I would be curious to know what AH understand by the term “reliability”.
"There are two configurations of planet gear within the current type design. In depth review of the design and service data showed that one configuration has higher operating stress levels that result in more frequent events of spalling, associated with rolling contact fatigue, while the other exhibits better reliability behaviour. By limiting the type design to the gear configuration with lower stress levels and better reliability and specifying a reduced life limit, combined with more effective oil debris monitoring procedures and other operational controls, an acceptable level of safety can be restored."
They state that one type has more frequent events of spalling but the certification of the H225 and Super Puma range MGB is reliant on magnetic chip detection. The problem identified by AIBN is that one 2nd stage planet gear fractured without prior warning by chip detection. Similarly the only warning on G-REDL was one single chip detected and referred to AH for advice. According to the AAIB Report 2/2011 section 1.18.3, the advice later given by AH when a chip of planet gear raceway was detected on another aircraft, G-REDN, in March 2011 (23 months post G-REDL and long after magnets were all removed from oil separator plates) was to continue to fly and monitor, which it did for another 87 FH until further chips were detected. It is fortuitous that G-REDN survived and it is therefore important to know what type, A or B, was its planet gear.
As far as published investigation reports go to date, no gear fragment recovered from either crash exhibited notable spalling. So, if operators replace these planet gears with ones that are even less likely to give prior warning by particles from raceway spalling, how exactly does that improve reliability? It seems there are 2 possible failure modes, one with spalling and one without.
What AH appear to be doing is attempting, desperately, to perpetuate the myth that the only cause of fatigue cracks can be spalling, and spalling extensive enough at that to be detected before a crack propagates to failure (which in the case of G-REDL the AAIB estimated the total time of crack propagation from initiation to failure as “possibly more than 100 flying hours”). The thing you have to ask yourself, punk…
Last edited by Concentric; 12th Oct 2016 at 10:54.
Reason: Corrected quoted date.