Twin Otter fuel problem

[maintenance105]

We are quite new to Twin Otters and have a fuel quantity issue. The system is pretty erratic with aft tank indication, cleaned all coax connectors and wiggled wires to see which probe has the problem. It is also hard to find a test set for the ancient system.
We tried using the dip stick to determine the quantity but it does not match the actual quantity. My theory is that the flapper in the collector tank is not letting the fuel level stabilize, even after waiting a while and having the aircraft level. It reads always too low. The vents seem to be fine, so is it possible for the fuel bladders to have detached themselves from the side walls for them to collaps?

[ettw]

Have you drained the sumps of water? I have had erratic fuel indications that seemed to have been caused by water adhering to and messing with the capacitance of the probes.

Cheers

ETTW

[GyvAir]

As ettw said, water collecting/condensing on and inside the probes causing indication problems seems to be common in that system, especially in high humidity environments. You may need to remove the probes and flush them with IPA to remove the water droplets.
I would think that even if the bladders had become detatched, the weight of the fuel would be more than enough to push them into place when full.

[maintenance105]

We have drained the fuel twice due to a fuel low level light coming on with more than 330 lbs and found #3 bladder flapper had an obstructed hole. I guess pulling the probes out and cleaning seems like the next logic step, thanks a lot
Does anyone know where to get the capacitors to make the dummy load for the little harness that simulates full fuel qty?

[FrozenWrench]

With everything you have said I would agree and say to clean the probes next. While you have them out if you have a Capacitence tester you can check each one and make sure they are in tolerance. Another way is to connect the meter to them and then slowly dip in fuel and make sure you get a consistent change of capacitance. The capacitors are standard and can be found at any electronics store, along with the smaller connectors. Generally a stuck flapper in the collector tank will keep the low level light on longer then it should but that wont change your fuel qty levels. The Junction Blocks also like to fail from time to time, have tried to repair them in the past but it has never worked, new is the way to go.

Hope this helps.

[robertw]

I had this exact problem on a number of aircraft that we imported from Russia and China about 10 years ago. We filled a 5 gallon bucket with Jet A and mixed in a 16 oz bottle of BioBor JF, placed the probes in the bucket (with the tops town) and let them sit over night. You should have seen the layer of dead microbial growth on the top of the bucket the next morning. Solved all of the fuel indication problems.

[HavaJava]

@robertw

That's interesting! We used to get this problem on Twin Otters in the Caribbean and I always wondered if it was due to bacterial growth. From what I understand Prist in JetA1 is not only for anti-icing but also anti-bacterial. We were operating JetA without Prist and that is what sparked the idea in my head but the AMEs never pursued it.

@maintenance105, do you operate with Prist in your fuel?

[EICAS]

If you are getting a low fuel level light at around 300-400 lbs. I would check your fuel motive line filter. If it is plugged the fuel ejectors in your #1 & 2 cell will not be moving fuel to the collector cells ( #4, 5)

[maintenance105]

@robertw

That's interesting! We used to get this problem on Twin Otters in the Caribbean and I always wondered if it was due to bacterial growth. From what I understand Prist in JetA1 is not only for anti-icing but also anti-bacterial. We were operating JetA without Prist and that is what sparked the idea in my head but the AMEs never pursued it.

@maintenance105, do you operate with Prist in your fuel?

we only operate with Prist in the winter, we have not had any problem with growth in the far North
I appreciate all the input, still looking for a dummy capacitor and connectors...

[david_351]

To find your bad probe, an easy way to do so, is to fill up a bucket with fuel, and place each probe into it, upside down, hooked up to the j block, and see which one does not read, or underreads. As far as your low level light coming on early that is almost always cause by a build up of debris in the motive flow strainers, pull them appart and give them a solid cleaning and ensure the motive flow line is free of obstruction.