A crack found beyond allowable limits, a new part delivered with an out-of-tolerance bore, damage that matches no case in the Structural Repair Manual (SRM): in each of these situations the shop can no longer proceed on maintenance documentation alone. It needs approved repair data, and therefore a sound technical file submitted to the organisation entitled to approve it. This article explains the concepts involved, who approves what under EASA rules, what a well-built file contains and when to bring in a structures engineering office.

Definitions not to confuse

Concession and deviation permit

Aerospace quality vocabulary separates two situations. A deviation permit authorises, before manufacture or work, a departure from a specified requirement for a limited quantity or period. A concession authorises, after the fact, the use or release of a product that does not meet requirements — as is, after rework or after repair. In both cases the departure affects an approved design: it can only be accepted by whoever holds responsibility for that design, or under a delegation they have formally granted.

MRB — Material Review Board

The MRB is the body (or process) that dispositions non-conformities: use as is, rework, repair or scrap. In production under Part 21 Subpart G, the approved production organisation (POA) must have procedures for handling production deviations and non-conforming parts; the arrangement between the production organisation and the design holder (point 21.A.133 and its AMC) routes any deviation affecting the approved design back to the design organisation. In maintenance, the term MRB is also used loosely for the process that handles out-of-limits damage. Do not confuse it with the Maintenance Review Board (MRBR), which concerns the initial maintenance programme.

Non-SRM repair

The SRM published by the type-certificate holder contains repairs whose data are already approved. As long as the damage is within its limits and the repair follows the manual strictly, no new design approval is needed. As soon as the damage exceeds the limits, the area is not covered, the material or configuration differs, or several repairs interact, the repair is non-SRM: a new repair design is required, and therefore an approval. The principle is set on the continuing-airworthiness side (point M.A.304): damage is assessed and repairs carried out using data approved by EASA or by an approved Part 21 design organisation, as appropriate.

Major or minor repair

Under point 21.A.435, a repair design is classified as "major" or "minor" using the same criteria as a change to the type design (point 21.A.91). The associated guidance lists, among indicators of a major repair: an appreciable effect on structural performance (static strength, fatigue, damage tolerance, flutter, stiffness), the need for extensive static, fatigue and damage-tolerance substantiation or testing, unusual methods or materials, the need to reassess the original certification data, or additions to the maintenance programme. A repair is minor when its effects are known and without appreciable consequence. The initial classification may change as the analysis progresses.

Who approves what

• SRM repair within limits: data already approved; the Part-145 maintenance organisation carries it out under its own procedures and issues the release to service.
• Minor non-SRM repair: classification and approval by an approved design organisation (DOA) holding the corresponding privilege in its terms of approval, or by EASA.
• Major repair: approval by EASA or by a DOA within the privileges recorded in its terms of approval (21.A.435(b)) — in practice very often the type-certificate holder's DOA, or a third-party DOA that applies to EASA for approval when it does not hold the privilege.
• Production concession: decision of the design holder (or its DOA) when the deviation affects the approved design, in line with the design/production arrangement.

A key point: a Part-145 shop does not approve a repair design under its maintenance approval. It executes and releases. An engineering office that is not itself a DOA does not approve it either: it prepares the substantiation, which the client's approved organisation reviews and approves. That is Kronos Engineering's position: our engineers produce stress notes, drawings and instructions in interface with the manufacturer's DOA or the third-party DOA chosen by the client, which retains responsibility for approval.

What a well-built repair file contains

• Defect description: precise location (station, stringer, frame), dimensions, nature (crack, corrosion, impact, manufacturing defect), photos, NDT results with method and sensitivity, history of adjacent repairs.
• Configuration identification: serial number, build standard, modifications and repairs already embodied, original materials and treatments.
• Static analysis: loads in the area, load paths before and after repair, margins of safety for members, fasteners and doublers.
• Fatigue and damage tolerance: effect of the repair on life and crack growth, associated inspection intervals and methods where the area falls under damage tolerance.
• Repair drawing: geometry, reinforcement parts, fasteners, pitch and edge distances, materials and specifications.
• Execution instructions: sequence, tooling, any special processes (for example cold working of holes or peening), surface protection, acceptance criteria.
• Inspections and continued airworthiness: inspections at embodiment, any repetitive inspections to be added to the maintenance programme, in line with the requirement of point 21.A.433 to provide the necessary data and instructions.

Lead times and common pitfalls

The lead time of a file depends mainly on the quality of the input data. The most frequent delays come from:

• Incomplete input data: damage dimensions not recorded, residual thickness unknown after corrosion blend-out, photos without scale or reference.
• Insufficient NDT: method unsuited to the defect, crack tips not confirmed, no inspection after damage removal. A file cannot substantiate a defect whose extent is unknown.
• Poorly established configuration: undocumented previous repair, embodied modification that changes loads or materials.
• Late classification: a file prepared as minor that turns out to be major changes approver and lead time. Agree the likely classification with the DOA from the start.
• Non-executable instructions: access, tooling or special process not available at the work site.

In an AOG situation, the aim is to run damage mapping, NDT and substantiation work in parallel, so that the DOA receives a complete file on first submission.

When to use an embedded structures office

An external engineering office makes sense when the flow of non-SRM damage exceeds in-house capacity, during a peak of heavy checks, for a specific skill (fatigue, damage tolerance, special processes) or when the MRO has no structures engineering cell. Two formats: a direct service (one file, one deliverable) or the secondment of an engineer embedded within the MRO, working under the host organisation's procedures and in interface with the relevant DOA. For major repair files, see also our Major Repair page.

FAQ

Does Kronos Engineering approve repairs?

No. Kronos Engineering does not hold a DOA, POA or Part-145 approval. We prepare the technical files (substantiation, stress notes, drawings, instructions); approval rests with the client's or manufacturer's DOA, or with EASA.

Is a non-SRM repair necessarily major?

No. It requires a new design approval, but its major or minor classification depends on its effects on the structure and the extent of substantiation needed, according to the criteria of 21.A.435 and 21.A.91.

What data should be provided to get started?

Aircraft or part identification, location and dimensions of the damage, referenced photos, NDT results, repair history of the area and the SRM chapter consulted.

Conclusion

A deviation, a concession or a non-SRM repair is won on the quality of the file: characterised defect, established configuration, complete static and fatigue substantiation, executable instructions. Our structures engineering office prepares these files for the approved organisation that approves them, as a direct service or embedded on site. Describe your case through our contact page: reply within 12 business hours.

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