1970-02-01

Design of Helicopter Transmission for “On-Condition” Maintenance 700587

This paper considers the reduction of helicopter “cost of ownership” by the process of reducing the frequency of transmission overhaul. Reasons for time scheduled overhauls (TBO) and their validity to helicopter transmissions are explored. Failure functions for several transmissions are shown and individual failure modes explored for indications of failure function trends beyond the current operating regime.The conclusion is reached that significant cost savings are possible with On-Condition maintenance.The only valid reason for retaining a TBO in the mature transmission is the presence of wearout type non-detectable safety-affecting failure modes. These generally take the form of structural fatigue originating from fretted joints or surfaces or slow wear in internal spline and clutches. These modes may exist in some of today's transmissions and, if so, represent the limiting TBO. The full potential of On-Condition operation may not be realized without safety oriented changes.These changes include concepts which minimize or eliminate fretting, provide fail safety via redundancy or are tolerant to damage. Further improvement in condition monitoring and early, concentrated, development testing are the necessary ingredients for On-Condition maintenance of tomorrow's helicopter transmissions.

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