Can somebody please help explain to me the difference between all of these. I understand that "Off/On time" is probably the Hobbs time which is determined by the time my battery master switch is turned on - but I do not understand the total AC time and the Offset. I am assuming that one of these is the time that my plane is in the air. Picture attached for reference, and I fly a Cessna 172R.

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2 Answers 2


There are different definitions for times in the U.S. FARs.

For example FAR §1.1 has a definition for flight time

Flight time means:

(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing; or

(2) For a glider without self-launch capability, pilot time that commences when the glider is towed for the purpose of flight and ends when the glider comes to rest after landing.

This would typically be your OUT and IN time.

The Hobbs meter (depending on how it is wired) indicates the hours the engine is running or the hours the airplane is flying.

FAR §1.1

Time in service, with respect to maintenance time records, means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing.

FAR §91.409(b)

Except as provided in paragraph (c) of this section, no person may operate an aircraft carrying any person (other than a crewmember) for hire, and no person may give flight instruction for hire in an aircraft which that person provides, unless within the preceding 100 hours of time in service the aircraft has received an annual or 100-hour inspection and been approved for return to service in accordance with part 43 of this chapter...

So the OFF-ON times are used for maintenance intervals.

I am personally unaware of total aircraft time or offset.

Most FMS installations have the ability to capture this data automatically. It is called the OOOI value or OUT-OFF-ON-IN times. Use the OFF-ON for maintenance and the OUT-IN for pilot logbooks.

  • 1
    $\begingroup$ I suspect 'offset' is related to replacing the meter. The meter was replaced at 3389 hours, so the aircraft's next service is due at 10617 hours, at which point the meter will read (10617-3389) = 7228 hours. $\endgroup$ Jan 2, 2021 at 9:20

A Hobbs meter that is started when you turn on the battery master is probably the least accurate method of determining Air Time or Flight Time.

The pilot in command is responsible to determine the correct air time and flight time for each flight they operate.

Aircraft rental companies, and individual aircraft owners, often make up there own procedures for recording air time and flight time that are not necessarily accurate or legal.

The FAA Air Regs make no mention of any sort of hour meter. The best source for recording air time or flight time is the clock in the aircraft or the watch on your wrist.

“HOBBS” time has no legal status. This is especially true since there is no standard on how an hour meter is activated. Some hour meters start when the battery master is turned on. Some start when they sense engine oil pressure. Some start when only when the aircraft is airborne.

“TACH” time also really has no legal standing as it is RARLEY accurate for flight time or air time.

The fact that Hobbs or Tach time are routinely used in pilot and aircraft log books is just an example of convenience and laziness.

“Off/On Time” represents the actual time on a clock at the moment takeoff and landing, and used to calculate the actual air time for a particular flight.

“Total aircraft time” represents the total accumulated Air Time or an aircraft.

“Offset” is just an attempt to try and correct for the inaccuracies of using hour meters to record air time or flight time.

  • $\begingroup$ There are some interesting distinctions made here. I'd be interested to know if they are traceable to, in order of preference: FAR's,, any FAA or ICAO publication, an approved set of opspecs. $\endgroup$ Nov 26, 2022 at 22:32

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