What’s the Hobbs meter?

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The Hobbs meter is an instrument that measures elapsed time and is widely used in the aviation industry. The meter is used for a number of time-related aviation record-keeping functions, such as logbook records, airframe hours, and engine running times. The Hobbs meter typically returns measurement increments of one hour and 1/10 of an hour and displays the results in a rotary odometer type reading. Also known as an hour meter, this instrument is electrically driven and can be activated in a variety of ways to record specific sequence-of-event periods according to the user’s unique requirements. Common activation methods to initiate the timing process include signals from the main switch, oil pressure, and undercarriage pressure switch.

Hobbs or hour meters are used in many industrial applications, most of which are focused on keeping track of the total running times of vehicles and machinery. One of the most common applications of the Hobbs meter is the compilation of time records in the aviation industry, where strict control is exercised over flight times and operation of various mechanical and structural parts of the aircraft. The airframe, engines and propellers of most aircraft are subject to very stringent service interval criteria, and hour meters are used to establish the total time between services. Equal attention is paid to the number of hours that pilots, particularly trainee pilots, log, another record for which the Hobbs meter is used. Aircraft rental companies also use these tools to calculate customer hourly charges.

Most Hobbs gauges found on older or smaller aircraft are round or square dial odometer style devices and panel mounted for easy cockpit reading. Recording of operating cycles is being integrated into electronic flight management software suites on many newer aircraft with a wide range of time recording functions available through various diagnostic menus. Unfortunately, individual meters can only record one stream of information at any given time. This requires careful thought about how and when the meters are activated because different operators use the registers for different purposes.

The meter, for example, can be activated when an aircraft’s master power switch is turned on. However, this has a tendency to only give a full power reading which may not indicate how long the engine ran or how long the plane was airborne. To get accurate engine hour readings, the Hobbs gauge can be activated by an oil pressure switch that will only close and turn the gauge on when the engine is running. It can also be triggered by inputs from landing gear or airspeed sensors that give an indication of actual flight times. Newer light aircraft such as the Cirrus line use a signal from the alternator to activate their Hobbs meters, providing accurate records of engine/propeller run time.

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