The Repair Evaluation Guidelines (REG) is a document developed by the Aircraft Manufacturer (Type Certificate Holder) and implemented by the Operator to ensure that an evaluation is performed of all repairs to the structure that is susceptible to fatigue cracking and could contribute to a catastrophic failure.

Repair Evaluation Guidelines (REG) are intended to assure the continued structural integrity of all relevant repaired and adjacent structures.

In August 1988, the Air Transport Association of America (ATA) and the Aerospace Industries Association of America (AIA) presented the FAA with a proposal to form the Aging Airplane Task Force (AATF), an international group comprised of technically qualified individuals representing the Airlines and aircraft manufacturing industry.

In 1992, this group became the Airworthiness Assurance Working Group (AAWG) and was chartered under the auspices of the Aviation Rulemaking Advisory Committee (ARAC). This group is composed of representatives from the FAA, aircraft operators, aircraft manufacturers, and representatives of other civil airworthiness authorities.

The original focus of the AAWG was on the large transport fleet and specifically on those aircraft that represented the older aircraft used in air carrier service in the United States.

The Large Transport Aging Aircraft program initially addressed the following eleven model aircraft –

  1. Airbus A300,
  2. Boeing 707/720,
  3. Boeing 727,
  4. Boeing 737,
  5. Boeing 747,
  6. British Aerospace BAC 1-11,
  7. Douglas DC8,
  8. DC9/MD-80,
  9. DC-10,
  10. Fokker F28, and 
  11. Lockheed L-1011.

The original program elements identified at the international conference became known as the five program elements. This includes:

  1. Corrosion program – Maintain corrosion to an acceptable level through the development of the Corrosion Prevention and Control Program (CPCP).
  2. Repair assessment program – Assess existing structural repairs to ensure adequate durability and frequency of inspections. Damage Tolerance methodology will be applied to future repairs. FAA issues NPRM (Notice of Proposed Rulemaking) for the damage tolerance assessment of the fuselage pressure boundary.
  3. Modification and inspection program – Reduce reliance on intensive structural inspection for known fatigue cracking problems by mandating modification of the aging aircraft).
  4. Supplemental Structure Inspection Program (SSIP) review. From 2003, the Airworthiness Limitation Item review superseded the Supplemental Structure Inspection Program review.
  5. Structural Maintenance Guidelines – Develop generic structural maintenance program guidelines for aging aircraft.

In 1992, the sixth task regarding Structural Fatigue Audit was added. The FAA has issued airworthiness directives on the eleven model transport aging aircraft to mandate corrosion prevention and control programs, modification programs, and to revise the supplemental structural inspection documents (as necessary).

The ATA has issued generic structural maintenance guidelines and some manufacturers have issued model-specific maintenance guidelines. The FAA did not mandate these maintenance guidelines.

In 1993, the AAWG recommended that a Widespread Fatigue Damage (WFD) evaluation would be carried out on each of the 11 models.

In June 1997, the FAA published the AC 91-56A which includes guidance to conduct a WFD evaluation. In order to ensure that no large transport category airplane will be operated beyond its DSG (Design Service Goal) unless a WFD (Widespread Fatigue Damage) evaluation has been done, the AAWG developed regulatory and advisory materials in December 1999.

The revised AC 91-56A, providing details to conduct the audit, as well as the corresponding NPRM has been submitted to the FAA for publication.

In March 1997, the AAWG submitted its final proposal to the FAA regarding the repair assessment program.

On January 5, 1998, a Notice of Proposed Rulemaking (NPRM) recommended by the AAWG to address the damage tolerance assessment of repairs was published for public comment.

In July 1997, the FAA published an AC 91-56A, also recommended by AAWG, to address the Structural Fatigue Audit of aircraft.

Although there was JAA membership and European Operators and Industry representatives participated in AAWG, the recommendations for action became based on FAA operational rules, which are not applicable in Europe. It was therefore decided to establish the European Aging Aircraft Working Group (EAAWG) on the subject to find a way to implement these Aging Aircraft activities into the JAA regulatory system, not only for the initial “11 Aircraft”, but also for other old aircraft and more recently certificated ones.

Finally, the FAA issued the Aging Airplane Safety Final Rule (AASFR) in February 2005. This rule requires the operators to use damage tolerance-based inspections and procedures to maintain the continued airworthiness of affected airplanes structures susceptible to fatigue cracking that could contribute to a catastrophic failure.

Sections 121.1109 (previous 121.370a) and 129.109 (previous 129.16) of the Title 14 Code of Federal Regulations (14 CFR) include supplemental inspection requirements to address the continued airworthiness of this type of airplane structure.

In April 2006, the FAA issued a Notice of Proposed Rulemaking (NPRM) 05-2006 enforcing manufacturers to support operators for compliance with the AASFR by defining the content of the list of affected structure, so-called Fatigue Critical Structure (FCS).

Finally, the FAA issued in December 2007 the final rule “Damage Tolerant Data for Repair and Alterations” followed by the publication of the new Part 26 sections –

  • 26.43 – Holders of type certificates – Repairs,
  • 26.45 – Holders of type certificates – Alterations and Repairs to Alterations,
  • 26.47 – Holders and applicants of Supplemental type certificates – Alterations and Repairs to Alterations, and
  • 26-49 – Compliance Plan.

These sections require the Design Approval Holders to develop certain information needed to support operators compliance with 14 CFR 121.1109 and 129.109. FAA also issued the AC 120-93 providing guidance for compliance to these new FAR part 26 sections.

At the same time, the EASA issued on the 26th December 2007 an Acceptable Means of Compliance (AMC) 20-20 that provides guidance in developing a continuing structural integrity program to ensure the safe operation of aging aircraft. It was confirmed during the Airbus Structures Task Group (STG) meeting 6 held in Sep-07 that the structures concerned by the repair assessment are only the metallic structures part of the FCS.

Purpose of Repair Evaluation Guidelines (REG) Document

The Repair Evaluation Guidelines (REG) document has been developed to comply with the requirements from FAR Part 26.43(e). Although not required by FAR Part 26.45(d), the REG has been found to provide the required level of safety and can be used to assess existing repairs made to alteration. The REG has been based on the guidelines provided in the relevant chapters and appendixes from the FAA Advisory Circular (AC) 120-93.

The REG is also in line with the relevant paragraphs of the EASA Acceptable Means of Compliance (AMC) 20-20 (effective 26/12/2007).

The Repair Evaluation Guidelines (REG) document is aimed at providing guidance to the aircraft operators to assess existing repairs on the aircraft structure in order to comply with FAA title 14 Code of Federal Regulation (14 CFR) 121.1109 and 129.109 (US-registered aircraft).

A repair assessment is necessary unless the operator can demonstrate that he has a process in place to ensure that all repairs performed on the Fatigue Critical Structures (FCS) are damage tolerant compliant.

As an option to a physical assessment, an assessment of airplane records to identify reinforcing repairs affecting FCS may be used. In order to use the records assessment, the operator will include in the Operator’s Implementation Plan (OIP) a process to demonstrate to its regulatory authority that its record-keeping system retains documentation (repair location, configuration, etc, and proximity to other repairs) of all reinforcing repairs affecting FCS made to the airplane over the in-service life of the airplane.

The demonstration process may include a physical survey of a percentage of the fleet or it may include a comparison of the results of an aging aircraft inspection and records review performed under 14 CFR §121.1105 to the aircraft record, or other acceptable means. Without such a satisfactory demonstration, a physical survey of all aircraft will be required.

When using an airplane records survey to identify reinforcing repairs to FCS, the operator needs a process to determine if an individual repair is a damage tolerant. If not, the operator will obtain the DTE (Damage Tolerance Evaluation), and any necessary DTI (Damage Tolerant Inspections), and incorporate the DTI into the maintenance program. If the record indicates that the repair is damage tolerant, the operator will verify that any required DTI has been incorporated into the maintenance program.

Any new repair performed (with or without reinforcement) affecting FCS must be considered for DTE and DTI must be recorded when required.

For educational purposes only – The information above is research-based and should not replace the original REG and other relevant documents.

References –

  • AC 120-93: Damage Tolerance Inspections for Repairs and Alterations 
  • AMC 20-20: Continuing Structural Integrity Programme 
  • FAR Part 26: Continued Airworthiness and Safety Improvements for Transport Category Airplanes
  • FAR Part 26 Subpart E: Aging airplane Safety – Damage Tolerance Data for Repairs and Alterations 
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