"Tach time" on a mechanical tachometer, is the number which corresponds loosely to operating hours of the engine, because it has counted engine revolutions. As Peter says, they generally read accurately at about 2300 RPM, and often say on the instrument case.

"Hobbs" meters, are simply electric clocks. They run when the electricity is turned on. This can be by the master switch, an oil pressure switch on the engine, or in a few cases, by a weight on wheels switch. If you're using the Hobbs reading to log time, it would be wise to know which of these hookups is used in the plane you're flying...

Are you sure these are called "hobbs" meters? From what I understand a Hobbs meter is simply a clock that starts counting when an electric current is applied to it. And then it counts "true".

Whether that electric current comes from the master switch being activated, or an oil pressure switch, or a landing gear switch, or an air pressure switch in the pitot-static system doesn't really make a difference as far as the hobbs meter is concerned. (Although it does make a difference to the pilots wallet.)

But a device that counts engine revolutions, and runs this through an (adjustable) set of gears so that it seems to run "true" at a certain RPM is called a tachometer, as far as I know.

To cool the turbo for one ;)

I use the flight log from the GPS for the airborne time. Starts logging at over 30 kts and stops at 30kts. Very easy.

Would not work for some of us. Depending upon the wind it would be stopping and starting a lot ;)

I've flown some stuff where I needed to put the nose down to get 30 knots!


I'm afraid that I'm very old fashioned - I write town engine start, brakes off, take-off, landing, brakes on, and shutdown time.

I then log engine time in the engine logbook, brakes-off to brakes-on in my flying logbook, and whatever is in use in the tech log.

GPS is nice, it gives me a cross check that I didn't make any stupid mistakes.

G

A while ago someone posted the results of some research on here. They fitted a thermometer to the turbo and looked at the temperature profile.

They found that the turbo was actually coolest just after touchdown. After all, on final approach the engine was hardly producing any power but was cooled by a significant slipstream. The turbo subsequently heated up while taxiiing, even at idle or near-idle power.

So "cooling down" the turbo at idle power might actually be a myth.

In any case, the interpretation of the two-minute cooling down period at my club (where we fly with Thielert 1.7 and 2.0s) is understood to include taxi time, as long as during taxi you don't exceed 1400 rpm. Since we generally need more than two minutes taxi time from the runway to the club, no further cooling down period is needed. (So far I don't think we've had any turbo issues.)

Nevertheless, if your POH prescribes a cooling down period you need to do so (I'm not going to tell you to disregard the POH or some other authoritative document), but you obviously do that cooling down not in its final resting place, do you? After the cooling down period you move the aircraft forward (a whole 10cm will do) to its "final resting place after the flight" in the meaning of the ANO. (Same for engine start: After starting, roll the aircraft forward 10cm so that it counts as "moves under its own power for the purpose of taking off" as far as the ANO is concerned.);););)

I use the EDM830, and you can see that even during taxy the TIT is 900-1000+ F. If you idle for a couple of minutes before shutdown then you normally get it down to below 800F, which has to be a good thing. Still I am sure there are a lot of myths, and as long as it is not oilburning hot then should be ok. I then pop the oil filler flap to let the heat out of the engine bay...

Yea, we taxy up outside the hangar, apply the brakes, then sit until TIT has cooled then shut down ;)