Originally Posted by
Geoffersincornwall
riff
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???).
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.
Geoffers
TBO is typically driven by the life limited component with the lowest fatigue life, be it a gear, a bearing, a shaft, what-have-you. The fatigue life is calculated based on the loads modeled and then the safety factors ...
A variety of other things arise which lead to "on condition" overhaul decisions, which means that something didn't reach the predicted MTBF/TBO. It is in the ability to detect "on condition" conditions -- wear, pitting, fretting, corrosion, ect. -- that an on condition decision is made. If conditions are hidden or masked, as appears to be the case with this crash, a surprise failure, rather than "graceful degradation" is what the aircrew are confronted with.
A TBO change as a design matures would, in a perfect world, be based on parts or components remaining serviceable beyond its initially predicted/calculated fatigue life.
I am not sure I understand your point on broad brush TBO changes. Over time I'd expect them TBO to go down, not up, as parts age, and wear, and as actual replacement intervals are discovered in service (as compared to design/calculated fatigue life).