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Rapid loss of fuel

ssturges

Active Member
Plane: RV9A IO320 stock per plans, Stock vans boost pump installed per Vans instructions. Number of hours on plane 102. No issues with fuel flow to engine during this incident or at any other time. Stock vent line and sniffle drain

A few weeks ago I had my tanks top off buy the fuel truck before flight. Unfortunately he over filled them to the point where when cap was installed fuel flood the wing, No big deal I cleaned it off.

Did run up, turned boost pump on for take off. After takeoff the tower had me do a series of 3 sharp 45 degree plus turns 1 right and 2 left for traffic avoidance. My fuel pressure with or without the boost pump runs about 37-40psi. I have the squawk limit to 40 so, I did here one squawk that indicated it got to 40 psi for a short moment and then dropped back down to 37 psi. I turned the boost pump off and I flew for 10 min or so to a near by airport and did 1 touch and go.

On departing this airport I look down and saw that I only 9 gallons in my right tank which was the active tank. I thought stuck fuel gauge lever. Rocked the wings and saw the fuel level bounce up and down. Ran for a bit on that tank and did not see further rapid fuel loss on the gauge. I switched tanks and proceeded on to my destination. On arriving I sticked the tank and I had indeed lost a bunch of fuel. Further operation of the plane has showed no further fuel loss.

So: what happened? Did the fuel siphon out the vent line on the steep turns after takeoff, because fuel was compressed into the top of the vent lines lines due to the overfill and start a siphon on the steep turns, this really seems like a physical possibility? Or does a IO320 fuel pump dump consider 40PSI an over pressure and dump the fuel?
 
The fuel pump does not have a mechanism to dump fuel, except for a failure of the fuel pump diaphragm which would show up as low fuel pressure, not high.

What fuel levels did you measure during the preflight, and what showed on the gauges during startup and taxi?
 
First, I don't know why you run 37 PSI on an IO-320 ?
That alone does not explain the loss of fuel and I am curious as to what the cause may be.
Since your tanks where overfilled, it is possible that your fuel cap was not properly seated and low pressure on top of the wing sucked out a good amount of fuel without you noticing it.
Steep turns and other maneuvers should not cause you to loose fuel or else a serious flaw needs to be corrected.
 
Fuel issues.......

A few weeks ago I had my tanks top off buy the fuel truck before flight. Unfortunately he over filled them to the point where when cap was installed fuel flood the wing, No big deal I cleaned it off.

Several issues I see here. NOT to be critical but instructive; I was not there at the time.......:rolleyes:

1) NO ONE has ever fueled any airplane that I have flown. I have been told the line person HAS to fuel the airplane and I tell them I'll be on my way without buying their fuel, which usually changes their mind, with occasional glances back to the office. Most of the time it isn't an issue and the line person doesn't mind not doing anything for a little while. It's a personal thing with me and my airplanes....not to mention Jet-A has never come near my fuel tanks and they have never been overfilled. And a fuel nozzle has never banged the inside of my fuel tank and no nozzle cap has ever banged into the wing. :)

2) For some reason, preflight has taken a back seat to just jumping in and going flying. I am NOT saying this happened here; just in general. Any time my airplane is on the ground having just arrived somewhere, I do a brief walk-around including pulling the tank lids and taking a peek inside before departure. If I have refueled, I tap the quickdrains to make sure water was not introduced into the tanks. Yeah, I know the water probably wouldn't get there that quickly but it makes me feel better. Again: Communing with my airplanes. SO: my airplane was on the ramp while I was in having breakfast with people walking up and looking at it. My abbreviated inspection tells me no one pushed on anything they shouldn't have, banged into a flap or other things people do. (I have had people push on my NO PUSH placard "just to see what it did" while I am standing there at fly-ins!:mad::mad:) I have a very good friend, CFI/ATR etc with tens of thousands of hours who once left our fuel stop with the fuel caps of his Cub still on the ground. Or somewhere.

3) sounds like the fuel was sucked out somewhere. It may not have been from the turns but from the tanks priming the vents and sucking air/fuel out. Just a guess but it had to go SOMEwhere......:confused:

IMHO...YMMV......
 
Are you sure they were “topped off”, if you pump fuel in fast enough it can’t displace the air quick enough and will “appear” full and spit fuel out the filler. You have to fuel an RV much slower than most aircraft to truly get the tanks full.
 
Are you sure they were “topped off”, if you pump fuel in fast enough it can’t displace the air quick enough and will “appear” full and spit fuel out the filler. You have to fuel an RV much slower than most aircraft to truly get the tanks full.

In the 9A that I am building, earlier builders complained that the holes in the ribs inside the fuel tank are sometimes too small and the fuel will burble out before the tank is filled. The vent cant release air fast enough either and the holes are too small to release air with fuel flowing past. I enlarged the holes in the ribs during tank assembly to help this situation

It sounds to me the fueler went too fast, the fuel burbled out and you thought it was full but it wasnt.

JMHO.
 
SNIP

It sounds to me the fueler went too fast, the fuel burbled out and you thought it was full but it wasnt.

JMHO.

The one time in 20 years I did not personally fill my plane - and I got shorted 5 gallons. The good news is the float gauges were well calibrated so I noted the error.

Always fill your own tanks.
Carl
 
Are you sure they were “topped off”, if you pump fuel in fast enough it can’t displace the air quick enough and will “appear” full and spit fuel out the filler. You have to fuel an RV much slower than most aircraft to truly get the tanks full.

This. It is common for me to need to wait for the fuel to settle for a minute or so and then add several more gallons, depending on how fast the fuel is being dispensed. Somebody who just topped them off and put the cap back on may not realize that after settling, there is room for several more gallons.
 
I checked the fuel after the fueler fueled the plane. I always install the caps or reinstall the caps after someone else fuels the plane. The caps were correctly installed.

I verified the caps were correctly installed when I landed. The second I noticed the fuel look I looked over at the cap to insure in was in place. In this case I installed the caps, hence I know the plane was fueled to the brim. I am very picky on fuel cap installation.

My tanks and caps are tight as when I pop the caps when someone over fuels the plane you get a small burst of fuel out the top from the vapor pressure. My tanks also vent fuel out the vent lines in case of an overfill when the plane rolls into the sun or has a temp increase. This was the case here I noted there was fuel dripping from the vents after the fueling and the caps had been installed.

I am based at large airport, you cannot self fuel your plane. I always supervise the fueling however, that does not mean they will not fill to the brim however in spit second. I agree that having someone fuel your plane or behind your back is not desirable.

Spec fuel pressure for an IO320 is between 12-45psi. I did not pick this fuel pressure everything is stock Vans/Lycoming. My fuel system runs at 36-40 both the engine pump and the boost pump.

The one thing I cannot remember is if I switched tanks during runup or taxi. I often do this on the case of an overfill to get a little bit of fuel out the tanks and vent tubes. In this case I might have start up on the left tank and switch to the right tank after runup and before taxi. In that case the right tank when have still been had fuel in the vent line which could have created a siphon on the steep turn.

My corrective action is don't let your plane be overfilled. If you are over fueled and have fuel dripping out the vent tubes don't do steep turns and burn some fuel out of each tank. Hard to believe I am the only person to have this happen. A lot of deep questions about how smart it to switch fuel tanks right before take off however :)
 
My tanks and caps are tight as when I pop the caps when someone over fuels the plane you get a small burst of fuel out the top from the vapor pressure. My tanks also vent fuel out the vent lines in case of an overfill when the plane rolls into the sun or has a temp increase.

I get the exact same thing.

I am based at large airport, you cannot self fuel your plane.

Never heard of anyplace with this as a regulation before, but maybe flipping the line guy a few bucks now and again might "persuade" him or her to let you handle the nozzle while he "supervises"? I'm with the previous poster...NOBODY fuels my plane but me. Only damage I've ever had was due to line guys.

Spec fuel pressure for an IO320 is between 12-45psi. I did not pick this fuel pressure everything is stock Vans/Lycoming. My fuel system runs at 36-40 both the engine pump and the boost pump.

IMO, this is WAY too high. I have the stock IO-360, and with the boost pump on, it's around 26-27. 24-25 with it off. 40 may be in spec, but why is it so much higher than everyone else's?

The one thing I cannot remember is if I switched tanks during runup or taxi. I often do this on the case of an overfill to get a little bit of fuel out the tanks and vent tubes. In this case I might have start up on the left tank and switch to the right tank after runup and before taxi. In that case the right tank when have still been had fuel in the vent line which could have created a siphon on the steep turn.

Plenty of people fill 'er up, taxi on one side, runup on the other, and take off without losing s**tloads of fuel. You have something else going on. Stuck drain valve? Leaking fitting somewhere? Running ungodly rich (not leaning the engine, or you *think* it's being leaned, but it's not)?

Vents shouldn't be creating a suction. If they're done right, they should be creating *higher* pressure in the vent line (open end facing forward).

Keep looking.

My corrective action is don't let your plane be overfilled. If you are over fueled and have fuel dripping out the vent tubes don't do steep turns and burn some fuel out of each tank. Hard to believe I am the only person to have this happen. A lot of deep questions about how smart it to switch fuel tanks right before take off however :)

I think you've jumped to a conclusion without adequate evidence. How would a (coordinated) steep turn offload any sizable amount of fuel?
 
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A few weeks ago I had my tanks top off buy the fuel truck before flight.

Knowing what was left in the tanks, I assume you had some idea of how much fuel was left you also know how many gallons you filled plus you verified visually that they overflowed according to your post.?

Fuel capacity in one tank is 18 gal and although there seems to be some validity to the following, its hard to believe that this phenomenon caused you to get only half a tank of fuel. A gallon or 2 maybe but not half your tank.
In the 9A that I am building, earlier builders complained that the holes in the ribs inside the fuel tank are sometimes too small and the fuel will burble out before the tank is filled. The vent cant release air fast enough either and the holes are too small to release air with fuel flowing past. I enlarged the holes in the ribs during tank assembly to help this situation


The siphon theory is also a bit of a stretch, possible but loosing 9 gallons in a matter of minutes? There is positive pressure on the vent lines so the siphon would have to overcome that pressure. Is your vent line in a low pressure area?

It's probably not related but a stock IO 320 fuel pump does not create more than 25PSI, none that I know of, what's going on there?

Not making accusations, just fuel for thought...
 
Well, it had to go somewhere. As I see it, the most likely scenarios are

1. tank not full to begin with
2. pumped overboard via fuel leak under the cowl somewhere
3. siphon from cap being loose/off
4. siphon thru vent line

#2 - If no fuel stains in the fuel system under the hood, then this is probably out.

#3 - Presumably you would know if you left the cap off or see massive fuel stains on the top of the wing.

#4 - Seems like this is pretty much impossible if built to plan. If the vent line is like it is on the 7, then then end of the vent is within about 1/2" of the top of the tank. Once it's unported, no more siphoning. No way you're going to be able to lose 8-10 gallons out of there.

You know what Sherlock Holmes said; once you eliminate everything else, whatever's left is the answer.
 
I have an IO-320 also. Just to second what has been said here many times, normal fuel pressure for me is 24-27 psi. I have also experienced a leaky fuel cap that streamed fuel with completely full tanks, but the fuel loss stopped within about a minute of takeoff and I was not able to discern any noticeable loss compared to what my totalizer predicted. I always fill my tanks to completely full (overflowing when cap is put on) and commonly do steep turns shortly after takeoff with no noticeable loss. The tanks will vent liquid onto the ramp due to thermal expansion if the aircraft is pushed out into the sun after fueling.

I don’t know what is caused your fuel loss, but I doubt it is the steep turns or the full tanks on a properly set up aircraft.

Good luck and interested to hear what it turns out to be.
 
What if the fuel vent line is loose at the inboard bulkhead fitting? Would that allow fuel to drain out of the inboard rib. I imagine you would see evidence though. If the fitting is loose on the inside, it could drain out the vent line. If the fitting was loose on the outside, it would drip out the wing root.

Also just for a data point, on my -4, I fill to the brim, it doesn’t need time to settle, the builder must have enlarged or added vent holes. On another -4 I flew recently, it would gurgle and take its time to settle. Neither plane would puke out the vent after overfueling, nor would they make a vapor hiss. I don’t understand how they could hiss unless the vents are blocked or holding pooled fuel somewhere.
 
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Fast Filling & not full

I stopped for fuel in SD a while back. Noted that my bill was less than expected. Checked caps for security and sumped the tanks. Figured out why the bill oddity on start up... they didn't fill the tank. Apparently tried to fill at full blast, saw the outboard section filling up, so stopped, put the cap on, and called it good.

RV6A with QB tanks.

Now I was only short a few gallons, not half a tank or something like this case.
 
Being an injected engine, does it return excess fuel to one of the tanks?

I can envision starting off on the non-return tank and having the excess fuel returned to the other tank, which is full, so it just goes overboard through the vent.
 
What if

1. tank not full to begin with
What if this theory holds "fuel"

Maybe ssturges can provide us with a few more details,pls.
We assume you are not the builder, your profile list says "newby"
If number 1. was actually what happened, your tank would have to have some deficiencies IMHO. Perhaps the builder failed to drill the holes in the middle of the ribbs? Maybe he only drilled the holes in the bottom.
If that was in fact the case the tank could possibly hold enough air to displace 4 or 5 gallons of fuel and once you made a steep turn the air bubble would escape upward and allow fuel to replace it in the root of the tank??
Just speculating without more information.
I am very interested to find out more about this and I do hope you'll come up with a plausible explanation and hopefully a fix to make sure it does not happen again.

Being an injected engine, does it return excess fuel to one of the tanks?
Ordinary injected IO 320 does not call for a return line.
Mine has a return line for other purposes and it returns 6GPH.
 
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OK, got to the bottom of this by looking at the EFIS logs. I may have lost a gallon out the vent but it was primarily an inaccuracy of the fuel tank sensors and how they are impacted by steep rolls. When the tanks are full the fuel readings also very course. Averaged over time the the fuel level matches that of fuel use reported by the red cube. After the steep rolls it took the fuel tank sensor almost 3 min to recover to the correct level. I have an AFS EFIS but I think the logs or display are not averaged and these are the actual readings from the sensor because they are so jumpy. Some of this could also be due to how long it takes the fuel to equalize between the ribs in the tanks after a steep roll. So I was basically faked out. It was just shocking at the time to look down after shortly take off and see such a low fuel reading.

I am the builder, 5 long years. It is the first plane I have build. Maybe I should change my profile to something other than a Noob. I am an engineer and pretty meticulous about things especially when they can kill me, hence my post here to understand this.

Thanks everyone for the suggestions. Best to ask questions and be safe. As a plug, if you don't use Savvy Aviation for analyzing your logs I suggest you look at it.
 
I have an AFS EFIS but I think the logs or display are not averaged and these are the actual readings from the sensor because they are so jumpy.

You can post the link to that flight's data in Savvy, so we could take a look at it. You say it's "jumpy"...be interesting to see what you mean by that. And what kind of fuel level sending units do you have, again?
 
A coordinated steep turn shouldn't matter to float type senders as the fuel shouldn't move outboard in a coordinated turn.

Capacitive senders OTOH, I'm less sure about. Recently doing a calibration on a buddy's new build - I expected no change in reading for the last 2 gallons before full as the wing root would already be full. But there was some change in reading. Huh? Perhaps pressure change was being detected?
 
A coordinated steep turn shouldn't matter to float type senders as the fuel shouldn't move outboard in a coordinated turn.

Capacitive senders OTOH, I'm less sure about. Recently doing a calibration on a buddy's new build - I expected no change in reading for the last 2 gallons before full as the wing root would already be full. But there was some change in reading. Huh? Perhaps pressure change was being detected?

Mine have always been very accurate no matter how full or empty. The idea behind capacitance senders is, from what I understand, that the measured capacitance is irrespective of attitude...that it should be the same in any orientation (all the fuel in one bay, or distributed evenly across the bottom, or top, or aft, or whatever). The two plates are measuring the capacitance of the total volume between them.
 
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