Fuel going from one tank to another while parked

[hish747]

So the last week I filled both tanks to the 40 gallon tabs on my Grumman Cougar. Took a short flight 30NM flight and put the plane away. When I returned this week. The right tank was half way from the tab to empty and the left tank was almost full. By the way, this seems to have started after I manipulated the fuel selectors to test whether they shutoff, cross-feed etc. The right engine never shut off. The left one shut off but it was a few minutes after shutting the left fuel selector to the off position. This makes me think that somehow the right tank was feeding fuel to the left. But how? Wouldn't gravity force them to equalize? Does this indicate a problem with the fuel selectors?

 

[Bell206]

Does this indicate a problem with the fuel selectors?

Yes. At least from what I can tell from a POH diagram online. However I wouldnt fly it again til you sort this out.

Wouldn't gravity force them to equalize?

No. Per the same POH, the electric pumps are upstream of the selectors and have check valves to prevent any back flow. So the tanks are not directly connected to each other. My guess on your short flight you ran both engines on same tank due to an internal selector crossfeed issue as shown during your ground selector test.

 

[Pinecone]

No, both tanks started at the tabs. One tank ended up above the tab. That is not running from one tank only.

Gravity can do this if the ramp is not level.

 

[Bell206]

No, both tanks started at the tabs. One tank ended up above the tab. That is not running from one tank only. Gravity can do this if the ramp is not level.

I believe he stated one tank was half full and the other almost full after the flight. Regardless, perhaps I missed something. So how would the fuel "gravity" transfer per the diagram below?

 

[TCABM]

Uncommanded crossfeed? No fun, no fly.

 

[Lindberg]

I believe he stated one tank was half full and the other almost full after the flight. Regardless, perhaps I missed something. So how would the fuel "gravity" transfer per the diagram below?


View attachment 114356

Maybe I misunderstand the diagram, but it looks like if one selector is on and the other is at crossfeed, then fuel can flow from one tank to the other.

 

[Bell206]

but it looks like if one selector is on and the other is at crossfeed, then fuel can flow from one tank to the other.

According to this diagram the x-feed supplies are direct to the opposite valve so for the LH tank to get "gravity" fuel would require the RH fuel back-feed through the LH side x-feed line, the LH pump check valve mentioned above also had failed, and the plane was on a 30+ deg lateral incline left wing down. However, I would have thought a bad pump check valve would have given him other fuel problems as well. I simply tried to tie his RH engine lack of shutdown problem to the tank mis-levels to give him a start point given the LH engine system appeared to be working properly.

 

[luvflyin]

So the last week I filled both tanks to the 40 gallon tabs on my Grumman Cougar. Took a short flight 30NM flight and put the plane away. When I returned this week. The right tank was half way from the tab to empty and the left tank was almost full. By the way, this seems to have started after I manipulated the fuel selectors to test whether they shutoff, cross-feed etc. The right engine never shut off. The left one shut off but it was a few minutes after shutting the left fuel selector to the off position. This makes me think that somehow the right tank was feeding fuel to the left. But how? Wouldn't gravity force them to equalize? Does this indicate a problem with the fuel selectors?

There’s one other thing to add to that logic.

 

[Dan Thomas]

According to this diagram the x-feed supplies are direct to the opposite valve so for the LH tank to get "gravity" fuel would require the RH fuel back-feed through the LH side x-feed line, the LH pump check valve mentioned above also had failed, and the plane was on a 30+ deg lateral incline left wing down.

A failed check valve is entirely possible, considering that boost pumps typically have a ten-year life, and most are run to failure rather than being replaced to avoid inflight failures and heart attacks.

The Cessnas would crossfeed with even one degree of slope. It doesn't take a lot, at least in them.

My 1947 Auster had gravity feed through two shutoff valves, and a sensitive check valve in each line to prevent crossfeeding. I wish Cessna had incorporated those check valves. Maybe they would have had to have an engine-driven pump to pull the fuel past the checks, as the Auster did.

The one disadvantage with the checks is that fuel stuck between them and the carb's float valve will expand with environmental heating and cause significant pressure in the system that could damage stuff.

 

[Magman]

The wife of one guy with an Apache would be upset when one engine would quit

due to fuel starvation. It seems the outboard tank would migrate via gravity to the

inboards. These in turn would fill and allow fuel to dump out the vent.

His solution was to add Tip Tanks!

There is an AD regarding the Fuel Selectors on this aircraft!

 

[Bell206]

A failed check valve is entirely possible,

True. But in the OP's context there's more going on than a leaking check valve given the details presented and the system setup.

 

[Mongoose Aviator]

The fuel switches in my Seneca were rebuilt. There were two main symptoms. With a third symptom more as confirmation.
1) On refueling there was a significant difference in how much each wing took.
2) Refueling with top-off and then sitting on the ramp with a very slight incline, the lower wing would leak a considerable amount of fuel.
3) Fuel switch moved to "OFF" position would not actually shut off the engine. Which is not something I normally do, so it never occurred to me to try this until the problem was already mostly diagnosed.
After the repair, everything has appeared to work as expected.

 

[flyingron]

Sometimes valves wear or just don't get into the right position. When I installed the MVP50 in my plane and was flying it back from the shop, I was a little nervous about the new electronic gauges so I kept the legs short (so I couldn't possibly have run the main dry). At my first stop I go to top off the main and find that it is full. Now, I know this is a new engine, but it has to have consumed some fuel. Sure enough, there's 15 gallons gone out of the aux tank. I hadn't fully closed the crossfeed. The control for that had been rerouted while replacing the wing, so it sat slightly different than what I was used to.

 

[luvflyin]

A failed check valve is entirely possible, considering that boost pumps typically have a ten-year life, and most are run to failure rather than being replaced to avoid inflight failures and heart attacks.

The Cessnas would crossfeed with even one degree of slope. It doesn't take a lot, at least in them.

My 1947 Auster had gravity feed through two shutoff valves, and a sensitive check valve in each line to prevent crossfeeding. I wish Cessna had incorporated those check valves. Maybe they would have had to have an engine-driven pump to pull the fuel past the checks, as the Auster did.

The one disadvantage with the checks is that fuel stuck between them and the carb's float valve will expand with environmental heating and cause significant pressure in the system that could damage stuff.

Cessna Cardinals, C177’s are notorious for that. Leaving the selector on Both is a no no. I was a slow learner. It wasn’t until the second time I siphoned gas out of a tank that I got it through my thick skull to put it on either Left or Right. I don’t recall a problem with 172’s.

 

[Dan Thomas]

Cessna Cardinals, C177’s are notorious for that. Leaving the selector on Both is a no no. I was a slow learner. It wasn’t until the second time I siphoned gas out of a tank that I got it through my thick skull to put it on either Left or Right. I don’t recall a problem with 172’s.

The 172 and 182 and 180 will also do it. Most any airplane with a "Both" position on the fuel selector. And many of those Cessna airplanes used a cam-style selector that shut both tanks off when the selector was Off, and did not allow crossflow in that position. Some older ones used a plug-type selector valve that actually allowed crossflow in the Off position even though the fuel flow to the engine was shut off.

 

[hish747]

No, both tanks started at the tabs. One tank ended up above the tab. That is not running from one tank only.

Gravity can do this if the ramp is not level.

Exactly. They both started at the tabs and then about 8 gallons flowed from right to left. I'm sure the ramp isn't quite level. This sounds a lot like what was going on with Mongoose's Seneca. The plane goes in for annual this coming week so this is high on the list of repairs. Thanks for all the helpful insights!

 

[Doug Reid]

The original post leaves me wanting more info. Did the pilot look inside the fuel tanks after landing to confirm levels after the short flight, or only a few days later. Were the fuel selectors turned off after the flight ? If not, what position were they in after flight ?

We have not considered a possible brain f@rt... I would fill both tanks again to same level, repeat short flight, confirming fuel selectors on, not xf, and look inside the tanks to confirm levels after flight. Then, if same result, I would investigate selector valves, etc. I would leave both fuel valves off after flight...that is just my advice.

I fly an old vee tail..5 fuel tanks, two fuel selectors, one fuel gage with three piano key switches to select proper tank on gage. I can easily see how a simple mistake could be made.

 

[brcase]

Fuel switch moved to "OFF" position would not actually shut off the engine

I have seen this issue in a 172. I watched a guy start the engine, taxi the full length of the 5500 foot runway, do a runup, wait a couple minutes at the end of the runway, line up and advance the power for take off and was just lifting off when the engine quit. Fortunately just landed straight ahead. The fuel selector was in the off position the whole time.

Brian

 

[Mongoose Aviator]

Fuel switch moved to "OFF" position would not actually shut off the engine.

I have seen this issue in a 172.

The quote from my post may have been taken out of context as this 172 issue mentioned was in no way similar to what I described.

 

[brcase]

The quote from my post may have been taken out of context as this 172 issue mentioned was in no way similar to what I described.

only related in that we are both talking about internally leaking fuel selectors. An issue that might not easily be found unless you try running the engine with the fuel selector in the off position.
The fuel cross feeding when in positions such a left or a crossfeed position might be a lot harder to detect.

Brian

 

[Dan Thomas]

I have seen this issue in a 172. I watched a guy start the engine, taxi the full length of the 5500 foot runway, do a runup, wait a couple minutes at the end of the runway, line up and advance the power for take off and was just lifting off when the engine quit. Fortunately just landed straight ahead. The fuel selector was in the off position the whole time.

Brian

This is due to poor maintenance.

The fuel strainer should be opened every 100 hours or annually for cleaning and inspection. This requires the fuel selector to be shut off. If the valve is internally leaking, fuel will continue to flow from the opened strainer onto the floor. It might be a persistent drip, it might be a trickle, it might be enough to get the airplane off the pavement.

So the mechanics are obviously slack here. The strainer either isn't being cleaned---and I have encountered strainers that I could barely get apart, bowls seized on by not having been serviced in a very long time---or they ignore the leakage while it's open.

Opening, cleaning, inspecting and reassembling the strainer takes maybe ten minutes.



The 172's fuel selector:



The cam (#4) pushes on the smaller steel ball (#15), to push the larger steel ball (#16) off the seat to open the valve. That seal is a rubber O-ring (#17). That O-ring, one for each side, left and right, ages and deteriorates and shrinks and cracks and crumbles and doesn't seal anymore.

Drain plug (#22) is supposed to come out every 200 hours and the accumulated crud that settles in the bottom of the valve drained out. That valve is the lowest point in the whole system. I have found those plugs in 40-year-old airplanes seized in there tight, never having been removed. The '96 and on 172s have a quick-drain there, and I put quick-drains in our legacy models. I found amazing amounts of goop and stuff in those valve after I got the plugs out. I had to drain the tanks so I could take the valve out to fix everything.

 

[Magman]

I’ve seen what Dan has mentioned.

One of the problems is the plug is square and requires a somewhat uncommon socket for removal. A 9/32 (?) in. square socket would be my preferred tool. Using hex sockets or vise-grips can deform the brass plug and complicate removal.