Annex 2 to Appendix 3 to AMC 20-20A Assessment of existing repairs

A DTI assessment process consists of an aircraft repair survey, identification and disposition of repairs requiring immediate action and development of damage-tolerance-based inspections, as described below:

1.         AIRCRAFT REPAIR SURVEY

A survey will be used to identify existing repairs and repair configurations on all FCS and provide a means to categorise those repairs. The survey would apply to all affected aircraft in an operator’s fleet, as defined in the maintenance programme, using the process contained in the REGs or similar documents. The procedure to identify repairs that require DTE should be developed and documented using CS 25.571 and AMC 25.571 (dependent on aircraft certification level), together with additional guidance specific to repairs, such as:

(a)       Size of the repair;

(b)       Repair configuration:

(1)       SRM standards,

(2)       other;

(c)       Proximity to other repairs; and

(d)       Potential effect on FCS:

(1)       inspectability (access and method),

(2)       load distribution.

See Paragraph 4 of this Annex for more details.

2.         IDENTIFICATION AND DISPOSITION OF REPAIRS REQUIRING IMMEDIATE ACTION

Certain repairs may not meet the minimum requirements because of cracking, corrosion, dents, or inadequate design. The operator should use the guidance provided in the compliance document to identify these repairs and, once they are identified, take appropriate corrective action. In some cases, modifications may need to be made before further flights. The operator should consider establishing a fleet campaign if similar repairs may have been installed on other aircraft.

3.         DAMAGE TOLERANCE INSPECTION DEVELOPMENT

This includes the development of the appropriate maintenance plan for the repair under consideration. During this step determine the inspection method, threshold, and repeat interval. Determine this information from existing guidance information as documented in the RAG (see Paragraph 4), the REGs or from the results of an individual DTE performed using the guidance in AMC 25.571. Then determine the feasibility of an inspection programme to maintain continued airworthiness. If the inspection programme is practical, incorporate the DTI into the individual aircraft maintenance programme. If the inspection is either impractical or impossible, incorporate a replacement time for the repair into the individual aircraft maintenance programme. The three-stage approach discussed in Annex 1to this Appendix may be used, if appropriate.

4.         REPAIR ASSESSMENT GUIDELINES

4.1.     Criteria to assist in developing the repair assessment guidelines

The following criteria are those developed for the fuselage pressure boundary, similar to those found in FAA AC 120-73 and previous JAA and EASA documentation. DAHs may find it appropriate to develop similar practices for other types of aircraft and areas of the structure.

The purpose is to develop repair assessment guidelines requiring specific maintenance programmes, if necessary, to maintain the damagetolerance integrity of the repaired airframe. The following criteria have been developed to assist in the development of that guidance material:

(a)       Specific repair size limits for which no assessment is necessary may be selected for each model of aircraft and structural location. This will enable to minimise the burden on the operator while ensuring that the aircraft’s baseline inspection programme remains valid.

(b)      Repairs that are not in accordance with SRM must be reviewed and may require further action.

(c)       Repairs must be reviewed where the repair has been installed in accordance with SRM data that has been superseded or rendered inactive by new damage-tolerant designs.

(d)      Repairs in close proximity to other repairs or modifications require review to determine their impact on the continued airworthiness of the aircraft.

(e)      Repairs that exhibit structural distress should be replaced before further flights.

4.2.     Repair assessment methodology

The next step is to develop a repair assessment methodology that is effective in evaluating the continued airworthiness of existing repairs for the fuselage pressure boundary. Older aircraft models may have many structural repairs, so the efficiency of the assessment procedure is an important consideration. In the past, evaluation of repairs for damagetolerance would require direct assistance from the DAH. Considering that each repair design is different, that each aircraft model is different, that each area of the aircraft is subjected to a different loading environment, and that the number of engineers qualified to perform a damagetolerance assessment is small, the size of an assessment task conducted in that way would be unmanageable. Therefore, a new approach has been developed as an alternative.

Since repair assessment results will depend on the model-specific structure and loading environment, the DAHs should create an assessment methodology for the types of repairs expected to be found on each affected aircraft model. Since the records on most of these repairs are not readily available, locating the repairs will necessitate surveying the structure of each aircraft. A survey form is created by the DAH that may be used to record key repair design features needed to accomplish a repair assessment. Airline personnel not trained as damagetolerance specialists can use this form to document the configuration of each observed repair.

Some DAHs have developed simplified methods using the information from the survey form as input data, to determine the damagetolerance characteristics of the surveyed repairs. Although the repair assessments should be performed by well-trained personnel familiar with the model-specific repair assessment guidelines, these methods enable appropriate staff, not trained as damagetolerance specialists, to perform the repair assessment without the assistance of the TCH. This methodology should be generated by the aircraft TCH. Model-specific repair assessment guidelines will be prepared by the TCHs.

From the information on the survey form, it is also possible to classify repairs into one of three categories:

When the LOV of the maintenance programme is extended, the initial categorisation of repairs may need a review by the applicant for the LOV extension, and the operator may need to ensure that these remain valid up to the new LOV.

4.3.     Repair assessment process

There are two principal techniques that can be used to accomplish the repair assessment. The first technique involves a three-stage procedure. This technique could be well-suited for operators of small fleets. The second technique involves the incorporation of the repair assessment guidelines as part of an operator’s routine maintenance programme. This approach could be well-suited for operators of large fleets and would evaluate repairs at predetermined planned maintenance visits as part of the maintenance programme.

The first technique generally involves the execution of the following three stages (see FigureA3(2)-1):

Stage 1: Data collection

This stage specifies what structure should be assessed for repairs and collects data for further analysis. If a repair is on a structure in an area of concern, the analysis continues, otherwise the repair does not require classification per this programme.

Repair assessment guidelines for each model will provide a list of structure for which repair assessments are required. Some DAHs have reduced this list by determining the inspection requirements for critical details. If the requirements are equal to normal maintenance checks (e.g. BZI checks), those details may be excluded from this list.

Repair details are collected for further analysis in Stage 2. Repairs that do not meet the minimum design requirements or are significantly degraded are immediately identified, and corrective actions must be taken before further flights.

Stage 2: Repair categorisation

The repair categorisation is accomplished by using the data gathered in Stage 1 to answer simple questions regarding structural characteristics.

If the maintenance programme is at least as rigorous as the BZI identified in the TCH’s model-specific repair assessment guidelines, well-designed repairs in good condition meeting size and proximity requirements are Category A. Simple condition and design criteria questions are provided in Stage 2 to define the lower bounds of Category B and C repairs. The process continues for Category B and C repairs.

Figure A3(2)-1: Repair assessment stages

Stage 3: Determination of structural maintenance requirements

The specific supplemental inspection and/or replacement requirements for Category B and C repairs are determined in this stage. Inspection requirements for the repair are determined by calculation or by using predetermined values provided by the DAH, or other values obtained using an EASA-approved method.

In evaluating the first supplemental inspection, Stage 3 will define the inspection threshold in flight cycles measured from the time of the repair installation. If the time of the repair installation is unknown and the aircraft has exceeded the assessment implementation times or has exceeded the time for first inspection, the first inspection should occur by the next ‘C-check’ interval, or equivalent cycle limit after the repair data is gathered (Stage 1).

The operator may choose to accomplish all three stages at once, or just Stage 1. In the latter case, the operator would be required to adhere to the schedule specified in the EASA‑approved model-specific repair assessment guidelines for completion of Stages 2 and 3. Incorporating the maintenance requirements for Category B and C repairs into an operator’s individual aircraft maintenance or inspection programme completes the repair assessment process for the first technique.

The second technique would involve setting up a repair maintenance programme to evaluate all the applicable structures as detailed in paragraph 1 at each predetermined maintenance visit to confirm that they are permanent. This technique would require the operator to choose an inspection method and interval in accordance with the EASA‑approved repair assessment guidelines. The repairs whose inspection requirements are fulfilled by the chosen inspection method and interval would be inspected in accordance with the approved maintenance programme. Any repair that is not permanent, or whose inspection requirements are not fulfilled by the chosen inspection method and interval, would either be:

(a)       upgraded to allow utilisation of the chosen inspection method and interval; or

(b)      individually tracked to account for the repair’s unique inspection method and interval requirements.

This process is then repeated at the chosen inspection interval.

Repairs added between the predetermined maintenance visits, including interim repairs installed at remote locations, would be required either to have a threshold greater than the length of the predetermined maintenance visit or to be tracked individually to account for the repair’s unique inspection method and interval requirements. This would ensure the airworthiness of the structure until the next predetermined maintenance visit, at which time the repair would be evaluated as part of the repair maintenance programme.

5.         MAINTENANCE PROGRAMME CHANGES

When a maintenance or inspection programme interval is revised, the operator should evaluate the impact of the change on the repair assessment programme. If the revised maintenance or inspection programme intervals are greater than those in the BZI, the previous classification of Category A repairs may become invalid. The operator may need to obtain approval of an alternative inspection method, upgrade the repair to allow utilisation of the chosen inspection method and interval, or re-categorise some repairs and establish unique supplemental inspection methods and intervals for specific repairs. Operators using the ‘second technique’ of conducting repetitive repair assessments at predetermined maintenance visits would evaluate whether the change to the predetermined maintenance visit continues to fulfil the repair inspection requirements.

6.         SRM UPDATE

The general section of each SRM will contain brief descriptions of damagetolerance considerations, categories of repairs, description of BZIs, and the repair assessment logic diagram. In updating each SRM, existing location-specific repairs should be labelled with appropriate repair category identification (A, B or C), and specific inspection requirements for B and C repairs should also be provided as applicable. SRM descriptions of generic repairs will also contain repair category considerations regarding size, zone, and proximity. Detailed information for the determination of inspection requirements will have to be provided for each model. Repairs which were installed in accordance with a previous revision of the SRM, but which have now been superseded by a new damage-tolerant design, will require review. Such repairs may be reclassified to Category B or C, requiring additional inspections and/or rework.

7.         STRUCTURE MODIFIED BY AN STC

The current repair assessment guidelines provided by the TCH are not always applicable to structure modified by an STC. Nonetheless, it is expected that all the structure modified by an STC should be evaluated by the operator and, if possible, in conjunction with the STCH. Point 26.370 of Part-26 requires the operator to amend their maintenance programme to address all such repairs, and a conservative extension of the TCH’s REGs to all STCs containing FCS can be envisaged to ensure that all repairs to FCS are identified. Subsequently, each repair can be subjected to a DTE, and DTI be provided with the support of a DAH. The STCH should conduct specific damagetolerance assessments of known published repairs (SRM, SBs, etc.) and provide appropriate instructions to the operator.

It is expected that the STCH will assist the operators by preparing the required documents. If the STCH is no longer in business, or is otherwise unable to provide assistance, the operator would have to acquire the EASA-approved guidelines independently. Ultimately, the operator remains responsible for the continued safe operation of the aircraft.

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