Hobbs meter is a genericized 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:
For general aviation, Hobbs time is usually recorded in the pilot's log book, and many fixed-base operators that rent airplanes charge an hourly rate based on Hobbs Time. Tach Time is recorded in the engine's log books and is used, for example, to determine when the oil should be changed and the time between overhauls. Tach (tachometer) time differs from Hobbs Time in that it is linked to engine revolutions per minute (RPM). Tach Time records the time at some specific RPM. It is most accurate at cruise RPM, and least accurate while taxiing or stationary with the engine running. At these times, the clock runs slower. Depending on the type of flight, Tach Time can be 10–20% less than Hobbs Time. Many organizations such as flying clubs charge by Tach Time so as to differentiate themselves from fixed-base operators by the fact that 10-20% less time recorded makes it 10-20% cheaper to fly (if the hourly rate is the same). In the case where flying clubs use Tach Time, many will charge a dry rate, thus requiring the renter to pay for fuel on top of the hourly Tach Time rate.
The Hobbs Meter is named after John Weston Hobbs (1889–1968), who in 1938 founded the company named after him in Springfield, Illinois which manufactured the first electrically wound clocks for vehicle use. World War II created the demand for aviation hour meters which led to the development of the original Hobbs Meter. The company was eventually renamed Honeywell Hobbs after being acquired by Honeywell International, who in 2009 announced plans to move manufacturing to Mexico.