G force generated by Bell412 aft crosstube collapse

[kopterkojak]

will some one please explain to me that if a Bell 412 EP comes in for a normal hover ldg at 4 ft,and the rear crosstube shears probably due to metal fatigue,how will G to the extent of 2.9 on an fdr get generated?regards kojak(reply [email protected]) thx

[Shawn Coyle]

The G can be generated pretty easily by the short, sharp knock that follows when the fuselage hits something solid. Very short duration.
I've had the crash lights come on in what I thought was a pretty 'normal' landing, but which investigation revealed could generate a G load in exactly the right axis to be enough to automatically switch on the 'hard landing' lights.
So 2.9 is not that unusual for something like the landing gear collapsing.
Hope that helps - if you want more discussion, please PM me or contact me at [email protected]

[Rigid Rotor]

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....:suspect:

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.

[zudhir]

Kojak, In this connection the following Bell OSN maaybe relevant. A coincidence that its dated just a few days before the incident.

OSN 412-10-37 dated 06 May 2010
TO: All Owners/Operators of Bell 412 series Helicopters
SUBJECT: HIGH LANDING GEAR AFT CROSSTUBE P/N 412-321-104
Bell Helicopter has recently investigated a failed high aft crosstube. The investigation revealed the failure occurred in the inspection window after accomplishment of the AAI Alert Service Bulletin AA-07109 (ref. BHT Cover ASB 412-08-129). Further analysis revealed the primer used was not in accordance with the specified primer of the Alert Service Bulletin. Use of primer other than that specified in the Alert Service Bulletin may negatively impact the visual inspection process by masking underlying cracks on the surface of the crosstube.
This OSN is issued to remind customers to only use the primer indicated in the AAI Alert Service Bulletin AA-07109 (ref. BHT Cover ASB 412-08-129). Use of alternate primer can significantly diminish the ability to detect cracks.