Understand what Hobbs and tach meters measure, why flight time differs from time in service, and how to keep consistent aircraft records.
Published by Aloft360, an aircraft management software provider. Editorial standards and corrections
A flight can produce several different time figures: an elapsed meter reading, tach time, pilot flight time and maintenance time in service. The numbers may be close, but they answer different questions.
Before choosing a field in a log or setting a rental rate, identify what the figure represents and how it was obtained. Keeping both meter readings is useful; treating either one as a universal answer is not.
A Hobbs-style meter records elapsed time while its triggering circuit is active. The installation determines when that happens. A meter may run with electrical power, oil pressure or another aircraft-specific trigger.
That distinction matters on the ground. Do not assume the meter in your aircraft starts with the master switch or measures only airborne time. Check the installation and explain the billing convention to anyone renting the aircraft.
A conventional recording tachometer accumulates time in relation to engine revolutions and a calibration speed. It may advance more slowly during low-power operation than during cruise. Electronic installations can use different logic, so the instrument’s documentation matters.
There is no reliable universal Hobbs-to-tach multiplier. A long taxi, repeated pattern work and a cruise flight can produce different relationships. Do not reconstruct required records by applying a generic percentage to a different meter.
| Purpose | What establishes the basis |
|---|---|
| Rental or member billing | The agreed rate and time convention |
| Pilot logbook | Applicable flight-time and logging requirements |
| Hours-based maintenance | The applicable requirement and aircraft records |
| Engine or component tracking | Manufacturer instructions and applicable requirements |
| Calendar inspection | The relevant calendar rule and completion record |
The FAA defines pilot flight time and maintenance time in service separately in 14 CFR 1.1. Neither definition says that every installed meter reading is interchangeable with the required time.
For 100-hour inspections, 91.409 uses time in service and specifies when the requirement applies. Establish a defensible recording method with your maintenance provider. Do not select Hobbs or tach merely because a software field defaults to it.
Either meter-based convention needs a rate that was calculated on the same basis. Comparing a tach rate with a Hobbs rate without accounting for the difference can mislead a renter.
For a simple illustrative example, a 1.3-hour billed interval at $150 per hour produces a $195 aircraft charge. The invoice should show the interval and rate. Whether fuel or instruction is included is a separate question, covered in the wet and dry rate guide.
When a renter questions a bill, the opening and closing readings should be easy to find. Define how staff correct a transcription error and how that correction affects the invoice.
A replacement meter may start at zero while the airframe has thousands of hours. Keep the replacement record and preserve the relationship between the display and the accumulated aircraft time. Do not overwrite the aircraft’s history just to make its total match a new display.
The same care applies when bringing an aircraft into a new app. Check the opening readings against the source records and label what each field means before logging the first flight.
Capture the required readings at the appropriate points, enter them consistently and review unexpected jumps or negative differences. Keep billing calculations separate from the maintenance basis when they differ.
Aloft360’s Hobbs, tach and cycles help explains its record fields. Use the inspection guide when configuring a tracked interval. For a shared aircraft, write the chosen conventions into the club’s or partnership’s operating notes so the next person enters comparable data.