Originally Posted by
MickG0105
Yes, it's decidedly difficult to see a discernible pattern. LN-RPK and SE-RET were built just over two years apart and both have cracks. You've then got LN-RNO and -RCN built just months either side of LN-RPK; no cracks.
Similar anomalies bob up in other fleets. Qantas's oldest B738, VH-VXA, cracks; -VXB, which came off the production line within a day and that actually has about 200 more cycles than -VXA, no cracks.
A few possibilities:
Inconsistent production practices, built on Friday or with a bigger hammer for alignment.
Number and magnitude of hard landings or other outside the norm conditions, reported or not.
Tolerance stackup issue where some combinations of within specification parts result in higher stresses.
Design should account for this but obviously something was missed somewhere.
Normal distribution of failures where some items will simply fail earlier than others, in some fields testing until a certain percentage of a batch of parts fail is used to quantify expected lifetime using statistical methods.
I suspect that Boeing is hard at work trying to determine the common factor(s), of course none of the above are mutually exclusive.