Fuel selector to off, How long should engine run?

[Michael Duncan]

I have an 1966 260B in which we've just completed the annual. As part of the annual we cleaned up and reassembled our fuel selector. The other day when I was doing the post mx run-up and leak check I switched the fuel selector to the off position. The engine ran for another five minutes and I finally just switched to another tank. My partner A&P says it may take longer to burn all the fuel out of the line. I was running at about 1500RPM. I thought it should shut off much sooner. Could someone please give me some advice on this. Just seems to me that heaven forbid I have an engine fire and my first action is to shut the fuel selector off it should stop all fuel to the engine quick! Thanks in advance for your advice.
Mike

[Pat Elliott]

sitting on the ground with no load, at 1500 rpm you're 'burning' about 2 gallons an hour.
How much fuel is in the float bowl, gascalator and lines? maybe a quart?
Given a quart at 2 gallons an hour= 7.5 minutes.
I had a CFI that would shut off the fuel in a 150 while his student was doing his pre-flight, it would taxi and get to the run up check list before it would die. funny to watch them try to figure out what happened.

but don't worry, a fire will burn that quart alot faster...

[Michael Duncan]

Thanks Pat, I think I just gave up too soon, we have fuel injection so I probably should have went just a couple more minutes. As for the fire part you are soooooooooooo right, I just try not to think about that.

[N3322G]

Mike,

I'm inclined to agree with Pat but should also mention a common failure for Twin, don't know about singles, selectors is when they start to fail, fuel seeps through the fuel selector. If fuel is seeping through your selector, the off position may not be really off.

It should never seep but when it does it is common to see fuel from aux tanks - higher from a gravity view - seep through the fuel selector to the main tanks so Auxes look down and mains look high. When it really lets go and the seep becomes a flow, the main tanks will overflow and you'll see a big puddle of fuel. Um, if you are thinking I've had this experience, the answer would be yes. Lost 5 gallons. Not something you want to see when you are someone else's ramp. But hey, that is what pre-flight is for!

BTW, I don't like fire either but do keep a halon fire extinguisher in the cockpit and weigh it every annual. So far this seems to have deterred any fire.

[Kristin Winter]

A 260B does not have a float bowl as it does not have a carburetor. The fuel injection system should quite fairly soon after shutting the fuel off. I wouldn't think that you have more than a half a cup or so of fuel in the lines. Call it a cup, and you have 16 cups in a gallon. The engine idling will burn at least 2 gph. That is 32 cups an hour, so about half a cup a minute. So I am thinking that it should run no more than 60 seconds, though someone with a calculator handy might want to check my math on that. If it takes longer than 60 seconds at idle, you may have internal leakage in the fuel selector valve.

[Michael Duncan]

Leakage in the fuel selector was what I was worried about and also being able to verify on the ground that I could access any tank without having to have that deafening silence in the air. The fuel strainer bowl is downstream of the fuel selector but I'm not sure exactly how much fuel it holds. Looks like it could be about a quart so I'm hoping I just didn't run it long enough with the selector in the off position. Pat and Kristin thanks so much for your input. Pat I do have a fire extinguisher in the aircraft under the right front seat easily accessible. I think I'm gonna do one more good ground run and this time I'll do two things. First I'll note the digital readout of each tank and make sure whatever tank I have selected thats the tank that is going down. Second I'll let the engine run no more than 10 minutes with the selector valve in the off position, if the engine starts to shut down between lets say six to eight minutes I'll take that as ok. Runs the full ten minutes then I'm pretty sure we've got some internal leakage. If anyone knows the exact amount the fuel strainer holds(I haven't been able to find it yet) please let me know. Once again thanks everyone for your help.

Mike

[JMH23]

Mike,
I have a 260B also. With the engine at a fast idle, it takes a while for the engine to die after selecting off, and while I have not timed it, my recollection is that it is more like a minute or so. I would be concerned if it took more than 2 or 3 minutes.

Joe Hand
N9023P

[Michael Duncan]

Finally made it back out to the hangar yesterday and as I should have known there must have been some operator error involved. Kristin you were right, I made sure when I selected fuel off I was in the detent and sure enough within 30 seconds the engine was shutting down. I had it running at appox 1900RPM and I tried it twice and worked both times. As Joe suggested I also made sure that I didn't have any migration issues from the Aux's to the mains and we don't. Thanks to all who replied your input was as always very helpful. Happy now we are finally getting the annual signed off and if we can get the weather to improve here we'll get our after mx test flt in and then we should be back in the flying business. Yeah!