Hobbs time vs tachometer time, explained. How each is measured, when to use Hobbs (billing, 100-hour) and tach (engine TBO), and how to set your records up.
Pilots use Hobbs and tach interchangeably in casual conversation. Mechanics, owners, and people writing checks know they aren't the same thing. The difference shows up in billing accuracy, when you'll need an engine overhaul, and what counts toward a 100-hour inspection.
A Hobbs meter is an elapsed time clock. It runs whenever the master switch is on (or whenever oil pressure is up, depending on the install). On most GA aircraft you flip the master, the Hobbs starts ticking, and it doesn't stop until the master goes off.
That means Hobbs includes engine run-up, taxi out, all airborne time, and engine-running taxi back to the ramp. One Hobbs hour is exactly 60 minutes of real time.
A tach time meter accumulates time as a function of engine RPM. There's a calibration RPM — typically 2,300–2,500 for a Lycoming or Continental — where the tach advances at 1.0 per real-world hour. At lower RPM (taxi, run-up at 1,000–1,200) the tach advances more slowly. At higher cruise RPM, it advances slightly faster.
A practical way to think about it:
Tach is not a clock. It's a proxy for engine workload. That's exactly what makes it the right measure for engine wear and the wrong measure for billing.
Billing students and renters. Hobbs is the industry standard for flight schools, flying clubs, and aircraft rental. It captures actual time used, including run-up and taxi, which fairly compensates the operator for fuel, wear, and aircraft availability during all of those minutes. Students expect to be billed on Hobbs and most management software defaults to it.
Tach billing produces slightly lower numbers than Hobbs because taxi and run-up time accumulates at a slower rate. A school billing on tach can post a more competitive-looking hourly rate, but the operator is undercharging for actual aircraft use. Some clubs do this anyway because the membership prefers it. That's a policy decision, not an accuracy decision.
100-hour inspections. FAR 91.409(b) says 100 hours of "time in service," and 14 CFR 1.1 defines that as airborne time only — wheels off to wheels on. In strict reading, Hobbs is too generous and tach is too stingy. In practice, almost every operator uses Hobbs against the 100-hour clock, and the FAA has not pushed back on that convention. If you want to be a stickler, use airborne time logged from the flight log.
Pilot logbook entries. Under 14 CFR 61.51, pilots log time toward currency and certificates. Most use the Hobbs delta because it's the easiest cockpit-time figure to capture. Tach is also acceptable as long as you're consistent.
Engine TBO. Lycoming and Continental publish TBO in tach hours. A Lycoming IO-360 with a 2,000-hour TBO means 2,000 tach hours, not Hobbs.
The gap matters more than people think. An aircraft that does a lot of short cross-countries with extended taxi will accumulate Hobbs faster than tach. By 1,950 Hobbs, the same airframe might show only 1,840 tach. That's a 110-hour difference, and at that point in an engine's life it's the difference between "approaching overhaul" and "comfortably out from it."
If you're tracking engine reserve dollar amounts (and you should be), the math should run on tach hours, not Hobbs.
Engine warranty calculations and AD compliance that reference manufacturer hours: also tach, by manufacturer convention.
The clean answer for any operation managing aircraft records: capture both Hobbs and tach in/out on every flight. Then:
| Use case | Use |
|---|---|
| Student or rental billing | Hobbs |
| Member self-service billing | Hobbs |
| 100-hour interval tracking | Hobbs (by convention) |
| Engine TBO tracking | Tach |
| Engine reserve $/hr math | Tach |
| Pilot logbook | Either, be consistent |
| Annual inspection due date | Calendar (not hours) |
Aloft360 records Hobbs and tach in/out on every flight log and lets you set inspection intervals against either. The 100-hour runs on Hobbs by default; engine TBO on tach. If you're using a calendar app and a spreadsheet today, the inspection due-date calculator is a free way to see what's coming up.
Older airframes sometimes have a tach but no Hobbs. You have two options.
Estimate. A factor of 1.05–1.10× tach hours approximates Hobbs for a normally-aspirated piston operated in typical cruise. That's accurate enough for billing in a small partnership where everyone agrees on the math.
Install one. A Hobbs meter is a small addition that pays for itself in billing accuracy if the aircraft is rented. If you're buying an aircraft to put on a leaseback or into a club, factor a Hobbs install into the deal.
Hobbs is a clock. Tach is a workload meter. Bill on Hobbs because it's fair and what people expect. Track engine life on tach because that's what the engine manufacturer measures. Capture both on every flight and you don't have to pick between the two.
For more on managing aircraft records cleanly, see aircraft maintenance tracking software and the wet rate vs dry rate breakdown.