Fuel Starvation in Turns

I have a PA18AS equipped with Piper and Micronair spray gear. The aircraft is used commercially to spray trees on cottage lots in Canada's lake playgrounds. On occasion, small target locations require repeated and extended high degree banks in the same direction. I learned the hard way that banking and fuel tank selection can create a potentially lethal situation. Example: if you do an extended left bank of approximately 30 degrees or more with left fuel tank selected, fuel will not load the off-set canister sufficiently and will drain causing fuel starvation and engine failure. A pilot died in a super cub up here recently circling an off runway landing site several times. A heads up for anyone driving a cub. I'm new to this site and apologixe if this issue has been addressed before.

How do you figure that high bank turns can cause fuel flow problems? When you are spraying, do you do lots of uncoordinated high bank angle turns near the ground?

I don't think that the fuel system really cares if you are in a banked turn or not, as long as the turn is coordinated. The header tanks should provide enough fuel to cover you in a uncoordinated turn, if the fuel sloshes away from the fuel pickup for a short period of time.

Whenever I am flying close to the ground, I always select the tank with the most fuel. I have always been under the impression that the left tank will flow better than the right tank, especially in steep climbs. So my normal procedure is to take off on the left tank, and when in level flight, switch to the right tank. Then the left tank is the one with the most fuel later in the flight. I also usually switch tanks when they get to about 1/4. I ran out on the right tank in the circuit once, when the float still looked like there was some fuel in the tank, switching tanks got everythink under control.

On a standard PA-18 fuel system, I've heard, NEVER seen it printed, but have heard that you should land and take off on the LH tank only. Any truth to this?? I agree with the above posting, if you're coordinated in the turn, fuel flow should be fine. In a coordinated turn, the forces are pulling you straight down thru the seat........same forces are being applied to the liquid (fuel) in the tanks. Just because the wing is down..doesn't mean the fuel flows to the lowest portion of the tank.
Brian

The right tank only has a fuel pickup in the rear of the tank unless it has been modified. The header tanks are supposed to take care of unporting a fuel pick-up. The Pacer/Tri-Pacer has had this problem of fuel starvation because the rt. rear fuel pickup goes down the rear doorpost, under the seat, up to the panel across and down to the fuel selector on the left side of the fuselage. I know people who have eliminated this problem by straigtening out the bends and giving a more direct path with less spots for air to get trapped.

Steve

Tanks

Another good reason to go with the headerless system. Just step back and look at the stock system. It's not all it's cracked up to be, no pun intended. Feeding out of both front inboard corners and both rear inboard corners of the tanks, you have 4 x 3/8" lines going to two T's then right into the selector. On "BOTH" you have all four corners of the tanks feeding the engine. This makes a lot of sense to me, am I talking to my self? There is just no way to starve the engine if you have fuel in the tanks! Crash

Yes, having both wing tanks feeding out of all four corners will keep fuel flowing to the engine in normal conditions, but so will a headered Cub. You could skid a headerless Cub in one direction long enough for the fuel to cross feed to the low wing and vent out the cap. Is this really nessessary?

It is fixing a problem that in the real world, doesn't exist. The headered system has been around a long time, operating with few problems, other than a few old persistant wives tales about how it works.

I've crossed up a Super Cub in some very interesting conditions, and it always feeds. Worst condition, feeding right tank, right tank low on fuel, skidding steep right turn. You need to hold this condition for about 3 minutes before the engine quites. If there is some condition you think will make a headered tank Cub quite any faster, let me know, and I'll try it out. I know that with my system, if I switch tanks, the engine will restart almost instantly.

However, I can't say that about the headerless system, and in the same condition, the headerless system may not even make it the 3 minutes. Yes Crash, I believe you can easily starve the engine with fuel in the tanks. Here's how....

For the sake of arguement, lets hold the headerless Cub up to the same scrutiny. Take your headerless Cub with both, or at least the left tank low on fuel, put it into the same steep skidding (top rudder) right turn and hold it there. All the fuel that in feeding the engine from the left tank is ALSO feeding through those big 3/8 lines, T's and vents into the right tank. Depending on how much fuel was in the left tank, at some point the last drop is going to cross feed into the right tank, and guess what?, the engine quites. Now you can't just switch tanks to a header that is full of fuel to restart the engine because you don't have any headers. All you can do is level the wings and wait for the fuel to start flowing from the right tank, through all those lines, all the way around the cabin, hope like hell it doesn't air lock somewhere, and restart the engine.


With the headered Cub, I know how long I have before it quites, and I know that when it does, I can get it back under power almost instantly.

With the headerless Cub, I know it's going to quit, but have no idea when, and when it does, I have no idea how long it will take to restart. And if it doesn't start quick enough and I crash, I have all those extra fuel lines wrapped around me that can break and soak me with fuel.

So what is safer? The headerless system tries to fix a problem that really isn't a problem, and the reality is that it creates a whole bunch of new problems.

There is another flight setup that makes the headered fuel system look better.

It goes like: flying on the right tank, low on fuel, begin a steep descent with full flaps. This could easily unport the tank, now you are just on the header tank. I imagine that the header is sized for this possibility, and you have several minutes of fuel available, while with no header, only the fuel in the lines.

Now go to the left tank, same situation, but wait, doesn't the left also feed from the front of the tank! Much less of a problem, and the header makes it even better.

If this analysis is correct, then take off and land on the left tank, and fly on the right tank until it is low. I will ususally run the right tank down to about 1/4 and switch to the left.

One question, at what fuel level would the right tank go unported during a full flap descent? Worst case would be max flap speed, and flying uncoordinated with the right wing low.

I just don't see the point on going to a headerless system, the stock system works fine, and I can manage the fuel much better that a cross connected system.

The unported condition in a full flap descent isn't much of a problem because to maintain this condition for any length of time would mean that the engine would be at idle or close to it, using very little fuel, and would run for a very long time on the header alone.

M1, the way you said you manage your fuel in your previous post is exactly the method I use, and it has always worked just fine.

This is a good thread!

thanks!

sj

Fuel Systems.

I agree with Super Cub MD, and M-1. I removed the right tank header last rebuild, but I also canted the tank in the wing as far as it would go. On a long steep descent it will unport with 1 1/2 quarts in the tank. I anticipate this and run it dry before the descent ever starts. My left tank has only one gas outlet, so I ran the header vent to the bottom of the sight gage, which takes care of the long descent. I can run it out of gas with half tanks however, with a flat prop and a very steep climb. The carburetor is the highest point in the system and it just quits like you threw the mag switches. The last thing I would want, is to be in a tight situation and have a little unmeasureable fuel in two tanks. re: Tales of the PA-18 / Case of the missing gas pump. However for those pilots trained in a both system like a Cessna, and who land on flat runnways with half tanks, the both system would probably be a lot safer. I believe that is why C.C. builds them the way they do. It's called INSURANCE. Have a Great Holiday. Jerry.

Nice topic to think about.

I do a lot of landings in sloped glacier spots. Usually the U turn on ground I would execute to the left and than stop the plane about 90° to the slop. The right wing points now up to the sky the left one down to the valley. Slopes my be as high as 20%. I sometime do hold that position for hours to go for a walk or skiing.

I always asked myself when coming back where is my fuel now? It seems to me that some does really shift from the upper tank to lower one. But I never found all the fuel in the lower one. If I cannot see the ball floating on the lower wing side I still guess its safer to use the upper tank for restart and start run because I think that the system is fed from the inboard corner of the upper tank better than of the maybe fuller lower tank. The inboard corner of the lower tank in fact could lay dry and not feed the header tank which exactly may bee consumed when I have completed the warm up and try to get into the air.

Never has this thing addressed by a instructor of mine so far. So I like to ask your guys, are I am right on that?

Hans

Excellent question Hans, yes, I believe you are doing everything right.

If you do actually have any cross feeding with the stock system, (there shouldn't be any at all), it has to be leaking through the valve internally. If it is, have the valve checked for proper function.

I'd make the left turn on top also. I would try to make the landing on the right tank, ensuring before landing that there is enough fuel in the right tank to do the warm up and take off. The right tank will feed better when parked on the slope. There really is no reason you couldn't change to the right tank while parked, it's just a old habit that I don't like to mess with the valve emidiately before takeoff.

Headerless

I have run my airplane with the headerless system and my other plane with the header tank system out of fuel on one tank then switched to the other tank and they both react the same, instant return of power. When parked on a slope, if you set the fuel selector 1/2 way between one of the tank selections (left or right) and the "OFF" position, or just the left or right tank only, it will not crossfeed. Like I said before, I have owned both systems and like the headerless system better. I like that it also weighs less, 4lbs (rear header tank full of fuel) less way aft of the CG, provides a lot more room under the panel (front tank removed), you can build your metal head liner with the back straight up and down instead of canted forward 5 inches (for clearence of the rear header tank). This allows for more room in the baggage area and also with the 3rd seat conversion your rear passanger dosn't have his neck cranked forward. Anyway to each his own. Personally, I won't build up another Cub with header tanks. Crash