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The Hobbs meter is an instrument measuring elapsed time that is used extensively in the aviation industry. The meter is used for a number of time-related aviation recordkeeping functions such as log book records, airframe hours, and engine running times. The Hobbs meter generally returns measurement increments of one hour and 1/10 of an hour and displays the results on a rotary odometer type readout. Also known as an hour meter, this instrument is electrically driven and may be activated in several ways to record specific event sequence periods according to the user's unique requirements. Common activation methods to start the timing process include master switch, oil pressure, and undercarriage pressure switch signals.
Hobbs or hour meters are used in many industry applications, most of which center around keeping track of total running times of vehicles and machinery. One of the most common applications of the Hobbs meter is time record compilation in the aviation industry where strict control is exercised over flight and operating times of various mechanical and structural aircraft parts. The airframe, engines, and propellers of most aircraft are subject to very rigorously enforced service interval criteria, and hour meters are used to establish total time between services. The same attention is paid to the number of hours pilots, particularly trainee pilots, log — another record for which the Hobbs meter is used. Aircraft for hire companies also use these instruments to calculate client hourly charges.
Most Hobbs meters found in older or smaller aircraft are typically round or square dialed odometer style devices and panel mounted for easy reading from the cockpit. The recording of operational cycles is being integrated into the electronic flight management software suites on many newer aircraft with a wide range of time recording functions available through various diagnostic menus. Unfortunately, single meters can only record one stream of information at any given time. This requires some careful thought as to how and when the meters are activated because different operators use the records for different purposes.
The meter, for example, can be activated when an aircraft's master power switch is on. This has, however, the tendency to only deliver a total powered-up reading which may not indicate how long the engine ran or how long the aircraft was airborne. To get accurate engine hour readings, the Hobbs meter may be activated by an oil pressure switch which will only close and start the meter when the engine runs. It may also be switched on by the inputs from undercarriage or airspeed sensors which give an indication of actual flight times. Newer light aircraft such as the Cirrus line use an alternator signal to activate their Hobbs meters, thus giving accurate engine/propeller running time records.