The fuel gauges were acting wonky (not reading full with full tanks). The A&P established that the problem was with he senders, and not the other things in the chain. So we sent the senders out for overhaul by Air Parts. After they came back, on the bench the senders seem to be working fine (reading 30 ohms when "full" and zero when "empty), so the A&P put them back in left it over night, filled the tanks. And now one seems to gone bad again. (Reads max 10 ohms, so the fuel gauge only reads about 1/3 full when the tanks are in fact full.)

So, the senders are being sent back to Air Parts for another look.

But it looks like, from the various threads on fuel gauges on the forums, fuel senders going bad seems to be a common problem.

So, is there a recommended solution? Are their replacement parts? Is continuously overhauling them when they do go bad the only solution? Is it legal to fly with a bad fuel gauge? (The plane has a Shadin Fuel Flow Gauge, so knowing how much fuel is left isn't a problem.)

Thanks.

Tomoharu

The service manual calls out a specific manner to re-install fuel senders - do you know if this was done? You can read it under the Home page Tech Tab on service manual.

Also, is the ground good for the electronic connection?

The Twin's senders operate in accordance with FAA specs. I rely on visible inspection prior to flight, fuel flow and digital Shadin fuel computer for closer fuel management. I always track range and fuel consumption by tank.

Hi Pat.

Thanks for the reply. I will forward the service manual section to the A&P to make sure that this was done in an appropriate manner. And we did verify the ground connection, since that seemed to be a common problem.

But we are pretty sure it's the fuel sender itself. After AirParts overhauled the sender in question, we tested it on the bench, and it performed the way it was supposed to. Then the A&P installed it in the tank, and it failed. So we pulled it out again, and retested the sender on the bench again, and this time it failed to work properly. So it is going back ti AirParts.

So, I too use a Shading fuel computer to monitor fuel flow and fuel remaining. That combined with a pre-flight check of the fuel gives me a good idea of how much fuel I have left.

But strictly speaking, that alone is not legal, as far as I understand--that the fuel gauge itself has to work properly. I hate to have to keep grounding the plane because of the fuel gauge. Are there sources of replacement fuel senders? Or do people just keep on overhauling them?

Thanks.

The manual has a drawing for a fixture to use to calibrate the fuel senders. An ohm decade box can be useful in trouble shooting the system. The micro switches at the fuel selectors are also know to have poor connections after years of use and wear.

Clarence

Hi Tomoharu

I'm having the same problem with one of our main tank sending units. It only reads 1/2 full even when it's full of fuel. The resistance measurements on the sending unit are correct outside the tank. The other main tank reading was fine, so we switched sides hoping it was somewhere in the wiring/selector switch/gauge. But the problem followed the sender to the "good" main tank. We have a 260B with the single fuel gauge and 4 position selector switch on the floor. The selector switch was replaced recently. The new parts were from Matt at Comanchegear.com.

We are shipping the sender to Air Parts tomorrow for rebuild. I'm not sure what the problem is though since it appears to be ok when we measure it outside the plane. There is side to side movement in the arm but that didn't seem to make a difference on the resistance measurements.

We searched all over trying to find a replacement sender, but none are available. The only option is rebuilding your old unit.

I'd rather buy a new one and be done with chasing ghosts. I noticed McFarlane has new "thick film laser cut ceramic resistors" that are supposed to be better. Unfortunately they don't have them for PA-24's. Wonder if they would ever consider making them for us.

Btw I heard that the fuel quantity gauge is "legal" if it reads empty when the tank is empty. Don't quote me though. Still, I'm considering our plane grounded until the problem is fixed.

Regards,
Jim

If the gages show 1/2 when the tank is full, it can not be a problem of the connection, for half is 15 Ohm and full 30 Ohm. Bad connection would show full at low level. Check your floater, if he is on the right level. If he has not enough uplift, he is to low in the fuel and shows too less fuel level. The floater can be too heavy or the specific volume is too low.

Hi Richard

Good point. We spoke to the Air Parts folks and they said they will check the float.

We noticed the float was "repaired" before and re-installed on the arm backwards, but I'm not sure if that would affect anything.

Regards,
Jim

Hi Jim,

I ended up sending the fuel sender back to Air Parts for overhaul. It came back and seems to be reading correctly for now (fingers crossed).

Don't know about the interpretation of the FARs regarding the fuel gauge. 23.1337(b)(1) says that the fuel gauge shave to read "zero" at level flight when the only fuel left in the tank is the "unusable" fuel. I think this is what everyone is referring to when the cite the "has to read zero at empty" rule.

But 23.1337(b) says "Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used." 23.1337(b)(1) is an additional requirement on top of that. On its face, 23.1337(b) seems to require a properly functioning fuel gauge, not just one that reads zero when the tank car empty.

From a safety perspective, I am reasonably comfortable with using the accurate Shadin fuel fuel flow gauge (combined with pre-flight inspection) to gauge the actual fuel left. But, the Shadin fuel flow gauge wouldn't catch a leaking tank, for example. The other day, I noticed blue streaks coming from the area around the access door to the fuel cap, so now I think I have a minor fuel leak.

Back to the shop. Oh the joys of owning an old(er) plane....

(Great plane when it's not in the shop, though...)

Tomoharu

You are citing FAR 23 requirements. No Comanche was certified under FAR 23. They were CAR 3 aircraft. You will find your answer in the old CAR 3 regulations.