Running a tank dry??

[Flybeech21]

The above post is exactlly how I operate as well.  Fuel pressure drops a significant time before the engine sputters.  That is the point when I switch tanks.  At least in my airplane.

Not to say that it is the best of ways to do it but, it is the way I do it and have been doing it in similar airplanes for many years.

Not running it dry is a respectable positions as well.

[jlunseth]

Two questions.  One, for the life of me, I can't understand why the "one tank or the other" switch design exists.  Some planes have a "both."  I have seen Cessna's with a "both." I understand that Cessna's have an overwing and therefore are gravity fed, but what is the magic in that as far as the switch is concerned?  Why not have a "both" position thereby eliminating the possibility that some small amount of otherwise theoretically useable fuel in one tank is "unuseable" because the pilot does not want to burn the tank to empty. If the concern is that the pilot should have the ability to isolate one tank in the event of bad fuel or other reasons, then a switch with "Both,""Right," "Left," and "Off" would do it.  And if we can't trust the pilot to not run the fuel out when the switch is on "Both," we can't trust the pilot who runs one tank dry not to forget and run the other tank dry also. 

I don't mean this to suggest either the practice of burning one tank empty or the practice of not burning it empty, I am wondering why not a "both" position on the switch, then the problem goes away.

Two, isn't there an issue with fuel injected engines, that if a line is run dry and fills with air, it may be difficult (or impossible in the time alloted before ground contact) to reprime the system and get the air out.  The POH for my M20K says it might be awhile, especially at higher altitudes where the boost pump is not very effective.

I am in the half hour camp also.  I have flown in turbulence with one tank significantly lower than the other, and every time it is rocked the plane feels like it wants to tip over.  Not a very comfortable feeling. 

[GeorgePerry]

I believe, althought admit I am not entirely sure, that the two separate systems with no interconnect (both) option was designed as a safety mechanism.  With two "almost" entirely separate systems if one fails or becomes contaminated etc, the idea was that hopfully the other is still good, the pilot can switch, get the motor re-fired and land safely.

[jlunseth]

That is partly my point.  The safety mechanism would still be there if the options are "Both,""Left,"and "Right."  The only issue is that the pilot would not immediately know which tank caused the problem.  But balance that against the possibility that a pilot, in the middle of a long trip, might lose track of his/her timing and forget to switch, running a tank dry?  And also that if the fuel in one tank is contaminated the chances are pretty good that the fuel in the other tank has the same stuff?  That would seem to me to be a pretty easy decision for a designer to make.

And in the bargain, you get rid of the whole issue of having to switch every "x" minutes to keep the aircraft balanced.  That alone would be worth doing it.   

Not trying to start an argument, just have always thought it was an odd thing not to do it that way.

[Greg_D]

The omission of a "both" switch on the fuel selector of a low wing airplane is pretty easy to explain if you can understand the physics behind it.  

On a high wing plane, with the fuel tanks in the wing, gravity will always force whatever fuel remains in either tank into the pickup for the selector.  As long as you have fuel remaining and don't have a blockage, the fuel pump will be supplied with fuel because of head pressure from gravity.  

If you had a "both" position on the fuel selector of a low wing plane, and one tank ran empty before the other, the air from the empty tank would get sucked into the pickup for the fuel selector along with a small quantity of fuel from the tank that still has some.  The pump will suck air before it sucks fuel, if given a choice.  This would cause the fuel pump to become air bound and stop pumping.

 

[jlunseth]

Thanks.  I get it.  So the useable fuel would be unpredictable.  It would depend on how well fuel was flowing from the "bad" tank.  It might quite ten gallons early, which might be right about the time you are coming in for a landing.