Hobbs meter

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Hobbs meter is a generic trademark for devices used in aviation to measure the time that an aircraft is in use. The meters typically display hours and tenths of an hour, but there are several ways in which the meter may be activated:

1. It can measure the time that the electrical system is on. This maximizes the recorded time.
2. It can be activated by oil pressure running into a pressure switch, and therefore runs while the engine is running. Many rental aircraft use this method to remove the incentive to fly with the master electrical switch off.
3. It can be activated by another switch, either an airspeed sensing vane under a wing (as in the Cessna Caravan) or a pressure switch attached to the landing gear (as in many twin engine planes). In these cases, the meter only measures the time the aircraft is actually flying. Metrics such as Time In Service and Turbine Actual Runtime are kept to monitor overhaul cycles, and are usually used by commercial operators under Federal Aviation Regulations Parts 135, 121, or 125.
4. It can be activated when the engine alternators are online (as in the Cirrus SR series).