Hobbs meters, also known as hour meters, are electrically driven instruments used to measure elapsed time. Time is measured in increments of one hour and 1/10 of an hour. Results are displayed on a rotary odometer like certain old clocks. These meters are used extensively on airplanes and in the aviation industry. There are a number of different applications that can use hour meters but a single meter can only record one stream of information at a time.
One of the main applications hour meters are used for is to measure the time a plane’s master switch is activated. These measurements are taken to monitor the number of hours pilots log, something that is especially important for trainees. Rented aircraft or aircraft for hire use Hobbs meters to calculate client charges. In a way it’s similar to how a taxi measures driving time, so they know how much to charge passengers. These meters also help enforce service interval criteria for an aircraft’s airframe, engines and propellers.
Linking a Hobbs meter to the master switch doesn’t measure the amount of time the engine has been running or how long the aircraft is airborne. To accomplish these feats there are a few different ways to turn on a Hobbs meter. To measure how engine operating time, oil pressure runs through a pressure switch that will turn on the meter when activated. This ensures the meter only runs when the engine is on. Another method is to attach a pressure switch to an aircraft’s landing gear. This method measure time only when the aircraft is actually flying.