I'm wondering about a scenario where a P92 has been flying for at least 30 minutes or so, at say 4,000 RPM and 2,000' AGL at standard temperature (I'm not sure if these details matter). With both selectors on, you would then shut off both fuel selector valves. With no fuel coming through the lines, how long would it take for the engine to starve and stop working?

Flight manual for reference: http://www.whangareiflyingclub.com/wp-content/uploads/2012/10/P92-Eaglet-LSA-flight-manual.pdf

  • $\begingroup$ If it isn't in the flight manual, what do you expect us to do? Your not likely to get a good answer based on calculations taken from existing charts if that is your expectation. If you are really curious, try it on the ground. 4000rpm ought to suck fuel roughly 4x faster than at idle. $\endgroup$ Feb 4 '21 at 16:31
  • $\begingroup$ yep that was my next step, but interested in what others like John K have come up with $\endgroup$ Feb 4 '21 at 17:00
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    $\begingroup$ No harm going to a safe altitude over an out of the way uncontrolled airport and trying it out! Most airplanes will run on the ground for several minutes with the valve off, the perfect amount of time to give you an engine failure on take off when the left the selector off, but that's with idle operation most of that time (if you do a long taxi then runup, it'll probably quit during the runup). Pretty sure any airplane won't run more than a minute or so on carb/gascolator fuel at cruise power. $\endgroup$
    – John K
    Feb 4 '21 at 19:03

It comes down to the fuel volume in the gascolator, lines, pumps, and float chambers of the Bing carburetors the Rotax 912 uses. Without knowing the precise numbers, we can make an educated guess.

So say it's burning 17 l/hr at cruise, which is 283 CC/min. I don't know the exact number, but say the Bing carb's float chamber holds 50 CC for a total of 100, and the gascolator/filter on the firewall holds another 100 cc of gas, maybe another 50 CCs on the lines between the gascolator and carbs, and another 50 CCs in the lines running to the shutoff valves. Add another 50 CCs for the pumps etc.

So say somewhere around 350 CCs of fuel is available once you shut off the supply, although the pumps won't be able to draw all of it due to the lack of venting at the opposite end at the valves, and it'll depend on the effectiveness of the fuel pump in creating a vacuum between the fuel and valve as it's drawn. Maybe it'll be able to draw 300, which will run the engine for about a minute, maybe a minute and a half at the most. Maybe quite a bit less.

So I would go with somewhere between 30 and 90 seconds.

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    $\begingroup$ I was pessimistic about a good answer coming out of this, but you have exceeded expectations! $\endgroup$ Feb 4 '21 at 19:10

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