Originally Posted by
JoeCool88
According to Airbus all the different stresses were taken into consideration. Due to the hardened surface of the bearing races and teeth,16NCD13 layer after a nitrid process, and the "weaker" material inside the rim (M50), stress usually peels of the hardened surface. Resulting in flakes, splinters etc. on the mag plug. Which gives an early warning of the degradation. What was new in the G-REDL and LN-OJF case is that only a very limited such surface particle production - if any at all - took place during all the hours where the crack has grown inside. After a lot of time spent trying to understand what happened, my last idea was resonance frequency. In the MGB you have a lot of shafts, gears etc. turning with different speeds, inducing frequencies into the system. What if, in addition to the mentioned stress, the planet gears are exited with a frequency closed to their natural frequency? In combination with a micro pitting in the bearing race, as starting point, this could explain the crack growth inside the rim. Consequently a slightly different rotor speed could lead to a different planet gear behavior. According to this theory a crack would not grow even if initiated by a pitting, if the MR Rpm is e. g. 263 instead of 266 or vice versa.
Hi Joe .... my comment is about ... "how the crack grows undetected"
I followed this investigation closely (read everything) back in the day ..... a small surface crack may not progress on its own unless there was a flaw in the original metal (rare in this case) ...... the problem is the rolling bearing is compressing oil at a very high pressure that causes hydraulic fracturing into the crack which causes it to keep growing over time.
Same physics that can split a granite rock when a drop of water seeps into a crack and freezes and expands.