Hobbs
Hobbs Time / Hobbs Meter
Real elapsed time recorded whenever the master switch (or oil pressure) is on.
A Hobbs meter is an elapsed-time clock that runs whenever the aircraft's master switch is on (or oil pressure is up, depending on the install). One Hobbs hour is exactly 60 minutes of real time.
Hobbs is the industry standard for billing — flight schools, flying clubs, and rentals charge by Hobbs because it captures actual time used, including run-up and taxi. It's also the conventional measure for tracking 100-hour inspection intervals.
For engine TBO tracking, tach time (not Hobbs) is the correct measure per Lycoming and Continental.
When it matters
Aircraft with extensive taxi time can show Hobbs hours noticeably higher than tach. Knowing which to use for what matters — billing on tach undercharges; tracking TBO on Hobbs over-tracks.
In Aloft360
Hobbs vs Tach explained