Hobbs vs. Tach Time: How to Bill Aircraft Rentals Correctly

Ask five flight schools how they bill rentals and you will get answers ranging from Hobbs to tach to block time, often with no clear reason for the choice. The two clocks measure genuinely different things, and picking the wrong one either quietly overcharges your students or quietly leaves money on the table. Here is what each clock actually measures and how to bill fairly.

What Hobbs time actually measures

A Hobbs meter typically runs whenever the engine is running, or in many installations whenever there is oil pressure. It is real-time, wall-clock time. Start the engine, the Hobbs starts. It counts taxi, run-up, the hold short line, the pattern, and shutdown, all at the same rate. Hobbs is the clock most schools bill on because it maps cleanly to how long the airplane was actually in the renter's hands, and because it is easy for a student to understand: the number went up by 1.4, you flew 1.4.

What tach time actually measures

A tachometer, by contrast, counts engine revolutions and is calibrated to record an hour at a specific cruise RPM. At low RPM, taxiing or idling, it runs slower than real time. At high cruise RPM it can run close to real time. The net effect is that tach time is almost always less than Hobbs time for the same flight, because a real flight includes a lot of low-RPM ground operation that the tach under-counts. Tach exists because that is how engine and airframe manufacturers want time-in-service tracked for maintenance.

Why the difference matters for billing

Because tach reads lower than Hobbs, billing on tach charges the student less for the same flight. That is not automatically wrong, but it has to be a deliberate pricing decision. If you set your rate assuming Hobbs and then bill on tach, you have quietly given every renter a discount and your margin is off. If you set your rate assuming tach and bill on Hobbs, you are overcharging. The clock and the rate are a pair, and you cannot change one without recomputing the other. The most common billing mistake I see is a school that switched clocks at some point and never re-priced.

The clock you bill is not always the clock you track maintenance on

Here is the subtlety that trips people up. Maintenance intervals and hour-based airworthiness directives are supposed to run on time in service, which for most operations is tracked on the tach or on a dedicated recording tach. So a school might legitimately bill rentals on Hobbs for simplicity while tracking the 100-hour and hour-based ADs on tach. That is fine, but only if the two numbers are recorded separately and never confused. If you record only one number and use it for both, your maintenance next-due math will be wrong. We go deep on that failure mode in AD and SB compliance tracking.

Record both, every flight

The clean practice is to capture both the Hobbs and the tach reading at the end of every flight. Bill from the one your rate is priced on, and feed the maintenance clock from the correct one. When your invoicing and your maintenance records draw from the same flight entry, the student's bill and the aircraft's next-due are both computed from the right clock automatically, and nobody has to remember which number goes where. This is a small discipline that removes an entire class of billing disputes and compliance errors.

Being transparent with students

Students notice when the bill does not match the tenths they wrote in their logbook, and an unexplained gap between what they logged and what they paid erodes trust fast. Whatever clock you bill on, state it plainly on the rate sheet and on the invoice, and show the start and end reading so the student can check the math. A rental that says "Hobbs 4521.3 to 4522.7, 1.4 at 165 wet" is self-explanatory. A rental that just says "1.4 hours" invites the question you do not want.

The short version

Hobbs is real time and reads higher. Tach is calibrated engine time and reads lower. Bill on whichever you like, but price the rate to match, record both readings every flight, run maintenance off the correct clock, and show the numbers on the invoice. Do that and rental billing stops being a source of friction. For where this fits in the larger operation, see what a modern flight school tech stack looks like.