Citabria - Uneven fuel use during flight

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poetpilot

New member
Has anyone else experienced the following, and if so, was it a fault that was easily fixed?

When we fly a long leg (e.g. say over 1 hour), we notice that the fuel is used entirely from the LH tank.

Once we stop and allow say 30 minutes standing time, the fuel evens out so that both tanks are even on contents.

We haven't yet flown it to a point where the LH tank physically runs dry (though it must have been close to it the other day!). If it runs dry, will it still get fuel from the RH tank ????
 
Which fuel vent system do you have? The most common, and current production, is a vent tube under the LH wing aft of a clear (usually) plastic ice deflector, this vent is tied into a line that goes thru check valves into the outboard ends of each tank the vent line from the RH tank is tied into the system with the line that runs just forward of the front spar carry thru structure at the top rear edge of the windshield. Your RH vent is clogged and MUST be opened before further flight.

Tom-
 
Hi Tom, if you are there,

..the engineer tells me he has checked venting from the RH tank by attaching a low pressure air line, and that if works through to the RH tank (not sure how he was able to determine that but he's the engineer, not me!)
 
It's common to have some degree of uneven fuel useage, if you fly beyond an hour does the other tank begin to feed?

On the ground empty the good tank, fill the one that does feed properly and see if it feeds.

Fly for an hour, land and IMMEDIATELY remove the cap from the offending tank and listen for the hiss of air that would indicate a vacuum.

It still sounds like a clogged vent, try switching fuel caps, the caps are not vented BUT if the entire vent system is clogged and one cap has a bad gasket that tank would continue to feed OK.

Tom-
 
OK Tom, will try those tips. A local engineer (not at my Citab's home base though) told me that this is a common problem with some ohis club's Cessnas.

He's had to drain fuel out through the main engine fuel pipe, watching the levels go down unevenly, but then as they get close to empty on one side, the other side sort of kicks in and bingo, it's taking fuel from the other all of a sudden.

Apparently Cessna put out an AD calling for venting fuel caps to be fitted on older models, but some of the aircraft still do it even with them.

Anyway, we will work away on the problem and will let you know if swapping caps does any good.

Thanks again!
 
The problem I've seen in Cessnas is, as your engineer described, uneven useage. Since the problem you described in your initial post was no flow from one tank in one hour a clogged vent is the likely problem.

Tom-
 
Tom,

I am not a mecanic, nor a computer person, I am not even sure where this memo will end up. I think that if you empty your good tank and you take off with a clogged tank the engine will stop soon because there is no air to replace the fuel that is supposed to go to your engine. So when you open the tank cap and you hear a hiss I think that it is not a good sign. I may be wrong. But if the tank cap is leaking then it will save your flight.
 
We've now checked the aircraft for correct venting (i.e. no blockages in the venting system) and have fitted brand new fuel caps, so we are now monitoring the situation.

At the moment it seems that it's running down evenly both sides, but I have yet to take her on a longer trip to verify.
 
HW-

You are correct that IF the vent system is clogged a non-vented cap that leaks around the gasket may save the day by allowing fuel flow.

However when the vent system is working properly there have been reports of reduced fuel flow from a tank that had a poorly sealed cap because the low pressure air on the upper surface of the wing created a partial vacuum in that tank and inhibited flow.

Tom-
 
HHW, Tom, et al:

Furthermore, a leaking/missing cap can empty that tank in a very short time, again due to the low-pressure area. Although I understood this, I was very surprised when it happened, to find one empty tank within about 15 minutes (over mountains too - thanks to Murphy!)

Regards,
 
Bob-

Years ago my partner left the cap off the Stearman tank after fueling, the stream of fuel being siphoned was immediately visible.

It takes a big hole to siphon BUT even a small air leak can cause pressure/feed problems.

Tom-
 
I had the uneven fuel flow problem the other day on a flight to Stockton, Missouri, in my '72 7ECA. Most all the fuel consumption was out of the left tank for the first hour, then right tank started to feed and the level came down. I recently installed new fuel caps from WagAero, but have not had a chance to fly and test yet. I have not checked for venting problems. I suspected a problem with a good seal on the gas caps. Any predictions on whether the new caps will fix the problem?
 
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