It sounds like the new materials being developed and becoming available will push the capabilities of new rotorcraft into areas where older types will struggle to compete, with evolutionary upgrades no match for fresh design (except perhaps where those types can demonstrate a long service history with very few if any major accidents).
Where will that leave aircraft such as the H225 and AS332L2? Were it not for two accidents, both apparently caused initially by fatigue fracturing of a planet gear of ‘current spec’ 16NCD13 material, we might not be discussing the merits or feasibility of changing this material and the H225 might have a healthier future. Was the planet gear material in the SA330 or AS332 up to L1 variant the same as in the L2 and EC225, I wonder?
In visualising what may have gone wrong in these 2 similar accidents in the moments following planet gear fracture, it has struck me that AH really put all their eggs in one basket depending on these gears not to fail.
My interpretation of the G-REDL accident and report 2/2011 is that it wasn’t simply the failure of one gear that brought the aircraft down. In photo figure 21, the other 7 gears look pretty serviceable and should have been able to accommodate the small increase in load share if that was the only design consideration.
It wasn’t simply that the ring gear split at the 5 o’clock position either, as seen in Figure 20. There would probably have been enough of the remaining attachment of the epicyclic casing at that moment to keep the mast in place and some measure of pitch control possible.
However, the way that successive bolting around the epicyclic casing would subsequently fail as trapped debris worked its way around the module, would lead to the lower bearing in the conical housing becoming displaced. Thereafter the loss of control and breakup sequence would have been very quick.
This thread discussion has wandered interestingly to a side issue of “run-dry” capability and, learning of some posters past military experiences, I began to understand there are other reasons apart from the condition of the North Sea why you might not want to make an immediate landing, for loss of oil or other cause.
It is not unusual in military aircraft to provide, at the cost of some increased weight, some ‘armoured’ protection for critical components against hostile action. Even civilian turbine engine casings will be designed to contain a certain degree of blade shedding.
So could not AH have provided some means of preventing larger items of debris from a fractured planet gear from becoming entrained in the gear meshing? I know there is not a whole lot of room between planets but I wonder if some sort of guarding built into the carrier design might delay the break up of the planet gear by bending forces; retain debris including rollers from escaping into the other workings of the gearbox; and keep debris from elsewhere out from the planet itself. The critical objective would be to keep debris out of the sun gear (especially) and the ring gear meshing. Planet gear tooth damage might be tolerant to an extent.
If this could buy just a few minutes to make an emergency descent and landing I think the extra weight would be worth it. Indeed, I am surprised that certification does not demand it.
To be fair to AH, the recommendations in AAIB report 2/2011 were to review the design of the planet gear, not the consequences that actually bring down the aircraft. If you don’t ask the right question you probably will not get the right answer.
If guarding of the planet gears is beyond engineering capability then could not the bottom of the conical housing be provided with independent location, in principal not unlike the upper lift housing? Some sort of external brackets between MGB and conical housing, 3 as a minimum and possibly close to M/R servos to limit deflection of a burst ring gear from breaking or jamming a servo?
If my questions sound a bit dumb it is because I am not an aviation ‘insider’. My contribution to aviation is mostly as moveable ballast, so if instructed to I will respectfully go and sit at the back and buckle up!
Last edited by Concentric; 11th Jul 2016 at 09:55.
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