Check out my reply in the Flight Test section - reproduced here for convenience--
You mentioned metal fatigue - was that the findings or just an informed guess?
If the metal analysis report indicates a fatigue fracture, its then certain that the cross-tube had weakened due to the fatigue and the last ‘knock’ was enough to break the weakened structure. The fatigue crack (if that was the reason) could have originated due to various reasons.
If the analysis report indicates an overload fracture – then its certain that the last landing was the sole factor and was hard enough to break the cross-tube all on its own.
Either way, you will perhaps need the metal analysis report to definitely conclude why the cross-tube broke – whether the structure was broken by that single hard landing or whether it had already been weakened and the hard landing was only the last contributory factor.
The FDR having recorded 2.9 Gs indicates that the landing was pretty hard indeed. I know a recent case of a skid-variant civil ALH (Dhruv) buckling its rear cross-tube after the pilot mismanaged a practice forward speed landing. The G value recorded in the FDR was around 3. However, the ALH’s cross-tubes are designed to deform around that G value to absorb the impact as a component of the overall crashworthiness capability of the aircraft (dictated by military requirements). So in that case, the skid cross-tube deformation was as designed.
If you are suggesting that the cross-tube broke off first in a normal landing and the G force was then generated by the fuselage impacting the ground – you will need the metal analysis report stating that the cause was fatigue fracture to back that hyothesis....
The 412 EP isn't designed to any military crashworthy requirements as far as I am aware - and that hard landing must have been enough to knock the stuffing off it!
In case of the ALH - it went through the mandatory checks of dynamic components and engines- everything found ok- the rear-cross tube that had buckled was changed and the helicopter is flying again.