Time: Hobbs, tach, flight ...

Logging time when flying is a bit more complicated than it should be. I’ve put together my thoughts; while I’m confident in the accuracy of this information, nothing here is legal advice and if you have any questions, I suggest talking to your local Flight Standards District Office (FSDO) and/or requesting a Letter of Interpretation from the FAA.

Different Types of Time

Hobbs

In many airplanes (especially rentals), there will be a Hobbs meter, that starts running when some event occurs (master switch turned on, oil pressure rises, or something like that). Most rental shops and flight schools will use this number for billing purposes. A $160/hr rental where 0.5 was clocked on the Hobbs meter will be billed $80.

Tach
JPI EDM-830

Every airplane will also have at least one tachometer (FAR 91.205(b)(4)), which is keeping track of rotations of the engine. Obviously, it's only counting when the engine itself is running, and it's almost certainly calibrated so that 1 hour of engine time matches one hour on your wristwatch at a particular RPM. In most fixed-pitch trainers (Cherokees, Cessna 172s, etc), this will probably be around 2300 RPM; in aircraft with constant speed propellers (Mooneys, Arrows, Bonanzas, etc), this will be set at a typical cruise power setting, likely 2500 RPM. Some digital tachometers will only start counting above a certain engine speed (e.g., the VM 1000 system in a Mooney I looked at once states: “Engine hours are accumulated any time RPM is greater than 1500”). Definitely worth knowing your plane!

The Mooney I occasionally fly has two tachometers, the “official” mechanical one and the one built into the JPI EDM-830 installed as a secondary set of engine instruments. The JPI can be configured as to what RPM will correlate 1:1 to the “wristwatch” hours of operation. (Buried in the JPI is also a Hobbs meter, but since the plane only gained that recently (it was upgraded from an EDM-700), I haven’t much used it.)

Flight

Finally, in many planes you’ll also find, somewhere, a “flight time” recorder. That Mooney has that in the Garmin GTX-327 transponder (accessible by repeatedly pressing the FUNC button); the Garmin automatically determines when the aircraft is flying (based on a squat switch, GPS ground speed, airspeed, or an altitude increase, depending on how it’s installed).

Other planes, such as newer Cirrus models, will have a flight time recorder alongside the Hobbs meter.

Which do you use?

Well, that depends on what you’re using the time for, and what type of operations you’re conducting.

Airplane Maintenance

FAR 91.417(a)(2)(i) requires maintenance records to include “[t]he total time in service of the airframe, each engine, each propeller, and each rotor.” FAR 1.1 defines “time in service” as, “with respect to maintenance time records, [...] the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing.”

Some aircraft manufacturers have addressed this explicitly. For instance, Cirrus aircraft from serial number 1863 on, include a “FLIGHT” hour meter next to the Hobbs hour meter, and the POH states:
FAR 1.1 defines time in service, with respect to maintenance time records, as “the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing.”

The #2 Hour Meter, located in the center console and labeled FLIGHT, begins recording when the airplane reaches approximately 35 KIAS and should be used to track maintenance time intervals as it more accurately records time in service than the #1 Hour Meter.
(p. 8-6, emphasis added). Additionally, e.g., FAR 91.409(b) refers to “hours in service” when determining when a “100 hour inspection” is due. Accordingly, I would record and use the “flight time” for tracking maintenance. The FAA, from what I’ve read, agrees:
  • MacLeod (2009) letter of interpretation (the owner or operator of an aircraft is required to “record[] and track[] in some form and manner the time in service of the airframe, engines(s), propeller(s), and rotor(s) from the moment the aircraft leaves the surface of the earth until it touches it at the next point of landing, as referenced in 14 C.F.R. § 1.1” (emphasis added)).

Pilot’s Hours

FAR 1.1 defines “flight time” (used for most things “pilot-related’) as: “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.” For logging flights towards ratings, etc., this is what you would use, and it includes time spent taxiing, etc., but not time spent sitting on the ground with the engine running, prior to the first motion of the aircraft:
 Etc. For this, I would use Hobbs if available, or tach - possibly with a multiplier. Clubs and flight schools, when billing for an aircraft without a Hobbs meter, will multiply tach time by, say, 1.2 or 1.3, to arrive at a “close enough” number reflecting aircraft operation time. This works for training flights that stay in the pattern, but according to my observations, for a typical cross-country flight, tach time is likely to be a percentage point or two off “wristwatch” time, if even that much, and I would use straight tach time in that event. (FWIW, the flight school that I occasionally rent a no-Hobbs Mooney from, does it exactly that way; pattern practice is billed at 1.2x tach, while cross-country flying is billed at straight tach time.)

That said, “Hobbs vs. tach” is discussed relatively regularly, with some confusion...

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