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9 Answers

Another question on logging flight time

Asked by: 24319 views General Aviation

I was just reading another post here titled, "How does an odometer differ from tach time?" and in reading some of the posted answers, it reminded me of a question I had as a student to whcich I never seemed to have gotten an answer.  When I first began my flight training it was in an airplane which I bought for training, and thankfully still own. It has never had a Hobbs meter, so my initial instructor just had me use the tach time, but told me that I should add .20 hrs to each hour of flight for logging purposes, to compensate for the tach running slower than a hobbs would, being that the engine spent a good portion of time at less than normal cruise power or at idle on the ground. I have long since abandoned this practice, since I wasnt totally comfortable with what I viewed as "Fudging" my hours, as well as it is much simpler to go by the tach. When I moved up to helicopter training, they were all equipped with Hobbs meters, and so naturally I use the hobbs for time to log, but when I still take out my PA-28 Cheorkee I continue to only log what I get from the tach. Am I cheating myself out of logable time?

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9 Answers



  1. Matt Farmer on Mar 08, 2011

    Forget all the meters (Hobbs or otherwise), the only legitimate way to log flight time is from “Start” to “Stop” and do the math (i.e. number of minutes/60).  In larger aircraft, as in the jet I fly, there are meters in the aircraft, but those are mainly used by maintenance.  There are 4 times we log on every flight: OUT (Start), OFF (Takeoff), ON (Touchdown), and IN (Stop).  The OUT, OFF, ON, & IN times are what is entered in the Journey Log & Technical Log of the plane, and these are the times the FAA would use in all flight/air time calculations.
    Basically: Flight Time = OUT to IN and Air Time = OFF to ON
    In larger aircraft some or all of these times are logged automatically. I know in smaller aircraft it’s a pain to write down these 4 times, but it is the proper way.
    FYI: Here is the FAA’s definition of “Flight Time” from Genreal Section 1.1
    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.

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  2. Wes Beard on Mar 08, 2011

    You are cheating some of your time but by 20% is hard to say.  I was also told the same thing during my training and never questioned the instructor.  When I became an instructor myself, I measured the tach time against an actual clock and determined that the hobbs time isn’t a really good indicator of how much time a person can log into their logbook.
     
    For a little history, a tachometer measures revolutions per minute or RPM.  At low RPM’s, the tach time is much slower than the real world clock.  At a single higher RPM, the tach time and clock are exactly correct and at a still higher RPM the tach time is moving faster than the real world clock.
     
    In my studies, a fixed pitch prop will in fact be around 20% different when going to the practice area AND coming in for landing practice.  During cross country flying, the percentage will go down to around 10%.  For a constant speed propeller, I have seen as little as a 5% difference between the tach and a clock.
     
    My advice is to start using a clock as a measurement of how much to log in the airplane.  Pretty easy to jot down starting and stopping times on a sheet of paper.

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  3. Matt Farmer on Mar 08, 2011

    Just an addendum to my previous response;
    In case you were wondering the Hobbs Meter measures the time battery or electrical system power is on in the plane. The Tach Meter measures the time the engine is running.
    Neither of which have anything to do with “Flight Time” or “Air Time”.  Think about it this way, you could start the plane and taxi around all day logging time on these meters, but you have not logged any “Flight Time” or “Air Time”.

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  4. Matthew Waugh on Mar 08, 2011

    As Wes points out – for measuring time you can’t beat a clock. A Hobbs, Tach etc. are just proxies for a clock.
     
    If you use a Hobbs you are “cheating”, it only measures in tenths, so sometimes you are counting time you did not fly and sometimes you are not counting time you were flying, depending on where in the “tenth cycle” the Hobbs meter was/is.
    Of course if you just use a clock to the nearest minute you are cheating, you should go to the nearest second. Of course there are sub-seconds so you’re cheating there if you don’t have an atomic clock handy…….
     
    For an accurate logbook, use a clock!

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  5. John D. Collins on Mar 08, 2011

    I use watch time or the clock in the airplane. As Matt answered, the definition of flight time is: (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.  So, once you start the engine, allowing the airplane to roll forward to test the brakes starts the clock.  After landing when the aircraft stops at the hanger or tie-down spot, stops the clock.  If you are recording anything less than this definition, you are reducing the legal amount of time that you are entitled to record.  In a training airplane with a fixed pitch propeller, using tach time can short you as much as 20%.  Using a Hobbs meter can also short the flight time, depending on how it is turned on and off.  Most Hobbs meter installations use an oil pressure switch to control the clock, but since the Hobbs only displays the time to the nearest tenth of an hour, it is possible that you can be shorted.  Many retractable gear aircraft have the Hobbs hooked to the gear up indication, so time on the ground is ignored, unless you forget to put the gear down, then you should stop recording the flight time when you slide to a stop!

     

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  6. James MacGregor CFI on Mar 09, 2011

    The hobbs starts when the engine starts (+oil pres) it ends on shutdown, this is the time I log. when you start the engine the plane WILL move (if only half a inch).
     
     As far is gaining a tenth or loosing a tenth, it will all balance out.
     
     Log the hobbs.

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  7. Kent Shook on Mar 09, 2011

    Thomas,
     
    If you don’t have a Hobbs, log the clock. Otherwise, I guarantee you’re missing out on lots of loggable time. My club has an airplane with a working Hobbs, an airplane with a broken Hobbs, and an airplane with no Hobbs whatsoever. We’re billed by tach time, and I log the clock.
     
    For a period of time when I first joined the club, I paid very close attention to the ratio between Hobbs and tach time for various flights. It varies depending on both the airplane and the type of operation. If you’re on a VFR cross country, you’ll be at high power settings pretty much the whole time you’re in the air, and your ratio will be fairly low (the lowest I ever saw was 1.07:1). At the other end of the spectrum, in the same airplane I would often see a 1.4:1 ratio on instrument training flights or hops around the pattern. Commonly, cross country flights would be around 1.2:1 and training flights in the 1.3:1 or 1.4:1 range. All this was on an airplane with a tach that measured in hundredths so it was a fairly accurate comparison.
     
    Like I said, though, it depends on the airplane too. We have a newer G1000 airplane that only has tenths on the tach, and I had one flight where I did three hops around the pattern that was 0.6 on the Hobbs and only 0.2 on the tach! (Yes, that was the plane with the working Hobbs.) 
     
    The bottom line: I have *NEVER* had a flight where Tach time was more than Hobbs time or Clock time, or even equal. So, YES you are cheating yourself out of a lot of loggable time if you’re logging tach time. Check your watch at startup and shutdown (I added this to my checklists) and log that.
     

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  8. Bill Strellson on Oct 30, 2012

    One option to consider if your plane doesn’t have a built in meter, are handheld GPS units. Some of them offer logging of flight time. I have seen a Garmin unit that did that. Or if you don’t have a handheld Garmin like me, there is an iPhone app called ‘Flight Time Logger’ that also logs your time automatically. It gives you both the OUT-IN times (called Block time in the app) and the OFF-ON time (called True Flight Time in the app). You need to first enter your stall speed, and then the app uses GPS speed and location to determine the times. The good thing with the iPhone app versus built in loggers is, that you can carry the device home and don’t have to copy the times onto paper in between. The app also counts how many landings you have done.

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  9. george on May 21, 2013

    Hello, i recently purchased a n registered aircraft but i live in trinidad and tobago. My question is 1. Can i log flight time with my aircraft with a Faa certified instructor here in trinidad? as i am learning to fly.
    2. What is the procedure? Do i need to sign up with a us flight school? and then fly back to trinidad and use my n registered plane with a FAA instructor? and then fly back to the us for the exams and ppl? Please email asap cbstltd@gmail.com

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