U.S. Department of Transportation Federal Aviation Administration Advisory Circular
Subject: Developing Data for Major Repairs of Turbine Engine Parts Date: 4/30/10 Initiated by: ANE-110 AC No: 33-9
1. Purpose.
a. This advisory circular (AC) provides information and guidance on developing the technical data needed for major repairs of critical and complex turbine engine parts. This guidance will help persons developing major repair data meet the requirements of Title 14 of the Code of Federal Regulations (14 CFR) part 43 to restore the engine to at least equal to its original or properly altered condition. This does not mean that a repaired part must be returned to a factory-new condition. However, the engine on which the repaired part is installed must remain airworthy and in compliance with the applicable regulations. For repairs of turbine engines and parts, the applicable requirements are Airworthiness Standards: Aircraft Engines (14 CFR part 33) and Fuel Venting and Exhaust Emissions Requirements for Turbine Engine Powered Airplanes (14 CFR part 34).
b. Additionally, this AC provides guidance to assist persons developing major repairs with developing a continued operational safety plan. It also helps them evaluate engine parts recovered from accidents and incidents prior to making a repair.
2. Applicability.
a. This AC provides guidance to all persons developing the technical data to substantiate major repairs for critical or complex turbine engine parts, including type certificate (TC) and supplemental type certificate (STC) holders.
b. This guidance 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. The FAA (“we”) will consider other methods of demonstrating compliance that a person developing repair data may elect to present. Terms such as “should,” “shall,” “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.
While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the applicable regulations. On the other hand, if we become aware of circumstances that convince us that following this AC would not result in
2 compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation as the basis for finding compliance.
c. This document does not change, create any additional, authorize changes in, or permit deviations from, existing regulatory requirements.
3. Related References. Please check the FAA’s website at http://www.faa.gov/regulations_policies/ for the latest revision of the following documents.
a. AC 20-62D, Eligibility, Quality, and Identification of Aeronautical Replacement Parts;
May 24, 1996.
b. AC 33.75-1A, Guidance Material for 14 CFR 33.75, Safety Analysis; September 26, 2007.
c. AC 39-8, Continued Airworthiness Assessments of Powerplant and Auxiliary Power Unit Installations of Transport Category Airplanes; September 8, 2003.
d. AC 43-18, Fabrication of Aircraft Parts by Maintenance Personnel; March 24, 2006.
e. AC 120-77, Maintenance and Alteration Data; October 7, 2002.
f. Order 8110.37D, Designated Engineering Representative (DER) Handbook; August 10, 2006.
g. Order 8110.4C, Type Certification; August 14, 2008.
h. Order 8110.54, Instructions for Continued Airworthiness Responsibilities, Requirements, and Contents; July 1, 2005.
i. Order 8120.2F, Production Approval and Certificate Management Procedures; January 30, 2009.
j. FAA Order 8120.11, Disposition of Scrap or Salvageable Aircraft Parts and Materials;
February 12, 1996.
4. Background.
a. The designs of many critical and complex turbine engine parts, hereafter referred to as “parts,” require the use of advanced design and test methodologies, comprehensive materials data, and refined manufacturing techniques to achieve high levels of safety, reliability and performance. A person developing a repair for a critical or complex part should have a thorough understanding of the part’s design characteristics, operational environments, operating requirements, and failure modes and effects. This knowledge is useful for determining if a repair to a part is major and for developing the appropriate technical data for FAA approval.
b. Our previous evaluations of major repair data packages for critical and complex parts have indicated that the data were not always adequate to support substantiation of the repair.
Data developed for major repairs of critical or complex parts may typically include rigorous analyses and tests to show the engine on which the repaired part is installed will be at least equal to its original or properly altered condition. An original or properly altered condition is an airworthy condition that existed before the need for the proposed repair. An airworthy condition means the aircraft conforms to its type certificate and is in a condition for safe operation.
c. This AC uses part categorization as an aid for persons to determine whether a part being repaired is a critical or complex part. In addition, this AC recommends certain information be included in major repair data packages to improve consistency and completeness. In Appendix 2 of this AC, we also provide templates for repairs related to various engine part families to assist repair developers in identifying the technical elements and regulatory requirements when developing and substantiating similar repairs, if determined major.
5. Part Categorization. This AC categorizes parts based on their most severe potential failure effect using various methods for assessing malfunctions and failure modes. To assess malfunctions and failure modes, repair developers may use the failure modes and effects assessment described in Appendix 1 of this AC or other acceptable analytic techniques. Refer to AC 33.75-1A for examples of other acceptable techniques.
a. Part Categories. The categories used in this AC are consistent with categories found in Order 8120.2 and AC 39-8, and used in AC 43-18. The part categories are:
(1) Category 1. A product, i.e. engine, or part(s) thereof, whose failure could prevent continued safe flight and landing; resulting consequences could reduce safety margins, degrade performance, or cause loss of capability to conduct certain flight operations. A Category 1 part, for the purpose of this AC and Orders 8110.37 and 8110.4, is a critical part.
(2) Category 2. An engine or part(s) thereof whose failure would not prevent continued safe flight and landing; resulting consequences may reduce the capability of the aircraft or the ability of the crew to cope with adverse operating conditions or subsequent failures. For the purpose of this AC and Order 8110.4, a Category 2 part is typically a complex part that may affect a critical part.
(3) Category 3. An engine or part(s) thereof whose failure would have no effect on continued safe flight and landing of the aircraft. The only consequence would be partial or complete loss of engine thrust or power (and associated engine services). For single engine applications, consider changing part categorization to Category 1 or 2 if complete loss of thrust could prevent continued safe flight and landing or reduce the ability of the crew to cope with adverse operating conditions or subsequent failures.
b. Tables of Potential Failure Effects. The following tables provide some potential failure effects for Categories 1 and 2. We also listed a few examples of parts whose malfunction or failure could result in one or more of the listed potential failure effects.
4 Table 1. Category 1 Engine Parts
Potential Failure Effects Part Examples
(1) Non-containment of highenergy debris.
(2) Concentration of toxic products in the engine bleed air intended for the cabin, and sufficient to incapacitate crew or passengers.
(3) Significant thrust in the opposite direction to that commanded by the pilot.
(4) Uncontrolled fire.
(5) Failure of the engine mount system leading to inadvertent engine separation.
(6) Release of the propeller by the engine, if applicable.
(7) Complete inability to shut the engine down.
Life-limited parts
Main engine mounts, with no redundant load carrying features
High pressure vessels (for example, casings subject to compressor discharge pressure and combustor pressure)
Containment structures
Fan blades
Fuel system shut-off
Primary structures (for example, structures that provide support and rigidity of the main engine backbone and for attachment of engine to airframe)
Thrust reverser control component if thrust reverser control component is part of the engine type certificate.
5 Table 2. Category 2 Engine Parts
Potential Failure Effects Part Examples
(1) Controlled fires (that is, those brought under control by shutting down the engine or by onboard extinguishing systems).
(2) Case burn-through where it can be shown there is no propagation to hazardous engine effects.
(3) Release of low-energy parts where it can be shown there is no propagation to hazardous engine effects.
(4) Vibration levels that result in crew discomfort.
(5) Concentration of toxic products in the engine bleed air for the cabin sufficient to degrade crew performance.
(6) Thrust in the opposite direction to that commanded by the pilot, below the level defined as hazardous.
(7) Loss of integrity of the load path of the engine supporting system without actual engine separation.
(8) Generation of thrust greater than maximum rated thrust.
(9) Significant uncontrollable thrust oscillation.
(10) Loss of protection such as loss of overspeed protection or loss of containment case capability.
(11) Effect or influence on a Category 1 part.
Rotating parts that are not life-limited (for example, compressor and turbine airfoils)
Accessory gearbox and internal components
Engine bearings
Spinners
Main engine mounts with redundant load carrying features
Static gas path parts (for example, vanes and seals)
Control system actuators
Combustion liners
Fuel nozzles
6. Developing Data for Major Repairs. To improve the quality and adequacy of data for major repairs of critical (Category 1) and complex (Category 2) parts, we recommend persons developing the major repair data include the following information in their repair data packages:
a. A description of:
(1) The part to be repaired, including the part number.
(2) The repair, including any associated limitations. For example:
• The extent of the damage the repair covers.
• How many times, if limited, the repair can be repeated on a single part.
b. The part categorization, if categorization is used.
c. The applicable sections of the regulatory requirements from the certification basis for the engine(s) that the repaired part is eligible for installation on.
d. An applicability assessment of any related service difficulty reports and airworthiness directives (ADs) to the part.
e. The complete step-by-step “how to” instructions for accomplishing the repair. This should include drawings and specifications, new materials and fabrication details, if any.
f. Technical data. The regulations require that when performing a major repair, the work be done in accordance with technical data approved by the Administrator. Technical data also includes substantiation data to show compliance to applicable regulatory requirements.
(1) Category 1 parts. We recommend that persons developing data in support of a major repair of a Category 1 part meet with the project aircraft certification office (PACO) to develop a substantiation plan before submitting a data package. The plan should outline the pertinent technical and regulatory requirements and testing, as needed to develop the data and substantiate the repair. In addition, the plan should identify process controls, inspections or tests, or both, required in the step-by-step procedures to mitigate the risks of improperly completing the repair.
Note: For major repairs of Category 1 parts, Order 8110.37, Designated Engineering Representative (DER) Handbook, requires DERs notify the PACO prior to approval of the data. Order 8110.4, Type Certification, requires PACO engineers notify the accountable directorate of a major repair of a Category 1 part and coordinate data approval with the certificate management ACO.
(2) Category 2 parts. We recommend persons developing data in support of a major repair of a Category 2 part use the templates in Appendix 2 of this AC or develop a new template, if applicable.
(a) Together with industry we developed the sample templates in Appendix 2 for typical repairs that were previously determined major. These templates may be used as aids to identify technical elements and regulatory requirements to be considered when developing your data and substantiating your repair for similar repairs to Category 2 parts, if determined major.
These templates are not all inclusive; rather, they are for a limited number of typical Category 2 parts.
(b) Persons using these templates should discuss any questions with the PACO. If a template is not available for a proposed major repair, consider developing a new template.
Using a format consistent with the templates provided will help ensure that your template identifies the applicable technical considerations and regulatory requirements. You should review the new template with the PACO early in the project to determine if any additional data will be needed.
g. An assessment of the applicability of the Instructions for Continued Airworthiness (ICAs). If ICAs are affected or needed, supplemental ICAs must be provided. Refer to FAA Order 8110.54, Instruction for Continued Airworthiness Responsibilities, Requirements, and Contents, for further information.
7. Developing a Continued Operational Safety (COS) Plan.
a. COS Management Plan. Repair approval holders and persons making a major repair must perform their work in accordance with approved technical data and applicable performance regulations. If service experience or FAA oversight audits show the repair data was inadequate, in error, or the work was performed improperly, the FAA will evaluate the safety impact and take appropriate action. That action may be to issue an AD, Special Airworthiness Information Bulletin (SAIB), or other appropriate document. In such a case, the repair approval holder or the person who performed the work, or both, may be requested to develop a corrective action program or a design change to restore safety to an acceptable level. Therefore, the FAA recommends that repair approval holders who develop data for major repairs and persons who use that data to perform maintenance develop a COS management plan.
b. Part Identification. To facilitate part tracking as part of a COS management plan, we recommend that persons proposing a repair provide supplementary part identification procedures.
(1) Any original part markings, such as the part number and serial number, should be maintained. Supplementary identification should be permanent and legible. It should include a name, trademark, or other symbol of the person performing the repair. Supplementary identification combined with adequate maintenance records ensures that if a problem occurs the affected parts can be identified, tracked, and, if necessary, removed from service.
(2) Parts subject to an airworthiness limitation should retain their original markings in accordance with § 45.14. When adding supplementary identification to the part, repair developers must ensure that their repair identification does not adversely impact the part. For example, do not apply repair identification on contact surfaces or near radii of parts because
8 these are typically areas of high stress concentrations. When impractical, i.e., the part is too small or applying the repair identification would compromise part integrity, we recommend entering part identification information in the part’s maintenance records.
c. Maintaining Records. Federal Aviation Regulations have specific minimum time requirements for the retention of certain maintenance records. Experience has shown that the maintenance records and the availability of repair data can be extremely valuable for accident investigation and corrective action development. Therefore, we recommend that the data approval holder and persons performing the major repairs, for example, repair stations, owner/operators, or air carriers, keep records that include at least the following:
(1) A record keeping plan to:
(a) Identify where repaired parts are in service or who the customer owners/operators of the parts are.
(b) Retain records of the work performed during the repair.
(c) Retain the technical data for the parts.
(2) Documentation of the quality control of the repair.
(3) A means to monitor the performance of repaired parts in service.
(4) A means to identify and isolate parts that do not conform to approved data or are unairworthy.
(5) The capability to analyze failed parts and identify root causes of failures in order to develop and implement corrective action(s) when needed.
8. Repair of Engine Parts Recovered from Accidents or Incidents.
a. We recommend persons repairing parts recovered from accidents or incidents inspect these parts for hidden damage prior to beginning a repair. Repair stations are required to perform this inspection per regulation, refer to § 145.211(c). This pre-repair inspection will ensure the part does not have hidden damage or has not been overstressed or overheated, which is not always obvious from visual or dimensional inspection.
b. The ICAs may be insufficient for this pre-repair inspection. Therefore, developing special instructions for evaluating repairability of such parts may be appropriate. We suggest repair developers review the part’s maintenance and operational records for relevant service information and verify the history and origin of the parts when making that determination. Parts that are determined to be unsuitable for repair should not be repaired and should be returned to the owner with the appropriate entry in the part maintenance record. Refer to AC 20-62 and Order 8120.11 for further information on the airworthiness and eligibility of such parts. Parts
9 otherwise eligible for repair which do not exhibit evidence of irreparable damage may be repaired.
Francis A. Favara Manager, Engine and Propeller Directorate Aircraft Certification Service
Appendix 1 10 APPENDIX 1. FAILURE MODES AND EFFECTS ASSESSMENT
1. A failure modes and effects assessment is a qualitative process, independent of failure rates and probabilities, by which each failure mode of a part in the engine system is analyzed. Some top-level functions typically considered in an engine failure modes and effects assessment are:
Maintaining structural integrity, including allowed overspeed and overtemperature exceedances;
Providing thrust or power;
Operating in inclement weather;
Providing customer bleed, power extraction, or both; and Meeting fuel consumption, exhaust gas temperature, vibration, emission or noise limits.
2. Each system and subsystem of the engine is broken down into its basic functions using a functional block diagram consistent with the Air Transport Association policy for identification and definition of systems.
3. The functional block diagram defines each system and subsystem, and all their functions, in the turbine engine. The experienced safety engineer performing the analysis determines the partto-part and part-to-system influences in both directions (input and output). The process flow is shown in figure A1.1 below:
Figure A1.1. Process Flow Diagram
Determine input functions from other engine systems Identify parts within the system being analyzed Determine functions provided by the part being analyzed Determine consequence of failure to provide the desired function by design intent Determine outcome on part, engine system, and the engine Categorize part by outcomes in Tables 1 and 2 of this AC
Appendix 1 11
4. The part categorization process is built around the fundamental understanding of the part function and its potential effects on physically or functionally mating parts or both. The fundamental premise in the categorization process, and in the physical operation and function of the turbine engine, is system interactions.
a. System interactions are influences a part, or a set of parts, can have on the turbine engine, propulsion system, or aircraft through form, fit, or function, where
(1) Form is the shape, size, dimensions, and other physical measurable parameters that uniquely characterize a part.
(2) Fit is the ability of a part to interface or interconnect, physically and functionally, at the common boundaries with another part or system of the engine.
(3) Function is the action(s) the part is designed to perform in the engine.
b. These influences may extend beyond the component being classified, may be direct or indirect, and may develop immediately or over time. Characteristics of these influences include:
(1) Direct influences, which are form and fit. These influences are based on physical contact or interface clearances between adjacent parts.
(2) Indirect influences, which are functional in nature. These influences are not based on physical contact, but may be aerodynamic, thermal, or vibratory.
5. The interactions where the consequence of failure is the furthest from the cause are the most difficult to identify. Many fundamental relationships in part interactions and subsequent system effects exist. Figure A1.2 below provides four examples.
Appendix 1 12 Figure A1.2. Part Interaction Considerations
Part Interaction Considerations
•Fluid interactions
• Air, fuel, oil
•Direct interactions
• Mechanical loading
• Aero loading
• Contact/wear
• Chemical
•Indirect interactions
• Vibration
• Acoustical
• Primary air
• Secondary air
• Control system
• Structural dynamics
• Sensor systems Stage 1 HPT Nozzle 1 2 3 4 Effective area/flow HPC press.
ratio Stall margin consumption Stall at limiting condition Fuel Pump Impeller Fuel flow schedule Fuel to Combustor Starting characteristic Altitude re-light capability Stage 2 HPT Nozzle Cooling flow to rotor Purge flow level Hot gas ingestion Critical part temperature & life Rotor Blade Loads and moments on disk Local dovetail loading Dovetail or slot bottom life LLP life impact Indirect Effect Indirect Effect Time Relationship Direct Effect