Fuel Draw

[km74]

If you are drawing from both tanks and one is drawing down faster than the other, when the one tank runs out will that cause any problems with vapour lock or anything else? I usually run with my fuel selector on both, and have never come close to this point but I am wondering what would happen. Does anyone have any experience with this? Thanks

Ken

[zero.one.victor]

My right tank empties quicker than my left. I have one worry,other than one wing becoming heavier than the other. If the right tank becomes empty inflight,and I make left traffic to land when I get to my destination,it's possible that the left tank outlet will suck air in a left turn. Remember,the right side is already empty.The engine could quit. To complicate things,in a ragwing the fuel line runs from the tank outlet up to the forward doorpost and then down. If the fuel level in the tank is below the high point of the line,which is where it turns to run down the forward doorpost,it will not feed once the siphon is broken in that line.You would have to dive the airplane in a nose-down attitude to get the fuel flowing again.
When I drained the tanks & had the wings off,I had to put about 15 gallons in the left side ( I have the extra left-side tank for a total of 25 gallons per side) AND elevate the tail to a level attitude before the fuel started to flow in the line.
I think this is why the ragwing has to have the fuel pump to be legal,though I have my doubts about the pump being able to put enough suction on the line to get the siphon going again without the fuel level being above the high point of the line.

Eric

[Watkinsnv]

I don't know about the 170's but the Cessna 140A that I had, the right tank would run empty and when the other got lower they would equalize. Lance

[GAHorn]

When running on both, as one tank lowers the other tank head pressure will tend to feed faster. If one tank does nearly empty the opposite tank will feed fuel into it with the fuel selector on BOTH. In other words, if one tank has more than an unuseable amount of fuel in it, and if the airplane is flown "coordinated" (with the ball in the center), then both tanks will share that fuel. Even if a tank is below it's useable fuel level I would want to make that fuel available as it sloshes around, if fuel supplies were low. I believe that when fuel supplies are low, that the safest mode of operation is with the fuel selector on BOTH. Remember that it is a limitation (required placard) that BOTH tanks be on for takeoff and landing in this airplane. Momentary uncoordinated flight is not a problem due to the length of the line and the fuel within the gascolator and carb fuel bowl acting as small header tanks.
(I'm told one reason ragwings feed faster from the right side is the increased head pressure from the greater quantity of fuel in those two tanks. When the total head pressure equalizes the two wings both feed again. I don't know if that's exactly true or not, but it sounds sensible.)
One reason the raqwing has the fuel pump is to overcome the forward fuel lines. That pump has more than adequate suction to deliver fuel.
If Eric had simply tried to start the engine, after the engine turned over 30 seconds or so the fuel would have been delivered via the pump.
The original reason for the prohibited takeoff warning below 1/4 tanks was due to the ragwing fuel line system running through the forward doorposts. In a descent such as when landing it shouldn't pose a problem but I wouldn't want to find out during a go-around. I'd keep the deck angle (pitch angle) pretty shallow and flat if I had to go around with minimum fuel in a ragwing.
As an observation and sidenote about the red area/no take off warning on the fuel gauges.... This is not an aircraft limitation. Notice that such a limitation is not listed in the type certificate, nor is there a required placard regarding this listed. It may be good practice. It seems like a good suggestion. But it's not a limitation.

[N2540V]

George,
" (I'm told one reason ragwings feed faster from the right side is the increased head pressure from the greater quantity of fuel in those two tanks. When the total head pressure equalizes the two wings both feed again. I don't know if that's exactly true or not, but it sounds sensible.) "

Ragwings with four tanks (two in each wing) still have the same fuel flow issue. I cannot see any differences in the fuel lines that can account for it.

Jim

[GAHorn]

Well, I'm sure glad I covered my A-- with that disclaimer!

[David Laseter]

On my 55 170B the left tank always empties considerably faster. I've thought I must habitially fly left wing high. I've been trying to figure this out for awhile. Don't think this is related but lately, I've found debris in the right tank sump on pre-flight. Can't seem to get rid of it. Looks like white crystallized something, like pieces of fertilzer? Anyway I'm going to flush that tank soon. Anyone ever seen this stuff?

I recently saw an inflight check-list that read, upon reaching cruise - switch to right or left tank.
What is the reason for doing this?

I've been switching to the right tank to balance the weight in flight, but this makes me nervous for 2 reasons.
1. That I'll forget to switch back to Both
2. The selector switch will wear out and break. Don't laugh at that, it happens I just read about a pilot whose fuel handle sheared off in his hands - leaving the valve stuck between tanks - on a flight over mtns. He landed a Bananza gear-up in a tight valley.
I've set my GPS up to give me a message, when it's initially turned on and every 15 mins thereafter, reading "Switch to BOTH Tanks".

I've never seen my tanks. Is there one 21 gal tank in each wing? and
(I don't care if you laugh) What's up with only being able to use 18.5 gals useful? I've forgotten why the last 2.5 gals is trapped in the wing?
Dave

[GAHorn]

Useable fuel is defined as fuel that is available at all flight attitudes. Since the 170A/B only has one exit line in the middle of the inboard end, a very steep nose up/down may deprive the user of up to 2.5 gallons. In straight and level flight it's possible to get all the fuel consumed, you just can't plan on it.
The white crystalline stuff could be anything. (I suspect someone has re-coated your fuel gauge float with incorrect shellac/varnish, but it could be corrosion or anything.) You should get it out and also drain/clean your gascolator.
Your fuel system requires at least one vented cap per AD. Normally that cap is installed on the right side, and that is why most airplane's right tank fuel levels drop first. Perhaps your cap is mistakenly installed on the left.
Other reasons may be because you are flying out of rig, or any of the other reasons that Cessna's sometimes appear to feed faster from one tank before the other. In actual fact, if flying on Both, both tanks feed the engine equally. But it may appear to be feeding from one tank faster because of fuel "transfer" through the single static line overhead which is centrally served by the gooseneck vent. If the gooseneck vent is clogged this can be exacerbated. What happens in those cases is that fuel leaves both tanks equally, but because one tank is vented by it's cap, then fuel can be siphoned through the vent line over to the opposite tank thereby refilling it. Therefore it appears the unvented tank is not draining. After a while the fuel level in the lower tank is sufficiently low that the siphon is broken and both tanks then appear to feed.
To confirm your gooseneck is clear and open to both tanks, remove BOTH caps FIRST, and use a siphon blow-gun connected to it drawing air. Listen at each fuel cap for the movement of air. This is preferable to simply blowing into the gooseneck because it won't introduce dirt to the system should one be blocked, and it won't endanger the fuel tanks/lines to rupture. If you don't have/can't borrow a siphon blowgun, then you can use a long piece of pvc hose that will just fit onto the gooseneck and hooking the other end to a funnel, use it as a speaking-tube to while listening at each fuel filler. (Whispers work best.) It's better to have a friend assist you in this latter attempt, so that he can stand beneath the wing in order to give greater audio defferential you can more easily distinguish (and also to avoid appearing to be talking fuelishly to youself.) (sorry-couldn't resist)

[David Laseter]

Thanks George!
I'll flush that tank / gascolator and check the fuel vent system when I get home. The wings were reskinned and I'm thinking something was done with that tank as well, just before the crystallizing debris started. I'll look back in the books.
Sure seems like there's always something! Flying back from Thanksgiving at Heber Springs, last week, the electrical system crashed. We finished the day being so thankful it didn't happen 20 hrs ago when we were SO dependent on the GPS, between Watson Lake and Fort Nelson.
Dave

[David Laseter]

George, I flushed that fuel tank real good and also pulled the gascolater and the srceen was clean. Nothing in bowl. Before I pulled the bowl, the gas drain was in front of the brace. When I put it back together, I turned the bowl to the back, so the drain is behind the brace. That way, since the bowl is tilted to the back, water would be right over the quick drain. Hope that's OK???
Thanks again for recommending the 6" safety wire twisters. I'm going to town with them. Drained both tanks!

[Dave Clark]

Dave

My drain is on the rear side of the gascolater.