PPRuNe Forums - View Single Post - EC225 crash near Bergen, Norway April 2016
Old 18th Oct 2016, 07:12
  #1593 (permalink)  
Geoffersincornwall
 
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riff
The manufacturers employ gearbox designers who presumably know about the shortcomings of making an epicyclic gearbox, which, if I have understood the situation, is one that harbours basic conceptual floors given the manufacturing tolerances currently achievable. Small imperfections are magnified by the very nature of the relationship between sun, planet and ring gears when under load. It seems that the design philosophy is that 'we know it's going to break but we will replace/overhaul it BEFORE it breaks. Premature failure will be detected by HUMS (or equivalent) and by monitoring for debris using a system that gathers debris on a sensor and indicates the presence of debris to the end user.

Seems to me (pilot remember) that a gearbox overhaul period is a major key element. It may also be that existing gear wheels are NDT'd and returned for another spell at the sharp end at overhaul and not swapped for a complete new set. The TBO, in my simple mind, should be a fraction of the period before a test article produces debris. Say... half that period. (what methodology to calculate TBO's is used currently???).

So that brings into question the certification criteria with regard to initial TBO and overhaul protocols. Presumably the authorities have their own experts in gearbox design so the manufacturers cannot 'snow' the pen-pushers with a scientific overload. Can we use the lessons of the 225 to change the certification criteria.

History can teach us some lessons. As a type enters maturity the TBO's are extended as much as possible so reducing the safety margins in pursuit of reducing the costs of ownership. At the same time new types put pressure on manufacturing facilities so older types may have their vitals farmed out to sub contractors. I think Sikorsky have had that as an issue over the years with the S61 and S76. Sub contractors can cut corners to make money, particularly when i comes to the detailed checking of components. Who's to know if you check one in five instead of every one. One in 100 instead of one in 10.

So history sets us up to fail unless the rules are changed or existing rules properly enforced.

Seems to me that we need to invest in a little more reality from every organisation involved.
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