Hobbs vs Tach Time: Difference for Pilots
Learn the difference between Hobbs and tach time, how each is used for rental billing, maintenance tracking, and pilot logbook habits.
Hobbs time and tach time both measure aircraft use, but they do not measure it the same way. That difference matters for billing, maintenance tracking, aircraft records, and your own logbook habits.
New pilots often hear both numbers during checkout and assume they are interchangeable. They are not.
What Is Hobbs Time?
Hobbs time comes from a Hobbs meter. In many training airplanes, the Hobbs meter records clock time when a specific aircraft condition is met. Depending on the installation, it may start with oil pressure, the master switch, an airspeed switch, or another trigger.
Many rental operators use Hobbs time for billing because it is easy to read and tends to track the time the airplane is being used by the renter. If the engine is running while you wait for a clearance, the Hobbs meter is often running too.
Hobbs time is usually displayed in hours and tenths. For example, 1.5 means one hour and thirty minutes.
What Is Tach Time?
Tach time comes from the tachometer and is tied to engine RPM. At a reference cruise RPM, tach time may run close to clock time. At lower RPM, it records more slowly. At higher RPM, it can record faster.
That means a lesson with lots of taxi, idle, and pattern work may show less tach time than Hobbs time. A flight with sustained higher power may narrow the gap.
Tach time is commonly used for maintenance tracking because it better reflects engine use than simple clock time. Inspections, oil changes, and other maintenance intervals may be tied to tach time, calendar time, or both.
Why the Two Numbers Differ
Picture a one-hour training block. You start the engine, taxi, wait at the runup area, fly several laps in the pattern, taxi back, and shut down. The Hobbs meter may capture most of that elapsed engine-running time.
The tachometer, however, records based on RPM. During taxi and idle, tach time accumulates more slowly. That is why tach time is often lower than Hobbs time in training aircraft.
The difference is not an error by itself. It is a result of measuring different things.
Which One Goes in Your Pilot Logbook?
Pilot logbook time should reflect the type of time you are legally logging, such as flight time, PIC time, dual received, night, instrument, or cross-country. Do not blindly copy Hobbs or tach time without understanding what your instructor and regulations require for the entry.
Many student pilots use Hobbs as a practical reference for lesson duration, especially when the school bills by Hobbs. But your logbook should still accurately represent the flight time and training received.
Ask your instructor how your school records time and how they want student logbook entries handled. Consistency and accuracy matter.
Which One Matters for Maintenance?
Tach time is often the maintenance reference. Aircraft records may require entries based on tach time because inspections and engine-related intervals are tied to aircraft use.
Some inspections are calendar-based, such as annual inspections. Others may be based on hours of operation, such as 100-hour inspections when applicable. Always check the aircraft records and maintenance requirements rather than assuming one meter answers every question.
Preflight and Postflight Habits
Before flight, compare the Hobbs and tach readings against the dispatch sheet or aircraft log. After flight, write down the new readings accurately.
If a number looks wrong, ask before leaving. A small recording error can create billing issues, maintenance tracking problems, or confusion for the next pilot.
Also pay attention to whether a meter appears inoperative. A broken Hobbs meter or tachometer should be handled according to the aircraft's equipment requirements, maintenance procedures, and school policy.
The Simple Difference
Hobbs time is usually closer to elapsed operating time. Tach time is tied to engine RPM and is commonly used for maintenance tracking.
For renters, Hobbs often affects the bill. For mechanics, tach often affects inspections. For pilots, both numbers deserve careful recording.
Good aviation habits are built from small accurate details. Hobbs and tach time are exactly that kind of detail.
Related Reading
Official References
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