Originally Posted by
Noiseboy
One point to note is that as well as removing any gearbox which has suffered impact from service, they also removed any having had a lightning strike.
The gearbox fitted in the L2 accident had sustained a lightning strike while fitted to a different airframe. Maybe they cannot categorically say it is the gear types because both gearboxes had encountered external factors which could contribute to the cause.
It probably takes an external damage event (e.g. like lightning strike or transport damage) to trigger premature spalling and propagation of cracks. The design itself seems to be OK as long as no such premature initiation occurs (otherwise more H225/AS332 helicopters would have lost their gearboxes - they accumulated massive fleet hours without such occurrences). It appears that it is simply not very tolerant against such external influences. And the combination of that particular bearing type producing higher peak stresses in the outer race plus an external factor was apparently sufficient in two instances to trigger this premature cracking. The really unfortunate part is that this damage mechanism isn't really properly captured by HUMS&Co. That is what makes it so dangerous.