My experience running a tank dry.

[Ron77]

Originally Posted by RV8JD

From AFP:

"WARNING

Airflow Performance and Bendix/Precision fuel injection systems are non-returning systems. In the event that a tank is run dry in flight, an air lock will be formed on the out let of the pumps. It is possible that the auxiliary pump will not pick up fuel, as the auxiliary pump cannot create enough air pressure to over come the flow divider opening pressure, thus displacing the air and resume pumping fuel. It is not recommended to run a fuel tank dry in flight without adequate testing and proper documentation of the procedure for this operation."

So try it safely before you need it.

So what is the conclusion about this warning. I really do not want to have an off-airport landing just because I unported a fuel pickup. Do I need to add a purge valve?

Thanks / Ron B.

[F1Boss]

Fellas, I had a C45 for ten years - that thing had 5 tanks - 6 if you really know the plane. My std OP was to take off on the mains (75gal ea) and swap to the nose tank (48gal) in the climb. So somewhere around 1hr I had to be ready as both engines were running off the same tank. I always caught that one, but the next tank was the aft wing tanks - with the L tank feeding the L engine and the R tank feeding the r engine. Both aft wing tanks held 25gal, so if done correctly, both engines would quit at the same time. The way to outsmart yourself is to switch one engine to its main for 5 min, then back to the aux, so at least BOTH wouldn't quit at the same time...except they did once: I had a non-familiar CP flying the thing at about 10500MSL, and my usual CP was in the left front seat. When the rt engine quit (right on time, I might add) my Fancy CP threw up his hands and said YOU HAVE IT! I put his hands back on the yoke and told him I was gonna restart the rt engine. My normal CP in the PAX seat started laughing - he said don't you notice something? I said no, not really - he said 'there is no yaw! BOTH engines have quit!' and continued laughing. Now my Fancy CP was really worried until the rt engine came back to life - displaying its power with a large yaw - and I got another YOU GOT IT! I put his hands back on the yoke and I told him I was now gonna start the LEFT engine and we would continue on our trip to OSH. I also asked him to put his feet on the rudder pedals..oh did we have some laughs about that trip - still do.

And I still run the left tank dry when flying cross country: the return to is the right tank.

My point is: run 'em dry so you know exactly what you have left - AND what's the point of carrying fuel around that you don't use? Most of the time it is a non-event - especially when you catch the fuel pressure needle wobble and do it without letting the engine quit.

I doubt you will have as much fun was we did on that OSH trip, but you get used to the quiet when the tank runs dry..

[odens_14]

Originally Posted by Ron77

So what is the conclusion about this warning. I really do not want to have an off-airport landing just because I unported a fuel pickup. Do I need to add a purge valve?

Thanks / Ron B.

I too try to consolidate my fuel in one tank when I need max endurance, but this has always worried me so I'm not running till my engine quits.

However, if you know you're about out of fuel in the tank, the fuel pressure will tell you you're out or almost out before the engine quits. The only time I've ran my tank "empty" I was trying to stretch a 700+ mile cross country to non-stop with a 30ish knot tailwind. I was watching the fuel section of my EFIS pretty close once I got below one gallon on the display. About a minute after it read 0 the fuel pressure started behaving erratically, presumably as the pickup was just beginning to suck a little air. That was good enough for me; boost pump on and switch tanks. The engine never showed any indications anything had changed.

[rv7charlie]

I'm another in the 'be prepared & don't sweat it' camp. I'd never tell someone else to do something they can't get comfortable with, but it's a very common practice in some circles. I flew a Swift for a couple of years that had aux fuel. SOP: Take off on main, switch to aux. Watch time, when fuel pressure needle wiggles, switch to main & run boost pump until pressure stabilizes. If you do get sloppy, it just gets a little quieter until you do the switch/boost pump thing, then the engine re-lights on its own.

Note that the equation may change a bit if you're using one of the 'new' boost pumps that use an auto style gerotor or roller-vane pump+regulator/bypass. In the alt engine world I frequent, the same pump is used, but with a separate regulator that returns to the tank instead of looping back to the pump inlet, as is done with the current crop of 'boost pumps'. One frequent practice is to notch the disc in the regulator valve slightly, so that a small quantity of fuel always flows back to the tank even if there's no pressure in the regulator to open the valve. This becomes an 'air bleed' if the pump ever sucks air. The pump has no problem moving air; this just gets the air out of the system quicker so that the pump can pull fuel back to its inlet. That trick won't work with the new a/c 'boost pumps', since they return the bypass to their own inlet.

The 'tank dry' stress can be eliminated by feeding the engine from just one tank. Separate transfer pump to move fuel from any aux tanks. This is how my alt engine (with automotive style injection) powered -7 will be set up. It's a very common setup, both in light planes and (I'm told) in turbine hardware.

Charlie

[dlloyd3]

I've seen the fuel pressure twitch giving time to switch tanks. I have also experienced in the same plane the engine quitting suddenly with no warning. Every time, just switched tanks and 5-7 seconds later the engine would be running. IO-550. A Bonanza has 37 gallon usable tanks. Every time one was run dry it took 40.6 gallons to refill. 3.6 gallons extra fuel on a long trip.
Pretty sure it was a 1965 Mooney POH that suggested fuel management on a long trip was run one hour fuel from one tank, switch and run the 2nd tank dry, then switch back to the first tank. Gotta look that up. Yep, so many words, that's what it says. Download an old POH on Mooneyspace. Used this method for many long trips.
I have not run a tank dry on my AFP injected RV7. Pretty sure it will restart with switching a tank as long as my mechanical pump is working. Pretty sure it will not restart if my mechanical pump is not working.

[KRviator]

I had cause to fly to Queensland and back today (7.5 hours total), and as is usual on long flights ran the tank dry. Only this time, I had the foresight to actually film it to prove there's nothing to be scared of, at least in my -9...

The engine setup is an ECI OX-340S, Sensenich GA prop, Rotec TBI and dual PMags.

The most obvious thing experienced is the loss of the exhaust note but switching tanks brings things back to normal in a matter of seconds. I did film it on the return leg, but it isn't as obvious as this one, as I switched tanks a few seconds earlier. And I am still trouble-shooting the alternator output hunting...

https://youtu.be/PSexkSe8gl8

[rv8ch]

You know your mission better than I do, but don't you want to balance the load a bit by changing tanks every 30/45/60 minutes or something like that? What I do is fly on the right tank while the minute hand is on the right side of my watch (minutes 0-29) and on the left tank when minute hand is on the left side (minutes 30-59). Seems that there is some additional (unnecessary?) risk in staying on one tank until it's dry, then changing tanks. Just my 0.000003 BTC.

[Paul 5r4]

With 41 gallons in the wings, I personally don't see a reason to let it run dry. For me, it's WAY out of my zone of comfort for the engine to sputter for whatEVER reason. I've read these post and learn something each time I'm on VAF. The thought that 6 in one is better than 3 in each and the only fuel that matters is the fuel ABOVE the pickup tube.... good lessons!
I keep an eye on fuel flow and try to keep the tanks close equal. I do think of problems when switching tanks and try to avoid it over vast areas of trees. I'll delay until theres a nice field/airport halfway close.
On a XCTY, I'll run the left tank low and like to arrive with no less than 5 or six in the right tank.... only because patterns are usually left. (I know in coordinated turns in doesn't matter).
I keep track at fill ups of what the red cube says and actual. Usually within 1 gallon and sometimes it's within a few tenths. Gives one the confidence to press on.
A look at times... by the time I've burned 30 gallons or so, for my plane that usually carries me about 3 1/2 hours... 650 miles... down the road. By then, I'll have the red lights going off in my head to empty MY tank!

[MConner]

To get max range, depleting a tank(s) completely has been common for decades.

It cannot be good for a motor to go from cruise power to off to cruise power again. I prefer to switch with a couple gallons left and then if I need max range, when I am within the GPS glide range ring of an airport, switch to the low tank and stare at the pressure gauge. I have been amazed at how long it can take to deplete that last little bit from the low tank. I may have to go back and forth as I overfly several airports but in the odd event I let it quit and even more improbable scenario that it fails to restart, I know I can dead stick. Remember Murphy lives in the ether.

This used to be a much bigger issue with old analog gauges but with our digital fuel flow totalizers, we may be splitting hairs to squeeze that last gallon.

P.S. I felt a little inferior when the RV-10 I bought was a 250 hp O-540 but I have been around a long time and all this debate about hot starting and AFP models and EFI makes my head hurt. It sure is nice to stroke the throttle a couple times and she starts. And if I run a tank dry, I am sure it will run again (barring Murphy’s mischief).

[Ron77]

Originally Posted by Ron77

So what is the conclusion about this warning. I really do not want to have an off-airport landing just because I unported a fuel pickup. Do I need to add a purge valve?

Thanks / Ron B.

Thanks for all of the thoughtful replies about fuel management, but my question is really about what happens if you unport a fuel pickup - for whatever reason. For example, say I had a fuel line crack at the connection to the pickup of the right tank and sucked air until the engine quit before I noticed a problem existed. Could I switch to the left tank and restart, or will the system be air-locked (as suggested in AFP's warning)?

Thank / Ron B.