As we all probably did, I watched and read a lot of coverage on the Bin Laden raid a year ago. What caught my eyes and ears was speculation on why the MH-60 Blackhawk crashed when attempting to land in the compound. We may never know the real cause, but the most-reported reason was that the aircraft entered vortex ring state, also known as settling with power.
This little YouTube video explains VRS, but the short course is that it happens when the rotor disc doesn't have enough clean air to generate sufficient lift to keep the aircraft from settlingit can be caught in its own wash or turbulence from surface features. It's the opposite side of the coin from translational lift. VRS is always a risk on the approach to landing.
Viewed from inside, it's a little like parachute mode in a light aircraft; a wobbly state of slow settling. It can be recovered similarly, by reducing the collective and pitching the cyclic forward to increase airspeed to get the disc into clean air. But that takes altitude and if there's not enough of it, a hard touchdown or crash will be the result. This video depicts what appears to be classic VRS. (Watch it to the end.) Sometimes, excess power can arrest the settling, but in this case, the trees intervened. And the heavier the aircraft, the more aggravated the settling. (The MH-60 was equipped with stealth hardware and was probably heavier than a standard Blackhawk.)
During one interview, former CJCS Adm. Mike Mullen said he watched the teams train for the mission in the Nevada desert with an exact mockup of the Bin Laden compound, but with a fence rather than the compound's 12-foot high walls. See the problem? A walled compound acts like a big bowl, reflecting the rotor wash upward and potentially making a mess of roiled air that would make VRS more likely. Since these teams have done these operations in the hundreds, the pilots would have known this, if they knew the target was walled in rather than fenced in. Mullen seemed to indicate that during training, they did not know who or what the target would be until very near the step off date.
That left me wondering if their briefing included information on the wallsI can't imagine it wouldn't haveand if so, did they decide the risk of unrecoverable VRS was low enough to proceed? Did they encounter higher density altitude than expected? Or are they good enough to just scoot in over the wall and plant it? That wouldn't surprise me in the slightest. Helicopter pilots, educate us.
One former spec ops pilot quoted in the Army Times said the stealth Blackhawks are equipped with anti-radar coatings on their windshields that may make it difficult to see with night vision gear. It's possible that VRS wasn't the cause at all, but that the helicopter simply clipped its tail rotor on the wall and lost control. Either way, you have to admire the pilot's training and skill to plant the nose in the dirt to keep a bad situation from getting worse. And to the SEALS for shrugging off the crash they'd just been in and getting on with business.
We should all do our day jobs as well.