Originally Posted by
Sallyann1234
Would there normally be sufficient stress on the casing to cause such a fracture, unless it had a manufacturing fault?
Surely the casing is a container, rather than a load-bearing component.
In practice it has both functions.
There will be an amount of oil in the gearbox but the front and rear faces of the alloy casing have mounts for the various systems driven by the gears.
These would include Fuel Systems, Oil Pump, Hydraulic Systems and ElectricalPower Generator.
Some of these can be reasonably heavy hence load bearing.
However, compared to the extremely highly stressed engine components the alloy gearbox casings would have low stresses.
It is possible that at the end of the manufacturing process the casing would be heat treated to normalise the stresses.