During a recent visit to assist a smaller international airline with their maintenance reliability program, the head of engineering — new to the role — asked me an interesting question: “How should I approach investigating these reliability alerts?”

Two engineers, the same aircraft, the same intermittent fault. One changes three components over a shift and the snag is still open when the aircraft leaves. The other reads for half an hour, spends an hour with a meter, replaces nothing, and finds a chafed loom behind a panel the fault isolation manual does not send anyone to.

The truck meter counts litres or gallons. The aircraft counts kilograms or pounds, because mass is what the wing lifts and what the engine burns. Between the two there is a multiplication, and the number you multiply by is density.