Tach time vs. Hobbs time (and how TachMinder uses each)

Every hours figure in TachMinder is labeled with its basis — tach or Hobbs — because they are two different measurements that answer two different questions. Mixing them up is one of the most common record-keeping mistakes in general aviation, and the app is built to make it impossible to do by accident.

Tach time

Tach time is engine-RPM-adjusted time. The tachometer's hour meter is geared so that one tach hour equals one hour of running at a reference cruise RPM. Run the engine slower than that — taxiing, pattern work at reduced power — and the tach accrues more slowly than the clock. That makes tach time a measure of how hard the engine has actually worked, which is why hour-based engine maintenance is typically counted against it.

Hobbs time

The Hobbs meter measures elapsed clock time while the engine runs. An hour on the Hobbs is an hour on your watch, whether you were at full throttle or idling on the ramp. Because of that, Hobbs time is typically higher than tach time for the same flying.

How TachMinder uses each

• Every figure is labeled. Anywhere the app shows hours, it tells you whether the number is tach or Hobbs. The two are never conflated or silently converted.
• Maintenance items declare their basis. Each hour-based maintenance item tracks which basis — tach or Hobbs — its interval counts against, so a 50-hour item counting tach hours isn't accidentally measured against the faster-moving Hobbs meter (or vice versa).
• The aircraft header shows both. Tach appears as the raw current tachometer reading, alongside the current Hobbs reading.

One thing tach time is not

The raw tach reading is the number on the meter — it is not "time since overhaul." A mid-life engine, a replaced tachometer, or an overhaul that didn't reset the meter all break that assumption.TachMinder presents tach as exactly what it is: the current tachometer reading. When you log flights, it's the change in that reading — the tach delta — that advances your hour-based maintenance clocks.

Practical takeaway

When you read a number in TachMinder, read its label first. When you enter a number, enter it against the right meter. If a shop invoice or logbook entry gives only one figure, note which meter it came from before recording it — future-you (and your IA at the next annual) will thank you.