225 cleared to fly in UK & Norway

[Concentric]

Originally Posted by SASless

I would suggest Door Four contains something lots of folks do not care to consider....that being the "root cause" is the basic design of the 225 MGB.

It is not capable of containing what should not be a catastrophic failure and fails in that regard due to its inability to transfer the load path to other parts of the gearbox following a failure within an individual module.

We know the existing detection and monitoring systems fail to give enough warning...so perhaps there must be a simpler explanation than what the Boffins are looking for.

The American MGB designs use "thru shafting" as a way of holding all the big pieces together unlike the AH 225 gearbox.

Or am I misunderstanding the various drawings I have been looking at?

Any of you design engineers able to help me with this?

In Root Cause Analysis, the design features and detection or monitoring system failings you describe above would be regarded as causal factors but not the root cause. Those causal factors certainly affect the final outcome and ought to be identified at the design stage, at least in any new design and in Failure Mode Analysis.

The architectural differences compared to US-made MGBs you describe could make a significant difference in mast retention; however the containment requirements are different to those on a turbine which can be shut down. As you know, it is not enough to just retain the mast you also need rotor rpm and pitch control, probably with some hydraulics. Broken gears and other shrapnel rattling around in a gearbox with no quick escape path could jam or seize the drive or damage blades or dampers. Freewheels on the input side are designed to allow for an MGB to rotate faster than an engine, not slower, and there is no output freewheel allowing the rotor to maintain rpm in an auto with a seized gearbox. The ‘Lack of Lubrication’ thread gives a graphic account of a Huey with a seized MGB.

Having said that, I do think the Blackhawk and possibly the similar S-92A MGB architecture would be less likely to seize if a planet gear fractured, which I am not sure has ever happened.

The root cause is what caused the bearing to initiate a crack in the first instance but then propagate it in a manner that the designers had not expected, i.e. different to spalling, leading to catastrophic fracture.


There is a curious statement on Airbus' accident investigation status page that: "We understand the sequence of events that led to the fatigue fracture, but the exact cause is still being investigated." That might infer they think they know 'how' but do not yet know 'why'.