I got sent this by a friend who has S-61 time and thought I'd post it to see what folks make of it.
Some observations based on the photographs:
1. The tail rotor was spinning when it hit the ground. It is impossible to tell at what RPM it was spinning upon contact... my guess is that it was decreasing or winding down.
2. The tail pylon damage is not consistent with a main rotor blade (MRB) strike.
The location on the tail pylon would have required contact at mid-span (on the MRB) vice the tip which would be unusual. Additionally, none of the MRBs appear to have damage.
The tail drive shaft cover has separated cleanly as opposed to being "chopped."
The pylon, itself, shows tearing at the top with wrinkling on the bottom (tensile mode of failure?) along with a horizontal bending moment to the left or port side of the aircraft.
3. The tail wheel has been severely deformed and displaced to the right indicating a possible left drift or clockwise yaw.
4. Different photo aspect shows initial tail wheel contact on runway delineated by puddle of fluid, ostensibly from the strut cylinder. Additionally, skid marks from tail wheel assembly indicate a left and back drift prior to coming to a stop.
Suppositions:
Aircraft was probably in a hover at onset of failure.
Engine failure unlikely culprit as these well-trained blokes would have set her down with little or no lateral drift or yaw.
Possible villains:
(1) Loss of tail rotor drive - one of the few emergencies in a twin that requires immediate action. Impetus would be to get aircraft on the ground before spin accelerated. Hard landing would be warranted with vertical component strong enough to tear tail pylon off. Residual tail rotor RPM would cause all five T/R blades to bend even if not being mechanically driven.
(2) Loss of tail rotor control (stuck pedal equivalent) - not quite as bad as loss of drive - can actually be flown in certain flight regimes, but in a hover it would be nearly as nasty as loss of drive.
(3) Assuming the hydraulic system in the British Sea King is similar to the Yank version... it could have been an auxiliary hydraulic hardover in the yaw channel which requires rapid corrective action (securing aux hydraulic system, if memory serves). Diagnosis would be problematic in a hover as there's less time before onset of a high yaw rate.
Bottom line: The fact that the lads kept it right side up and the crew was able to sprint for the nearest exit attests to the best possible outcome under the circumstances.