Other posts didn't answer my question. In A&P school months ago they told us the difference and can't find where I wrote this down. One of them was used to find engine time and the other was used for aircraft total time.

  • $\begingroup$ I wrote in my post that it didn't answer what my question was. $\endgroup$
    – aaronstran
    Jan 7 '16 at 20:37
  • 1
    $\begingroup$ Then you will need to clarify how it doesn't answer your question. The answer explains that one is based on engine revolutions and the other is based on regular time, which should explain which relates to engine time and which relates to aircraft time. $\endgroup$
    – fooot
    Jan 7 '16 at 20:48

Generally the engine time is measured using the Tach (and is what the engine is serviced based on) while the airframe time is measured off the hobbs meter. This is based on the idea that the hobbs is running when then plane is "on" (usually when oil pressure is present) but the tach time measures engine wear time. In theory the engine wears slower at a lower RPM.

As for maintenance this should be elaborated on a bit since most people actually measure this incorrectly sometimes. Tach time is always running however it does not always run at a 1 Tach Hour = 1 Clock Hour rate. Generally the tach timer is set up to run 1 Tach Hour = 1 Clock Hour at 70% power (or some other often used cruise power). Any RPMs below that will run the tach timer slower and any above it will run it faster. Tach time is generally accepted for engine service since it directly measures engine time (it actually measures RPM count) but for service on the rest of the airframe there are a few options. When it comes to a Hobbs meter there are various ways they can be connected. I was always taught it was based on oil pressure however some planes seem to have it wired to the master switch and more importantly others are wired to a squat switch on the gear. The FAA defines Time In Service as

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.

Lets take the 100 hour inspection which is required after 100 hours of Time In Service

(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 or has received an inspection for the issuance of an airworthiness certificate in accordance with part 21 of this chapter. The 100-hour limitation may be exceeded by not more than 10 hours while en route to reach a place where the inspection can be done. The excess time used to reach a place where the inspection can be done must be included in computing the next 100 hours of time in service.

As such a hobbs meter connected to a squat switch would actually be the most representative of Time In Service which is when hourly maintenance needs to be performed.

  • $\begingroup$ -1. All maintenance (airframe & engine & else) is based on flight time, not engine run time. For single engine aircraft (and many twins) both airframe and engine (and all other components) times are measured by tach time. Hobbs time starts ticking when the engine is started. Tach time starts ticking at high RPMs (such as at take-off). Since the FAA bases maintenance on flight time, the tach time is a more accurate representation of flight time. $\endgroup$
    – Steve H
    Jan 8 '16 at 15:09

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