Fuel Flow Discussion, Moved from Float Mounting

[Nev]

Very interesting. Incidentally, when I was doing fuel flow testing on the ground we discovered something that has potential to make someone's day more memorable than you'd otherwise want. Starting with an empty tank and pouring 20 liters (5 Gallons) into the tank, nothing came out where we had disconnected the fuel line at the firewall. We stood there scratching our heads, checked that the fuel selector was on the tank we had added the fuel to. But nothing. Eventually we discovered that by moving the fuel selector, fuel would start flowing. It appeared to be the action of moving the selector that somehow restarted the fuel flow. It didn't matter if it was from BOTH to the tank with fuel in it or VV, or to any other position then back to the tank with fuel in it. At the time we reasoned that it was an improbable thing to occur inflight. However the scenario that you have tested may have emptied the fuel line with the tank unported and fuel sitting at the end of the tank (ball out to the right). Then when it "re-ports" it's in the situation we discovered during flow testing. It's possibly the situation that Brooks described above.

[TimTall]

I didn't run the low tank all the way dry. There was nothing showing on the gauge in the three point attitude so there wasn't much in the tank, but enough to keep it running. Wings level or empty tank high didn't make a difference. The engine ran with no hiccups. It wasn't until i went wing low on the low tank that I got it to quit, and then only after several seconds.

With such little fuel in the tank it wouldn't take much to unport. I did a few shorter sloshes and there was no issues. The lines would have definitely unported for a few seconds but I guess the fuel replaced the air quickly enough that the engine was never starved. I do have a mechanical pump with a vapour return circuit so that may have got rid of the air instead of sending it on to the fuel distributor.

It seems like there's at least 15 seconds in the lines at low power settings. In fact the math supports it. 5' of AN6 line at 5 gal/hr should give about 12 seconds of fuel. At idle at 2 gal/hr that's 30 seconds. There's about 5' of fuel line above the level of my engine mounted mechanical fuel pump in flight attitude so it seems this is probably about accurate.

[Nev]

I wanted to see what it would take to unport the lines and cause the engine to quit and to see how long it would take to re-light.

I love this sort of testing, excellent work and thanks for sharing.

Did you notice any difference with the ball out to the side of the low tank (remaining fuel at far end of tank and unporting) compared with the ball out to the side of the fullest tank (low tank well ported but possibly completely empty) ?

[TimTall]

I accidentally left my fuel valve in the both position while parked on a cross slope the other day. I ended up with about 12 gallons of fuel in the left tank and about 3 in the right. I figured it was a perfect opportunity for a test.

I climbed out on the left tank to 5000' agl and set up an orbit overhead the field. I then switched to the right tank. In level flight 3 gallons of fuel doesn't even register in the fuel gauge, so I'm running below what I would ever want to see in real world conditions.

I wanted to see what it would take to unport the lines and cause the engine to quit and to see how long it would take to re-light. I did a few orbits on the single tank at cruise settings simulating an out of trim condition. I was right wing low by enough that it was feeling pretty uncomfortable. I held this for about 5 minutes with no signs of fuel starvation.

I was more interested to learn what would happen in an aggressive slip in a landing configuration though. Suppose I'm low on fuel at the end of a flight and I have to slip to get in and I've forgotten to select both tanks for landing. How long would I have before the engine quits, would there be warning signs, and how long would it take to make power again?

I slowed down to approach speed, 60mph, full flaps, idle power. I did a few shorter slips and climbs back to altitude until I got it to quit. It took almost 30 seconds in a maximum slip to starve the engine. That's a very long slip. At least 500 vertical feet. I tried a few at higher power settings to simulate a situation like lifting a wing for a passenger to take a photo where I'd be purposefully out of trim with power on. At a slow cruise setting it took about 15 seconds to quit.

In all cases the engine quit abruptly but smoothly. There was no noticeable stutter or surging. During the idle test it barely even got quieter. As expected, EGT's dropped off the scale and fuel flow went to zero but nothing else changed.

It took what felt like forever for the engine to re-light, in reality it was around 20 seconds each time after returning to level flight. Not a problem at 5000' feet but far too long at 100'. The engine restarted smoothly with just a little bit of surging for the first few seconds. Turning on the boost pump as soon as i rolled level helped by several seconds but it made the restart rougher.

By this point i didn't have much gas left in the almost empty wing. I would like to test if turning to the fuller tank would speed up the re-light. The higher head pressure may help. I was down to almost no fuel by the last test. An extra 6" of head pressure isn't much but it might make a difference. Although in my scenario, reaching for a fuel valve on short final, on what is most likely about to be a deadstick landing, is probably not a good idea.

I'm running a different fuel system to most of you but the fluid dynamics should be similar. The air in the lines needs to be replaced with fuel. In my case that takes about 20 seconds apparently.

Overall I learned that i'll continue to take-off and land on Both tanks but there is some margin for a mistake. 30 seconds is a long slip. Even in the mountains there's not many situations where that would be necessary in a Bearhawk. It slips very well but it can also descend like a rock in a straight line if it needs to.

[Bcone1381]

I agree with Grant....valuable information. I want to know what happens when fuel sloshes. I have experimented a bit with clear tubing in the shop. What follows is a bit of data, experimentation and art of imagination to maybe help me understand this dynamic..

Cessna's gravity system on C-172 and C-182 shows the designers made changes over time. The venting of the fuel lines that exit the tank seemed to be an interest.. The creation of an air bubble must be happening inside a fuel line when fuel sloshes and unports a tank bung. The bubble is an invisiable, silent, disappearing restriction to fuel flow. I see Fuel flow being hitting the bubble restriction and slowly descending around it. A long bubble slows flow considerably.

The Bubble wants to rise up inside a vertical section of the tubing but the fuel flowing down the line tends to keep the bubble suppressed inside of the fuel line. Its worse in a horizontal section of the fuel line. Venting gives the bubble an escape route out of the tube. I imagine the Cessna designers found getting that bubble restriction to move out of a line that is horizontal also to be more difficult than getting it removed out of a line that is vertical. Look where they put the bubble escape route.

This is a C-172N fuel system diagram.

Screenshot 2024-10-28 at 6.59.54 AM.png

[Bissetg]

I think there is valuable information in this. As he described the unporting referred to is a function of the fuel sloshing to the other end of the tank. The mitigation being to keep more fuel in the tanks, both selected (I’m not sure if he had the option of selecting both) and the ball on the middle I guess. I still try and keep an hours fuel up my sleeve for the unexpected, such as having to provide assistance to someone else or act as a comms relay in the event of an emergency of another aircraft.

I’ve been really impressed with the fuel system on the B model and that the gascolator seems to get fuel from either tank when at relatively low levels.

I’ve also found the stronger rudder pedal springs have made it a lot easier for folks to keep the ball in the middle and not over control on the rudder causing said sloshing.

[Nev]

Very interesting fuel and engine failure discussion with Alaskan Bush Pilot Will Johnson.
If it starts at the beginning, skip to 58:30.

https://m.youtube.com/watch?v=1yx1d_2qntM&t=3510s

[Bcone1381]

For the first time reader of this thread....Today it is 2023, consider jumping to post #174 for Matt has discoverd and shared with us a different cause of fuel starvation.

[kestrel]

I have really appreciated this forum over the years as sometimes I feel "alone" down here. My goal is to provide accurate information to the group. I am not trying to convince anyone of what happened.

Until September 29, 2019 I flew with the assumption that in a high wing plane flying in BOTH was the safest mode....similiar to what others have said here. I believe I did NOT unport a tank. I would explain it as this; flying uncoordinated caused a pressure differential between the two tanks. Instead of fuel flowing (at the selector) forward through the firewall to the engine driven pump, the pressure differential was enough to cause the fuel to flow back up to the other tank instead. Basically it overcame gravity and starved the engine. Jon told me he was able to replicate this twice. You will not be able to replicate this on the ground unless you have some way to cause a pressure difference in the tanks. Additionally this will not occur just because one tank is dry as air will not flow downhill when there is fuel in the other tank that will push it back up (when running on BOTH).

When I purchased the plane the fuel selector only had "R", "L" and "closed". I accept full responsiblity because I made a modification (installed a selector with BOTH) without doing my homework to see how this modfication would affect everything (again, I was trying to make the plane safer and made a few posts about this modification on the forum trying to make the correct decision). To me this is the rule I did not follow and paid for it:
If two or more tanks
have their outlets interconnected they shall be
considered as one tank and the air space in the tanks
shall also be interconnected to prevent difference in
pressure at the air vents of each tank of sufficient
magnitude to cause fuel flow between tanks.

Thankfully I was the only one in the plane. I was not hurt. A small seatbelt mark on my neck (install shoulder harnesses!). My daughter asked my how I felt soon after the accident. I replied "Blessed and pissed!". We are in the process of repairing the plane. If anyone would like to volunteer to come down, I am looking for some good rivet bangers. I'll throw in a free flight over the Amazon rainforest in a RV6!

Back when I first read this, I was not convinced that it was the correct explanation. I don't believe that fuel will flow "up" to the lower tank causing starvation of the engine.

I do believe that there is a very simple failure mode where a blocked fuel cap vent will result in the fuel for that tank not being available. A cross vent would provide a secondary vent for that tank. I don't believe that is what happened here.

My guess was that there was some other factor that had not yet been identified. While there is still room for reaching an incorrect conclusion, the new data about the fuel pump feels much more like a correct explanation.

It is my opinion that a number of engine failures are pointed at and blamed on mystical boogie men when there is actually a much simpler explanation that is missed because some fact about the system or circumstances hasn't been identified. Maybe in this case that missing fact has been found.

[kestrel]

Please do not be sorry! Objective data is far more useful than unsubstantiated conjecture and can help all of us. Thank you!

[MattS]

So almost 4 years after my engine out and hard landing we have some more information. We overhauled the engine and had some wing repairs to complete. After 12 hours or so of flying after getting it back together, I had a couple times when the engine sputtered. It always came back strong but could tell something wasn't correct. It's a Cont IO360 and therefore has a pretty extensive procedure for measuring and adjusting the fuel pressure. Doing this test, we could not get the correct pressures. Wouldn't quite hit the full PSI requirement and was fluctuating alot. We started troubleshooting and found out if we removed the electric boost pump from the system, we could get the correct pressures and it was stable.

Back up 5 years to when we first overhauled the engine and put it on floats. We had the engine sent off to an OH shop and when it came back I had moved 1,000 miles away and wasn't on-site. The team installed the overhauled engine, but when they went to start it, the electric boost pump was leaking. They found another pump in the hanger, hooked it up, it provided the correct pressures to prime and seemed to work. Looking back, we didn't have a history on this pump and we should not have used something this critical without knowing more about it. This engine does not use the electric boost pump for anything except starting and emergency procedures- so 99% of the time it is just a pass through. However we believe that it was intermittently affecting the fuel flow and this is what caused the engine out. With a new/correct pump installed, we were able to complete a positive fuel pressure test and after 10 flights (albeit staying close to the airport!) the fuel flow and pressures are much more stable (with no sputtering!)

Sorry for all the confusion over this situation. I was hesitant to post this and possibly drag up more junk, but also wanted to be very transparent, hopefully set the record straight, and maybe someone can learn from my mistakes. Blessings!

[Nev]

MattS do you recall was the remaining fuel measured after being drained, or was the remaining quantity read from the sight gauges ?

It was just a random thought, but I wondered if there any chance that fuel was becoming trapped in the right auxiliary tank ? You made mention early on of the right wing sitting lower.

One thing that has me stumped is that the airplane always sits with the right wing much lower than the left (8 to 10 inches at the wing tip).

After the mishap I saw that 10 litres was found in the left tank and 65 litres in the right. If both mains held 10 litres each at the end of the journey, then the additional fuel in the right side is approximately the quantity that one auxiliary tank would hold.

[MattS]

Yes we have the auxiliary tanks.

[Nev]

MattS I was wondering if auxiliary tanks were fitted in PR-ZJO ? (I assume they were for a flight of this length).

[Redneckmech]

Hi Nev,
Sorry for the delayed response but I tried to take some time to puzzle over this to come up with a solid answer. Unfortunately, the more I dug into it, the more variables I thought of. I am not sure if there is any one definitive answer since there are so many variables that can affect the pressure seen by the fuel vent, take for example if it were a twist in the wing it could affect the pressure distribution over the fuel vent differently than if the rigging were off, which would be different than if the fuselage were misaligned with the wings, which in turn would be different than if one wing were a a slightly different angle of attack, etc..
I guess the short answer is that I really do not know.