Airplanes may be equipped with a Hobbs meter which is an electric instrument which starts from an oil pressure sensor and reads standard hours. The tach on an aircraft engine also has an hour meter which reads time as a function of power settings. ( Ex. One hour at 75% power setting equals 3/4 hours Tach Time.) Usually aircraft are rented by hobbs time and serviced by tach time.
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PS. Hobbs is a brand name of an hour meter..