Before the introduction of the pressurized fuselage, a mechanic had the capability of designing and implementing a repair for just about any part of the airframe. Wooden ribs, steel tubes and the early monocoque aluminum skins — if they were broken, they could be repaired. As aircraft got faster and flew higher, cracking became a serious issue. Primary structure patches grew larger and uglier, and often required repetitive inspection and replacement. Patches on control surfaces required rebalancing to prevent possibly catastrophic results from flutter or vibration.
Recognizing the need for tighter control of the repair process and to keep the aircraft within the original certification parameters, the major/minor classification rule was adopted by regulatory agencies worldwide. Repairs or alterations that meet the definition of “major” would require approved data and regulatory agency involvement, essentially providing engineering oversight of the repair process to ensure the aircraft is returned to an airworthy condition.
For the maintenance technician, the major repair adds complexity, time and effort to the repair task. Once discovered, damage or cracking must be addressed immediately, and if it is found to require major repair, approved data are required. As maintenance technicians, we like to think that we can do anything, but approving data is out of reach. This privilege is withheld by theand can be delegated to Designated Engineering Representatives (DERs) or the recently created Organization Designation Authorization (ODA) Unit Members.
In many countries, the role of the maintenance technician is described as an engineer or maintenance engineer. So why not have maintenance technicians approve data? Well, in the eyes of the FAA, an engineer has more specific scientific knowledge. Even though the laws of physics and chemistry have not changed, the way they have been applied to aircraft design has. Aircraft skins are relatively thinner than they were years ago, and aerodynamic forces have increased. Addressing a crack with a simple patch may not be sufficient to prevent further cracking. A worst-case scenario is that the patch will induce unwanted stiffness that leads to further damage that becomes undetectable because it is hidden. This is where structural engineers who have scientific knowledge of loads, stresses and material strength are crucial to developing an airworthy repair scheme.
On Aug. 15, 1985, adeparted Tokyo for Osaka with 509 passengers and 15 crew onboard. About 12 min. after departure, upon reaching cruising altitude, the aft pressure bulkhead failed with an explosive decompression that severed the vertical stabilizer and all hydraulic lines to the tail. The aircraft began to oscillate out of control and crashed into a mountainous region outside of Tokyo. All but four people onboard perished. During the investigation, it was discovered that back in 1978, the aircraft was involved in a tail strike. The pressure bulkhead was repaired, but instead of using a single patch with a double row of rivets, the repair team elected to use two patches with a single row of rivets. Analysis by engineers on the investigation team calculated that this repair had a fatigue life of 10,000 cycles. On the day of the accident, the patches had completed more than 12,000.
While many technicians question the need for approved data, it is clearly stated in the regulations that this is necessary. To help the busy maintenance manager and senior technician navigate the world of major repair design and approval, we spoke with experts to explain how DERs (and ODA Unit Members) can help keep your aircraft safe and airworthy.
Major Until Proven Minor
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.
The Role of the DER
Engineers who can create approved data are known as Designated Engineering Representatives or the recently created designation of Organization Designation Authorization (ODA) Unit Member. There are many different specialties such as electrical, propulsion and flight test, but those who specialize in structural design and repairs are trained and skilled in creating structurally sound repair schemes. Where the OEM provides you with an approved repair scheme and analysis, they need to provide you with an FAA Form 8110-3 (or 8100-1 from ODA), signed by a properly qualified engineer.
While dealing directly with the OEM simplifies the process, it is not always an option for older or out-of-production aircraft. When you hire a consultant DER, however, it is up to you to ensure that the person is properly qualified. To verify a DER's status, you can visit the FAA's website at: www.faa.gov/other_visit/aviation_industry/designees_delegations/designee...
The repair approval process begins with the damage assessment. While the damage may at first appear obvious, the mechanism of the damage may cause hidden distress. A trained and experienced engineer can assist with determining if a deeper inspection is required. “When you find damage, you want to know if the damage is isolated to the immediate area, or if the load has traveled farther away from the impact site,” said Steve Crawford, of Crawford Aviation Services Inc. Crawford, CEO of the Bella Vista, Ark.-based company, has over 36 years of experience supporting structural repairs and modifications. “As an example, is it really just the tip of the horizontal stabilizer that incurred damage from the hangar door while under tug power, or did the load travel all the way into the horizontal stabilizer front spar, where it intersects with the pitch trim actuator screw jack? An experienced engineer can recommend further inspection or disassembly, which will become part of the repair drawing,” he added.
In addition to assessment, the repair design needs to take into consideration possible changes to Chapter 5 Inspection Intervals of the AMM, Instructions for Continued Airworthiness. “When the aircraft was certified, crack growth and propagation studies were performed by the OEM, and a repair patch may change the load characteristics, which could add or revise the existing inspection program,” Crawford added. Design considerations for repairs need to address possible effects on aerodynamic performance and flutter, even weight and balance. Most maintenance technicians may not have the expertise to make that determination. “A good DER will be able to determine the certification basis of the aircraft and design both a thorough damage assessment inspection, repair and address any possible Chapter 5 Inspection Interval impact,” Crawford added. During the assessment, be sure to take note of anything that looks unusual. Document it and communicate it, because it may be important to detect hidden damage. This is especially important for damage to composite structures because they may not deform the way a metal structure does.
Even though aircraft structures are well understood, increasing concerns over continued airworthiness have driven the minor repair almost into retirement. Any type of damage needs to be thoroughly investigated and classified. If your OEM or service center has a mechanism to provide you with an approved repair scheme, by all means use it. Hire a consultant DER that is properly qualified and experienced on your aircraft. An improperly designed and installed repair can have both safety and long-term value repercussions. Although it may take longer to execute, take the time to have a thorough assessment performed and an approved repair scheme developed by a qualified engineer. BCA