Thanks Fortyodd2.
The fact that the Eurocopter technical bulletins state that the capacitance fuel probes need cleaning and that some inspected probes worked correctly after cleaning, suggests that the probes could be getting a build up of bacteria. This can cause water droplets to be retained between the probe's concentric tubes. This in turn would give an incorrect capacitance reading, an incorrect remaining fuel reading and a false sense of security for the pilot.
Fuels
Larger aircraft use multiple probes to determine the location of the fuel surface across the whole tank. An incorrect reading from one probe would show up as a fault. The Boeing 777's ultrasonic system also uses an inverted probe to look for the water/fuel interface.
In response to Bill4a's query about water in the fuel tank, this generally takes four forms that I am aware of:
- Water pooled on the bottom of the tank below the fuel (what one is generally looking for when taking a fuel sample).
- Water molecules sitting between the fuel molecules throughout the fuel.
- Condensation on the tank components in the ullage (area above the fuel).
- Water droplets free floating in the fuel due to fuel sloshing.
- Water trapped by bacteria or by tank hardware.
If the problem is water droplets sitting between the probe tubes on a clean probe, then the very act of getting at the probe to inspect it may cause the droplets to be washed off or dislodged before the evidence can be seen.