The short answer
Tach time is engine time recorded by the tachometer, accumulating in proportion to engine RPM. It's calibrated to advance at one hour per clock hour only around cruise RPM — at lower power settings it accrues more slowly — which makes it the usual reference for engine maintenance intervals.
Why tach time runs slower than Hobbs
A recording tachometer counts engine revolutions and converts them to hours using a reference RPM near cruise power. At full cruise it matches the clock; at taxi or idle RPM it might accrue at half rate. Over a typical flight with taxi, climb, and descent, tach time totals roughly 80–90% of Hobbs time.
Because it tracks how hard the engine actually worked, tach time is the customary basis for oil changes, 100-hour inspections, and engine time like SMOH. Pilot flight time for logbooks, by contrast, is usually recorded on Hobbs or block time.
Related terms and reading
Keep every hour and inspection straight
Maggneto tracks engine times, inspections, and ADs from your actual logbooks — so terms like these become numbers you can act on. Browse the full maintenance glossary.
Frequently asked questions
Is 100-hour and annual maintenance based on tach or Hobbs time?
Most operators track engine and inspection intervals on tach time, but the aircraft's own records govern — whichever meter the logbooks and inspection program consistently reference is the one that counts. Record both at each event to keep the history clean.