Aircraft Maintenance: Your aircraft has a secret past

My first aircraft was a 1975 Grumman AA-5 Traveler. It was the aircraft that kicked off my passion for aircraft maintenance, and my career as a mechanic, STC manufacturer, and educator. I absolutely loved working on it, even when that meant lying upside down with my arm bruised and buried in a fuel tank struggling to fix a leak. On that particular day, I was stumped by a structure inside the fuel tank that didn’t match the maintenance and parts manuals for the aircraft. There was a suspicious-looking angled doubler inside the tank secured by rivets (rivets are extremely rare on the Grumman bonded airframe). There was no record of it in the maintenance logbooks.

Rock dug through the company records and quickly pulled out a file for my particular aircraft’s serial number. He set me up at an old microfiche machine, and let me peruse the entire history of the construction and delivery of my Traveler. It was absolutely fascinating. Contained in the files were many of the expected items, such as order sheets, build configuration, airworthiness documents, and others. However, there was a particular set of documents that were especially enlightening: Non-Conforming Material Reports (NCMRs).

Despite the desire to build aircraft similar to the way we build cars, aircraft manufacturing volumes simply don’t come close to making this possible. Cirrus Aircraft, for example, produced an amazing 629 aircraft in 2022. However, in the same time frame, Ford Motor Co. delivered nearly 2 million Ford F-150s. In spite of Cirrus Aircraft’s great production rate, the automation, modularity, and standardization that are the norm in the automotive world simply do not work for aircraft. Aircraft are largely hand-built and, when mistakes happen on the assembly line, they are repaired. The company cannot afford to simply toss out a wing or a fuselage just because someone drilled a hole in the wrong place or dropped a hammer, or a defect was discovered.

Enter the NCMR. Anytime a discrepancy is found during the manufacturing process, it needs to be evaluated, and a fix must be designed, approved, implemented, inspected, and documented. It’s simply a fact of life in the world of aircraft manufacturing, and it’s more common than you might imagine. It’s also not limited to general aviation. Based on the official findings released from the investigation of the recent Boeing 737 Max door plug incident, the door plug was removed at the factory so that damaged rivets could be replaced on the forward frame of the left mid-exit door plug. Although Boeing may use different terminology, this was a version of the NCMR repair process. A manufacturing defect (non-conforming item) had to be repaired on the line.

In the case of my Grumman, I found the specific NCMR that explained the doubler in the fuel tank. Delamination was found during the manufacturing process for the wing. An engineer designed a fix and a doubler was installed inside the fuel tank area using rivets. I also found several other NCMR documents detailing repairs done on the assembly line to various other parts of my airplane. It was remarkably enlightening and, coupled with logbooks and maintenance manuals, completed my understanding of the aircraft.

So why, you might ask, does this matter? As aircraft owners and mechanics, we only have direct access to the maintenance logs for the aircraft. NCMR documentation and the entire history of our aircraft prior to the issuance of its airworthiness certificate are proprietary to the manufacturer. However, it’s important that we know that our aircraft were probably not perfect when they left the factory; instead, they were airworthy. They may not have conformed exactly to the manufacturer’s drawings and design because they were made by hand, by humans. In every case in which they did not conform, the situation was evaluated and addressed in a manner deemed to be airworthy.

These variations, however, remain with the aircraft throughout its life. And it is quite possible that we will encounter them during maintenance and repairs. In some cases, this might lead a mechanic to question the validity of an undocumented repair, or be forced to modify a replacement part that simply does not fit as it should. In these situations, consider reaching out to the factory for help. You may find that they know a little more about your particular aircraft than you do. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.