Definition
Three clocks run on every training flight and they record different things.
The **Hobbs meter** is a simple elapsed-time counter, usually wired to oil pressure or the master switch, so it runs whenever the engine is running. It therefore records something close to the regulatory definition of flight time in 14 CFR § 1.1 — pilot time that "commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing". Taxi, run-up and holding all count. Because it is the most generous of the three and the easiest for a renter to verify, it is the usual billing clock.
The **tachometer hour meter** counts engine revolutions and converts them to hours against a calibrated reference RPM, typically somewhere near cruise. At cruise power it runs at roughly real time; at taxi, descent or idle it runs slower. A one-hour flight with a long taxi and a lazy descent can put appreciably less on the tach than on the Hobbs. That is not a fault — it is the meter doing its job, which is to measure engine wear rather than wall-clock time.
**Time in service** is neither of them. 14 CFR § 1.1 defines it, for maintenance records, as "the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing" — wheels-off to wheels-on, excluding all ground time. And 14 CFR § 91.409(b) requires the 100-hour inspection "within the preceding 100 hours of time in service". So the legally binding maintenance clock is the shortest of the three, and it is the one no common meter measures directly.
Most operators resolve this by tracking tach as the practical proxy for time in service and applying a conversion factor, or by recording block and air times from the flight record. What matters is that the school knows which clock each number came from.
Why It Matters for Flight Schools
For a flight school the gap between these clocks is not academic — it is margin, and it is compliance. Bill on Hobbs and maintain on tach and the two drift apart at a rate set by how much ground time the operation has, which is precisely what differs between a busy towered circuit and a quiet uncontrolled field. A school that treats tach hours as interchangeable with time in service is over-running its inspection interval in the direction that matters, because tach usually reads higher than true air time.
The reverse error is a revenue one. A school quoting a wet rate calculated from tach-hour costs but billing on Hobbs hours is charging on a bigger number than its cost model assumed — which flatters the margin until someone reconciles the two. Neither error announces itself; both show up as a slow divergence between the maintenance forecast, the invoice total and the engine reserve.
How Aviatize Handles This
Aviatize records the meter readings a flight actually produced rather than a single generic "hours" figure, so the number billed and the number driving the maintenance forecast are traceable to the meter each came from. Because bookings, invoices and the maintenance record are the same system, a school can see when its billing clock and its inspection clock have drifted apart instead of discovering it at the next 100-hour.
Frequently Asked Questions
- Why is tach time lower than Hobbs time?
- The tachometer counts engine revolutions against a calibrated reference RPM near cruise, so it only runs at real time when the engine is at that power setting. Taxi, run-up, descent and idle all turn the engine more slowly, so the tach advances more slowly than the wall clock. A Hobbs meter is a plain elapsed-time counter that runs whenever the engine does, so it always reads at least as high.
- Which clock does the 100-hour inspection run on?
- Neither Hobbs nor tach. 14 CFR § 91.409(b) requires the inspection within the preceding 100 hours of time in service, and 14 CFR § 1.1 defines time in service for maintenance records as the time from the moment the aircraft leaves the surface of the earth until it touches it at the next point of landing. That is wheels-off to wheels-on, excluding all ground time, and no common meter measures it directly.
- Which clock should a flight school bill on?
- Most bill on Hobbs, because it is unambiguous, easy for a renter to verify, and closest to the regulatory definition of flight time. The important thing is consistency between the clock used to build the rate and the clock used to invoice: a rate costed from tach hours but billed on Hobbs hours overstates margin, because Hobbs reads higher by however much ground time the operation carries.