Hobbs time counts clock time while the engine runs; tach time counts engine revolutions normalised to cruise RPM, so it runs slower at low power. Schools typically bill students on Hobbs but schedule maintenance on tach — tracking both per flight is essential.
The Hobbs meter records clock time while the engine runs (or oil pressure is up): one hour of engine operation equals one Hobbs hour, whether taxiing or at full power. The tachometer time counter accumulates in proportion to engine RPM, calibrated to run true at cruise power — so at taxi and low-power flight it under-counts, typically making tach time 10–20% lower than Hobbs for the same lesson.
The standard training-fleet convention exploits the difference: bill students on Hobbs (captures all engine time, matches what the customer experienced) and schedule maintenance on tach (better proxy for engine wear). That requires recording both meters per flight — and never mixing them up when computing inspection due points.
Why it matters for flight schools
The Hobbs–tach spread is quietly one of the most consequential numbers in training economics. On a busy trainer the gap can exceed 100 hours a year: revenue is earned on the higher counter while 100-hour inspections and TBO consume the lower one — effectively stretching maintenance intervals relative to billed hours. Fleet decisions, hourly rates and engine reserves all sharpen when the school knows its actual Hobbs-to-tach ratio per aircraft rather than assuming one.
How FlightLogger handles it
FlightLogger records both Hobbs and tach per flight automatically as part of the journey log — billing runs on one, FL Maintenance tracks due items on the other, and the ratio is there for anyone pricing hours.
Frequently asked questions
Which meter should maintenance intervals use?
Whichever the maintenance programme specifies — for most piston trainers, tach time (or recorded flight time) drives 100-hour and TBO tracking because it better reflects engine wear. The critical discipline is consistency: pick the counter the programme names and never substitute.
Why is tach time lower than Hobbs?
The tach counter is geared to RPM and calibrated at cruise power, so it runs slow during taxi, idle and low-power flight. Training profiles with lots of ground time and pattern work show the biggest spread.