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Unusable fuel quantity

KatanaPilot

Well Known Member
Just performed the FF tests for inspection, as well as calibration of the fuel senders/gauges (G3X through GEA).

We leveled the airplane for the calibrations and pumped all we could out of the tanks. From the fuel supply tank, we pumped out 1 gallon into a one gallon can and 5 gallons to a 5 gallon can (to their respective full lines) to "calibrate" the supply pump gauge.

Doing that, we found that we could only get about 39 gallons in to the tanks versus the specified 42 gallons. Either we have about 3 gallons of unusable
fuel or the supply tank gauge and our calibration of the gauge is way off.

The left tank has a flop tube which I assume could be a less effective pickup than the stock Van's.

I did search and what I found were most folks had ounces of unusable fuel and some got more than 42 gallons into their tanks. Anyone else have gallons unusable like it appears we do?
 
The way I check unusable fuel is to drain and sump the tank (or do this the first time any fuel is put in the tank), level the plane, pour in a measured 5 gallons and pump it back out, through the fuel servo or carb supply line, using the boost pump just as the engine does. Measure what comes out, and the difference is the unusable fuel. I also time it so I get an idea of the max capable fuel flow of the pump. What you've done is measured the capacity of the tank, and I have no answer as to why it is different than Van's numbers. Your unusable fuel should be about .5 gallon per side.
 
I initially tried calibration with 5 gallon fuel containers, and later re-did it with the airpark 100LL fuel pump system.

I found that the "full" marks on my 5 gallon and 2 gallon containers were way off. :)

Your problem is probably in the input system, not the tank.
 
Thanks All

We have tried several methods of measuring unusable fuel and both have led to the same result - several gallons unusable. I still suspect the flop tube may be part of the problem.

The airplane was leveled and we used the electric fuel pump to drain each tank until we heard the pump start to run dry.

The first time we did this we only used the meter on the fuel supply tank and with completely dry tanks and fuel system - we pumped 41.2 gallons. We pumped them out and only drained 38 +- gallons.

Same approximate result yesterday.

Next time I will completely drain the tanks, weigh the airplane, fill the tanks and weigh again. Might be a little more accurate, but not convinced accuracy of scales is any better than that of the pump meter. Plus density of fuel changes with temperature, so weight/gallon comparisons might not work too well.

QB tanks from Van's, doubt they are for an RV-6. Made in Mexico I think, so I guess anything is possible.
 
So if I read it right, you measured the IN from a 100LL fuel dispenser, and measured the OUT with he "full" marks on 5 gallon containers?

If that is correct, it's an error in your OUT measurement.
 
Flop tube

Ok, I hear you keep mentioning the Flop tube...
All your totals are for both tanks.
Do you have numbers for each tank?
That would show if one tank was different.
Remember the Vans SB about the fuel pickup coming loose?
Maybe you do have a problem internally....
While you have the tanks empty, you could stick a borescope up the drain hole and look around for issues.
A missing flop tube or pickup tube on one tank could cause your issue.

On the measuring topic, I bought a pitcher from a farm supply store that was 2 gallons, marked well up the side and used that to get accurate measurements.
 
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So if I read it right, you measured the IN from a 100LL fuel dispenser, and measured the OUT with he "full" marks on 5 gallon containers?

If that is correct, it's an error in your OUT measurement.

I get your logic but when most of us can measure unusable fuel in ounces, it wouldn't matter what you used "out". It is going to be an obvious difference between a partial gallon and several gallons.
 
Ok, I hear you keep mentioning the Flop tube...
All your totals are for both tanks.
Do you have numbers for each tank?
That would show if one tank was different.
Remember the Vans SB about the fuel pickup coming loose?
Maybe you do have a problem internally....
While you have the tanks empty, you could stick a borescope up the drain hole and look around for issues.
A missing flop tube or pickup tube on one tank could cause your issue.

To add to this, on my 6, the normal pick up can be seen without aid thru the drain hole. It should be laying right on top of it.
 
Add a couple of gallons, pump til no flow, pull the quick drain & measure what's left.

Then remove the flop tube.

:)
 
I get your logic but when most of us can measure unusable fuel in ounces, it wouldn't matter what you used "out". It is going to be an obvious difference between a partial gallon and several gallons.

Not if you have a 10% error in the "full" mark on a 5 gallon container. :)

Some of mine were that far out.
 
Next time I will completely drain the tanks, weigh the airplane, fill the tanks and weigh again. Might be a little more accurate, but not convinced accuracy of scales is any better than that of the pump meter. Plus density of fuel changes with temperature, so weight/gallon comparisons might not work too well.

I discussed accuracy of the pumps with my airport manager. He told how they were calibrated and the legal standards. I would trust the pump above all for delivering a known volume. It less than a tenth of a percent. Probably changes as soon as it is dispensed, though.

3 gallons sounds like a lot.

I took a gallon jug and put in a measured gallon of distilled water (by weight), marked the jug and then used it as the measurement container for fuel. Using the mark and then weight used the density found from that for unusable. Poured in the gallon and pumped out what would come. Did each tank separately. Shut off pump when hearing cavitation, did not wait for depletion or pressure drop. Just to be consistent. The unusable was higher for the first tank-first test, just due to filling the system. There were just ounces left in each tank, not flight usable, but good for that condition. Tail low, tail high and level conditions all nearly the same result. The stock pickups are right on top of the drain. I can see mine.

If you do the tanks separately then the issue of the flop tube should be clear.

BTW - Although the gages are calibrated to 17 gal, I still don't know exactly how much fuel my tanks hold. The level on a dipstick that was extrapolated from 17 gal shows more than 21, high probability it is wrong. I am going to have to drain and tow the plane to the pump when weather permits. Even then, I am not sure what the definition of "full" is. Is a measured distance from the upper skin?

Let us know what you find.
 
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Add a couple of gallons, pump til no flow, pull the quick drain & measure what's left.

Then remove the flop tube.

:)

Unfortunately, the tanks are now full from the calibration. At some point, I will again pump them out and then remove the quick drains.

I'm not overly worried about this right now - too busy getting ready for first flight. I will get it figured out though.

Flop tube is staying given the amount of effort made putting it in and a certain young pilot son of mine who actually likes negative G's. Or so he thinks... ;)
 
When I perform these tests, I use a calibrated container to pore fuel into the tank in 1 or 2 gallon units, this insure I have an accurate record of what went in. I also have my 5 gallon containers calibrated so I have a fairly accurate idea what is in or drained into that container during the test.
I have found in 7/7A tanks that they can actually hold slightly more than the stated 21 gallons per side, fuel filled to bottom of filler ring, overfilling during operations would not be desirable as the vent circuit would get filled.

I do the test on one tank at a time (level) as fuel flow could be affected by differences in the plumbing & pick-up & if there is a notable difference, I would go back & find out why. This portion of testing validates the plumbing.

For us in Canada, the required fuel flow test is performed not level, but at the anticipated most extreme angle of attack, to test the delivery (pump or gravity) system to insure engine fuel volume needs are meet.

---
For the OP's concern of the fuel left in the tank after level testing, I would be concerned that the pick-up tubes are set too high in the tank, check if the rigid pick-up was bent to the right position & if there is some reason the flop tube is being held off the bottom skin.
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For fun I googled "unusable fuel" & got the following: (BillL basically stated the same)

In aviation, usable fuel is the fuel on board an aircraft that can actually be used by its engines. The opposite of usable fuel is unusable fuel.[1]

The unusable fuel figure is calculated for an aircraft fuel tank in "the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank".[2]

The figure usable fuel is used when calculating or defining other key figures of an aircraft such as MTOW, zero-fuel weight etc.[citation needed]

Usable fuel is the total amount of fuel in an aircraft minus the fuel that cannot be fed into the engine(s): fuel under the pump-intake, fuel behind ribs of a tank, fuel in lines between the tanks and the engines etc. As this figure is calculated/defined for a plane in level flight it is possible that the engines of an aircraft run dry (out of fuel) even when the amount of usable fuel is still above zero, such as if the wings are not level and/or the angle of attack is higher or lower than when cruising. The inverse is also possible; in some conditions, fuel can continue to be fed to the engines when the usable fuel is below zero


It would be informative to get a DAR's (& MD-RA's in my jurisdiction of Canada) official interpretation of 'Unusable Fuel' is.
 
Unfortunately, the tanks are now full from the calibration. At some point, I will again pump them out and then remove the quick drains.

I'm not overly worried about this right now - too busy getting ready for first flight. I will get it figured out though.

Flop tube is staying given the amount of effort made putting it in and a certain young pilot son of mine who actually likes negative G's. Or so he thinks... ;)

I think a few people have dropped clues here...is the flop tube properly installed, with anti-rotation and anti-hang-up brackets, and did you at least audibly confirm it was not hanging up when you built the tank by moving the tank in all axes and listening for the tube to move appropriately?

A fuel pickup problem on a new airplane sounds...bad. Make sure you don't have one.

BTW, my unusable fuel quantity was less than single fuel strainer on one side, and something like 2-1/2 fuel strainers' worth on the other. I just pumped out the fuel in the tank using the boost pump into containers, then used the strainer to sump whatever was left. Easy peasy, don't know why everyone is making it so hard with measuring/weighing fuel going in and out and up and down and all that.

ETA: BTW, you might try to simplify your procedures a bit...quit worrying about how much is being pumped IN to the tank for a while, save that for your fuel level calibration task. Just put a few, say 5, gallons into the tank, then pump it out with the boost pump until the pump "runs dry". Pull the quick drain and see what comes out...that's your unusable fuel.

If you're into the gallons range of unusable fuel per side, something is wrong.
 
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QB tanks

I think a few people have dropped clues here...is the flop tube properly installed, with anti-rotation and anti-hang-up brackets, and did you at least audibly confirm it was not hanging up when you built the tank by moving the tank in all axes and listening for the tube to move appropriately?

Post 5 above states he has QB tanks!
This makes it subject to workmanship error, especially since it is an internal part issue. The main reason I suggested using a borescope in the fuel drain to look around.
 
I found that the "full" marks on my 5 gallon and 2 gallon containers were way off. :)

Yup. The best way to measure fuel quantity with high precision is by weight. Even a cheap bathroom scale eliminates several sources of systematic error that can foil visual estimation. To wit, gasoline will expand by about 3% from freezing to 80 degF; the container, who knows. A nice digital scale will have better than 0.1% accuracy over this temperature range.
 
Post 5 above states he has QB tanks!
This makes it subject to workmanship error, especially since it is an internal part issue. The main reason I suggested using a borescope in the fuel drain to look around.

Ah, righto...yep, borescope (will most of the common ones fit in that opening?)
 
Yes, QB tanks. Flop tube installed at Synergy, anti-hangup brackets installed per the plans.

Easiest "fix" would be inaccurate meter on buddy's fuel trailer.

Doubtful there is a problem with the pickup in the QB right tank.

I'll get it figured out. Thanks for the opinions.
 
I completed my fuel flow calibrations, float level calibrations, and checked unusable fuel this past Saturday. Very happy with 26 GPH on the electric pump. 2oz of fuel left per tank or less, there was so little the pump could not pump its not even worth mentioning.
If anyone is a gallon or more of unusable fuel there is an issue, assuming the pickup tube is not on the bottom inside of the tank????
 
it was earlier mentioned that weight measurement is the most accurate. that may be true but not helpful here. your system leaves fuel behind by volume, not weight.your unusable may be 6lbs on a cold day and 5lbs 14oz on a hot day.
 
it was earlier mentioned that weight measurement is the most accurate. that may be true but not helpful here. your system leaves fuel behind by volume, not weight.your unusable may be 6lbs on a cold day and 5lbs 14oz on a hot day.

Either way that's still too much - almost all RV's have an unusable in the 1-2 digit ounces (by weight OR volume) range. Gallons is not an acceptable unit of measure in this case, something else is hokey.
 
I agree with a previous post. I would not make the first flight until I fully understood the discrepancy. The downside is too great.
 
Problem Solved

Today I pumped out all of the fuel using the electric fuel pump, then pulled the sump drains. Although the fuel remaining was not a couple of ounces, the total unusable fuel for my airplane with a flop tube is less than 0.25 gallons. While the right side pickup in the QB tank may be a little higher than desired, I find this result perfectly acceptable.

The problem clearly was twofold - the meter on the portable fuel tank was inaccurate and the "full" mark on the 5 gallon can I used to measure the fuel pumped out is also way off. So the fuel "in" and "out" were not measured properly. But the unusable fuel is within a reasonable range.
 
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