Bell 412 EP aft crosstube &G relationship
 

Gentlemen(and ladies) anyone can tell me what and how 2.9 G can be generated in a normal hover ldg where the rear crosstube then breaks and the h/c skids slightly left?regards kojak Pl reply at kopterkojak@ hotmail.com thx

When you stress a part, engine, transmission, rotor blade, cross tube, ect..... it probably won't fail right away. Chances are it will fail much later on some poor unsuspecting soul who is just out doing his job within all rules and guidelines. That is why it is good to work with trustworthy folks who fly and maintain the machines. If you are not sure if something was normal or abnormal, discuss it with your mechanics and pilots.
Not sure if this applies to your question. Several "firm" landings could be cumulative.

I've never heard of a 2.9G landing being described as "normal".

I had a front cross tube fail on a UH-1H during a normal landing several years ago while I was in the Army. They (Army) sent the parts to a lab and said that there was a small discontinuity in the tube wall and it failed due to fatigue.
The landing was an approach to the ground, the aft part of the skids touched down first and as the A/C rocked forward the front tube snapped at the support. There was no hard landing.

Wasn't saying 2.9G landing is normal. I was saying you never know what was done before you got in the ship that causes it to break when you are operating within all rules and guidelines.
Kind of like a rotor blade coming apart in flight when you are in a normal cruise configuration, or a turbine wheel disintegrating in normal green power limits

mfriskel, I wasn't implying that you said anything of the sort... I'm referring to the OP's statement that a "normal" landing from the hover resulted in a (presumably) recorded 2.9G collision... because with that much force, I can't think of a better term to describe it :)

bell 412 Ep crosstube & G [email protected]
 

Folks, shawn gave me the best on Test Pilot/Engs section.It is a normal approach ,hover and touchdown.AFTER touchdown, crosstube snaps resulting in 2.9 g, then boom on tailguard and 2nd spike of 1.9 g.My Q to him has been G is acc which needs movement-hence lateral skid shift to port,however 2nd sound 1.9 g had a 2nd lat shift to generate 1.9 G?regards kojak

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....: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.