Eurocopter is working to reassure operators over the safety of the EC135's fuel probes in the light of an EASA emergency airworthiness directive (EAD) relating to the type.

The EAD relates to an update of the aircraft's flight manual covering procedures should the low fuel warning lamp illuminate and clarifying the correct operation of the light twin-engined helicopter's fuel pumps.

The safety regulator's move comes on the back of problems, traced to malfunctioning fuel probes, on several helicopters operated by Bond Air Services in the UK and two other undisclosed companies. This could lead to an overestimation of fuel on board, Eurocopter has warned, although the low fuel lamps - indicating 8-10min flight-time remaining - continue to function.

However Eurocopter stresses that the emergency nature of EASA's alert is simply to ensure swift compliance with the flight manual changes rather than indicative of an unsafe condition with the aircraft.

Eurocopter is also issuing alert service bulletins - accompanied by an EASA airworthiness directive - covering the EC135 which mandate operators to check the fuel probes on the helicopter within four weeks. If errors are detected then operators are advised to remove, clean and dry the components before refitting them.

The ASB cautions that tests have confirmed that "the affected fuel content probes generate erroneously high signals if contamination with water occurs". It also warns that sediment in the fuel can cause false readings from the probes.

However, Eurocopter is unsure as to the source of the contamination. "We have some ideas but we absolutely need to go deeper into our investigations and to a large extent the fleet-wide check will help," says chief technical officer Jean-Brice Dumont.

"Should it be purely limited to a specific operation we will be able to define a specific measure tailored to the customer."