Deviation Log: Pass It On

If you’ve ever owned an old car or an old house, you’ve come across things you can’t make sense of. A switch that does nothing. A wire hanging free with no terminus in sight. Two fuel filters in series. A door that opens to reveal a wall. The reason for “the thing” has been lost to time. It’s the work of a previous owner.

Homebuilt airplanes aren’t immune to this. In fact, every homebuilt is born with anomalies. It’s the nature of homebuilding. Hopefully not a dangling wire or a functionless switch, but things that don’t conform to what others may expect to find in a particular design. Or any design.

Obvious changes to a design typically announce themselves. Large tires. An added cowl bump or cutout to accommodate an oversized oil cooler or alternate engine. A modified instrument panel. Less obvious changes can leave future owners and A&Ps scratching their heads, asking themselves, “Why?” In this category I place hardware substitutions (or additions), added or altered structure, control linkage changes, and added or altered engine components. Things that have no obvious reasons for being. Things that may be a Band-Aid for a mistake elsewhere, or poor workmanship.

Builders, or a non-builder who modifies a homebuilt, can do future owners a favor by recording their deviations. Making notes in the construction documentation may seem like the obvious approach, but I have seen (too many) homebuilt airplanes change hands without the original construction documentation. I have also seen documentation lost to floods, fires, and storm damage.

Engine and airframe logs may, at first blush, seem like great places to record changes and deviations, but those logs often don’t come into existence until the aircraft is done. Even then they might not be used to record some changes, like why a fuel pump and pressure regulator were added to a gravity-fed fuel system. Was that something that was done from the first, or was it in response to a fuel delivery issue found after the airplane was put in service? The answer may be locked away in the head of the aging builder or lost forever to their passing.

A dedicated deviation log, begun the moment you begin your project, can serve the aircraft and its owners for decades to come. An ideal log would live with the airframe and engine logs. A simple notebook will do. Or an airframe log with masking tape emblazoned with the word “Deviation” placed over “Airframe.”

Now that you’ve got a place to record deviations, you need a system. Tabs that segment a logbook or notebook into categories (structure, controls, fuel system, cooling, etc.) would make identifying deviations easy. Hopefully you don’t have so many deviations a tabbed notebook is needed. Chronological entries may not be ideal, especially if you are working an engine installation at the same time you’re building the tail. Maybe a compromise would be if each entry had a highlighted one- to three-word subject line, making scanning the log entries easy.

Finally, what is recorded is where the real value of the log comes in. A simple “Added four rivets to gusset 44B” communicates nothing. A reason is needed. You need to place the future owner/inspector in your head. You need to tell them why. In a deviation log, it may look like this:

Now we’ve got something to hang our hat, and our confidence, on. (Speaking of confidence, a simple Watch List can keep a lost washer from becoming a serious concern.)

The benefits of a deviation log go beyond helping future owners and mechanics understand what they are looking at. Projects can get drawn out for years, even decades. The log may even help you remember the “why” of something you did years earlier. And when it’s time to sell, the log may add value to your aircraft. Maybe not in actual dollars, but in buyer confidence. A well-documented build will be more attractive to potential buyers.