Most Hobbs meters use a sensor that checks the oil pressure (left sensor). In some instances, it is connected to a squat switch on the landing gear (right sensor).
Like it or loathe it (and most of us loathe it), the Hobbs meter is a familiar instrument. It’s a simple clock that counts off time in hours and tenths of hours. Six minutes equals a tenth of an hour. That time is used to calculate what you pay to fly the airplane.
In most airplanes, the Hobbs—more properly known as an hourmeter—is connected to a sensor that checks the engine for oil pressure. When oil pressure rises above a certain level, the Hobbs begins tracking your time. In this case, its function isn’t affected by engine rpm or gear retraction. In some aircraft, however, the hourmeter is wired to a gear or squat switch so that it begins operating when landing gear is retracted.
Few Hobbs meters are connected through the master switch, so shutting that off won’t cause the meter to stop.
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