re post and article at
http://www.pprune.org/rotorheads/204...ml#post6163118
Grumpytroll says: (as endorsed by FH1100 Pilot)
1. The author is pushing the VRS hard here but, according to his report, the aircraft had a forward speed of 80 knots when it first contacted the ground. That speed removes the first and biggest reason for getting into VRS. Zero or low forward airspeed.
Try to think "post-VRS aftermath recovery profile" here when considering the CV22's arrival at circa 80kts. You can do the same nasty thing after/during a classic helo recovery from a vortex ring encounter - due to having to lower the nose cyclically and fly out of the condition near the ground. A horrendous descent rate develops and you meet, unwillingly and unexpectedly, with terra firma. Not hard to envisage this scenario in either a helo or tilt-wing. Especially not hard if you've personally had a close encounter. The VRS recovery profile descent rate is akin to a full-blown autorotation.... pre first flare. However the big difference is that, in the VRS scenario, there's unlikely to be the stored rotor energy to achieve a flare. The recovery maneuver is trying to re-energize the rotor by escaping recirculation. I'm guessing that it's the same terminal phase that the USAF CV22 pilot was in. He just ran out of height but luckily he'd thought out his pre-planned response and had the sense and presence of mind to bump the nacelles forward and go for forward speed (rather than emulate the hapless Marana crew by applying power and risking further (rolling?) development of any incipient hybrid asymmVR trend.
(Land a commercial transport aircraft with a 17 knot tailwind....
Here I think we must concede that it's the wind at height on the approach that's more conducive to setting the "arcing over" stage for VRS, rather than the wind component at ground level (sorta chalk and cheesish).
...but with 17 knots blowing at your back, the possibility of a brown-out is very likely and may have contributed greatly to the events that followed regardless if the fault was eventually that of the aircraft.
Drawing a very long bow here methinks (see ETL commentary by FH1100). Quite realistically, the stage was set for this accident much much earlier than that at which brown-out might be encountered. It was quite fairly discounted by the Board of Inquiry as a factor. Not likely to affect vision during a high R.o.D. 80 knot run-on "landing". Concede that - or lose credibility.
Overall, because the concept of incipient/hybrid VRS in a tilt-rotor requires quite a bit of extrapolated imagination, it's a mite indigestible. But you can be sure that it is uppermost in the mind of any V22 flight-crew, particularly if extraneous factors like TOT and "can do" pressures leads them into an unstable approach. Even if it wasn't uppermost, any sign of VRS/asymmVR "departure" (to use the FW autorotative pre-spin term) would make it so.
There will always be tilt-rotor accidents. The unfortunate aspect of this one is the "disappearing" of the recorder. It's starting to smell that same flavor as did the process leading to the earlier (2002?) dismissal of an MV22 Sqn Commander for fudging his fleet's serviceability and operational readiness figures. That investigation never went far enough up the ladder to nail the initiator of the cover-up. The charade would appear to be continuing. "Remember guys, we're all agreed. If we lose one over there, the first priority is to secure the recorder to keep its data out of the "wrong hands". Make sure the relevant personnel are briefed accordingly."
http://www.pprune.org/military-aircr...56-osprey.html
http://www.pprune.org/rotorheads/881...tor-hover.html
http://www.pprune.org/rotorheads/881...tml#post843574
http://www.pprune.org/rotorheads/116...er-merged.html
http://www.pprune.org/rotorheads/142...lt-rotors.html
Unfortunately the very lengthy post-Marana thread appears to be no longer on Pprune. (that I can find anyway).