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M20K - usable fuel or fill to the max


NicoN

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There is a certification requirement that running a tank dry, then switching tanks, the engine restart in 10 seconds for a normally aspirated, 30 seconds for a turbocharged.  I think those were the times, don't remember any other action such as turning on the fuel pump is mentioned.   I bet those would be a long 30 seconds.   I would expect higher altitudes would take longer to restart than lower altitudes.

Many years ago, Richard Collins wrote in Flying magazine, ATC gave him an unexpected turn while at FL190.  He was told a Mooney had run a tank dry and would be descending thru his altitude. He said there were some anxious moments but the engine restarted before going thru 190. Ever the wit, Collins said the Mooney pilot wouldn't do that again.

For me, many times down low. Usually took about 5 seconds to restart, about the time needed to switch tanks, pull the prop back some, pull some throttle, push in the mixture a little and reach for the mixture.  Each time running a tank dry, a little over 40 gallons would go back in that tank, matching the fuel totalizer. Tank was 40 gallons total, 37 useable.  On a max range trip, I always preferred to land with 14 gallons in one tank as opposed to 7 in two.

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The one time it happened to me I was waiting for it, but it still came a little sooner than expected. I think my time to the fuel switch was milleseconds. The engine just stumbled a little and we were back in action. I didn't like it even though it was intentional. I don't think I will be doing that again, the endurance in my aircraft is so long (nearly 7 hours) there is just no reason. I would not try it above 12k at all though, you are banking on being able to restart before the turbo impeller slows down too much. If that happens it is going to be a long wait.

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I do it because I'd like to know what to expect if it happens unexpectedly. Having run tanks dry several times and always intentionally, I'm confident that if it happens unexpectedly, I'll be able to address the issue calmly and deliberately as it's not a new thing.

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I’m NA but I know when the fuel gauge says zero I have 15 until the fuel pressure starts to fluctuate when that happens I have a couple minutes until the engine quits. Having intentionally run the tank dry enough times I know there is no reason to go past this fluctuations there essentially no fuel left. I my J it would surge for a little while before it quit and fire right up as soon as I switched tanks, it had a lot of obvious warning. My S just quits and takes a few seconds to fire up, not much warning so unless your paying attention it’s a little more alarming. It’s nice to know exactly how much fuel you have left a get the bonus 3 gallons of unusable fuel out of the tank you run dry.

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