I strongly believe that speculation should be left to the investigators, but there are some theories put forward with which I cannot agree. Having reviewed my tantrum above, I thought that I should break a personal golden rule about speculation in order to show that I am not just intent on criticizing other theories. I was also qualified on the UH-60, but I do not have extensive mountain flying experience. I have a copy of the footage and have been able to repeatedly view it frame by frame but there is some critical information missing on this accident, the most important of which is wind direction/velocity and gusts. IMHO all speculation will be weakened (including mine) without this info.
My tape shows that the aircraft was hovering OGE with its right side toward the slope. This seems a reasonable thing for a right hand seat flying pilot to do so he/she could use the crevice as the only available reference point in the white wall above and below the aircraft, have an escape route down and left, and put minimise the hoist to ground distance as the hoist is fitted to the right hand side. Seems smart enough to me. Note that they had already hoisted a rescue crewman and gear into the site, thus they were lighter at the time of the accident, than during the previous hovering. This would indicate that there was initially sufficient hover power margins assuming that they did not refuel between hoist insertion and the accident. There is not enough clarity to see if the hoist was deployed during the accident – i.e. wether they were attempting to lift up to 3 survivors (the capacity of the hoist is about 600lbs) or wether they were just hovering when the sequence began to look ugly.
The aircraft begins a GENTLE turn to the right, toward the mountain face, but who knows if it is uncommanded or due to wind gusts? Maybe the pilot wanted to do it? At 11,000 ft OGE, I believe it is probable that the aircraft had insufficient left pedal to arrest the right turn, as per B. Sousa’s comments above. This is NOT LTE in the classic vortex induction sense, rather it is similar to having insufficient pitch at its max deflection, sometimes (incorrectly) referred to as “overpitching the tail rotor”. Running out of left pedal is not unheard of in a Black Hawk, and in fact they had a pitch link rigging change incorporated into Australian ones for a while to increase the max available tail rotor pitch (subsequently removed for other reasons I believe).
At this point there is a slight nose down pitch (sorry to blow your theory here Lu) although this may be an illusion due to camera angle. There appears to be no movement of the stabilator Shy Torque, and I think it may be safe to exclude it as a cause because it would be full down in the hover anyway, so movement would only be stabilator up which I think would cause a nose UP not down. Possibly the pitch down is due to the ricurculation caused by the slope acting on the front of the disc. Maybe, knowing the vis out of the 60 is terrible, MAYBE the nose down can be accounted for by the pilot trying to stay visual with the reference point for the reasons that Shy Torque expresses so well above. Who will be able to say this definitively? Probably not even the crew. A flight data recorder would be the only answer here – are they fitted to the AFRES Pave Hawks?
So, it has turned gently right to face the slope, the aircraft appears to pitched a couple of degrees nose down and the situation is getting ugly. The conning angle does not appear to be rapidly increasing, there is not puff of smoke or debris, and there is no sudden altitude drop evident from the camera angle, thus I believe a sudden engine failure is unlikely. There is a possibility of gradual conning angle increase but the camera angle and time frame from this point to impact does not allow a definitive statement on this aspect. At this point, the nose pitches up and as a result, the aircraft starts sliding backwards and losing altitude, although not losing too much height due to the ground slope. There is a sudden pitch nose down bringing the aircraft almost level , but still descending and now losing height as well, and the refuel probe followed by the rotor disc impacts the slope and the tumbling begins.
It is the sudden nose pitch up which most convinces me that speculation about vortex ring is misplaced here SASless. It is very unlikely (though not entirely impossible) that the aircraft would be in vortex ring given that it was less than 50 ft above the slope in a steady state hover. I presume you are thinking that there was some form of sudden updraft because the aircraft did not appear to be descending prior to the sequence – it was in a hover. If there was an updraft, then less power would be required to maintain a hover, thus reducing the likelihood of vortex ring. Also the UH-60 is not very susceptible to vortex ring, but when it gets into the state it has very little cyclic control, yet the accident aircraft clearly pitched up away from the ground slope indicating that there was control.
IMHO, therefore, there are HEAPS of possible causes. I have listed just some of them. I think the right turn is critical to the sequence, as there appears to be a slight increase in conning angle (though not definite as discussed above). These aspects would indicate a power deficiency of some description – i.e. main rotor bleed due overpitching due to a couple of the following possibilities:
1. Wind downdraft causing excessive collective application to be used to maintain height,
2. Wind directional change causing more left pedal to be demanded, therefore more torque required,
3. Pilot reacts to uncommanded right turn by applying collective to get away from ground (disorientation a possible factor here too),
4. The footage suggests that a catastrophic or sudden engine failure is unlikely, but does not rule it out entirely. However, nothing in the footage rules out a low side governor failure (although my experience has only been of highsides in the Black Hawk as per their great design). The low side would be accompanied by main rotor bleed, and running out of left pedal.
I am sure there are more possibilities, such as during the turn the pilot loses visual on the reference point and becomes disorientated. But lets look at what happens after the turn: the aircraft pitches very nose up. Because the aircraft turned, it has turned away from its escape route – downslope. Thus the escape route is behind so maybe the pilot tries to go that way to avoid the survivors on the ground, and to get some height to reduce power. Now sliding backwards quite rapidly, the aircraft gets a large flap back (in this case “flap forward”) and pitches nose down so rapidly the pilot is unable to avoid contact with the ground, or maybe he/she does not even see the ground due to white out, or maybe he/she is already trying to level the aircraft when the flap forward strikes thus exacerbating the control input.
Hopefully I have shown that there are many possibilities not all of them fall on the shoulders of the pilots – and perhaps the investigation will be better than our speculation???????
The pilots and crew were trying to help other people. They were trying their best.
Hats off.