EC135 Fuel Sensing Probes
An EC135 operator practices a stringent policy of checking both the bulk fuel supply and the aircraft fuel tanks for water every morning before the commencement of operations. They use the standard ‘Shell’ detecting capsules, which will detect water in aviation fuel at 30 ppm (the recommended ICAO standard).
They are fully aware of Eurocopter SIN 2535-I-28, and have been for some time.
Yet potentially serious fuel indication problems still occur with their aircraft.
At present, there is no requirement by the manufacturers Master Servicing Manual (MSM) to inspect or function check the fuel quantity indication system at 500 or 1000 hrs. The fuel sensing probes are an ‘on condition’ component (that means they are not subject to time-life or overhaul).
So, if the probes correct operation is so susceptible to water, why is there no requirement to check them at regular intervals? There is always water in aviation fuel, it may be only a couple of ppm, but it’s there. I would even go as far as to ask if the probes are fit for purpose?
The EC135 Technical Briefings from Heli-Expo 2011 (pg24) and 2012 (pg29) are of some interest.
When Eurocopter talked about a ‘worse case’ in their last SIN relating to fuel indication, they were talking about this……
Both transfer pumps off, 170 kg in main tank, 47kg in No 1 supply tank, 30 kg in No 2 supply tank. FUEL amber caution illuminated and both No 1 and No 2 FUEL LOW red warnings illuminated (I can't get the photo to paste).
If the No 1 supply quantity indication was correct, the No 1 FUEL LOW light wouldn't be on. There was actually 13 to 21 kg less fuel in the No 1 supply tank than the display was indicating. A 27 - 44% error.