Could you please explain how this counter works (in the picture)? I thought that it starts with propeller (so propeller starts moving so this counter), but seems it registers the pure flight - from take off to landing (is it so?). I just started lessons and of course I‘ll ask FI, but since I’m doing my homework now - your advice would be helpful. I don’t see this gauge in POH.


  • $\begingroup$ Hi Sergey, where did you find this picture? My DA-20 POH is buried somewhere so can't check, but my memory coincides with your statement that it does not exist there. $\endgroup$
    – Jpe61
    Feb 11, 2022 at 12:21
  • $\begingroup$ Hi. It exists in our flight school aircraft for sure, but you are right it’s not in flight manual. $\endgroup$
    – Sergey
    Feb 11, 2022 at 12:22
  • 1
    $\begingroup$ Picture is from school materials. $\endgroup$
    – Sergey
    Feb 11, 2022 at 12:24

3 Answers 3


Your photo is not very clear but the gauge appears to be nearly identical to this VDO Hour Meter with sweep second hand. VDO is a German company and “betriebsstunden” means Hours of Operation in German.

The factory installed hour meter in DA-20 aircraft is prone to failures and I suspect your flight school added this VDO hour meter as a replacement. Most single engine aircraft use engine oil pressure to activate the hour meter.

enter image description here

  • $\begingroup$ THank you! Yes, it's this gauge. $\endgroup$
    – Sergey
    Feb 11, 2022 at 18:33
  • $\begingroup$ Dang you mr. Sowsun, you beat me to it. This meter type is actually a rather old design, found a picture of one in a 25 year old excavator, but hey, if it ain't broke, don't fix it 😃 $\endgroup$
    – Jpe61
    Feb 11, 2022 at 19:45

That would in all likelyhood be a Hobbs meter. Hobbs meters are used to count operating hours, and there are actually quite many different ways it can be activated:

  1. master switch is on (plane is "electrified" but engine not necessarily running)
  2. alternator switch is on (engine is running and alternator is switched on)
  3. oil pressure (engine is running, no need for electrics)
  4. airspeed sensing vane (airplane is running, but not necessarily airborne)
  5. a pressure switch attached to the landing gear (airplane is airborne)

(list adapted from Wikipedia)

All of the above serve pretty much the same purpose, to keep track of the operating hours of the plane, but each has it's own "niche". For example verification of engine hours is important with regard of service intervals. Airspeed would give a pretty good reading on civilian flight hours, counted as from "stop to stop", "airborne" method would track "military flight hours" as they are measured as actual flying time (at least in my jurisdiction).

The specific thing with these meters is, that they have no user interface, so the pilot (or any other person for that matter) cannot tamper with the reading. The count is accumulative since the plane left the factory.

As for which type the meter is in your specific case, cannot be determined with the information you have provided.


That is a Tach Hour meter, if you look close at the image it even says "Tach Hours" under the number read out. This measure engine "tach" hours. Typically they are normalized to the cruise RPM of an engine. In other words if your DA-20 has a cruise RPM of 2300 then when you are running it at 2300 RPM the meter will work on a 1:1 ratio of hours. If you run the engine slower, the meter will clock hours slower and if you run it faster it will clock them at faster than your base rate.

There is a good overview on the different between tach time and hobbs time here.

Real world example from my past few flights. My club charges per tach hour so here are two examples of bills:

I went up to do some pattern work the other weekend. I put 1.1 on the hobbs but spent a chunk of time taxiing on the ground and coasting into some nice landings so I only ended up getting charged for 0.2 hours on the tach as I was well below cruise RPM for most of my flying.

On the contrary I took the same plane on a long XC a few weeks before and put 6.2 hours on the plane round trip and somewhere in the range of 5.9 tach hours since most of the flying was at cruise power.

  • $\begingroup$ In other words, this counts engine revolutions and (mechanically) divides by a figure that represents cruise revolutions per hour. $\endgroup$
    – Zeiss Ikon
    Feb 11, 2022 at 16:47
  • $\begingroup$ When I squint, I see “engine hours”. It is highly unlikely this gauge is showing tachometer hours that vary with engine RPM. It is most likely an “Hobbs” style hour meter activated by oil pressure. $\endgroup$ Feb 11, 2022 at 18:12
  • $\begingroup$ Thank you for detailed answer. In our club plane is charged based on this gauge. I have block time for FI (it's clear) and this one which was not very clear to me. $\endgroup$
    – Sergey
    Feb 11, 2022 at 18:31

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