Hobbs vs Tach Time: A Pilot’s Guide to Both Meters

Pilot inspecting cockpit instruments showing Hobbs and tach meters

Hobbs time and tach time both measure aircraft usage, but they measure different things. Hobbs time records total elapsed operating time, typically in hours and tenths of an hour, and starts when the engine runs based on oil pressure, the master switch, an airspeed switch, or a squat switch. Tach time, by contrast, counts engine revolutions and converts them to time calibrated at cruise RPM, so it runs slower during taxi and idle than a wall clock does. The practical result: tach time runs 10–20% lower than Hobbs time on most flights, with the gap widening on pattern-heavy or low-power training sessions.

Here is why both numbers matter:

Hobbs time approximates FAA flight time as defined in FAR 1.1 and is the standard for billing at flight schools and FBOs.
Tach time reflects actual engine strain and is the reference for maintenance intervals, oil changes, and time between overhauls.
The gap between them is not an error. It is the natural result of measuring two different things.
Logging both after every flight is standard practice across general aviation, letting you catch meter malfunctions early and keep maintenance records accurate.

What is Hobbs time and how is it measured?
What is tach time and how does the tachometer measure it?
How do Hobbs and tach time actually differ?
How pilots and mechanics use both times in practice
Best practices for accurate logging with both meters
Key Takeaways

What is Hobbs time and how is it measured?

The Hobbs meter is an hour meter, a device that tracks elapsed time in hours and tenths of an hour. Honeywell, which now owns the Hobbs brand, defines it simply as “a gauge or instrument that tracks and records elapsed time.” An hour and a half shows as 1.5 on the display. That simplicity is exactly why flight schools and fixed-base operators (FBOs) rely on it for billing.

What triggers the meter varies by aircraft. Hobbs meters can be activated by an oil pressure switch, the electrical master switch, an airspeed switch, or a squat switch on retractable gear. Each activation method records a slightly different slice of time:

Oil pressure switch: Starts when the engine fires and stops at shutdown. This most closely matches FAR 1.1’s definition of flight time.
Master switch: Starts when the electrical system comes on, which can add pre-start and post-shutdown time.
Airspeed switch: Records only airborne time, excluding taxi and runup entirely.
Squat switch: Activates when the aircraft lifts off the gear, also capturing airborne time only.

Because the activation method affects what gets recorded, two aircraft of the same type can show different Hobbs readings for the same trip. Before you rent or fly an unfamiliar aircraft, confirm how its Hobbs meter is wired. Misunderstandings about meter activation are a common source of billing disputes and maintenance errors.

The Hobbs meter is not a legally required instrument under 14 CFR 91.205, but nearly every general aviation aircraft has one installed because it gives pilots and operators a reliable, easy-to-read measure of operating time.

Infographic comparing Hobbs time and tach time

Pro Tip: Ask the FBO or flight school which trigger activates the Hobbs meter before your first flight. An oil-pressure-activated meter starts billing the moment the engine runs, including runup time, while an airspeed-activated meter does not.

What is tach time and how does the tachometer measure it?

The tachometer does not measure time directly. It counts engine revolutions and converts that count into a time value calibrated to a reference cruise RPM. At cruise RPM, tach time runs like a standard clock, clicking off a tenth of an hour every six minutes. Drop below that reference RPM during taxi or slow flight, and the tach clock slows down proportionally. Push above it at high power, and it ticks faster.

Close-up of aircraft tachometer dial showing needle

Think of it this way: if the reference RPM is 2,500 and the engine idles at 1,250, the tach clock runs at half the speed of a wall clock during that idle period. That is not a malfunction. It is the meter doing exactly what it is designed to do, which is reflect how hard the engine is actually working.

Here is how tach time behaves across common flight phases:

Taxi and runup: Tach time accumulates slowly because RPM is well below cruise reference.
Climb: Tach time may run close to or slightly above clock time depending on power setting.
Cruise: Tach time and clock time align closely at the calibrated reference RPM.
Pattern work: Tach time falls noticeably behind Hobbs time due to repeated power reductions.
High-power operations: Tach time can briefly exceed clock time.

Tach time better reflects engine wear than simple elapsed time because it accounts for how many revolutions the engine has actually turned. A 100-hour inspection interval tied to tach time means the engine has genuinely accumulated 100 hours of proportional wear, not just 100 hours of sitting with the master switch on.

Under 14 CFR 91.205, the tachometer is a required instrument for all general aviation aircraft. That requirement reflects the FAA’s recognition that engine monitoring depends on accurate RPM data, not just a running clock.

How do Hobbs and tach time actually differ?

The core difference is what each meter measures. Hobbs time is a clock that runs when a specific condition is met, regardless of what the engine is doing. Tach time is an RPM counter that converts revolutions to time, so it is sensitive to power setting throughout the flight.

Pilot logging flight times in an aviation logbook

A training flight with a 30-minute ground delay, a full runup, several pattern laps, and a normal taxi back will show a noticeably higher Hobbs reading than tach reading. A cross-country at sustained cruise power will show readings much closer together. Tach time is typically 10–20% lower than Hobbs time, with the gap largest on short, low-power flights.

Factor	Hobbs time	Tach time
What it measures	Elapsed operating time	Engine revolutions converted to time
Activation trigger	Oil pressure, master switch, airspeed, or squat switch	Engine RPM via tachometer
Runs slower at idle?	No, runs at clock speed	Yes, proportional to RPM
Primary use	Billing, pilot logbook	Maintenance intervals, engine logs
Required by FAA?	No	Yes (14 CFR 91.205)
Typical difference	Reference point	Generally lower than Hobbs

Flying clubs sometimes bill by tach time rather than Hobbs time, which can make the hourly rate appear lower because fewer hours are recorded. The catch: many clubs using tach-based billing charge a “dry rate,” requiring pilots to pay for fuel separately on top of the hourly charge. Knowing which system your club or FBO uses changes how you calculate the true cost of a flight.

Neither meter is more “correct” than the other. Hobbs suits operational billing; tach suits engine wear assessment. Treating them as interchangeable leads to billing errors, missed maintenance, or both.

How pilots and mechanics use both times in practice

Flight schools and FBOs almost universally bill by Hobbs time because it closely matches the time a renter has the aircraft. Most flight schools use Hobbs for billing and the tachometer for engine service intervals, which is why you are typically asked to record both readings at the end of every flight.

For pilots, the practical logging breakdown looks like this:

Pilot logbook: Log flight time per FAR 1.1, which an oil-pressure-activated Hobbs meter approximates well. Do not blindly copy either meter reading without confirming what your regulations require for the specific type of time being logged (PIC, dual received, cross-country, etc.).
Aircraft flight folio: Record both Hobbs and tach readings before and after each flight. A growing gap between the two over time can signal a meter malfunction.
Engine logbook: Tach time drives entries here. Oil changes, 100-hour inspections, and time between overhauls (TBO) are all tracked against tach hours.

For maintenance personnel, tach time is recorded in engine logbooks to determine oil change intervals, inspection due dates, and overhaul scheduling. Airworthiness Directives (ADs) may specify compliance based on hours of operation, and accurate Hobbs and tach logs are primary inputs for calculating when those ADs come due.

Some inspections are calendar-based, like annuals. Others, like 100-hour inspections for aircraft operated for hire, are hour-based and tied to tach time. Mixing up which meter applies to which interval is a real compliance risk. An aircraft that has accumulated 98 tach hours since its last 100-hour inspection is two tach hours away from a required stop, regardless of what the Hobbs meter shows.

Best practices for accurate logging with both meters

Logging both readings consistently is the single most reliable way to catch problems early. Recording both Hobbs and tach time after every flight is industry standard precisely because a diverging gap between the two often signals a meter issue before it becomes a compliance problem.

Here are the practices that keep your records clean and your aircraft airworthy:

Record both readings before and after every flight. Pre-flight and post-flight entries let you verify the meters advanced correctly during the flight.
Know your aircraft’s meter activation. Confirm whether the Hobbs meter runs on oil pressure, master switch, airspeed, or squat switch. That knowledge affects how you interpret billing and what counts toward your logbook.
Cross-check the gap regularly. A consistent 10–15% difference between Hobbs and tach on training flights is normal. A sudden change in that ratio warrants a conversation with your mechanic.
Never assume one meter covers everything. Tach time governs engine maintenance; Hobbs time governs billing and pilot logbook entries. Each has its lane.
Verify AD compliance intervals against tach time. When an AD specifies an hour-based compliance threshold, confirm whether it references tach hours or total time in service, since those are not always the same.
Use digital tools to reduce manual entry errors. Automated flight data import eliminates the transcription mistakes that accumulate over hundreds of flights.

Accurate recording of both meter readings is not just administrative. These figures are primary inputs for calculating compliance with ADs and mandatory inspections, and errors in the log can translate directly into airworthiness violations.

Squawkfree’s Flight Intelligence feature auto-imports flight data, including Hobbs and tach readings, directly from ADS-B without manual entry. For flight schools managing a fleet or co-owners sharing a single aircraft, that automation removes the single biggest source of logging errors: human transcription. Squawkfree also tracks AD compliance against your actual flight hours, so you know exactly where each aircraft stands against every applicable directive.

If your logbooks are paper-based or partially digitized, Squawkfree’s logbook digitization service uses human-verified OCR to convert historical records into a searchable digital format, giving you a complete, accurate baseline for both Hobbs and tach time going forward.

Pro Tip: After any maintenance event that resets an engine interval, verify that both the tach reading and the maintenance entry agree before the aircraft returns to service. A mismatch between the logbook entry and the meter reading is one of the most common sources of premature or overdue inspection calls.

Squawkfree

Key Takeaways

Hobbs time and tach time serve distinct roles: Hobbs tracks elapsed operating time for billing, while tach time measures engine revolutions for maintenance, and logging both accurately is required for FAA compliance and airworthiness.

Point	Details
Hobbs measures elapsed time	Activated by oil pressure, master switch, airspeed, or squat switch; used for billing and pilot logbooks.
Tach time reflects engine wear	Calibrated to cruise RPM; runs slower at idle, making it the standard for maintenance intervals.
Typical gap is 10–20%	Tach time is usually 10–20% lower than Hobbs, with the largest gap on pattern-heavy or low-power flights.
Log both after every flight	Recording both readings catches meter malfunctions early and supports AD compliance tracking.
Tachometer is FAA-required	Under 14 CFR 91.205, the tachometer is a required instrument; the Hobbs meter is not, but is nearly universal.

Hobbs vs Tach Time: A Pilot's Guide to Both Meters