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Loss of fuel pressure with 11 gallons in right tank


Pascal B

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Hello, I'm flying a CT 180 as a european Microlight. With a carb-fed Rotax 912 ULS (100hp) .The aircraft has 2 wing tanks selectable left/right or both on. Left tank was empty and selected OFF. Took of on full right tank (confirmed by dipstick), after 10 minutes warm-up on the same tank. After about 20 minutes at normal cruise (4900rpm, all Ts and Ps in the green) at 2500ft AMSL in level flight got a LOW FUEL PRESSURE warning. Switched on electrical back-up (yes, we are allowed to install one of those). No result. Switched to both tanks, shook the aircraft around a bit to slosh fuel and maybe unblock something. No joy. By this time the engine had stopped from fuel starvation and I had to look for a field. A last restart attempt got the engine running at idle but still no fuel pressure and  no power. Managed to glide into a freshly seeded and sodden field without breaking the aircraft despite sinking 2 inches into the mud. E-Prop tips damaged but the rest ,even the wheelpants were ok. After a deep breath, checked the gascolator which was full of fuel.

The aircraft had to be dismantled and transported back to base. After a change of underwear, the search for an explanation began. There were 11 gallons remaining in the right tank. Vent was clear. Fuel flowed freely out of the tank, fitting was taken off, fuel pickup inside was clear, fuel return line correctly fitted. Downstream there is a first filter in the luggage compartment (fuel lines run down the back trough central tunnel, only return lines go back up inside the cockpit frames). Filter was clean as a whistle. Fuel selectors opened and shut correctly, line was free down to the gascolator which is the lowest point of the system. No sediment or water in the bowl, mesh filter clean. Next in line comes the electric fuel pump, which on the bench managed to lift fuel from more than 1 m up at a flow rate of at least a gallon and a half per minute. The mechanical pump comes next in line and also works as advertised. The fuel lines were all replaced in october 2022. The aircraft has since flown around 55 hours, with regular switching between tanks (every 30 min) on long crosscountry flight without any issues. I use mostly 98 unleaded or 100 LL. Temperature the day of the incident was just above freezing, so i don't think it was a vaporisation issue. Aircraft was flown in very high summer temperatures without any overheating/vapor lock problems. So, I am at the end of my wits. One last step will be to inspect the inside of the tank with an endoscope.

I would be grateful for any helpful hints or comments

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Possible that there is a hose routing issue that led to a line getting kinked. Over time it can gradually close up especially if its exposed to heat cycles.

Does the mechanical pump allow fuel to be pushed through when it is not operating? It's supposed to be able to flow some.

Do you have a fuel flow sensor? Is it permitting fuel to flow?

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Yes, there is a fuel flow sensor fitted (red cube) and it allows fuel trough. Same for the mechanical pump which allows fuel flow trough towards the carbs but blocks the return, as it should. I thought about a kink as well but during the renewal of the fuel lines i took great pains to avoid any sharp bends and lines are well secured and cannot move excessively. As for the heat cycles possibility, i have flown at both warmer and colder temperatures without any problems....

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1 hour ago, Pascal B said:

 As for the heat cycles possibility, i have flown at both warmer and colder temperatures without any problems....

I never liked these kinds of logical statements, since everything works until it doesn't. It's not working now, right? You already went for the easy stuff, now it's time to consider more difficult possibilities and things that could have been issues in the making.

Anyways, you said you had a full gascolator. Does that mean you opened it and let it flow for a while and got more out than what it could hold?

I also don't know what a "CT 180" is, but I do know how the rotax fuel system works. Since you lost fuel pressure, but had fuel in the gascolator, that still makes me believe a hose issue or faulty fuel pump.

How did you determine the fuel pumps operating characteristics? How is the condition of the actuation rod that connects with the eccentric ring in the gearbox?

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Even though he has a fuel tank selector the panel and or instrument with his ball may not be perfect and the wings are flat like ours. So just like ours here in the USA many CT owners have one tank that drains faster than the other due to tank design. So like us we may fly 1/2 ball out towards the low wing to keep fuel flow even during flights. If the fuel gets too low like 11 gals. in the right tank and 2 gal. in the left then fly one whole ball out to the left wing. This will ensure fuel for the engine and will transfer fuel from the right to the left if the fuel selector is in a both setting. For us in the USA we don't have fuel selectors on most CT's (the 912iS engine has them).

I this particular case if the fuel selector was on one wing with the fuel and the trim was off even (if the ball was centered on the panel) then this time the fuel was pushed to the outboard part of the fuel tank and away from the inboard part of the wing causing fuel starvation.

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Pascal, do you have fuel sight tubes at the wing roots that you can see while in flight?  If yes, did you get a look at the right tank sight tube when the engine stopped to see if fuel was present at the root?  That would get at Roger's question about the plane flying in a right-wing-low slip when the engine stopped (such a slip might result in "unporting" of the fuel outlet at the wing root due to the effect of gravity).  The more fuel in the tank, the less likely such unporting would be in a slip.  

BTW, what is the fuel capacity of a CT 180 tank?

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Starting with a full right tank and being able to un-port and push fuel away after 20 minutes of flying would be insane for the CTs I know. You'd have to be flying sideways.

But, it's not unreasonable path to think about, it only takes a little air to increase the risk. A twin took off from an airport near cincinnati a couple decades ago. Lost the critical engine on takeoff and Vmc rolled during climbout. They were in a rush and it's believed that they turned a corner really hard during taxi and pulled air into the system that couldn't purge. It was long enough ago that it doesn't show up in google searches so I can't provide the accident info anymore, I don't remember enough details to find it again.

 

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Hi, thank you for the suggestions and questions. The CT180 is the ancestor of the series, with longer wingspan and manual flaps. It was rebranded as CT2K with some improvements. I don't know if it was ever sold in the US but the fuel tanks are the same as in the CTSW, ie 65 litres (approx 17 US gallons) per side. They do have the same sight tubes which show full after approx 7 gallons. And my sight tube showed full when the starvation occured. The aircraft was trimmed for cruise so the ball was centred. Unporting of the fuel pick-up can happen during turns when you are down to the last 3 gallons but not with 11 on board. I did not record the quantity of fuel in the gascolator, without tools,  I just checked the drain valve. There was fuel coming out and not trickling. But I did not leave it running for more than a few seconds as i was standing in a field. Concerning the mechanical fuel pump issue ass raised by anticept, i just checked the pump valves for flow. The mechanical rod point is valid but irrelevant if as in my case the electric back-up pump would have been able to push fuel through the mechanical pump had there been any fuel reaching it. I have had a mechanical fuel pump failure on another Rotax-engined plane and the electrical pump (same type as in my plane) restored normal running within 10 secs.

It is true that things work until they don't. But this plane has spent 1655 hours in the air with the same fuel system configuration. So if one day, things go wrong, you would at least expect to find some smoking gun. I will try Madhatters suggestion of probing the fuel lines instead of just checking for flow. But i think this will have to wait til after Christmas. It is very cold in the hangar...

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Hi Pascal,

"The aircraft was trimmed for cruise so the ball was centred."

People with CT's issue here in the US. We've been discussing this since 2006. Then I did some research and testing. The issue is the wing design, but more important you may fly with the ball centered like they do here, but that doesn't make it correct. The panel and the instrument probably isn't dead on. Many here that have the same issue and lets say the left wing drains and the right doesn't so they fly 1/2 ball out to that low fuel wing. That trims it up even though the ball says out by 1/2 ball.

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I agree that the ball isn’t a precision instrument and I do have the same issue as you guys in that on long flights the left tank drains faster than the right one. Which is why I normally alternate every half-our by shutting each tank off in turn. But here I was flying on an almost full right tank straight and level and even a full side-slip descent would not have unported the fuel pickup.

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Pascal, I'm not familiar with your fuel valve.  You say the empty left tank was selected "off".  Is that how the handle is set, or is the right tank selected "on"?  And if that is the case, I assume fuel can only flow from the right tank?

What was the purpose of switching so that a known empty tank is now in the fuel flow system?  Was that a reflex or trained response or did you have a specific objective in mind?

Is it correct to assume the tanks could cross level only through the drain tubing - no other hose to permit cross-leveling?

Put another way, is there any way that fuel that could not get from the right tank to the engine could get from the right tank to the left tank to the engine?  (I doubt if this is possible and I only ask to eliminate it as a possibility.)

You have posed quite a puzzling problem.

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Hello Jim. A short description of my fuel system: fuel flows from each wing tank down to an E-shaped manifold in the back of the center console between the seats. The wing tanks connect to the outer branches of the E, the center branch to the fuel line going forward to the gascolator. Each of the lines (left tank,right tank,main going to engine has its own shut-off valve. If both tank valves are open at the same time, tanks can cross-level through the E-shaped manifold, which is how I ended up with a full right tank and an almost empty left tank. The hangar floor is uneven and only the main line was shut off  after last flight. Knowing that the left tank was empty, its valve was set to OFF to avoid getting an airlock. I did warm the engine for 10 minutes, take off and cruise for 20 minutes with the right tank fuel valve ON , the main going to the engine ON , and the left tank fuel valve OFF. This is a normal configuration used in cruise flight to balance out the fuel contents because you choose from which tank the engine is supplied and prevent unwanted draining from one tank to another.

The purpose of switching ON a known empty tank is part trained response,part desperation, but I first tried the electrical back-up pump for 20 seconds without getting any pressure. Since evidently no fuel was reaching the engine from the right tank, i figured i had nothing to loose by opening a tank which though registering empty on the sight tubes might still contain a few useful drops. And it took me longer to type this reply than i had to assess the situatioun in the air... Once you are at 900 feet AGL, you have to stop fiddling with valves and pumps and prepare for the landing

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The CT2K had a few models sold in the US. It has the honor of being the aircraft of choice by the FAA to do the initial round of LSA training flights for ASIs and DPEs at Oklahoma City back in the day.

Since you said that opening the gascolator had a steady strong stream of fuel, that means there is no obstruction prior to it.

I still think your issue is after the gascolator.

You said fuel lines were replaced oct 2022. What about the pump? Does it have a dampener ball on top on one of the fuel lines? That has a rubber diaphragm and also should be replaced.

Im thinking maybe something came apart and sent bits of debris. Anything found in the carb bowls? Lines taken apart and scoped internally?

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The carb bowls are clean and the float weights within tolerances. Anyway, the problem announced itself with low fuel pressure warning, not with rough running.The mechanical pump was not replaced at the time the fuel lines were renewed. As it is a sealed type, I cannot open it to check the state of the membrane or the valves without cutting it open. I am unaware of any dampener ball that should be present and I have worked on several non-certified Rotax engines. Still, as I have checked that fuel is able to be pushed through the mechanical pump by the electrical pump, that should have been able to keep the engine running if the issue was pump failure. Since I was able to restart the engine after gliding for approx 1 minute and it ran only at idle, I think that was time enough for some fuel to trickle into the gascolator but not enough to sustain normal operation. So I still suspect that the problem lies upstream of the pumps. I will scope the fuel lines section by section to make sure there is no internal issue that could have acted ass a flap without leaving telltale residue, but that will have to wait until after the holydays😇

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Fuel pressure is picked up at the fuel distributor on top of the engine. It's after the fuel pump, which branches to both carbs, the fuel pressure sensor, and a final line that is supposed to return fuel to the gascolator or fuel tank.

Since you lost your engine power, something has to have clogged or choked off fuel somewhere around here. Since your gascolator was flowing nicely when you tested, that means it comes after it.

It's possible you missed the debris that came out, or again a hose pinched off and you don't see it. It's also entirely likely that the cause was "shaken loose" when you hit and could have been anywhere in the system.

The fuel pumps are part of the 5 year change. The valves and the diaphrams weaken over time. Since it's a diaphram pump, the valves see a lot of movement. Even if you can push fuel through it now, it doesnt mean there isn't a problem, and it seems like you are stuck at the stage of using the "parts cannon"

My other recommendation is never run an aircraft with a tank dry. One reason is you could take in air during a forward slip to land and have engine issues on a go around.

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I certainly concur that i will never again run an aircraft with one tank dry or even below 3-4 gallons. And fitting a new pump as a precaution is cheap insurance. But it is the nagging thought of not knowing what happened that is really bothering me. If you have not identified the danger, how can you know which precautions to take? The battle plan now is to go trough the fuel lines from wing tank to carb system section by section to look for any mechanical obstruction or undiscovered kinks in the areas where the lines are hidden from view. Then put the wings back on, put some fuel in and measure the quantity arriving by gravity at the gascolator for each tank separately. If those values are satisfactory, i intend to run the engine first on only the new mechanical pump, before putting my electrical pump and its bypass loop back in line and then finally do a ground run at cruise setting for at least 30 min. (there are no neighbours...) But this will have to wait until the new year. Merry Xmas to all who have contributed ideas, I will keep you posted about the results.

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6 hours ago, Anticept said:

Since you lost your engine power, something has to have clogged or choked off fuel somewhere around here. Since your gascolator was flowing nicely when you tested, that means it comes after it.

From what he said I don't think his flow test at the gascolator was of sufficient length to determine where the restriction is. A partially blocked line before the gascolator would cause fuel starvation, yet allow the gascolator to refill and provide a solid stream of fuel for just a few seconds.

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i cannot get any decent flow for even a moment if my fuel shutoff is closed. it also flows with a weak stream if it's down to a couple gallons left.

i do agree to come back to this, @skunkworks85 suggestion to check fuel vent might be the answer.

i would say take the gascolator bowl off including screen and put a bucket underneath. have more buckets ready. bypass electric pump. set fuel selector to right tank, watch fuel flow for poor or decreasing flow. let it drain to empty and observe if debris is in the bucket.

repeat for left side. flow should be similar.

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