Tach
Tachometer Time
Time measured by engine RPM — advances at 1.0/hr only at the engine's calibration RPM.
Tachometer time is recorded as a function of engine RPM. At a calibration RPM — typically around 2,300–2,500 for a Lycoming or Continental — the tach time meter advances at 1.0 hour per real hour. At lower RPM (taxi, run-up), it advances more slowly. At higher cruise RPM, slightly faster.
Tach is not a clock. It's a proxy for engine workload — which makes it the correct measure for engine wear, manufacturer TBO recommendations, and AD recurrences specified in engine hours.
For billing, Hobbs is the standard; tach undercharges for the taxi and run-up portion of every flight.
When it matters
Track both Hobbs and tach on every flight if you can. Billing runs on Hobbs, engine economics run on tach.
In Aloft360
Hobbs vs Tach explained