PA-30 Unuseable Fuel Question

[Clarence Beintema]

There are instructions in the maintenance manual for building a fuel sender calibration stand. Same tool for main and auxiliary senders.

Clarence

[Zach Grant L1011jock]

David,
As with many things that were not originally designed with precision as a primary concern, you will find that by applying something that is so designed, you will not find any greater accuracy than the sum of the whole allows. I hate to be this way but it should be accepted as "good enough" because that is as right as it was ever designed. Applying an accurately reading calebrated microchip controlled electronic device to a 50 year old 20 cent variable resistor with a float on the end of it that is stuck in a tank with little true concern other than to generally reflect the level of the fuel in it (uncompensated for density or attitude, or any other possible error) is still only going to give you the most accurate result possible from the entire system. Aviation has lived by the adage, measure with a micrometer, mark with a grease pencil, and cut with an ax for many years. You are simply trying to rationalize your micrometer measurements of the ax hacking at this point and I am afraid you will never be satisfied with the result until you realize that is what you are doing.

Zach

[SLIMDREDGER]

Zach: Loved your post. I have a couple of soft pine wood sticks marked with a ball point that are calibrated based on what was pumped into the tank. I find they check very well with what I buy to refill from different levels. Bone simple, not high tech, but with my plan to always land with an hour of fuel, it works. Al Powers N88AP

[David Powell-Williams]

Hi Zach:

Thanks for your post.

Perhaps I should clarify why I originated this thread:

My TC has the AeroSpace Logic digital fuel quantity display installed by the previous owner, so I inherited it.

As an EE I fully understand the impossibility of realizing a highly accurate display, given the restrictions imposed by the OEM float/potentiometer senders. However, I don't think it unreasonable to try to get the same accuracy as the original analog gauges, such as +/- a quarter tank amount or slightly better. I have yet to achieve same, even though I have rung out the wiring, including ground connections.

I do check my fuel levels with a dipstick prior to each departure. It's religion The Shadin integrators / totalizer are a second and reliably accurate check.

The AeroSpace issue will likely boil down to the inability of my resistive senders to consistently produce (roughly) the same value for a given fuel quantity over time. I'd be happy with a delta to the dipstick values of 10 to 20 percent. I expected to achieve same by having the senders overhauled. Silly me

Thanks to all who have responded. If and when I resolve this, I'll post a contented note.

David.

[Zach Grant L1011jock]

David,
In thinking about the non linear response to the initial tank fill you are experiencing, you may have a mechanical issue vs electrical. Could your sender be installed backwards, ie it is resting on the bottom of the tank on the high side, not the low side, and thus the fuel must reach a higher level before moving the float? Also, is the aircraft truly level when you are filling? I have never seen a float gauge not move for 3 gal. Up to about a gallon of non movement can be seen pretty regularly on aircraft depending on tank/float condition etc., but you should be seeing a re stance change almost immediately upon adding fuel. The rest will be celebrating as best you can with the equipment provided.

Zach

[David Powell-Williams]

Zach:

Thanks for the continued thoughts on my issue.

I installed the overhauled quantity senders such that the floats were inboard. Recall that we are talking about the main tanks only on the TC. AFAIK, the mounting holes for the senders will only permit this orientation. Sound right?

I leveled the a/c in pitch using wing jacks and a bubble level across the fuselage leveling screws. I did not have to make any adjustments in roll.

As I'm sure you know, there are mechanical stops on the sender float arms. I make the dangerous assumption that an overhaul by Air Parts would include a trip to the jig, as described in the maintenance manual. If your experience says that it takes only a small amount of fuel to lift the float, something is clearly wrong with my installation.

As I poured the fuel into the empty tank, I did find it strange that it took 3 USG to change the resistance, i.e. move the float upwards.

Incidentally, I found a lot of information on the (gasp) BeechTalk forums. Seems many Bo owners have the AeroSpace Logic system and are generally positive in their comments. One member there has developed an alternate calibration procedure since the manufacturer's procedure of adding 2 USG at a time can fall victim to the inherent non-linearity of the sender as its 'button' travels up the wire-wound resistor.

If you haven't already glazed over, the alternate procedure is to disconnect the electrical connection to the sender and monitor its resistance with a DVM. Add the fuel in 1 USG allotments while recording the resistance value. Graph the resulting curve of volume vs resistance and smooth out the outliers. Connect the fuel indication system to a decade box or 10 turn pot and repeat the manufacturer's procedure using the smoothed graph resistance, not the float. Reconnect the sender.

My neighbor in the hangar has an Ercoupe. The fuel tank has a float that pushes a wire up into the pilot's view. "Never fails", he claims.

You are thinking - enough already. Just connect the OEM gauges and be done with it.

Good idea

Cheers,

David.

[Zach Grant L1011jock]

David,
Well so far I see nothing wrong with any of your procedures, and under perfect conditions with a new airplane they would probably work. A couple of points you should verify that might make the situation clearer. First, I would do as the brand B boys have suggested and test the prob for a linear resistance throughout its travel. If it is showing a slow response through the first bit of travel, there is your problem. Also, I would use a bore scope to look in the tank with the sender in situe and verify the float actually rests on the bottom of the tank when empty. If there is a wrinkle in the bladder or the arm is bent such that the mechanical stop on the sender is preventing the float from reaching the bottom of the tank, that could certainly be an issue as well. Some "adjustment" to the highly precise (insert sarcasm here) float arm to allow for low readings of the gauge would be in order. My preference is to have the inaccuracy be at the top end of the spectrum. Having the fuel gauge dwell at full for 20 minutes is easier to interpret than having the engine continue to run for 20 minutes after showing empty, and guessing when the silence will arrive! Regarding the sender passing through the jig, well it may have, but that jig may not mirror the exact dimensions of your individual, hand built, possibly many times repaired aircraft, if you get my drift.

Zach

[Douglas Dodson]

I have a different system (AuRacle) but the calibration process is the same as being talked about here.

First, the fuel senders have stops. They PREVENT the float from riding on the top AND bottom of the tank. I presume this is to prevent the float from wearing a hole in the bladders. They should be adjusted per the maintenance manual in the jig described there. Its tedious, but not difficult.

Second, in a simple float system such as ours, there is no way to accurately read both full and empty. The dihedral of the wing makes it impossible. The regs of course require it to be accurate at empty, so full won't every be possible. As a result, my gauges show 30 gallons for a long time then suddenly drop to 26 gallons. They read fairly accurately below that with the usual erratic behavior as bad spots in the sender are found. They are generally accurate though.

In our system, as you found, it takes a few gallons to get the float to float. Part of that is it shouldn't be touching the bottom. And it takes some depth to float the float. Just getting wet isn't enough. So its 3 gallons. It has always been that, but now we know it well!

On my electronic gauges, the accuracy at 5 gallons is about right. It then drops to zero over the next two gallons. This is to be expected since the calibration is 2.9 real gallons indicates zero, and 5 real gallons indicates 5. Same as on the high end... 26 real gallons reads 26, 26.5 real gallons reads 30. The float hit the stop at 26.5 (ish).

Operationally, this should not be much of an issue. Here is how I max range my airplane. Takeoff and climb on the mains. Get to cruise and switch to the aux's. As the aux's near the bottom, say 5 gallons remaining, I note the fuel used number in the totalizer. From there I calculate the fuel used number that should be exhaustion. The aux gauges go to zero shortly after that but I switch back to the mains based on the totalizer. The remainder of the flight is on the mains of course and you shouldn't get too close to the 3 gallon mark before landing.

Note also the levels show lower than truth in a descent. I hate that! But there it is!

I have consistently been able to get to .5 gallons or less in the aux's without causing fuel starvation.

Doug Dodson
PA30, ATP, A&P, CFI, Flight Test Engineer

[David Powell-Williams]

Well, it's been a while. Earth's crust and this thread have cooled substantially since my last post.
For those born this century, here's a recap:
The previous owner of my TC had the AeroSpace Logic 6-tank fuel indication system installed. He warned me that it displayed varying fuel quantities, mostly unrelated to reality. The displayed values changed significantly when I hit a bump on the ground, in the air or when experiencing the feather-like caress of a typical Twinkie landing. I needed to rely upon the trusty dipstick and the Shadin integrator display.

Not to be dismissed so easily, I sent the fuel senders out for overhaul and recalibration (twice). I tried two different methods of calibrating the AeroSpace unit but got little improvement operationally. I found a slightly loose DB-25 connector attaching the sensor and power cables to the rear of the display instrument. Eureka! (but that didn't fix it either).

So, on this year's annual my mechanic and I started pulling out the wiring from the display mounted on the upper left of the panel and proceeding under the floor toward the sensors. That would be one wire for each of the 6 tank sensors plus power and ground. Being as how the original TC analog fuel guages were fed from switches at the floor fuel selectors, the OEM wiring brought the fuel sensors from the wings to these switches. I was also found on the floor when we discovered a total of 12 'knife' connectors splicing the wires enroute from the AeroSpace display to the fuel selector area. All professionally done but a squadron of bad ideas.

Unlike the original Piper fuel quantity system, the AeroSpace microprocessor-based unit uses a very low voltage on the sensor wires, something akin to a DVM in the resistance measuring mode. Any change in the path resistance from the fuel tank sensor to the digital display resulted in a changing fuel quantity, just as if the fuel tank float had moved vertically. The 'knife disconnect' connectors and associated crimps were providing that varying resistance path from time to time.

I began this thread to discuss the Unuseable Fuel quantity in the TC main tanks and how it should be treated by a modern digital fuel display. As several contributors have stated, the 3 USG unuseable quantity is there to ensure positive flow to the engine under all normal flight attitudes. Perhaps it's not a coincidence that a properly rigged main tank sensor does not begin to float until 3 USG (roughly) are in the tank. Given the ASL calibration procedure, this results in a zero fuel reading when the level decreases to 3 USG.

With all offending connectors removed, the AeroSpace Logic fuel display is both stable and accurate. It's a pleasure to be able to view the fuel quantities in all tanks simultaneously. The microprocessor firmware and associated calibration procedure allows most of the non-linearity of the ancient OEM fuel sensors to be ironed out. All in all, a great upgrade and now a happy camper.