Most piston airplanes have two time meters, and they rarely agree. After a one-hour lesson the Hobbs might show 1.2 and the tach 1.0. Which number you use changes what a flight costs and when maintenance comes due.
What each meter measures
Hobbs meter
A Hobbs is an electric clock. It runs in real time whenever its switch is on. Depending on how it's wired, that's when:
- the engine has oil pressure (the most common), or
- the master switch is on, or
- the airplane is moving (an airspeed or squat switch, less common in light aircraft).
One hour of running = one hour on the Hobbs. It counts taxi, run-up and waiting at the hold short line.
Tachometer (tach) time
The tachometer shows engine RPM, and most include an hour meter that's driven by engine speed. It records a full hour only at a reference RPM (often around normal cruise RPM). At lower RPM, during taxi, run-up and descent, it accumulates more slowly.
That's why tach time is almost always less than Hobbs time for the same flight. For training flights with lots of ground time and pattern work, the gap is commonly 10 to 20 percent. On a long cross-country at cruise power, they're close.
Which one is used for what
Billing rentals and instruction
Most flight schools and rental operators bill aircraft time on the Hobbs, because it reflects the whole time the airplane was in use. Some bill on tach and charge a higher hourly rate to compensate. Neither is wrong, but the rate and the meter go together: a $180/hr tach rate and a $160/hr Hobbs rate can cost about the same.
Instruction is normally billed on the instructor's time, including briefing, which is separate from either meter.
Partnerships and clubs
Partners usually agree on one meter in the partnership agreement. Tach is popular because it tracks engine wear more closely; Hobbs is simpler to explain. Either works if the hourly rate is set accordingly.
Maintenance intervals
14 CFR 1.1 defines time in service as the time from when an aircraft leaves the surface of the earth until it touches down. Many owners and operators use tach time to track inspection intervals, the 100-hour inspection, oil changes, recurring ADs and engine time since overhaul, because it's recorded automatically and stays close to (and usually above) actual flight time. Whatever you use, use it consistently and record it in the logbooks.
Logging pilot time
Pilots log flight time in their own logbook. Many use the Hobbs reading as a convenient approximation; the regulations define flight time for pilots as from when the aircraft first moves under its own power for flight until it comes to rest after landing.
Recording time accurately
Disputes over meter readings are one of the most common sources of friction at flight schools and in partnerships. A few habits prevent them:
- Record both meters at the start and end of every flight, not just one.
- Take a photo of the meters when you shut down; it settles questions later.
- Check the starting reading against the last recorded end reading before you fly. A gap means someone didn't log a flight.
- Use one place to record readings that everyone uses: a dispatch sheet, an app, a shared log.
Frequently asked questions
Why is tach time lower than Hobbs time?
A Hobbs meter runs in real time whenever it's on, while the tach hour meter records a full hour only at a reference engine RPM. During taxi, run-up and descent at lower RPM, the tach advances more slowly.
Do flight schools charge Hobbs or tach time?
Most flight schools bill aircraft rental on Hobbs time. Some bill on tach time at a higher hourly rate. Check the school's rate sheet to see which meter it uses.
Which time is used for maintenance inspections?
Many operators track inspection intervals, oil changes and engine time on the tach. The key is to use one meter consistently and record it in the maintenance records.
How much difference is there between Hobbs and tach?
It depends on the flight. For training flights with taxi and pattern work, tach time is often 10 to 20 percent lower than Hobbs. On long cruise flights the two are much closer.