Tach vs Hobbs: Why Your Maintenance Tracking Matters

If you run a flight school, you track engine hours. But which hours? The answer matters more than most operators realize. Using the wrong time reference for maintenance intervals can mean performing inspections too early (wasting money) or too late (risking non-compliance). The difference comes down to understanding what the tachometer and the Hobbs meter actually measure.

What the Hobbs Meter Measures

The Hobbs meter is a simple elapsed-time counter. On most training aircraft, it starts when oil pressure registers — meaning it runs whenever the engine is running. Some installations wire the Hobbs to the master switch instead, but oil-pressure activation is the most common setup. One Hobbs hour equals one real-time hour, regardless of what the engine is doing.

This means the Hobbs is running during taxi, run-up, holding short for traffic, and shutdown — all time where the engine is running but the aircraft isn't flying. In a busy training environment at a towered airport, it's common for 15-20 minutes of Hobbs time per flight to be spent on the ground with the engine at idle. That adds up fast across a fleet.

What the Tachometer Measures

The tachometer (tach) meter measures engine revolutions, not time. It's calibrated so that one tach hour equals one hour of engine operation at a specific RPM — typically cruise RPM (around 2,400 RPM for a Lycoming O-320 or O-360). At lower RPMs, the tach runs slower than real time. At higher RPMs, it runs faster.

During ground operations at idle (around 1,000 RPM), the tach accumulates significantly less than real time — approximately 40% in a typical training engine, though this varies by powerplant. During cruise at 2,400 RPM, tach time and real time are approximately equal. During a full-power climb at 2,700 RPM, tach time actually exceeds real time slightly.

The practical result: tach time on a typical training flight is 10-15% less than Hobbs time. Over 100 hours of Hobbs time, you might only accumulate 85-90 tach hours.

Why the Distinction Matters for Maintenance

FAR 91.409 requires a 100-hour inspection for aircraft used for hire (which includes flight training). The regulation says “100 hours of time in service.” Time in service, per FAR 1.1, is the time from the moment an aircraft leaves the surface until it touches down. That's neither Hobbs nor tach — it's flight time.

In practice, most schools and maintenance shops use tach time as the reference for 100-hour inspections. The reasoning is that tach time more closely approximates actual engine wear than Hobbs time, because it accounts for the reduced wear rate during idle and taxi. Some shops use Hobbs, which results in more frequent inspections and higher maintenance costs.

The 10-15% difference between tach and Hobbs isn't trivial. On an aircraft that flies 800 Hobbs hours per year, that's the difference between eight 100-hour inspections (using Hobbs) and seven (using tach). At $1,500-$2,500 per inspection, you're looking at $1,500-$2,500 in unnecessary maintenance cost per aircraft per year by tracking the wrong reference.

The 100-Hour Inspection and the 10-Hour Overfly

FAR 91.409(b) includes a provision that allows aircraft to exceed the 100-hour limit by up to 10 hours for the purpose of flying to a location where the inspection can be performed. The excess time must be included in the next 100-hour interval — so if you overfly by 5 hours, the next inspection is due at 195 hours, not 200.

This overfly allowance creates a management challenge. Your scheduling system needs to know not just when the inspection is due, but when the hard limit (100 + 10 = 110 hours since last inspection) is reached. Between 100 and 110 hours, the aircraft should show a warning. Past 110, it should be grounded until inspected.

Manual tracking inevitably introduces errors. A tach time gets transcribed wrong, an update gets missed over a weekend, and suddenly an aircraft is past its overfly limit with three students booked on Monday. The tracking system needs to update automatically as part of the flight lifecycle — not as a separate step someone remembers to do.

Common Mistakes Schools Make

• Mixing time references. Some maintenance items are tracked in tach hours, others in calendar time (like the annual inspection), and nobody realizes the references don't line up. Maintenance items get done early or late depending on which reference someone checks.
• Not recording tach times after each flight. If tach time only gets recorded when someone remembers, you don't have accurate hours for maintenance forecasting. The whole system depends on consistent data entry.
• Ignoring the AVIATES items. The 100-hour inspection gets all the attention, but the AVIATES checklist (Annual, VOR check, Inspections, ADs, Transponder, ELT, Static system) includes items on different intervals. Missing an AD compliance date is an airworthiness issue, not just a scheduling inconvenience.
• No integration between maintenance and scheduling. Maintenance tracking lives in one system (or binder). Scheduling lives in another. The dispatcher doesn't know an aircraft is 3 tach hours from its 100-hour, so they book it for a 4-hour cross-country.

Automated Tach-Based Tracking

The solution is a system that captures tach time as part of the postflight flow — the pilot records the ending tach time when they shut down, and the system automatically updates all maintenance intervals. When a 100-hour inspection is due in 12 tach hours, the scheduling system shows a yellow warning on that aircraft. When it's past due but within the 10-hour overfly, it shows an amber alert. Past the overfly limit, the aircraft is automatically blocked from booking.

This level of automation gives dispatchers real-time visibility into fleet maintenance status — always current, always accurate, with every interval recalculated the moment a flight closes out.

SQWKRuses tach time as the primary reference for all maintenance tracking. AVIATES compliance, 100-hour inspections with overfly awareness, and automatic interval recalculation on every postflight — integrated directly with scheduling so your dispatch board always reflects reality. Request a demo.