A clear explanation of Hobbs time vs. tachometer time in general aviation — how each is measured, which is more accurate for billing and engine tracking, and how to decide which to use.
Hobbs time and tach time are often used interchangeably in general aviation, but they measure different things. The distinction matters for billing, maintenance tracking, and engine TBO calculations.
A Hobbs meter is an elapsed time clock. It runs whenever the master switch (or oil pressure, depending on the installation) is on. On most GA aircraft, the Hobbs starts when the master switch is activated and runs continuously until it's turned off.
This means Hobbs time includes:
Hobbs time is real clock time — 1.0 Hobbs equals exactly 60 minutes of elapsed time.
A tachometer meter records time as a function of engine RPM. At a specific "calibration RPM" — typically 2300–2500 RPM for a typical Lycoming or Continental — the tach meter advances at 1.0 per real-world hour. At lower RPMs (taxi, run-up), it advances slower than real time. At higher RPMs (cruise), it may advance faster.
This means:
Tach time is not real elapsed time. It's a proxy for engine workload.
If you're billing members or students by the hour, Hobbs time is generally fairer to both parties:
Tach time tends to produce slightly lower billing numbers than Hobbs, since taxi and run-up time accumulates at a slower rate. Some schools use tach time precisely for this reason — it makes their hourly rate look more competitive.
For billing purposes, Hobbs is the industry standard for flight schools, flying clubs, and rentals. Most students and members expect to be billed on Hobbs.
This is where it gets nuanced.
FAA airworthiness directives and manufacturer limits typically specify time in hours SMOH (since major overhaul) or hours since new. The question is: what kind of "hours" does the manufacturer mean?
Most engine manufacturers — Lycoming and Continental included — specify tachometer hours for their TBO (time between overhaul) recommendations. A Lycoming IO-360 with a recommended TBO of 2,000 hours means 2,000 tach hours, not Hobbs hours.
The practical difference: on an aircraft that does a lot of short flights with extended taxi time, Hobbs hours will accumulate faster than tach hours. An aircraft might show 1,950 Hobbs but only 1,840 tach — a 110-hour difference that matters when you're approaching TBO.
For 100-hour inspections, FAR 91.409 refers to "100 hours of time in service." Under 14 CFR 1.1, "time in service" means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing — essentially airborne time only. In practice, most operators and mechanics use Hobbs time for 100-hour tracking, and this is broadly accepted.
Under 14 CFR 61.51, pilots must log training time and aeronautical experience used to meet recency and certificate requirements. Most pilots log the Hobbs difference because it's the most practical measure of total time spent in the cockpit.
Some pilots log tach time because it aligns with what's in their aircraft's maintenance records. Neither is wrong, but you should be consistent and know which you're using.
Here's a practical decision framework:
| Use Case | Recommendation |
|---|---|
| Student billing | Hobbs |
| Member/rental billing | Hobbs |
| 100-hour interval tracking | Hobbs (by convention) |
| Engine TBO tracking | Tach (per manufacturer) |
| Pilot logbook | Either, but be consistent |
| Annual inspection due date | Calendar (not hours) |
If you're managing a fleet or even a single aircraft, you'll want to track both Hobbs and tach time per flight. This gives you:
In Aloft360, both Hobbs in/out and tach in/out are recorded on every flight log entry. Aircraft maintenance intervals can be set based on either, with automatic due-date calculations. The 100-hour tracks against Hobbs by default; engine TBO tracking can use tach.
Some older aircraft have only a tach meter and no Hobbs. In these cases, you can estimate Hobbs from tach time using a conversion factor based on typical cruise RPM. Many operators use a factor of 1.05–1.10x (tach hours × 1.05 ≈ Hobbs hours) for a normally-aspirated aircraft operated in typical cruise.
If you're buying an aircraft and it has only a tach meter, installing a Hobbs meter is a minor addition that simplifies billing and record-keeping considerably.
Hobbs time is the standard for billing. Tach time is the appropriate measure for tracking engine life against manufacturer TBO recommendations. Logging both on every flight gives you accurate records for each purpose. For more on managing aircraft records, see our guide on aircraft maintenance tracking software.