When you discover structural damage, you need to evaluate it and collect as much data as possible to start developing a repair scheme. You want to take measurements, photographs and sketches and reference the position in accordance with either the maintenance manual or structural repair manual. Your next step is to determine if the damage is considered major or minor. For minor repairs, you can develop your own repair scheme with acceptable data such as the structural repair manual or Advisory Circular AC 43.13-1B. But if the repair falls into the major category, you will need an approved repair scheme. You can use existing data and have the local FSDO approve it for you, but many inspectors will require you seek DER approval first.

While all repairs require attention to detail and skilled workmanship, in the eyes of the FAA they are divided into two categories: major and minor. According to FAR Part 1, Definitions, a major repair is one that:

(1) If improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics or other qualities affecting airworthiness; or (2) Is not done according to accepted practices or cannot be done by elementary operations.

While this is a very broad description, FAR Part 43, Appendix A gives more detail about the specific parts of the aircraft, and the specific types of repairs that are considered major: strengthening, reinforcing, splicing and manufacturing of primary structural members or their replacement, when replacement is by fabrication such as riveting or welding. To the technician, the key difference between major and minor is that the major repair will require an approved repair scheme and a 337 Form.

Many OEMs provide structural repair manuals that have some approved structural repair data within, but you need to verify that the data are applicable to your specific aircraft and damage. You can reference a similar approved repair, but you need to convince the FAA that it is applicable. The safest bet is to have an engineering evaluation completed and an approved repair scheme along with it.

Decades ago, it was common for technicians to treat most repairs as minor until proven otherwise. As we have grown wiser about the dangers of improper repairs, as well as become an even more litigious society, that concept has taken a complete one-eighty. With the recent high-profile cases of fuselage structural failures on Part 121 aircraft, the FAA is putting the process under even tighter scrutiny. “The trend is going to be that you are going to have to put time limitations on repairs, or provide failure analysis for every single repair on your airplane,” said Bob Beaumont, Designated Airworthiness Representative (DAR) and president of Southport, N.C.-based Air Conformity LLC, a global provider of aviation technical services. He is also a contributor to a number of NBAA maintenance committee activities. “For many operators, old repairs are being replaced and analysis provided. The trend is to return the airplane back to its original condition, and this is costly,” he added. This is especially important for the residual value of the aircraft: Poorly documented repairs are red flags to potential buyers.

The FAA also has become more cautious when it comes to dealing with approval of 337 Forms, often requiring a complete structural analysis for any hull penetration or major repair project. “The FAA treats every field approval like a mini STC, and it has become accustomed to seeing a complete data package,” said Woody Cottner, vice president of engineering at Wichita-based Global Aviation Technologies, an engineering, consulting and manufacturing firm with over 60 years in combined aviation experience. “We made a decision to treat all of our projects this way. It helps the approval process and answers any questions that may arise from the field inspectors,” he added.

One key concept to be developed over the last few decades is damage tolerance. This is the ability of a crack to propagate slowly enough to be detected, and even missed on two regular inspections before it reaches its critical length. “Any pressure vessel penetration we do receives a damage tolerance analysis,” Cottner said. “This will show if there are any crack propagation issues, and that the fasteners, etc., are the appropriate size, and that the repair does not generate stress hot spots during the cycles of compression and contraction,” he added.