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Aero-Tips 12.12.06

Logbooks. Rental fees. Inspections. Overhauls. Taxes. There are a number of elements of flying and operating aircraft that are dependant on aircraft time. Rules, regulations and suggestions tell us to base most everything on the amount of time the aircraft has run... but there's nothing in the "regs" that says how to judge that time.

There are three "types" of aircraft time. Each has advantages; under Federal Regulations we can choose the type of time that best suits us.

Hobbs time. A Hobbs meter (a trade name) is a digital counter that records sequentially how long an airplane has been operating. Hobbs meters measure time accurately when full electrical voltage is available.

• In most fixed-gear airplanes the Hobbs meter is wired to the battery master switch. Even if the engine isn't running, the Hobbs is recording actual clock time as long as power is applied.
• In many retractable-gear airplanes the Hobbs meter is wired through landing gear squat switches. It records time only while the airplane is airborne.
• In some unusual situations a Hobbs meter might be wired to sense other things, for instances, when the engine oil pressure is above a certain limit. A Hobbs wired this way would measure actual clock time when the engine is running.

Hobbs meters are generally used for airplane rental time, and often for time that's put in the pilot's logbook. If flying a retractable-gear airplane with the Hobbs wired through the gear system, this would cheat you of civilian-loggable time during ground operations, although it would save you money on airplane rental.

Tach time. Most propeller engine tachometers have an "odometer"-type recording capability with time displayed on the face of the instrument. Tach time is usually set to be accurate clock time when the propeller speed is above some point, for instance 2250 rpm. Since tach time records slower when the engine is running slower, it's not usually used for rental time or time in a pilot's logbook. But because tach times often does not build as rapidly as Hobbs time, it's to an owner's advantage to use tach time for judging time between 100-hours inspections (for commercially used aircraft), repetitive Airworthiness Directives and engine overhaul.

Actual clock time. Since the regs don't tell us how we're supposed to measure time, we can also use actual clock time. Record engine start and engine stop times, and use these to your advantage. Since there is no permanent record in the airplane, and because it will usually record more time than either a Hobbs meter or a tachometer, it's not convenient to use actual clock time for aircraft inspection intervals or tax purposes. It is advantageous, however, to use actual clock time for time in your pilot's logbook, if your goal is to log as much time as legally possible.

Be consistent. Use whatever time best serves your needs. But be consistent. Especially is you're recording aircraft time for some tax purpose, but also to satisfy the FAA if ever arises a question of pilot time, airplane inspection or AD intervals, use the same type of time for each type of log entry.

Aero-tip of the day: Know the "types" of aircraft time, and use each type appropriately to your advantage.

FMI: Aero-Tips