The short answer
Hobbs time is elapsed clock time recorded by a Hobbs meter, which typically runs whenever the engine is running (often triggered by oil pressure or the master switch). It counts real hours and tenths — taxi, run-up, and flight all accumulate at the same rate.
How the meter works
A Hobbs meter is a simple hour counter. Depending on installation it starts with oil pressure, engine start, or squat-switch/airspeed logic, and it always advances at real-time rate: one hour of meter time per hour of clock time, regardless of engine RPM.
Because it captures everything from startup to shutdown, Hobbs time runs faster than tach time in normal operation — commonly 10–20% higher. Flight schools and rental operators usually bill by Hobbs because it reflects total occupied time.
Hobbs vs. tach for maintenance
Maintenance intervals are commonly tracked against tach time (which is proportional to engine RPM), while pilot logbooks and rental billing usually use Hobbs. Knowing which meter your maintenance records reference — and recording both at each service — avoids interval drift between the two clocks.
Related terms and reading
Keep every hour and inspection straight
Maggneto tracks engine times, inspections, and ADs from your actual logbooks — so terms like these become numbers you can act on. Browse the full maintenance glossary.
Frequently asked questions
What is the difference between Hobbs time and tach time?
Hobbs time counts real clock hours whenever the engine runs, at the same rate regardless of power setting. Tach time counts revolutions and is calibrated so it accrues at 1:1 only near cruise RPM — at idle and taxi it accrues slower. Hobbs typically reads 10–20% more than tach over the same flying.
Which time should maintenance be tracked by?
By whichever reference the aircraft's records and inspection program use — most piston maintenance intervals are tracked on tach time, but the key is consistency. Recording both meters at every maintenance event keeps the history unambiguous.