Originally Posted by
Concentric
HC,
From an engineering perspective it ought to be appreciated that the main shaft in the 225 gearbox is multi-functional. It is supported in 3 bearings, 2 near the top very close together (possibly even contacting) and one near the bottom but above the oil pump drive. That bottom (radial) bearing appears much bigger than needed to react oil pump loads only. It locates the main shaft radially and reacts a moment due to the eccentric contact between the input drive and the axis of the main shaft. Indeed the proof of that lies in the cyclic bending stress that propagated the fatigue cracks, severing the shaft above the lower bearing. If the fatigue loading had come from the oil pump drives the shaft failure would be expected below the bearing.
With the shaft broken not only is drive lost to the oil pumps but the 2 upper bearings and their supports are subjected to loads which I very much doubt they would have been designed for. Evidently, and fortuitously, they survived long enough for the ‘precautionary’ landings on water but can anyone say how long they would have lasted without lubrication and cooled only with a 30 minute spray of glycol?
Yes I know all that, but the fact remains that the sheared shaft only caused loss of oil pump drive and the remaining bearing showed no signs of distress. My point is that you could present this in one of two ways, either as a loss of oil pump drive which, with functioning emlube, allows 30 mins at Vy (which of course probably doesn't get you very far, 40nm still air minus the time taken to land). Or you could present it as "main shaft sheared, rotors nearly fell off shock horror" except of course that the rotors didn't nearly fall off.
It's all down to how emotive you want to be.