Manufacturing defects have been suggested as the root cause of an in-flight failure of a crucial gearbox component that contributed to the 10 May ditching of a Eurocopter EC225 in the North Sea.
A special bulletin issued on 13 July by the UK's Air Accidents Investigation Branch details the problems suffered by the helicopter, G-REDW, operated by Bond Helicopters, forcing it to ditch in the sea 20nm (37km) east of Aberdeen with 12 passengers and two crew on board.
The key component in the drama was the bevel gear vertical drive shaft. Formed of two separate pieces welded together, this cracked in two after fractures spread from a hole drilled at the weld. As a result, says the report, "the lower part of the shaft moved downwards...causing the pinion to disengage partially from the oil pump drive gears".
"At this stage, the lower part of the shaft was no longer being driven," it says.
This, in turn, caused the failure of the gearbox's primary and back-up lubrication systems. Although the pilots operated the emergency lubrication system, a warning light indicating the latter's failure illuminated shortly after it was activated. The correct procedure in this situation is an immediate landing, says the report, which the pilots duly performed at 11:14 local time.
The cracks in the drive shaft propagated from a 4.2mm hole drilled through it. Investigators found tooling marks, a spiral scratch "that ran along the length of the hole" and that the countersunk portions at either end were "outside the design specifications and there were a number of 'scoops' in the inner countersink," it says. A post-accident examination of 18 bevel gear shafts by Eurocopter showed "variability" in the geometry of countersinks "and a number were found to be outside the design tolerance".
Tests by the investigators on a bevel gear shaft initiated and propagated a crack from a 4.2mm hole "after it had been deliberately corroded under laboratory conditions prior to the test", the report says.
However, Eurocopter disagrees with the AAIB's analysis. It says: "Laboratory testing to date indicates that the manufacturing quality of the vertical shaft hole and of the countersinks are not the cause of the crack."
Its fatigue tests confirm the "component shows wide strength margins and that the dimensions of the shaft remain correct", it says.
Eurocopter believes it has identified the root cause of the failure and is conducting further tests to confirm this.
The incident was also the first in-service use of the emergency lubrication system. This takes bleed air from the left-hand engine and uses it to inject a water-glycol mix from a reservoir into the gearbox. Investigators have so far been unable to find a reason for the illumination of the emergency warning light, as the majority of the system appeared to be functioning properly.
AAIB notes, however, that "there is some preliminary evidence that the in-service reliability of some of the components of this [emergency lubrication] system is lower than that assumed in the System Safety Assessment for certification.
"The work in this area is ongoing," it says.