Hobbs vs Tach Time: How Flight Schools Bill and Log Aircraft Hours

A student flies a one-hour lesson, the instructor reads 1.3 off the Hobbs meter, and the invoice charges 1.3 hours. The aircraft's maintenance log, meanwhile, records 1.0 hours toward its next inspection. Both numbers are correct. They measure different things, and a school that treats them as interchangeable will either overbill students or run an engine past its inspection interval.

What the Hobbs Meter Measures

The Hobbs meter records elapsed time, usually triggered by an oil pressure switch or a squat switch when the aircraft is running or airborne. In most trainers it starts the moment the engine produces oil pressure at idle and stops at shutdown. That means taxi, run-up, holding short, and any time spent idling on the ramp all accumulate Hobbs time.

Because Hobbs time captures the full block of engine-running time, it tracks closely with what the FAA defines as flight time: the time from when an aircraft moves under its own power for the purpose of flight until it comes to rest after landing. The exact definitions of flight time and time in service live in 14 CFR 1.1, and the gap between them is the whole reason two meters exist.

What the Tachometer Measures

The tachometer time recorder counts engine revolutions and converts them to hours, calibrated to a cruise RPM. At full cruise power the tach runs close to real time. At idle and low power settings it runs slow, sometimes accumulating only half an hour of tach time for a full hour on the Hobbs.

That under-counting is deliberate and useful. Tach time approximates how hard the engine has worked, which is why maintenance intervals are usually tracked against it. The oil change, the 100-hour, and the time between overhauls are all engine-wear events, and a meter that discounts idle time reflects wear better than one that counts every minute the prop is turning.

Billing: Why Schools Usually Charge Hobbs

Charging on Hobbs time is the default for wet rentals and dual instruction. The student pays for the time the engine was running on their behalf, which they can verify by reading the meter before and after the flight. It is transparent, and transparency reduces billing disputes.

A minority of schools bill on tach time, or on tach with a multiplier, to avoid charging students for long taxi delays at busy fields. Both models are defensible. What is not defensible is switching between them without telling anyone, or quoting a rate "per hour" without specifying which hour. Publish the basis in writing and apply it consistently across every aircraft and instructor. Inconsistent billing is one of the quiet revenue leaks covered in our guide to improving flight school operations.

The number that lands on the invoice should come straight from the booking record, not from a slip of paper that gets transcribed three times before it reaches accounting.

Maintenance: Why Inspections Track Tach

Aircraft used for instruction given for hire require a 100-hour inspection in addition to the annual. That 100 hours is measured in time in service, defined as the time from when the aircraft leaves the ground until it touches down at the next landing. Many schools approximate time in service with tach time because the tach already discounts ground operation, though the two are not identical and a maintenance program should state which one it uses.

The practical consequence: a dispatcher who tracks only Hobbs time reaches the 100-hour limit late, because Hobbs accumulates faster than tach or time in service. Overshoot the limit by even a few tenths and the aircraft has flown for hire while out of inspection, which grounds it and creates a compliance problem. Conservative schools set an internal warning several hours before the legal limit and block the aircraft in the booking system before it can be dispatched again.

Where the Two Meters Drift Apart

The ratio between Hobbs and tach is not fixed. It depends on how the airplane is flown. A primary student grinding out touch-and-goes at a towered field generates a wide spread, because pattern work means repeated power reductions and time holding short. An instrument student flying approaches with a long taxi back each time produces the same effect. A commercial student on a 300-mile cross-country at cruise power sees the meters track almost one to one.

This matters for forecasting. If you bill Hobbs and inspect on tach, revenue per maintenance hour runs higher for aircraft doing a lot of pattern work and lower for cross-country machines. Schools flying mostly primary students reach inspections later in calendar terms than their Hobbs revenue would suggest, which is worth knowing when scheduling downtime.

Building Reconciliation Into Dispatch

The failure mode is almost always the same: numbers captured on paper, entered late, and never reconciled. The fix is to log both the Hobbs and the tach reading at the moment a lesson closes, against the specific booking, aircraft, and student. Done that way, billing pulls from Hobbs, maintenance pulls from tach, and neither has to be reconstructed from memory at the end of the month.

This is the same data discipline that makes currency tracking reliable. A booking record that already captures Hobbs time, tach time, and landings can answer billing, maintenance, and regulatory questions from one source instead of three. A modern flight school scheduling platform like HangarOS should make both readings part of the flight-close step rather than an afterthought.

Treat the two meters as what they are: one measures what the student owes, the other measures what the engine has endured. Keep them separate on purpose, capture both every flight, and the questions that usually surface at the worst time stop surfacing at all.