Surge Tank Sensors

Please reference rotate.aero/forum/767-28fuel/401-ata-28-21-pressure-fueling for more info on pressure fueling overflow protection.

We had an aircraft that would not accept fuel (open the fueling valves) without pressing the "reset" button at the fueling panel. When this button is used protection provided by the Fuel Level Sensing Card (M586) is bypassed.

Usually, when this fault occurs you have one of the two surge tank probes not feeding a capacitance signal to the card.

There is a extender card for testing the probes.
  
     
  
Co-ax cables are connected to a fuel quantity test box on the Hiz and LoZ "Tank Unit" inputs. (The indicator plugs are used for simulation.)
  
     
  
For a 767, you want to see around 7pF to 8pF. We had one reading correct and one not.
     
  
Switching from capacitance to resistance, showed a HiZ to Shield short on the low reading probe.
  
     
This reading (all meg readings) should be open or the high MegOhm range.

To isolate further we disconnected the probe.
  
     
  
Resistance was still low. We then disconnected the plug at the wing root to isolate the wing harness. After disconnect, the resistance reading showed open.
  
     
  
Our problem was the wing harness. 

It's a tough replacement as it runs into the center tank, through the main tank, and then into the surge tank.
  


 

Looks like a brutal fix. Fuel is the worst system on the airplane by far to work on.