Or look at your watch? Write down engine start time, then engine shutdown time.
That's how we did it when towing banners. Cub's don't have electrical systems
or hobbs meters, and when you're flying along at 1900 rpm's, the tach time isn't
exactly accurate either.

That's the way I do it today, with my watch, though by the puzzled
looks I get when I'm converting minutes to decimals, I think most
everybody else must use the tach.

My first flight instructor used watch time, and as with so many
things, the way you learned it when you were green is the way you keep
doing it.

all the best -- Dan Ford
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