Originally Posted by
alf5071h
The ALF 502 had a history of problems with the turbine bearings (4-5), these involved oil flow - overheat, oil coking, maintenance, filter clogged, ...
More often these issues resulted in a bearing failure, which in isolation was not a major problem - other than engine failure! However, because the 4-5 bearings were a combined package, so that if one or particularly both size it is possible to shear the main shaft due to a differential ‘clamping effect’.
The certification regulations on ‘containment’ require a narrow conical zone of airframe structure and systems which can tolerate debris exiting beyond the engine containment structure.
A major contribution to this safety requirement is that the engine main shaft should not break, not allowing turbine discs to move rearwards or change off axis angle, i.e. anything which can lead to tangental blade shedding or even disc failure outside of the certificated zone.
With continuing problems, the AFL 502 was subject to mandated modification requiring the redesign and change of the 4-5 bearing package.
AFAIK all engines were been modified with a new single pack / combined bearing.
The LF 507 used a similar (identical) rear bearing, but I do not know if these engines were required to be modified; it would be surprising if not.
Thanks for the background.
As for external damage, as above, I can only recall one other adjacent engine failure due to uncontained blades - Aviasca.
Yes, I'd heard of the Aviacsa incident, resulting in the famous "missing engine ferry" photo, though that can't be one of the two-engines-out events that the previous poster was referring to, as it wasn't an N2 failure and it was ages ago (about 25 years, I think).