Definition
Tach time — short for tachometer time — is measured by a recording tachometer that accumulates hours proportionally to engine RPM. The instrument is calibrated so that one tach hour equals one real-time hour only when the engine is running at a specific RPM, typically the normal cruise setting (around 2,400 RPM for many training aircraft). When the engine operates below that RPM, such as during taxi, run-up, or pattern work at reduced power, the tach advances more slowly than real time. Conversely, at full-power operations the tach may advance slightly faster. Because training aircraft spend a significant portion of each flight at reduced power — taxiing, performing ground checks, and flying in the traffic pattern — tach time for a given flight is almost always lower than the corresponding Hobbs time. The difference can be meaningful: a one-hour Hobbs flight might register only 0.8 to 0.9 tach hours depending on the power settings used. Tach time is the standard unit the engine manufacturer uses for maintenance intervals such as oil changes, top overhauls, and time-between-overhaul (TBO) limits. This makes sense from an engineering standpoint because engine wear is more closely correlated with RPM and power output than with simple elapsed time. A minute of idle produces far less wear than a minute at full power, and the tach captures that difference.
Why It Matters for Flight Schools
Flight schools that bill by tach time rather than Hobbs time are effectively passing a discount to students and renters, since tach hours are fewer than Hobbs hours for the same flight. Some schools use tach-based billing as a competitive differentiator, while others prefer Hobbs billing for its simplicity. In either case, the operations team must track both values because maintenance intervals are tach-based regardless of the billing method. Misunderstanding the difference between Hobbs and tach can cause confusion for new students comparing prices across schools. A school quoting a lower tach rate may actually cost the same per flight as a school quoting a higher Hobbs rate. Flight school managers need to communicate this clearly in their pricing materials.
How Aviatize Handles This
Aviatize supports both Hobbs-based and tach-based billing configurations at the fleet level, so each aircraft can be set up according to the school's preference. When a flight is logged, dispatchers or instructors enter both Hobbs and tach readings; the system automatically applies the correct rate to generate the invoice. On the maintenance side, Aviatize tracks cumulative tach hours per airframe and engine, triggering alerts when service intervals approach. This dual-tracking ensures that billing and maintenance stay aligned without requiring manual cross-referencing of logbooks and spreadsheets.