The European Aviation Safety Agency issued an emergency airworthiness directive on 17 May relating to Eurocopter EC135 helicopters after a crack was detected on the lower hub-shaft flange of a main rotor hub shaft. The cause of the cracking has not been determined yet.
This, says EASA, could lead to further crack propagation "possibly resulting in main rotor hub failure and consequent loss of the helicopter". The agency had issued a previous directive on EC135 helicopter drive system cracking, but since then "further cracks have been detected on the lower hub shaft flange of two other helicopters during accomplishment of the pre-flight checks".
It adds: "It has been determined that the identification of deformed safety pins may not be sufficient to detect the cracks on the [main rotor hub] shaft."
EASA requires, for EC135 and EC635 series helicopters, repetitive pre-flight visual inspections of the main rotor blades attachment area in the upper and lower hub-shaft flanges, as well as repetitive visual inspection on the upper and lower hub-shaft flanges and the blade bolt area.
Initial inspections must be carried out within three days of the directive. After that, the flight time between any two pre-flight inspections must not exceed 6h.
Once initial inspections have been carried out, the accumulation by a main rotor hub of 400h since since its first installation on a helicopter should trigger further a further checking regime, and if cracks are found the hub should be replaced.
This warning about Eurocopter drive train cracking comes in the wake of a successful controlled ditching on 10 May of a Bond Helicopters Eurocopter EC225 (G-REDW). The crew ditched the helicopter off Aberdeen when gearbox oil pressure dropped because of a failure in the oil pump drive.
Use of flotation devices when the aircraft was set down on the water enabled the helicopter and all on board to be recovered.
An exhaustive inquiry by the UK Air Accidents Investigation Branch is still continuing into the sudden catastrophic failure of the epicyclic gearbox module in a Bond Eurocopter AS332L2 Super Puma (G-REDL) in April 2009, in which all 16 people on board died when the rotor completely separated.