PA-30 Unuseable Fuel Question

[David Powell-Williams]

In searching the forums, I see nobody has raised the 'unuseable fuel' question for almost 13 years. Perhaps I shouldn't disturb a sleeping dog but ...

I'm having grief with highly inaccurate fuel quantity displays on my PA 30 with Osborne tips. The tank senders are the standard float / potentiometer types and the display is the AeroSpace Logic digital fuel quantity system. The inaccuracies are on all tanks but let me focus on the mains:

In a first attempt to rectify this issue, I sent both main tank senders to AirParts for overhaul. Upon return, we jacked the aircraft to level laterally and longitudinally. We siphoned all possible fuel out of both mains and then held the sump drains open until all fuel flow stopped.

At this point, I put my lighted boroscope into the tanks and saw nothing resembling fuel anywhere.

So now the question: Is the 3 USG 'unuseable' fuel still in the tank at this point, lurking somewhere? There can't be 3 USG in the lines from the sump drain to the engine, can there??

My thinking is that, after draining the tank as above, the next 3 USG of fuel put in become the 'unuseable' quantity, leaving the following 27 USG (on a good day) to completely fill the tank.

I'd really appreciate guidance from those more knowledgeable.

Thanks, David.

[Randy Johnson]

Have removed my comments as they were incorrect

[JIMICS2452]

Unusable fuel is the amount of fuel required in the tank to prevent unporting of the fuel pickup when maneuvering, as in for a landing pattern. It is the amount that must be deducted from the tank capacity when calculating fuel requirements and reserves. It does not define the fuel left if a tank is run dry in level flight.

[Kristin Winter]

Jim is correct. The amount of unusable fuel is a certification requirement that corresponds to a requirement that the tanks feed under certain maneuvers.

[David Powell-Williams]

Thanks to all that replied to my question. Very much appreciated.

The reason I asked is because it relates to trying to calibrate my AeroSpace Logic 6 tank fuel quantity display.

Their procedure calls for draining the relevant tank and then adding the unuseable fuel. This becomes the zero fuel level. After this, the procedure continues by adding 2 USG at a time until the tank is full. The instrument knows the tank is full when there is no further increase in its fuel quantity measurement and terminates the calibration.

Following the above, I completely emptied the tank, then added 3 USG as the unuseable amount and proceeded with the calibration. This caused an argument with my A&P because he felt the unuseable was still in the tank, even though I had sumped it dry.

Therefore on the other main tank, I started out doing it his way and called the sumped dry tank quantity the zero amount. Now, here's the catch: The first 2 USG added as part of the procedure was not sufficient to move the sender float up. Therefore the AeroSpace unit determined the tank to be full and terminated the procedure.

I had to add another gallon (now 3 USG) before the float moved and I could complete the project.

Theoretically, my gauge will show full as ~27 USG and empty with 3 USG remaining. I wish it were so. The gauge quantity display is still hopelessly inaccurate, even though both senders were overhauled last week.

If anyone is interested in the AeroSpace Logic display and its calibration procedure, an excellent 10 minute video is at: http://www.aerospacelogic.com/index.php ... s_id=50#50


Thanks, David.

[N3322G]

David,

Lots of things can make the fuel senders be wonky. Not having a good ground is very common for lots of Twin electrical problems in my 45 years around and now owning 22G. Ancient wiring is another to the sender, to the gauge, to the selector. Matt Kurke of Comanchegear.com fame sometimes has replacement wafers that go underneath the selector that also can impact operations.

[JIMICS2452]

Is the sensor (float) going all the way to the bottom of the tank? The gap would be uncounted fuel. Or it may not be measuring the last of the fuel due to the dihedral of the wings. It sounds like you will be close the "unusable fuel" amount when the system says you are empty.

[David Powell-Williams]

Jim,

I will have to check if the float bottoms on the tank floor. I hope that the rebuild of the senders by AirParts would have included setup in an appropriate jig. It is interesting that my installation requires 3 USG of fuel to begin lifting the float and changing the resistance from the zero position. Is it a coincidence that 3 USG is the unuseable quantity or is there some real science going on here?

If I find that my sender float is on the bottom at empty, it will take 3 gallons of gas to start the AeroSpace calibration and the result will be a gauge that shows empty with 3 USG remaining in the tank.

If the float does not contact the tank bottom at empty, I could bend the arm to cause it but that would have consequences at the tank full end and likely mess up any sender certification from AirParts.

As you say, the next step is to see where the float bottoms out.

Thanks much,

David.

[N3322G]

Forgot to check that the service manual install instructions have been followed. I know there are some specific and if I recall correctly iterative install instructions.

[David Powell-Williams]

Hi Pat,

The service manual procedure calls for draining and then adding fuel one-quarter tank-full at a time (roughly 7.5 USG in the mains). However, that procedure assumes the use of the original analog gauges. My bird has the AeroSpace Logic digital display. It is capable of much better resolution but is limited by the accuracy of the sender units, which are the original OEM offerings (overhauled).

Unfortunately, the calibration of the AeroSpace display cannot proceed until the tank float starts to move upward and, in my present situation, that is at 3 USG. Thus 3 gallons becomes the 'zero fuel' level for the display.

I can live with this but, as a new owner, I wondered what was done previously by those who have installed the same display.

Thanks very much for your reply.

David.

[N3322G]

Hi David,

Guess I should have been more specific. I'm referring to the sender check in paragraph 9-15 - checking its profile and resistance etc. They may have been perfect leaving Air Parts but then they get packed and shipped and handled so its something to check.

hope this helps.

BTW, I use the original gauges as a gross indicator of fuel but rely on the Shadin digital fuel computer for all fuel tracking wrt all 8 operational tanks.

[David Powell-Williams]

I did a resistance check on the overhauled senders prior to re-installation. They were a few ohms off the Piper spec and a few ohms different from each other. I discounted this because the AeroSpace calibration looks at the relative resistance change as each quantum of fuel is added. It does not operate on the absolute resistance value which would be important on a non-computerized analog display.

I also have the Shadin totalizer/integrator and agree with you - it's very accurate if kept updated when fuel is added.

As far as the AeroSpace display goes, I'm a little obsessive about installed 'things' working properly. Of course, I make an exception for myself.

David.

[N3322G]

David,

I see a great friendship in our future I too like the little and big things to work right. Wait until you read some of Charles posts - he is just a focused on having things right as we are ... and so are many others.

[JIMICS2452]

One of my favorite sayings, from an old AI that taught me a lot - "What part of good enough is right?"

[David Powell-Williams]

Jim:

A good question, for sure.

By posting here I was trying to find out what was 'right' when one is using the AeroSpace fuel display. I had hoped someone who has this system might be flushed out and pumped for information.

David.