AC 25.933-1
8.5.2 Minimization.
Minimization guidance provided below is for fragments from axial flow rotors surrounded by fan flow thrust reversers located over the intermediate or high-pressure core rotors f (Figure 4).
Figure 4. Cross-Section of Typical High-Bypass Turbofan Low and High Compressor with Fan Thrust Reverser
8.5.2.1 L arge Fragments.
For the large fragments defined under paragraph 8.5.1.1 of this AC, the thrust reverser retention systems should be redundant and separated as follows:
8.5.2.1.1 Ring Disks Compressor Spools.
Retention systems located in the outer barrel section of the thrust reverser s hould be separated circumferentially (circumferential distance greater than the 1/3 disk fragment model as described in AC 20-128A) or axially (outside the ±5° impact area) so that a 1/3 disk segment cannot damage multiple redundant retention elements and allow thrust reversal (in other words, deployment of a door or translating reverser sleeve half). Retention systems located between the inner fan flow path wall and the engine casing should be located axially outside the ±5° impact area.
8.5.2.1.2 Deep-Bore Disk Spools and Single Disks.
Retention systems should be separated axially with at least on e effective retention element located outside the ±5° impact area.
AC 25.933-1
8.5.2.2 Small Fragments.
For the small fragments defined under paragraph 8.5.1.2 of this AC, thrust reverser retention systems should be provided with either—
8.5.2.2.1 At least one retention element shielded in accordance with paragraph 7(c) of AC 20-128A or capable of maintaining its retention capabilities after impact, or
8.5.2.2.2 One retention element located outside the ±15° impact area.
9 MIXED CONTROLLABILITY/RELIABILITY OPTION.
If the proposed airplane design might experience an unwanted in-flight thrust reversal outside the “controllable flight envelope” anytime during the entire operational life of all airplanes of this type, then the applicant must show reliability compliance outside the controllable envelope, and should take into account associated risk exposure time and the other considerations described in section 8 of this AC. Conversely, if reliability compliance is selected to be shown within a given limited flight envelope with associated risk exposure time, then outside this envelope controllability must be shown taking into account the considerations described in section 7 of this AC. Mixed controllability and reliability compliance may be shown in accordance with guidance developed under sections 7 and 8 of this AC, respectively.
10 DEACTIVATED REVERSER.
The thrust reverser system deactivation design should follow the same “fail-safe” principles as the actuation system design. The applicant should evaluate the design following the guidance in paragraph 8.1 to show that an unwanted in flight thrust reversal is not anticipated to occur on a deactivated thrust reverser system. The effects of thrust reverser system deactivation on other airplane systems, and on the new configuration of the thrust reverser system itself, should be evaluated according to paragraph 8.2 of this AC. The location and load capability of the mechanical lock-out system (thrust reverser structure and lock-out device) should be evaluated according to paragraphs 8.3 and 8.4 of this AC. The evaluation should show that the level of safety associated with the deactivated thrust reverser system is equivalent to or better than that associated with the active system.
11 DEMONSTRATING COMPLIANCE WITH § 25.933(a)(2).
For thrust reversing systems intended for in-flight use, compliance with § 25.933(a)(2) must be shown for unwanted in-flight thrust reversal, as appropriate, and applicants may use the methods specified in sections 7 through 10 of this AC. While the guidance in section 7 is clearly relevant to § 25.933(a)(2), it is unlikely to be sufficient for showing compliance with § 25.933(a)(2) as it relates to intended use of reverse thrust in flight.
Given the rarity and novelty of such designs, development of an acceptable means of compliance for intended use should be done on a case-by-case basis. (See AC 20-166.)
AC 25.933-1 Appendix A Appendix A. Definitions The following definitions apply for the purpose of this AC. Also, see referenced ACs for other relevant definitions.
A.1 ANALYSIS.
See AC 25.1309-1B.
A.2 ASSESSMENT.
See AC 25.1309-1B.
A.3 CONTINUED SAFE FLIGHT AND LANDING.
The capability for continued controlled flight and safe landing at an airport, possibly using emergency procedures, but without requiring exceptional pilot skill or strength or causing serious injury. Some airplane damage may be associated with a failure condition, during flight or upon landing.
A.4 CONTROLLABLE FLIGHT ENVELOPE AND PROCEDURE.
An area of the normal flight envelope where, given a procedure defined by the applicant and accepted by the FAA as appropriate for the condition, the airplane is capable of continued safe flight and landing following an in-flight thrust reversal.
A.5 DEACTIVATED REVERSER.
Any reversing system that has been deliberately inhibited prior to flight such that it is precluded from performing a normal deploy/stow cycle, even if commanded to do so.
The deactivation procedure is sometimes referred to as a “lock-out” procedure.
A.6 EXCEPTIONAL PILOTING SKILL OR STRENGTH.
Skill or strength capabilities that exceed that of the anticipated pilot population. See § 25.143 and AC 25-7D.
A.7 IN-FLIGHT.
That part of airplane operation beginning when the airplane is no longer in contact with the ground during takeoff and ending when the airplane again contacts the ground during landing.
A.8 LIGHT CROSSWIND.
A-1
AC 25.933-1 Appendix A An up-to 10-knot wind at right angles to the direction of takeoff or landing, which is assumed to occur on every flight. See AC 25-7D.
A.9 LIGHT TURBULENCE.
Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude (pitch, roll, and/or yaw), which is assumed to occur on every flight. See AC 25-7D.
A.10 NORMAL FLIGHT ENVELOPE.
An established boundary of parameters (velocity, altitude, angle of attack, attitude) associated with the practical and routine operation of a specific airplane that is likely to be encountered on a typical flight and in combination with prescribed conditions of light turbulence and light crosswind.
A.11 QUALITATIVE ANALYTICAL PROCESS.
See AC 25.1309-1B.
A.12 QUANTITATIVE ANALYSTICAL PROCESS.
See AC 25.1309-1B.
A.13 SYSTEM.
See AC 25.1309-1B.
A.14 THRUST REVERSAL.
A physical spoiling or redirection of all or part of engine airflow from the forward thrust position.
A.15 THRUST REVERSER SYSTEM.
Those components installed on the airplane that spoil or redirect the engine airflow to decelerate the airplane. The components include all of the following:
• Engine-mounted hardware.
• Reverser control system.
• Indication and actuation systems.
• Any other airplane systems that have an effect on operation of the thrust reverser system.
A-2
AC 25.933-1 Appendix A A.16 TURBOJET THRUST REVERSING SYSTEM.
Any device that redirects the airflow from a turbojet engine so as to create reverse thrust. Systems may include any of the following:
• Cascade-type reversers.
• Target or clamshell-type reversers.
• Pivoted-door petal-type reversers.
• Deflectors articulated off either the engine cowling or airplane structure.
• Targetable thrust nozzles.
• A propulsive fan stage with reversing pitch.
A-3
AC 25.933-1 Appendix B Appendix B. Best Practices This appendix provides the Aviation Rulemaking Advisory Committee (ARAC) Powerplant Installation Harmonization Working Group (PPIHWG) recommendations that the FAA considers best practices. Similar information is included in European Union Aviation Safety Agency (EASA) Acceptable Means of Compliance (AMC) 25.933. Thrust reverser system designers and airplane operators may consider these best practices when they design the system or establish the maintenance process for the system.
B.1 MANUFACTURING/QUALITY.
Due to the criticality of the thrust reverser, manufacturing and quality assurance processes should be assessed and implemented, as appropriate, to ensure the design integrity of the critical components.
B.2 RELIABILITY MONITORING.
An appropriate system should be implemented for the purpose of periodic monitoring and reporting of in-service reliability performance. The system should also include reporting of in-service concerns related to design, quality, or maintenance that have the potential of affecting the reliability of the thrust reverser.
B.3 MAINTENANCE AND ALTERATIONS.
The following material provides guidance for maintenance designs and activity to assist in showing compliance with sections 7 through 10 of this AC. (Also see §§ 25.901(b)(2) and 25.1529 (appendix H)). The criticality of the thrust reverser system requires that maintenance and maintainability be emphasized in the design process and development of the maintenance control program, as well as subsequent field maintenance, repairs, or alterations.
B.3.1 Design.
Design aspects for providing adequate maintainability should address the following:
B.3.1.1 Ease of Maintenance.
The following items should be considered:
B.3.1.1.1 It should be possible to operate the thrust reverser for ground testing/troubleshooting without the engine operating.
B.3.1.1.2 Lock-out procedures (deactivation for flight) of the thrust reverser system should be simple and clearly described in the maintenance manual.
Additionally, a placard describing the procedure may be installed in a conspicuous place on the nacelle.
B-1
AC 25.933-1 Appendix B B.3.1.1.3 Provisions should be made in system design to allow easy and safe access to the components for fault isolation, replacement, inspection, lubrication, and so forth. This is particularly important where inspections are required to detect latent failures. Providing safe access should include consideration of risks both to the mechanic and to any critical design elements that might be inadvertently damaged during maintenance.
B.3.1.1.4 Provisions should be provided for easy rigging of the thrust reverser and adjustment of latches, switches, actuators, and so forth.
B.3.1.2 Fault Identification and Elimination.
B.3.1.2.1 System design should allow simple, accurate fault isolation and repair.
B.3.1.2.2 System design personnel should be actively involved in the development, documentation, and validation of the troubleshooting/fault isolation manual and other maintenance publications. The systems design personnel should verify that maintenance assumptions critical to any SSA conclusion are supported by these publications (for example, perform fault insertion testing to verify that the published means of detecting, isolating, and eliminating the fault are effective).
B.3.1.2.3 Thrust reverser unstowed and unlocked indications should be easily discernible during pre-flight inspections.
B.3.1.2.4 If the airplane has onboard maintenance monitoring and recording systems, the system should have provisions for storing all fault indications. This would be of significant help to maintenance personnel in locating the source of intermittent faults.
B.3.1.3 Minimization of Errors.
Minimization of errors during maintenance activity should be addressed during the design process. Examples include physical design features, installation orientation markings, dissimilar connections, and so forth. The use of a formal “lessons learned” review early and often during design development may help avoid repeating previous errors.
B.3.1.4 System Reliability.
The design process should, where appropriate, use previous field reliability data for specific and similar components to ensure system design reliability.
B.3.2 Maintenance Control.
B.3.2.1 Maintenance Program.
When the applicant develops the initial maintenance plan for the airplane, they should consider, as necessary, the following:
B-2
AC 25.933-1 Appendix B B.3.2.1.1 Involvement of the manufacturers of the airplane, engine, and thrust reverser.
B.3.2.1.2 The compatibility of the SSA information with the certification maintenance requirements (CMRs), airworthiness limitations (AWLs), maintenance review board report, master minimum equipment list (MMEL), and so forth.
Note: A CMR is a required scheduled maintenance task established during the design certification of the airplane systems as an operating limitation of the type certificate (TC) or supplemental type certificate (STC). The CMRs, including the interval, result from the thrust reverser SSA. For information on CMRs, see AC 25-19A.
B.3.2.1.3 Identification by the manufacturer of all airplane system maintenance tasks in which foreseeable errors could result in an in-flight thrust reversal preventing continued safe flight and landing. The manufacturer should take all practical actions to minimize the potential for such error, and the operator should consider these tasks when identifying and documenting the required inspection items for the airplane system.
B.3.2.1.4 The complexity of lock-out procedures and appropriate verification.
B.3.2.1.5 Appropriate tests, including an operational test, of the thrust reverser to verify correct system operation after the performance of any procedure that would require removal, installation, or adjustment of a component, or disconnection of a tube, hose, or electrical harness of the entire thrust reverser actuation control system.
B.3.2.2 Training.
The following items should be considered when developing training documentation for the thrust reverser system and other interfacing airplane systems:
B.3.2.2.1 The reason and the significance of accomplishing critical tasks for the thrust reverser and interfacing systems as prescribed. This would clarify why a particular task needs to be performed in a certain manner.
B.3.2.1.2 Instructions or references as to what to do if the results of a check or operational test of the system do not agree with those given in the airplane maintenance manual (AMM). The manual should recommend some corrective action if a system fails a test or check. This would help ensure that the critical components are not overlooked in the troubleshooting process.
B.3.2.1.3 Emphasis on the total thrust reverser and interfacing system s training by a single training source to preclude fragmented information without a clear understanding of the thrust reverser system. This training concept should be used in the initial airplane system training and subsequent retraining.
B-3
AC 25.933-1 Appendix B B.3.2.1.4 Inclusion of airplane system fault isolation and troubleshooting using the material provided for the respective manuals.
B.3.2.1.5 Evaluation of the thrust reverser and interfacing system s training materials to assure consistency between the training material and the thrust reverser system maintenance and troubleshooting manuals.
B.3.2.3 Repairs and Alterations.
The original airframe manufacturer should provide, per § 25.1529, Instructions for Continued Airworthiness essential to ensure that subsequent repairs or alterations do not unintentionally violate the integrity of the type design approval for the original thrust reverser system. Additionally, the original airframe manufacturer should define a method of ensuring that this essential information will be evident to those that may perform and approve such repairs and alterations. One example would be maintaining the wire separation between relevant thrust reverser control electrical circuits. This sensitivity could be communicated by statements in appropriate manuals such as the wiring diagram manual and by decals or placards placed on visible areas of the thrust reverser and/or airplane structure.
B.3.2.4 Feedback of Service Experience.
The maintenance process should initiate the feedback of service experience that will allow the monitoring of system reliability performance and improvements in system design and maintenance practices. Additionally, this service experience should be used to assure the most current and effective formal “lessons learned” design review process possible.
B.3.2.4.1 Reliability Performance.
Operators and manufacturers should collaborate on these items and provide authorities visibility as required (e.g., refer to § 21.3, § 121.703, or § 135.65, as applicable):
• Accurate reporting of functional discrepancies.
• Service investigation of hardware by manufacturer to confirm and determine failure modes and corrective actions if required.
• Update of failure rate data. (This will require coordination between the manufacturers and airlines.)
B.3.2.4.2 Improvements Suggested by Maintenance Experience.
This will provide data to update the following effectively and periodically:
• Manuals.
• Troubleshooting.
• Removal/replacement procedures.
B-4
AC 25.933-1 Appendix B B.3.2.5 Documentation and Procedures.
The following considerations should be addressed when preparing and revising documents and procedures for the instructions for continued airworthiness to support the thrust reverser in the field in conjunction with §§ 25.901(b)(2) and 25.1529 and appendix H to part 25.
B.3.2.5.1 Documentation should describe a rigging check, if required after adjustment of any thrust reverser actuator drive system component.
B.3.2.5.2 Documentation should describe powered cycling of the thrust reverser to verify system integrity whenever maintenance is performed. This could also apply to any manual actuation of the reverser.
B.3.2.5.3 The AMM should include the reasons and the significance of accomplishing critical tasks.
B.3.2.5.4 The AMM should include instructions or references as to what to do if the results of a check or operational test do not agree with those given in the AMM.
B.3.2.5.5 Provisions should be made to address inefficiencies and errors in the documentation identified from—
• The validation process of both critical and troubleshooting procedures.
• Input from field.
• Operators’ conferences.
B.3.2.5.6 Development of the documentation should be a coordinated effort between the thrust reverser, engine, and airplane manufacturers, as well as operators. This is especially important in the areas of—
• AMM.
• Troubleshooting.
• Fault isolation.
• Maintenance data computer output.
• Procedure validation.
• MMEL.
B.3.2.5.7 Initial issue of the documentation should include the required serviceable limits for the complete thrust reverser system.
B-5
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