are used only in the sense of ensuring applicability of this particular method of compliance when the acceptable method of compliance in this document is used. If the FAA becomes aware of circumstances in which following this AC would not result in compliance with the applicable regulations, the agency may require additional substantiation as the basis for finding compliance.
2.2 Applicability of § 25.933(a).
The requirements of § 25.933(a) apply to turbojet reversing systems, also known as thrust reversing systems. Section 25.933(a)(1) applies to reversers intended for ground operation only, while § 25.933(a)(2) applies to reversers intended for in-flight use. The guidance in this AC applies only to unwanted thrust reversal in flight. Thrust reverser failure conditions during ground operations, such as failure to stow or deploy as commanded, unlatching of reverser cowling, and unstow or deployment short of reversal, are addressed by §§ 25.901(c) and 25.1309 and associated guidance.
2.3 Applicability to Reversers Certified Under 14 CFR Part 33.
The airworthiness regulations are structured to allow the applicant to certify thrust reversers either at the airplane level under part 25, or at the engine level under part 33.
The aircraft installation requirements, such as § 25.933(a) still apply to the thrust reversing system, even if certified under part 33 with the engine type design. The applicant for airplane type certification may coordinate with the engine type certificate holder to determine whether any of the requirements of § 25.933(a) can be met coincidentally with showing compliance to part 33 requirements.
AC 25.933-1 3 RELATED DOCUMENTS.
3.1 Regulations.
The following 14 CFR part 25 regulations are related to this AC. You can download the full text of these regulations from the Federal Register website at Electronic Code of Federal Regulations, jointly administered by the Office of the Federal Register (OFR) of the National Archives and Records Administration (NARA) and the U.S.
Government Publishing Office (GPO). You can order a paper copy from the U.S.
Superintendent of Documents, U.S. Government Publishing Office, Washington, D.C.
20401; at Government Publishing Office, by calling telephone number (202) 512-1800;
or by sending a fax to (202) 512-2250.
• Section 25.4, Definitions.
• Section 25.111, Takeoff path.
• Section 25.121, Climb: one-engine-inoperative.
• Section 25.123, En route flight paths.
• Section 25.143, Flight Controllability and Maneuverability—General.
• Section 25.147, Directional and lateral control.
• Section 25.149, Minimum control speed.
• Section 25.161, Trim.
• Section 25.251, Vibration and buffeting.
• Section 25.253, High-speed characteristics.
• Section 25.302, Interaction of systems and structures.
• Section 25.571, Damage-tolerance and fatigue evaluation of structure.
• Section 25.901, Installation.
• Section 25.903, Engines.
• Section 25.1155, Reverse thrust and propeller pitch settings below the flight regime.
• Section 25.1305, Powerplant instruments.
• Section 25.1309, Equipment, systems, and installations.
• Section 25.1322, Flightcrew alerting.
• Section 25.1529, Instructions for Continued Airworthiness.
• Section 25.1535, ETOPS approval.
• Part 25, Appendix K, Extended Operations (ETOPS).
AC 25.933-1
3.2 Advisory Circulars.
The following ACs are related to the guidance in this AC. Please refer to the latest version of each AC referenced in this document; they are available on the FAA website at FAA Advisory Circulars and on the Dynamic Regulatory System.
• AC 20-128A, Design Considerations for Minimizing Hazards Caused by Uncontained Turbine Engine and Auxiliary Power Unit Rotor Failure.
• AC 20-166A, Issue Paper Process.
• AC 25-7D, Flight Test Guide for Certification of Transport Category Airplanes.
• AC 25-11B, Electronic Flight Displays.
• AC 25-19A, Certification Maintenance Requirements.
• AC 25.901-1, Safety Assessment of Powerplant Installations.
• AC 25.1309-1B, System Design and Analysis.
• AC 25.1322-1, Flightcrew Alerting.
• AC 120-42B, Extended Operations (ETOPS and Polar Operations).
3.3 Industry Documents.
Aerospace Industries Association (AIA) Report, Criteria for Assessing Transport Turbojet Fleet Thrust Reverser System Safety, Revision A, dated June 1, 1994.
4 BACKGROUND.
4.1 General.
4.1.1 Most thrust reverser systems are intended for ground operation only. Consequently, applicants generally size and develop them to provide high deceleration forces while avoiding ingestion of foreign object debris, efflux impingement of airplane surfaces, and airplane handling difficulty during the landing roll. Likewise, airplane flight control systems are generally sized and developed to provide lateral and directional controllability margins adequate for handling qualities, maneuverability requirements, and engine-out VMC1 lateral drift conditions.
4.1.2 In early turbojet airplane designs, the combination of control system design and thrust reverser characteristics resulted in control margins that allowed the airplane to be capable of recovering from unwanted in-flight thrust reversal even on ground-use-only reversers; this capability was required by the versions of § 25.933 established by amendments 25-40 and 25-72.
4.1.3 As the predominant configuration of large transport category airplanes has developed into a high-bypass-ratio twin-engine-powered model, control margins affording
1 See 14 CFR 1.2. VMC means minimum control speed with the critical engine inoperative.
AC 25.933-1 recovery from an unwanted in-flight thrust reversal have decreased. Whenever and wherever thrust reversal is intended, the applicant should maintain focus on limiting any adverse effects of thrust reversal. However, when showing compliance with § 25.933(a)(1) or § 25.933(a)(2), the FAA accepts that applicants may either provide assurance that the airplane is capable of continued safe flight and landing during and after any in-flight thrust reversal event (the airplane is “controllable”), or that the unwanted in-flight thrust reversal event is not anticipated to occur (by showing compliance with § 25.1309(b) assuming the airplane is not capable of continued safe flight and landing during and after any thrust reversal in flight).
4.1.4 The FAA, in several amendments to § 25.933, has sought to limit either the effect or the likelihood of an unwanted thrust reversal during flight on thrust reversers. However, service experience has shown the need to further amend requirements for ground-useonly thrust reversers, since previous methods did not always result in robust designs, and it was not practical for applicants to directly comply with those methods for some designs. The current rule, and this related advisory material, are intended to allow applicants to provide assurance in a manner that recognizes there may be limitations in the airplane design, thereby maximizing both the design flexibility and safety provided by compliance with the rule.
4.2 Minimizing Adverse Effects.
4.2.1 The primary purpose of reversing systems, especially those intended for ground operation only, is to assist in decelerating the airplane during landing and during an aborted takeoff. To accomplish this safety objective, the reverser needs to be quick-acting and effective in producing sufficient reverse thrust. These parameters often result in design characteristics (actuator sizing, efflux characteristics, reverse thrust levels, and so forth) that, in the event of thrust reversal during flight, could cause significant adverse effects on airplane controllability and performance.
4.2.2 If the effect of the thrust reversal occurring in flight produces an unacceptable risk to continued safe flight and landing, then the applicant must design the reverser operation and deactivation system to prevent unwanted thrust reversal to comply with § 25.933(a)(1) or § 25.933(a)(2). Alternatively, for certain airplane configurations, it may be possible to limit the adverse impacts of unwanted thrust reversal on airplane controllability and performance such that the risk to continued safe flight and landing is acceptable (discussed later in this AC).
4.2.3 For turbojet reversing systems intended for operation in flight, per § 25.933(a)(2), the reverser system must be designed to adequately protect against unwanted in-flight thrust reversal.
4.2.4 Advisory circular 25.1309-1B and AC 25.901-1 provide guidance for developing and assessing the safety of installed systems at the design stage. This methodology may also be applied to the installed reverser system, which includes all of the following:
• Reverser.
AC 25.933-1
• Engine (if it can contribute to thrust reversal).
• Reverser motive power source.
• Reverser control system.
• Reverser command system in the flightdeck.
• Wiring, cable, or linkage system between the flightdeck, the reverser control system, and the engine that is used for reverser control or if its failure can contribute to thrust reversal.
4.2.5 Accepted removal, deactivation, reinstallation, and repair procedures for any element in the reverser system installation should result in a safety level equivalent to the certified baseline system configuration.
4.2.6 Qualitative assessments conducted pursuant to aspects of this AC that include evaluating human actions, such as maintenance tasks or airplane operation, should be done taking into account potential human errors. (See AC 25.1309-1B for information on errors.)
4.2.7 Data necessary to determine the level of hazard to the airplane in case of in-flight thrust reversal, and conversely data necessary to define changes to the reverser or the airplane to eliminate the hazard, can be obtained from service experience, test, and/or analysis.
These data also can be used to define the envelope for continued safe flight.
4.2.8 There are many opportunities during the design of an airplane to minimize both the likelihood and severity of unwanted in-flight thrust reversal. These opportunities include design features both of the airplane and of the integrated engine and reverser system. During the design process, consideration should be given to the aircraft stability and control design features, while preserving the intended function of the reversing system.
4.2.9 Some design considerations, which may help reduce the risk from in-flight thrust reversal, include:
4.2.9.1 Engine location to—
4.2.9.1.1 Reduce sensitivity to efflux impingement.
4.2.9.1.2 Reduce effective reverse thrust moment arms.
4.2.9.2 Integrated engine and reverser system design to—
4.2.9.2.1 Optimize integrated engine and reverser system integrity and reliability.
4.2.9.2.2 Inhibit in-flight thrust reversal of ground-use-only reversers, even if commanded by the flightcrew.
AC 25.933-1
4.2.9.2.3 Rapidly reduce engine airflow (in other words, auto-idle) in the event of an unwanted thrust reversal. Generally, such a feature is considered a beneficial safety item. In this case, the probability and effect of any unwanted idle command or failure to provide adequate reverse thrust when selected should be verified to be consistent with § 25.901(c). Also, some reversers are designed so that, at higher power, airloads help prevent deployment. (See paragraph 4.2.9.2.11 of this AC.) Therefore, for such reversers, an auto-idle feature may not have a net benefit.
4.2.9.2.4 Give consideration to the airplane pitch, yaw, and roll characteristics.
4.2.9.2.5 Ensure an undamaged reverser will respond to any anticipated flightcrew command as required by § 25.1309(a). As specified in AC 25.1309-1B, this should include any anticipated crew actions in response to abnormal conditions. Failure of the reverser or the propeller pitch control to respond to pilot commands were contributing factors in various accidents, such as the Cranbrook Boeing Model 737 accident in 19782 and the Luxair Fokker Model F27 accident in 2002.3
4.2.9.2.6 Consider effective efflux diameter.
4.2.9.2.7 Consider efflux area.
4.2.9.2.8 Direct reverser efflux away from critical areas of the airplane where the efflux could be hazardous to the airplane. The thrust reverser pattern associated with high airspeed and high engine power caused significant airflow disruption over the upper wing surface and resulted in loss of airplane control in the Lauda Boeing Model 767 accident in 1991.4
4.2.9.2.9 Expedite system detection of unwanted thrust reversal and provide for rapid compensating action within the reversing system. Consider incorporating an auto-restow capability for unwanted thrust reversal.
However, care should be taken to assure the design features facilitating restow capability do not unacceptably contribute to an unwanted stow or deploy failure condition. For example, a latent failure of the directional control valve might reverse an auto-restow feature and result in an autodeploy command.
4.2.9.2.10 Optimize the use of aerodynamic forces to keep an unrestrained reverser from deploying.
2 See the FAA Lessons Learned from the Civil Aviation Accidents webpage, Cranbrook Boeing Model 737 Accident 1978.
3 See the FAA Lessons Learned from the Civil Aviation Accidents webpage, Luxair Fokker Model F27 Accident 2002.
4 See the FAA Lessons Learned from the Civil Aviation Accidents webpage, Lauda Boeing 767-300ER Accident 1991 6
AC 25.933-1
4.2.9.2.11 Optimize the use of aerodynamic forces to overpower the reverser actuation system preventing a powered deployment under operating conditions where use of reverse thrust is prohibited. Areas to consider might be airspeeds well above maximum V15, or engine power levels above those where deployment is necessary to affect acceptable rejected takeoff (RTO) performance.
4.2.9.3 Airframe/system design to—
4.2.9.3.1 Maximize aerodynamic control capability.
4.2.9.3.2 Expedite detection of thrust reversal and provide for rapid compensating action through other airframe systems.
4.2.9.3.3 Consider crew procedures and foreseeable responses.
4.2.9.3.4 Minimize the potential for flight and maintenance crew error.
4.2.10 Any acceptable means of showing compliance with § 25.933(a) should ensure that the influences that could render the finding invalid are identified, and that acceptable means for managing these influences are defined. To this end, adequate design assurance and continued airworthiness features should be provided, including means to monitor and report in-service experience relative to thrust reverser system safety and effectively respond to any conditions that may substantially invalidate the finding.
4.2.11 The use of systematic “lessons learned” reviews early and often during design development that document how the design accounts for previous service experience may help avoid repeating errors and take advantage of previous successes. For some “lessons learned,” see FAA Memorandum, Criteria for Assessing Transport Turbojet Fleet Thrust Reverser System Safety, dated June 1, 1994,6 and the FAA Lessons Learned From Transport Airplane Accidents website.7
4.3 Coordination With Engine Manufacturer.
4.3.1 The applicant should coordinate with the engine manufacturer to identify any aspects of the thrust reverser design that the engine manufacturer is addressing. This may include aspects that will be included and certified under the part 33 type design. Since it is permissible for the thrust reverser to be certified under part 33 requirements, the applicant may want to coordinate with the engine manufacturer to identify how any of the requirements in § 25.933(a) could be met with the part 33 compliance showing. This may include information on areas such as structural integrity, system reliability, fault
5 See 14 CFR 1.2. Maximum V1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate -stop distance.
6 See the FAA Memorandum, Criteria for Assessing Transport Turbojet Fleet Thrust Reverser System Safety , June 1, 1994 on the internet at FAA Files.
7 See footnote 3.
AC 25.933-1 tolerance, maintenance, and operational assumptions. As such, when planning to certify a thrust reverser or any of thrust reverser system components at the part 33 level, the applicant may wish to coordinate with the engine manufacturer when applying AC 25.933-1 guidance for those aspects of the thrust reverser the engine manufacturer is addressing.
5 DEFINITIONS.
See appendix A for terms and definitions that apply to this AC.
6 DEMONSTRATING COMPLIANCE WITH § 25.933(a)(1).
Sections 7 through 10 of this AC provide guidance on specific aspects of complying with § 25.933(a)(1), according to four different means or methods:
• Controllability. (See section 7.)
• Reliability. (See section 8.)
• Mixed controllability/reliability. (See section 9.)
• Deactivated reverser. (See section 10.)
7 CONTROLLABILITY OPTION.
The “controllability option” provides assurance that the airplane is capable of continued safe flight and landing during and after any in-flight thrust reversal event, which is considered “unwanted” for a ground-use-only thrust reverser. The following paragraphs provide guidance regarding an acceptable means of showing compliance with § 25.933(a)(1)(i).
7.1 General.
7.1.1 To show compliance with § 25.933(a)(1) by demonstrating that the airplane is capable of continued safe flight and landing following any unwanted in-flight thrust reversal (the “controllability option” provided by § 25.933(a)(1)(i)), an applicant should account for structural integrity, performance, and handling qualities as described in this AC.
7.1.2 To identify the corresponding failure conditions and determine the probability of their occurrence, a safety analysis should be carried out using the methodology described in AC 25.1309-1B. The reliability of design features, such as auto-idle and automatic control configurations critical to meeting the following controllability criteria, also should be considered in the safety analysis.
7.1.3 Appropriate alerts and/or other indications of the thrust reverser system and failure conditions should be provided to the crew, as required by regulations such as §§ 25.1141(f), 25.1305(d)(2), 25.1309(c), and 25.1322. (See ACs 25-11B, 25.1309-1B, and 25.1322-1.)