Aircraft Repair Assessment & Facilitation – Sofema Aviation Services (SAS) www.sassofia.com considers the process of assessing & managing aircraft Repairs.
Introduction
Repairs are a concern on older airplanes because of the possibility that they may develop, cause, or obscure metal fatigue, corrosion, or other damage during service.
This damage might occur within the repair itself or in the adjacent structure and might ultimately lead to structural failure.
The damage-tolerance evaluation of a repair would be used in an assessment program to establish an appropriate inspection program, or a replacement schedule if the necessary inspection program is too demanding or not possible.
The objective of the repair assessment is to assure the continued structural integrity of the repaired and adjacent structure based on damage tolerance principles.
In general, repairs present a more challenging problem to solve than the original structure because each repair is unique and tailored in design to correct particular damage to the original structure.
Unlike the performance of the original structure where the behaviour may be predicted from tests and from experience on other airplanes in service, the behaviour of a repair and its effect on the fatigue characteristics are generally not known to the same extent as for the basic unrepaired structure.
Repair Assessment
Although the cause of an airplane accident has never been attributed to properly applied repairs using the original repair data, repairs may be of concern as time-in-service increases for the following reasons:
- As airplanes age, both the number and age of the existing repairs increase.
- Along with this increase is the possibility of unforeseen repair interaction, autogenous failure, or other damage occurring in the repaired area.
- The continued operational safety of these airplanes depends primarily on a satisfactory maintenance program (inspections conducted at the right time, in the right place, using the most appropriate technique).
- To develop this program, a damage-tolerance evaluation of repairs to flight-critical structures is essential.
- The longer an airplane is in service, the more important this evaluation and a subsequent inspection program become.
Damage Tolerance Methodology
The damage-tolerance methodology has evolved gradually over the last 20 or more years.
- Some existing repairs described in the airplane manufacturers’ Structural Repair Manuals (SRM) were not designed to current standards.
- Repairs accomplished in accordance with the information contained in the early versions of the SRM’s may require additional inspections if evaluated using the current methodology.
- Because a regulatory requirement for damage-tolerance was not applied to airplane designs type certificated before 1978, the damage-tolerance characteristics of repairs may vary widely and are largely unknown.
Aircraft Repair Considerations
When a repair is carried out, which is essentially outside of the Structural Repair Manual (SRM) criteria or is carried out to a primary structural element, it is necessary to receive airworthiness approval for the repair.
FAA and EASA Repair Design Data Approval Process.
(a) FAA shall approve design data in support of major repairs in accordance with:
- FAA Order 8110.4 Type Certification;
- FAA Order 8110.37, Designated Engineering Representative Guidance Handbook; FAA Order 8100.15, Organization Designation Authorization Procedures; and
- FAA Order 8300.16, Major Repair and Alteration Data Approval”.
- Minor repairs are made in accordance with “acceptable” data, in accordance with 14 CFR Part 43.
(b) EASA shall approve design data in support of repairs in accordance with EASA Part 21 Subpart M-Repairs and EASA’s procedure Airworthiness of Type Design. A design approval shall be issued for all repair design data.
Boeing Repairs
In the case of Boeing Aircraft, the repair having been designed in accordance with Part 21 Design Standards or designed by Boeing the repair is approved by the issue of FAA 8100-9 FAA form approving the repair.
Boeing may also issue Form FA 8100-9 for any repairs which affect Airworthiness Directives.
Airbus Repairs
In the case of Airbus Aircraft, there is a similar process which is called a RDAS (repair design approval sheet).
- The scope of Part 21 subpart M (21A.431) is the approval of the repair.
- This means the elimination of damage and/or restoration to an airworthy condition of in-service aircraft. In Airbus DOA, a RAS is the issuance of the Repair Design Approval as per Part 21A.437.
- A Repair design Approval Sheet (RAS) is dedicated to structural damage, mainly ATA structure chapters (52 to 57) and interface with systems like flight controls of ATA 27. The Airbus process for system damages is called a Technical Adaptation (TA).
Boeing/Airbus Repair Differences
The difference between Airbus and Boeing is that the FAA form 8100-9 is issued under Organization Designation Authorization (ODA) which is issued to a company (in this case Boeing).
The company then decides who to appoint as the individual members.
- A DER cannot sign the 8100-9 form unless they are also an ODA appointed designee but the DER still uses the 8110-3 form to show compliance to the FARs. Either way, both forms are considered FAA “approved” data whereas the RDAS is issued under DOA privilege.
- Design data used in support of repairs must be approved or accepted, as appropriate, by the exporting authority (State of Design).
The following describes the process that shall be followed by FAA and EASA so that repair design data can be approved or accepted.
- Repair designs requiring the production of new parts that would constitute a design change are not eligible for acceptance under these Implementation Procedures.
Note: It is permissible to fabricate parts that will be used in the repair of the individual aircraft, engine, propeller, or appliance.
EASA Requirements related to Blend Out Repairs
Question – With respect to blend out repairs, is it required to record the depth and area dimensions of material removed during a blend out repair, or is it sufficient to simply record that the damage has been repaired as per the SRM?
Answer – Yes, the dimensions of the damage and the removed/remaining material should be recorded. This is very important information in order to assess whether further damage (adjacent or at the same spot) at a later stage would be allowable or not. In addition, it is a safeguard measure in order to be able to determine, during audits, whether the person correctly determined that the damage was within limits.
Next Steps
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