Airplane Flying Hours Calculator

Mastering Flight Time: Hobbs, Tach, and Logbooks

For every pilot, from the student taking their first discovery flight to the seasoned airline captain, logging flight time accurately is a ritual of the trade. Generally, general aviation aircraft are equipped with meters that track usage, but understanding exactly what time you are recording is critical for both legal compliance and aircraft maintenance. Our Airplane Flying Hours Calculator is designed to simplify the math of converting Hobbs meter start and end times into the precise decimal hours needed for your logbook.

Hobbs Meter vs. Tachometer: What's the Difference?

If you rent aircraft or own one, you have likely noticed two different hour meters. Knowing the difference can save you money and keep you out of trouble with the FAA.

The Hobbs Meter

Named after its inventor, John Weston Hobbs, this meter is electrically activated. In most aircraft, it runs whenever the master switch is on and the oil pressure is up (or simply when the master is on, depending on the installation).
Usage: Hobbs time is typically used for rental billing and pilot logbooks. Since it runs while you are taxiing and doing your run-up, it reflects the "Block Time" (time from when the aircraft moves under its own power for the purpose of flight until it comes to rest).

The Tachometer (Tach Time)

The Tachometer records time based on engine RPM. It is calibrated to record one hour of "time" only when the engine is running at a specific high RPM (cruise power). At idle (taxiing), the tach meter spins much slower than real-time.
Usage: Tach time is used for maintenance inspections (like the 100-hour or annual inspection) and "Time in Service" for airworthiness directives. It accurately reflects the wear and tear on the engine.

The Decimal System in Aviation

Aviation rarely uses minutes for logging; it uses decimal hours. This makes adding up columns in a logbook significantly easier. The standard conversion is based on 6-minute intervals, where every 6 minutes equals 0.1 hours.

Minutes	Decimal Hours
1 - 6	0.1
7 - 12	0.2
13 - 18	0.3
19 - 24	0.4
25 - 30	0.5
31 - 36	0.6
37 - 42	0.7
43 - 48	0.8
49 - 54	0.9
55 - 60	1.0

How to Log Flight Time Correctly

1. Use the Right Meter: If you are renting, you will almost always pay based on the Hobbs meter. Therefore, you should log Hobbs time. This ensures your logbook matches the rental receipts, which is helpful for audits or checkrides.
2. Do the Math: Always subtract the START time from the END time.
Example: End: 1456.2 - Start: 1454.8 = 1.4 Hours.
3. Check for Errors: If the meter seems broken (e.g., you flew for an hour but the Hobbs only moved 0.1), notify the FBO or owner immediately. Never estimate flight time in your logbook if a meter fails without noting the discrepancy.

Why Accuracy Matters

For student pilots, every 0.1 hour counts toward the minimum requirements for a certificate (e.g., 40 hours for Private Pilot). Shortchanging yourself by rounding down could leave you 0.1 short before a checkride. Conversely, rounding up aggressively (logging 1.5 for a 1.2 flight) is falsifying records and can lead to certificate revocation.

For aircraft owners, tracking Tach time accurately ensures you don't overfly mandatory maintenance intervals. Overflying an AD (Airworthiness Directive) recurring inspection by even an hour can render the aircraft unairworthy.

Conclusion

Logging flight time is more than just paperwork; it is the diary of your aviation journey. Whether you are building time for a commercial rating or simply enjoying the freedom of the skies on weekends, using tools like our Hobbs Flying Hours Calculator ensures your records are precise, professional, and audit-ready. Safe flying!