21.3.1 Approval of repair processes

 Repair processes approval

Civil sector: For this the approach of the US American FAA will be exemplary.
The development of a repair process which can be approved/certified from the responsible agencies (e.g., FAA, EASA, Lit. 21.3.1-1) respectively its „administrator“, presumes at the developer special capabilities, resources and features (Fig. "Requirements for repair", Lit. 21.3.1-2). These must be proved for the certification of the developer (applicant). „The administrator can authorise a company or a private persons with the test, with the proof, with tests and inspections as well as with the approval. However the directionary decision can not be transfered”.

For the supervision of the repair developments by the aviation authority designated specialists, for example FAA appointed Designated Engineering Representatives (DERs) can be used (Lit. 21.3.1-3). They are familiar with the for the opproval/certification responsible agencies and the processes and care for a specified activity. For this they generate the necessary documents for the submission at the approval, ordered by the shops. Concerned are experimental/test designs, observations/results, reports ansd so on. Narurally also the quality asssurance must be developed respectively tested and prooved. Further they support informations to the organisation of the developer.

In the predominant cases of repair developments, it will be necessary to involve the OEM in time (Fig. "Not by the OEM approved repairs"). Only he possesses the sufficient knowledge about the design of the components , operation loads, potential failure mechanisms, material features, production processes and lifetime. That makes his estimation /assessment during the approval process, respectively at the approving authority important. A subsequent objection by the OEM can have estensive consequences (Fig. "Not by the OEM approved repairs").

The sequence, the approval of the process and the application must be in every respect according to the prescriptions. Is this not the case, the repair parts for example are classified as Suspected Unapproved Parts (SUPs, chapter 20.2.1). Accompanying documents/shipping documents inform about status of a part (for new parts see Fig. "Identifying not approved parts").
For a distinct application approved repair processes can not be transfered at other, also seemingly comparable parts or applications without the agreement of the agencies (Fig. "Approval for other engine types and missions"). This is quite understandable, if the part specific operation loads are considered (e.g., short haul or long distance operation).

At the Risk and Revenue - principle in the civil sector, the developer as partner during problems with new parts, different to military projects, bears also his part of the consequences (costs, juridicial). That is also true for repairs.

Military sector:
In this case seem to be national differences in the sequence. In the following two typical scenarios will be considered, however there is no claim for universal validity.
A criterion may be, if the military carries itself out repairs and its development. In such a case development, tests/prooves and approval may be in own responsibility.
Will the repair adopted by a civil producer, he at least will carry out development and laboratory proofs under attendance of a governmental supervision. Then the testing in the aeroengine at the test rig or in the aircraft (military tests) as also the evaluation/assessment and the approval, are in the hands of the operator.

Because in the military sector the approving institution is also simultaneously the operator and responsible for tests/proves. Different to the civil sector, depending from the contract, the approving position bears also its part of the consequences.

 Requirements for repair

Figure "Requirements for repair" (Lit. 21.3.1-1 and Lit. 21.3.1-2): The The instruction Federal Aviation Regulation Subtitle No. 36 (SFAR No.36) of the US Federal Aviation Administration FAA can be applied as universally valid model also for other countries(Lit. 21.3.1-1). It regulates the repair of aviation components/parts. To these belongs the procedure during development and approval/certification (Fig. "Approval of a repair") of repairs respectively repair processes.

 Approval of a repair

Figure "Approval of a repair" (Lit. 21.3.1-2): The development and approval of repair processes is a demanding and responsible task (Fig. "Risks during development of repairs"). The instance with the decision making, not delegable discretionary decision is the approving aviation authority (here FAA). The developer of a repair in specifications is also called „applicant“. This can be the OEM, an operator (airline) or an independent repair developer/repair shop. A repairshop must comply requiremennts for the approval by the authority. This especially concerns the organisation with the qualification of the personnel and the documentation (instructions, manuals; frame below). The repair shop can use a so called Designated Engineering Representative (DER) for the supervision of a repair development and as contact ot the approving agency. This is an experienced person, certified by the aviation authority. For this the necessary proofs/certificates and expertises must be submitted.

The OEM is integrated in the approval process. He advises during the development, helps during definition and realisation of the testing in the aeroengine and evaluates the suitability. With this he has for the approving authority at least an important consultant function.

 Damaging changes during repair

Figure "Approval of a repair".1: Even if the parts to be repaired are not critical changed in its properties by the operation (Ill. 21.3.2-2), pocesses of the new parts production can be problematic. So the development of a repair must consider, if the used part differs from the new part, through a combination of technologies (e.g., coatings) and elements (e.g., thread inserts). With this the production processes of the new parts are only limited applicable.
An example are rub in coatings in compressor casings. Do those consist of silicone rubber which is filled with glass beads, they can swell in cooling lubrication fluid used for machining. This is obviously reversible, but is at least a temporary decrease of the adherence must be expected.


The condition of a part for repair must consider a possible deterioration of integrated components respectively technologies by the repair process.

 Approval for other engine types and missions

Figure "Approval for other engine types and missions" (Lit. 21.3.1-2): Indeed the experience in operation and processing plays an important role for the application of a repair during an approval (Fig. "Requirements for repair" and Fig. "Approval of a repair"). However this is not sufficient to introduce the repair without a further approval at the other parts by the responsible agency. This is also true if this part is comparable, respectively a version or the same part, in an other operation mission. Such a limitation has important, not at least safety relevant reasons.

It is clear, that the lifetime relevant operation load can differ markedly in hight and type for parts of the same function (e.g., HPT rotorblades) in different aeroengine types.

Understandable is also, that for the same aeroengine type the thermal fatigue of the hot parts because of the number of start cycles in short haul markedly differs from long haul.

Even the flight route, e.g., the altitude of a frequently approached airport can represent a markedly difference, especially because of the starts.This has influences on the load of the repair and with this at the lifetime of the part. This applies even for the failure mechanism (e.g., thermal fatigue instead of creep), which was the basis of the suitability proof of the repair testing.

So special experience is needed, which makes an evaluation by the design responsible OEM necessary. This is a requirement for the approval of an, in other cases already successful repair (Fig. "Not by the OEM approved repairs").

 Not by the OEM approved repairs

Figure "Not by the OEM approved repairs" (Lit. 21.3.1-5): In an Airworthiness Directive (AD, Fig. "Maintenance instructions and specifications") the responsible aviation agency (FAA) demands for specified part numbers (PN) and serial mumbers (S/N) of a certain aeroengine type a dimensional check of the fan blade in the transition to the root platform (sketch left). This part zone is regarded by the OREM as critical. That led to the following requirements:

The inspection must be carried out by a repair shop, approved by the OEM. Reason for this approach:

Citeation: „This AD results from a report that a repair station created their own repair and performed it on 520 1st stage fan blades, without approval from …(the OEM).”
With this obviously increased the fear of the OEM about the risk of the exit of a fan blade fragment (uncontained failure).

The OEM demands for the approval of a repair station a source demonstration of the repair process.
Blades which are identified must be disassembled and send to a shop which is approved by the OEM for the dimensional check. After a sufficient result they can be reused. This action may be connected with an extensive effort (logistics, time, costs).


21.3.1-1 „Vergleichende Studie betreffend die amerikanische Federal Aviation Administration (FAA) sowie die vorgeschlagene Europäische Agentur für Flugsicherheit (EASA)“. Europäisches Parlament, Generaldirektion Wissenschaft, page 1-12.

21.3.1-2 Federal Aviation Regulation SFAR No.36, page 1-6.

21.3.1-3 Federal Aviation Administration, DER-Antrag auf Zulassung„ „General Regulatory Criteria”, page 1-13.

21.3.1-4 P.Hoeveler, „Reparieren statt wegwerfen“, Zeitschrift Flug Revue, April 2007, page 83-85.

21.3.1-5 Airworthiness Directive FAA 2005-26-09 Amdt.39-14430, „Pratt & Whitney JT9D-7R4 Turbofan Engines”, 21.12.2005, page 1-4.

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