Maule Fuel Starvation

[1:1 Scale]

The problem as I see it is this (and it affects both left and right fuel tanks): a climb out (say maximum performance take off) with 1/3 of a tank or less fuel unports the front fuel uptake. A climb (and perhaps a turn out) enhances the rear fuel line low point that occurs beneath the rear part of the door. On my 220C, fuel tank was pressurized by the forward facing air vent on top of the fuel cap. The pressurized air then rushes down the unported front fuel take-up blocking fuel from entering the line from the rear line. The air continues down the path of least resistance into the carburetor. The resulting engine stoppage is abrupt and sudden. Note that all this happened to me as I was on the left tank.

Safe flying to all!

Peter

Hi Peter,

I'm not an engineer, but I'm not sure this theory works. If the front pickup is un-ported, the fuel level in the front line should still be level with the rest of the fuel in the tank. And as far as the tank being pressurized, that pressure is also acting on the fuel that's remaining in the tank, meaning both pickup points have equal pressure on them, the front just has fuel further down inside the fuel line. The only time the engine would start sucking air is when the fuel level in the lines gets to the same level as the fuel valve.

Think of a closed, circular manometer with a T at the top and a restricted T at about the 4-5 o'clock position. If you fill it half way with a fluid, you can pressurize it from the top T and rotate it, and the fluid will remain level between the two tubes. But if you get the lower T above the fluid level, it will start blowing air.

Likewise, if you have one dry tank, the other has 10 gallons in it, and your fuel selector is on "both", will your engine suck air? I don't think so- the weight of the 10 gallons of gas is going to push the air back to the empty tank (assuming everything is flowing properly).

Your original post caught my attention because during the course of my rebuild, and getting ready to start my engine for the first time, I put 5 gallons in my left tank, but didn't get any fuel flow into the gascolator. I assumed the system would self prime from the weight of the fuel. It wouldn't flow until I opened up the bottom sump drain and "burped" the system, then suddenly the fuel rushed through the selector valve and filled the gascolator. I'm still not totally clear as to what's going on there.

[Dkuber]

This thread is intriguing.

I have not experienced the “fuel starvation” described. However I have circled to land with 6 gal in the low wing which I was on (10 in the other) and had it sputter when I decided to go around. Tried pumping throttle, then I immediately switched fuel tanks, and kicked on aux pump. The go around never happened, I was lucky it was wet mud and spun the plane sideways for the drag. Stopped 90 degrees right at the end.
Man does she slow down quick when sideways! I also decided to install L,R,Both after that.

I have drained my fuel tanks quite a few times, both sides. New dipsticks for different tires, skis, gear as well as repeatedly attempting to have a working fuel gauge. I have always put gas in without ever having an issue that needed to burp it.

What size fuel lines do you have?

M4-220

[yanknbank]

Dkuber, your situation is quite a bit easier to understand. Prolonged descending bank without a both selector could very easily cause this issue. Even with a relatively full tank, both ports could become uncovered, and the engine would quit once the low wing fuel lines were used up. A header tank would likely prevent this.

If not descending, a reasonably shallow bank should keep the port covered still with the higher aoa to keep level.

This is the primary reason I'd like to add a both to mine.

[Dkuber]

Dkuber, your situation is quite a bit easier to understand. Prolonged descending bank without a both selector could very easily cause this issue. Even with a relatively full tank, both ports could become uncovered, and the engine would quit once the low wing fuel lines were used up. A header tank would likely prevent this.

If not descending, a reasonably shallow bank should keep the port covered still with the higher aoa to keep level.

This is the primary reason I'd like to add a both to mine.

Yeah, I get it and completely understand what happened to me. Mine was a quick steep turn and decent. Probably only 200’ over the 500’ strip checking for water and kept it tight probably 50-60 degree turn. I had been circling the other direction trying to keep fuel lines fed, however I flipped for wind and it was just enough time. It did make me swap the fuel selector out and now I pretty much run both unless I am draining a tank for other reasons in flight. I will admit after swapping the fuel valve it took me a while to get used to “both” and not knowing exactly how much I had per side. But I did get used to it.

Just figured I would toss it out there. I know PCH mentioned straight & level. If my low consequence learning lesson helps someone else reading this, awesome.

As I said before, never had to burp mine.
I do wonder if there are different sized fuel lines in some of these birds, could see a smaller line trapping air bubbles. Was this bird factory 220 or install later? Is there a difference in fuel line size/routing between the smaller engine options?
More than most, this thread does get me thinking about potential causes though. Line size/routing/fittings or pinch point, debris etc.
I appreciate y’all’s willingness to share. I know it must have been difficult to write up PCH, but thank you for doing it.

[yanknbank]

Didn't mean to sound disparaging if I did. It's good Intel for posterity. We should all be trying to learn from others consequences, lest we be unfortunate enough to have to learn from our own.

[1:1 Scale]

I have drained my fuel tanks quite a few times, both sides. New dipsticks for different tires, skis, gear as well as repeatedly attempting to have a working fuel gauge. I have always put gas in without ever having an issue that needed to burp it.

What size fuel lines do you have?

M4-220

Thanks for that bit of info. My lines are new, stock size (3/8", I believe). I've installed an Andair fuel valve with the "both" position, and plan to use that setting for takeoff and landing, but also intend to at least understand what's going on with the initial fuel flow after filling from a dry tank.

[PCH]

Hi Everyone:

Thank you for taking the time to make your posts. They have been very illuminating.

Regarding the fuel lines, as far as I could tell, the fuel lines were all original 1972 with the under seat modification having been made. Kirk in his post references Trimmer's fuel system for the Pacer. Interestingly in the illustrations, it appears that the fuel line forward of the fuel selector is a larger diameter than the lines going into the selector. On my Maule, all the fuel lines are the same diameter. I wonder what difference, if any, this makes.

Dkuber's situation is interesting. I have always assumed that I unported one or both of the left tank's fuel inlets on climb out and turn to crosswind after doing a touch and go. Dkuber's situation makes we wonder if I didn't unport on my turn to base or final thus putting a bubble into the line and thus setting up the fuel system for a failure on climb out. 1:1's comments on having to "burp" the fuel system before getting fuel to flow is also interesting. It makes me wonder if trapping an air bubble and given the right conditions, that would be a set up for a total fuel starvation scenario. Upon inspection, my fuel lines and gascolater forward of the fuel selector were completely empty of fuel. By the way, Dkuber's incident is the third one I know about that occurred at about 6 gallons. 6 gallons is just under 1/3 of a tank. which is the magic number of the Pacer AD.

The reason for my original posting is to hopefully allow someone to avoid having had the same accident I had.

[Dkuber]

FYI
My engine “recovered” and ran smooth a second or so after I stopped sliding. I am pretty confident if I had tried to take off any longer it would have died from low fuel pressure. It almost died as it was. I had been cautious about the fuel being drained being in the high wing, then a momentary change in plans almost turned bad. Get the both fuel selector.

Same here, just hope to help someone else before they get the experience.