The UK Air Accidents Investigation Branch (AAIB) has explained why the emergency lubrication system sent a “failure” warning–while working nominally–to the crews of the two Eurocopter EC225s that ditched in the North Sea last year. Following a modification in 2010, the internal wiring of some switches was changed, “owing to an error in the specification [Eurocopter] sent to the switch manufacturer,” the AAIB wrote in a special bulletin released earlier this week.
Therefore, the system had a pressure switch configuration that resulted in illumination of the main gearbox emergency lubrication warning 30 seconds after it was activated. Such an indication requires the pilots to land immediately.
In both incidents, the bevel gear shaft cracked, causing a loss of main lubrication. The emergency lubrication system was operating normally, the investigators emphasized. They added that Eurocopter issued an Alert Service Bulletin on February 22 “to modify the wiring on the helicopter, to be compatible with the pressure switches.” EASA simultaneously mandated the bulletin.