Is there any guidance, official Cessna or otherwise, that states how long at a specific RPM, it is expected that the fuel feeding from one tank,will be exhausted before the newly selected tanks fuel takes over?
I've never seen anything official on this, but to convince pilots of the danger of changing tanks just before takeoff, I often perform the following demonstration (on the ground) which you can try on your own aircraft.
Set the RPM used for engine checks (usually around 1600-1800). Then switch the fuel off, and start a stopwatch. Most people are surprised how long the engine will run with the fuel off.
The last aircraft I demonstrated this with was a Cherokee 180 which ran normally for 60 secs, before the fuel pressure started dropping, then a further 30 secs before it stopped.
I have done this same demonstration in a C172, and the result was about the same, although I don't remember the actual times.
The point I try to make with this demonstration, is that if you change the fuel tank just before takeoff, and the new tank isn't feeding, the engine will run for just long enough to get you airborne,
So, If you change tanks just before takeoff, you need to run the engine, at the test RPM, for
at least as long as you would normally take to test the mags etc. to be sure that it is running on the new tank. (This could be even longer on other types)
MJ
