When you need approved data, you need a qualified engineer.
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 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