Originally Posted by
riff_raff
Here's something interesting to consider regarding the level of technology used to design the typical main rotor gearbox. Take the example of the AS332 MRGB failure in April 2009. The gearbox was designed prior to 1993. And the accident investigation was performed in 2011, around two decades later. If you take a look at the very sophisticated finite element fracture analysis study work described in the 2011 accident report, you'd appreciate that this level of analysis capability was far more advanced than what was available to the gearbox designers in 1993.
The various types of failure modes and the fundamental process behind each one, have been understood by gearbox designers for many years. But remember that the gearbox conforms to a design that was certified many years ago, and represented the acceptable level of safety and reliability required by government regulations.
Whilst it may well be that the gearbox conforms to a design certified several years ago it should also be remembered that following two bevel gear shaft failures, both resulting in precautionary ditchings with no serious injury, Airbus Helicopters extensively re-designed that shaft and these have now been retro-fitted into all of the offshore fleet.
So it would appear there is a precedent at AH for today’s analytical methods to be used to analyse stress distribution in older components and to redesign them and have them approved against (original?) certification requirements.
The big question is, did AH make any design changes to the 2nd stage planet gears following the 2009 accident and
AAIB Safety Recommendation 2009-075 to “review the design, operational life and inspection processes of the planet gears” ? The TBO for the epicyclic module appears to have remained at 2000FH.
Last edited by Concentric; 27th Jun 2016 at 18:43.