Originally Posted by
Agile
take the AS350, from the B to the B3, power went from 650hp to 1000hp, MTOW from 4300Lb to 6700Lb (EC130). All they did is add reinforcement plate at the root (fastening to the fuselage) of the tail, but nothing diferent in between.
For reference, the 350/355 tailboom structure, while of the same monocoque construction, has a completely different mounting system and would be considered a bit more rigid than Bell’s method. However, even 350/355 system had their share of t/boom issues and failures.
Originally Posted by
helispotter
My current 'guess' is that one point in the breakup may have looked something like this (rotor facing aft is mostly omitted from sketch & gap in tailboom is simply due to graphic limitations of my drawing package):
Agree. The vertical fin offset will also drive the tailboom in that same direction.
As to the t/boom failure itself, one needs to understand that the strength with monocoque construction is from the skin and not the internal structure per se. So given the t/boom failed just aft of the intercostal structures, any loss to the structural integrity of the skin in that general area would cause the t/boom structure to collapse under load.
This is why most OEMs have no or minimal damage allowance for t/boom skins and especially near load transition areas. And for reference, the 4 ea. intercostal hat sections immediately forward of the t/boom failure point and inline with the 4 mount bolt fittings, are what transition the flight loads from the 4 mount bolts and rings to the t/boom stressed skin.
Regardless, tailbooms have always been an Achilles heel for Bell as far back as the UH-1 and 206A. Even the 214ST was known to bust t/boom bolts, skins, and pylons on occasion.
there is a hint that some of the vertical stabilizer may also have been laying near the horizontal stabilizer parts at the pier. If so, it is in at least two parts (with one showing the black-gold-white paint scheme):
Yes, the vertical fin is there in 2 sections with the lower section still attached to a portion of the TR GB mount casting that also includes the lower TR GB cowling still attached to its mount ring on the fin. Thinking out loud, given the clean breaks on the vert fin and hori stab, it appears the tailboom assembly suffered multiple MR strikes under power. So it is quite possible those blade impact forces on the tailboom assy may just have been enough to cause the deck beam attachment to fail resulting in the loss of the MR and drive assy. At least when compared to other similar 206 deck failures I've seen in the past.
As to the Van Horn blades collective bounce issue, even Bell had similar issues with this phenomenon with factory blades and as I recall even an accident or 2 as well. There can be various reasons behind collective bounce, and I believe if this was a more common occurrence with VH blades than Bell blades, then there probably would have been more regulatory action taken.