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 the FAA and 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, a Japan Airlines Boeing 747 departed 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 Boeing 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.