A maintenance log is testimony of an unusual kind, taken before anyone knows there is going to be a question. The person filling it in is not defending anything and is not trying to make a point. They are writing down what they did today so the next shift knows, and then they go home. Years later someone comes along with a question nobody thought to ask at the time, and the entry answers it, written by somebody with no reason to shade anything.
What a good record does
A good record closes a question. Once the entry exists, and there is reason to trust it, that question is answered and the next question can stand on it. What no record reaches stays open. It can be guessed at and argued over, but until somebody who was there wrote it down at the time, guessing is all it is, and a careful person marks the question open and moves on.
That sounds simple on the page. In practice it meant training myself against my own memory. I have a good memory and it has been wrong more times than I can count. I have stood in front of a machine sure I had serviced it in March, gone to the log, and found my own writing telling me it was February and the part number was not the one I remembered. The log was right. It usually is. I learned to write things down at the time and to believe the writing later, even when it disagreed with what I remembered.
A worked example
Years ago, at a maritime outfit, there was a gearbox that kept losing a seal. Every few months the oil would show up under it and the gearbox would come off the line for a reseal. Everyone on the floor knew why. It ran hot in the summer, they said, and the heat cooked the seals. It was the kind of thing everyone agreed on so completely that nobody had ever checked it.
The entries had the answer already. When I read them straight through, the failures were not clustered in the summer at all. They were spread across the calendar almost evenly. What they did cluster around was a change nobody remembered making. A couple of years back the shop had started fitting a cheaper replacement seal from a different supplier, and every failure since had come within a few months of one of those seals going in. The original seal, the one they had fitted for a decade before that, had almost never let go. Nobody had cooked anything. Somebody had bought the wrong part, and everyone's memory had supplied the summer to explain it.
A plot is a record
When I started writing mysteries I did not have to learn a new skill. I had to take an old one and turn it around. A fair-play plot is a record the reader is allowed to read. Everything the detective will eventually use is entered at the time it happens, in plain view, looking like an ordinary day at the shop. The reader walks past it because it looks like the rest of the day. But it is on the page, written down when it happened, and when the solution comes, it comes out of the log the reader has been reading all along. A reader can go back, read the entries again, and find the answer had been filed the whole time.
I spent a career teaching people to trust the written entry over the thing they were sure they remembered. Now I write books that ask a reader to do the same, and then reward them for it. It turns out to be the same job. I just get to invent the gearbox now.