Originally Posted by
ColdWhiskey
My helo knowledge is very limited, but I do know that a ‘retreating blade stall’ can occur after an abrupt pull up, especially when operating at high gross weight and high speed. The result of such, is loss of lift and a rolling motion to the left.
Could the pilot have suddenly realized his terrain situation, and initiated an abrupt pull up, and inadvertently stalled? This could account for the left turn and sudden altitude loss.
Another thought. Has this model of helicopter ever had an instance of the rotor disc contacting the tail boom, after an abrupt pull up?
1. Retreating Blade Stall: Not likely the culprit here. If anything the abrupt cyclic input would result in a transient droop in rotor RPM, which could be exacerbated in high/hot/heavy conditions (not the case here). Rotor blade stall is typically something you'd expect to encounter in a high speed, power-on dive, not while performing an aggressive climb to clear a cloud layer. Thinking about this phenomena as you would a typical fixed wing airplane stalling isn't going to do you any good. Simply increasing AOA in helicopter (read: nose attitude in relation to the horizon), no matter how abruptly, is not going to result in a stalled condition. If the climb is continued and conditions are right (i.e. high/hot/heavy situation) power required will eventually exceed power available and the aircraft will begin to settle.
2. Rotor disc contacting the tail boom: The fully articulated rotor head found on the S-76 is more efficient and survivable than something you might find on aircraft with an underslung rotor system (see AH-1W, UH-1N, Bell 206, etc). In negative G situations, the possibility exists for underslung systems to "flap" resulting in something called mast-bumping or even the main rotor blades impacting the airframe. With the fully-articulated head, this is not something that would really ever be a concern. If you wanted to, you could unload the head significantly with little to no impact on aircraft performance.
Many things could have happened here, but looking at the weather, the flight path, and hearing the ATC communications in the minutes before the crash, it's easy to look towards spatial disorientation as a likely culprit. Reference the crash of the Army H-60 carrying the MARSOC team off Panama City a couple years back, and you'll see similar things (even with 2 pilots). A strong cyclic pop into the clouds resulting in what would have to be an immediate transition to an aggressive IFR scan is a recipe for disaster, especially in a single piloted aircraft. Having felt the effects of severe spatial-d many times in many types of helicopters, I can attest to the fact that it is not something easily recognizable. Couple that with close proximity to mountainous terrain and the results can, and have been, disastrous.