What are TACH and HOBBS times?
Airplanes have timers in them that are used to track operating time for maintenance and billing. For example, most small aircraft engines are overhauled after reaching 2000 tach hours. The Tach timer is connected to the tachometer, which indicates how fast the engine and propeller are turning in revolutions per minute, just like your car. This timer is set by the factory to be equal to normal clock time when the engine is at 2300 rpm (Piper) or 2400 rpm (Cessna). Therefore, when the engine is operating at low power levels, common during training, the Tach timer operates slower than clock time. The Hobbs timer records clock time whenever the engine is operating. Generally, Tach time is 85% to 90% of Hobbs (clock) time. So a billing rate of $99 per Tach hour is equivalent to $84 to $89 per Hobbs hour. So, the difference is very important when comparing the cost of flying!
