Blackhawk Accident

[Gibbo]

Just saw some footage of a Blackhawk accident on morning news. The accident occurred on a mountain side, in the snow; end result is aircraft coming to rest on its side after a number of rolls through the snow.

The clip provides an incredible, clear view of the accident.

I hope that all involved are OK.

Gibbo

[Heliport]

Sadly, according to a CNN, one of the crew is in a critical condition.

GOVERNMENT CAMP, Oregon (CNN) A mission to rescue trapped climbers atop Mount Hood turned into disaster Thursday when a military helicopter crashed on a snowy slope.

The chopper, an Air Force Reserve Pave Hawk, slowly twirled out of control and crashed onto the mountainside. Its rotor blades broke apart and the chopper rolled over several times as it slid downhill.

The helicopter was carrying four people, one of whom was in critical condition after the crash, said Krista Vasquez of CNN affiliate KGW-TV, which captured the crash live.

At least one person was thrown from the aircraft and was run over by the chopper as it rolled in the snow.

The rescuers had been trying to reach three climbers who died and four others who were trapped in a crevasse after they fell while on a climbing expedition earlier in the day, a Clackamas County fire official said.

The climbers were with others in a group just 800 feet from Mount Hood's 11,000 foot summit when they fell, Clackamas County fire Capt. Jamie Karn said. Weather conditions in the Cascade Mountains at the time of the accident were "absolutely beautiful -- a clear, sunny day," Karn said.

No details on the ages or nationalities of the climbers were available.

Rescue workers on the mountainside went immediately to the wreckage to tend to survivors.

"We had one of our rescue helicopters go down, we're not sure of their condition," said Oregon Air National Guard Sgt. Nick Watt. "A rescue operation for the downed chopper was under way."

Retired Air Force Maj. Gen. Don Shepperd told CNN the gleaming white snow could have diminished the pilot's depth perception, causing him to maneuver the helicopter too close to the mountainside. At such a high altitude, he added, it may become difficult for a helicopter to gain needed lift.

http://i.cnn.net/cnn/2002/US/05/30/oregon.mthood.accident/vert.crash.sequence.jpg

[Heliport]

HH-60G Pave Hawk
http://i.cnn.net/cnn/SPECIALS/2001/trade.center/interactive/aircraft/helicopters/hh60.seahawk.jpg

Primary function: Combat search and rescue and military operations other than war in day, night or marginal weather conditions
Speed: 184 mph
Armament: Two 7.62mm machine guns
Crew: Two pilots, one flight engineer and one gunner

Source: U.S. Navy

[ShyTorque]

Heliport,

Your picture is actually of a Seahawk, I think.

The Pave has a refuelling boom on the stbd side of the nose and the normal tailwheel at the extreme rear of the fuselage.:)

[RW-1]

I just saw the video on CNN.

My condolances to the families of the crew.

This is just sad.

(Yes, ST, its a seahawk Pretty are they not? :) )

[Heliport]

I lifted the pic from the CNN site without checking.
Humble apologies. http://www.stopstart.btinternet.co.uk/sm/notworthy.gif

http://www.afreserve.com/images/aircraft/hh60.jpg


http://www.afrc.af.mil/news/afrcnews/9392.gif

Thrust: 1,560-1,630 shaft horsepower, each engine
Height: 16 feet, 8 inches
Speed: 184 mph
Maximum Takeoff Weight: 22,000 pounds
Armament: Two 7.62mm machineguns
Length: 64 feet, 8 inches
Ceiling: 14,200 feet
Range: 445 statute miles; 504 nautical miles (unlimited with air refueling)
Crew: Four (two pilots, one flight engineer and one gunner)
Date Deployed: 1982


Now that we have the pic resolved, I hope the crew, especially the man critically injured, will be in our thoughts and prayers.

[B Sousa]

Sad stuff indeed. Same thing (UH-60), almost to the elevation happened on Mt Shasta ( California) a few years ago.
Unknown to me the cause of either accident, but Im betting Altitude/Experience plays a big factor, even for such a good machine.

[Steve76]

Or maybe loss of situational awareness? Trying to hover facing a snow covered mountain isn't easy.

[B Sousa]

Steve 76..... Could be your right, but I finally saw the pictures and as you see the aircraft facing the mountain is because what looks like it spun around. That sort of indicates Loss of tail rotor Effectiveness and back to Altitude/Experience...
Where is Nick Lappos or Lu Zuckerman (with one N) for an opinion... but again I think Nick will have to stay away from this based on where he works....Understandably so....

[nomdeplume]

Latest BBC reports:
All crew in a serious condition - one thought to be critically injured.
The exact cause of the crash is still unclear - not surprisingly.
The weather was sunny and winds were calm. The helicopter dropped off a rescuer and equipment above the crevasse and then started to lose altitude. Its refuelling probe got caught in the snow, according to officials - causing the helicopter to career sideways down the mountain. The rotor blades sheered off and the helicopter rolled over 300m (1,000 ft) before coming to rest.


Does anyone have experience of high altitude ops in snowcovered mountains?
Please no speculation about the possible cause of this sad accident - just general comments about difficulties faced in those conditions.

Hope all the crew pull through. :(

[Joker's Wild]

We can start with the obvious I suppose. Whiteout and loss of tail rotor effectiveness.

On an overcast day in snow covered hills there is little or no depth perception when working near the ground and this has been the primary cause in far too many helicopter mishaps.

Loss of tail rotor effectiveness is something most mountain pilots will have on their minds while conducting high alt ops, particularly when operating near gross weight for that altitude. I can personally atest to being very short final for a peak or pinnacle on a few occasions only to find my left foot can no longer go any further forward. Round about that time the helicopter simply wants to start rotating to the right. If one is lucky and the aircraft gives enough advance warning, the pilot may have room to lower the collective, fly away and try again.

I can't comment on high altitude resuce ops though so will leave it at that.

My two cents.

[Fly Stimulator]

Video clip currently available at Sky News (http://www.sky.com/skynews/article/0,,30000-10429425,00.html)

[OldAg84]

It appeared the nose dropped/was pushed over rather abruptly and the pilot tried to back off and get some breathing room. There was apparently minimal wind and clear skies, whateverthat means - Can any pilots tell me-in general terms-


How linear is the loss of control authority with altitude?

How could/would tail rotor effectiveness affect the pitch?

How do mountian winds behave that close to the mountain? Could mountain winds force a nose over close to the side of the mountain? I'm guessing yes- but I'm curious for better info.

Respectfully submitted.

Thanks.

Again, my thoughts are with the crew (and climbers).

Disclaimer- I'm not a punter nor a journolist.

[tgrendl]

Hopes for a speedy recovery for all involved.


Watching the video it appears the pilot intentionally avoided the area occupied by ground personnel/ rescuers to his (or her) own detriment.

Could have been much worse.

[RW-1]

Looked wobbly to me prior to the LTE rotation, possible they pulled in more power then available and drooped Nr ?

Can go with loss of situational awareness in those conditions.

[B Sousa]

Just an example to talk about tail rotors. I flew a few thousand hours in Hueys. In fact just about all models. Hueys were in two different airframes for different reasons and various engines and rotor systems. In the civilian world they are both hueys, but in airframes. 204 or 205. The 204 being the smaller of the two.....
Anyway, the UH-1H is the better hauler and designed for just that. The UH-1 (C) or (M) was usually used as a Gunship. M being the latter designation until replaced in the inventory by the AH-1.
Bottom line when some M Models were used for hauling (minus guns etc) and further up in the Hills, pilots began to run into problems. It had a shorter tailboom, and different rotor system, which made all the difference in the world. My first IP would take me up to an LZ in the Mountains and We would do 50' OGE hover turns. Sometimes it would not turn. An important lesson for sure. So much so that if I were flying at Altitude with an (H) pilot and in an (M), the first thing I would do would be to go to some Ridgeline that was safe at High Altitude and have them try and land like they would in an (H). Everything would look good until down at the last few yards. I could see the guy was trying to shove the pedal through the chin bubble and got ready for the spin.... It almost always happend and then I would take it over the other side of the ridge and gain control....It was a good lesson for those guys that even with the same engine as the (H) the difference in tailboom/Tail rotor position made all the difference in the world.
I have found that at Altitude one must be more aware of things such as this and leave avenues of escape should this begin to occur. I understand its not always possible. Enough rambling, Back to Blackhawks....

[I. M. Esperto]

I've been to Mt. Hood. Calm conditions are very rare. Generally it is windy, so I'm not sure of the reports of light/calm conditions.

[SASless]

The final report should be interesting to read....I watched the video clip a couple of times over a few hours....and a few thoughts came to mind. My view of the thing seemed to show the aircraft in a high...OGE hover facing the slope....he had performed one successful hoist and was back for a second one....the aircraft began to accelerate downwards, at an ever increasing rate, the main rotor blades seemed to develop a noticeable coning angle, the nose dropped and began to rotate to the right.....my impression was he may have encountered settling with power (vortex ring state) and was trying to fly out of it when the mountain got in the way. High altitude rescue flying remains a hazardous business.....my hat is off to those that take those risks in order to save others. Sikorsky sure built a tough bird to survive all that as well as it did.

[B Sousa]

Settling with power is a good thought. Facing the slope and trying to turn out of the situation. Coning of the blades a possible indication of an attempt to climb through, but as in Settling only increasing the problem....
Interviews with the crew will be the only sure way to verify....
Heres to a speedy recovery for those guys as that was one big snowboard......

[ShyTorque]

From a previous life where I flew SAR, sometimes on the S-70 (and I have flown a few hours in the Pavehawk too), I have two lines of thought but no theories.

Whiteout is quite a major problem in this type of situation. Once you face a slope like this with nil detail you have no external visual pitch, roll or yaw cues. Facing a steep slope the horizon is well above normal eye-line and possibly out of view altogether. You then need to fly instruments whilst operating adjacent to the slope, with the obvious problems that this may cause. A movement that would be corrected instantly if visual cues were available might develop into something far more serious very quickly.

Secondly, The Blackhawk series have a moveable stabilator. At low speeds this automatically moves downwards at the trailing edge (like a large flap) and back up as airspeed increases again. If the automatic facility fails there is a manual reversion. From experience, it produces a large nose down pitching moment if airspeed increases too far with a large amount of downwards droop.

Thankfully, as the crew have survived we should know what happened at some later stage.

Best wishes and a speedy recovery to all involved :)

[Lu Zuckerman]

To: Bert Sousa

I stated the following on another thread about taking off from an elevated deck. I think the situation might be similar relative to the rotor dynamics.

Being a theorist and not a pilot I open myself to some flack on this but I will repeat what I stated in a post I made a long time ago. This post is in response to the advice given about hovering the helicopter to the edge of the flight deck placing half of the disc in ground effect and the other half out of ground effect.

The UH-60 was hovering over an inclined surface and facing into the rising ground. This placed the forward edge of the disc over the higher surface and the opposite edge of the disc was over the declining surface. This could have caused an imbalance of the downward airflow causing a variance in the lift (ground effect) being generated across the disc. This imbalance could have forced the helicopter backwards and in the process it tried to maintain its' relative position to the declining ground surface thus sliding backward and downward while airborne. If the pilot tried to compensate for this backward movement he may have caused the nose to drop and the imbalance of the ground effect was exacerbated increasing the rearward movement. At least it is a theory.

:confused:

[helmet fire]

First and foremost, my deepest sympathies to the crew and families. You were out there trying to save lives. I wish a speedy and full recovery to all concerned.


Lu, Lu, Lu, Lu, …….:rolleyes:

Your lack of understanding of aerodynamics is sometimes frightening – and this time it is definitely in that category.

You did say “At least it is a theory”. But you are wrong. It is just crap this time. Why?

1. If your theory was not crap, why didn’t the helicopter roll left, away from the slope before turning it’s nose toward the slope? :rolleyes:

2 How did they hoist in someone before the accident without rolling away from the slope? :rolleyes:

3. How does anyone in a helicopter rescue people from slopes? :rolleyes:

Even if we entertain your theory for a nanosecond, why have you YET AGAIN neglected flapping to equality? Have you ever seen a ground effect diagram using a steeply sloping surface? Have you seen or heard of recirculation? Can you please attempt to explain how a helicopter tries “to maintain its' relative position to the declining ground surface” as you claim? WTF?


:eek: :eek:

[helmet fire]

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.

[sling load]

I can't comment on the accident, could be any number of reasons, my own experience in winching at high altitude in the snow are these,

I always had a crewmember toss out an orange smoke flare at the point of the winch and if able to determine the wind, another one up wind nearest the ridge line.

Wind travels over smooth snow so quickly, and it can really surprise you if you are not prepared for it.

I always attempted to have an exit path that allowed for a pedal turn with the airframe if it was about to go into a max pitch maximum pedal hover, ie starting to settle, turn away and attempt to add power back to the rotor and fly down the hill. sometimes you couldn't do it, but I always tried to allow for it.

Depth perception is a real problem in snow, and I can really sympathise with any pilot who has ever experienced it.

It sounds like the crew were on the ball if they only winched in one person per trip, especially with the power that a Blackhawk has. I hope they are all going to get through it. It could have been much worse, that blackhawk is like a brick outhouse!

[ShyTorque]

I haven't seen the full footage as my browser only showed a short sequence of stills BUT from the situation described by HelmetFire, I had a further thought or two.

A sudden and large gust coming from behind a 'hawk in the hover could easily cause a nose-up pitching moment because the airflow over the stab (at the full down position as advertised)would cause the tail to be deflected downwards. The pilot would attempt to correct this by forward cyclic. Because of the low (negative) airspeed the stab would have no input to motor up.

If there had been a loss of yaw control about the same time too, as the aircraft tried to weathercock into wind (ran out of tail rotor authority). In the latter situation it would rotate to the right away from the main rotor direction of rotation. Most pilots sensing a loss of control in the close proximity to a hill (and probably by now also in whiteout conditions if the only visual reference had been lost ) would probably pull a handful of collective in an attempt to fly clear. This would cause the coning angle to increase as described. As the aircraft came round into wind it would be out of trim in pitch (too much forward applied because of the previous attempt to prevent the nose-up pitching). This could easily result in the long probe on the nose striking the hill and the rest is history.

I look forward to the report on this one, I hope we can learn from it. Thank goodness the guys are safe.

[heedm]

I fly SAR in mountains so have some training and experience relevant to this. Helmet fire wrote lots of good information on this. Just want to highlight a few of the relevant points.

Don't hover into the mountain. When you're nose into the mountain you lose references, cut off escape routes, increase your hoist altitude, and may decrease rotor clearance (depends on hoist location).

Small, smooth control inputs. Whenever the rotor disc is tilted from the vertical position more power is required to maintain hover. You may decrease that requirement by gaining forward speed, but initially it increases. If you are hovering in high DA aggressive control inputs may preclude you from maintaining altitude. Try establishing a stable IGE hover and then move the cyclic aggressively-ish. I was able to land a 206 from a 4' hover doing this...center cyclic and you pop back up to the hover.

Cross check instruments. Most of the illusions that are specific to mountains and most that are a result of snow fields can be countered by referring to and trusting in your instruments.

Provide hover references. If the victim is in a uniform field of snow then drop colored smokes, equipment, FOs, whatever you have to create a hover reference. If that just isn't possible, it's probably safer to hover where there are references and have the ground team move the victim.



I'm sure the experience in this group could increase this list, I just included the ones I could think of that I believe may pertain to this incident.
_____________


Regarding the incident, the biggest question I have is why did the helicopter yaw to the right prior to descending? Right yaw occurs only for a few reasons: commanded yaw, application of power, wind and/or tail rotor concerns, autopilot/SAS problems.

After the yaw towards the mountain loss of reference and horizon would be a big concern. If there was an autopilot type problem, then the movements after the yaw may also be uncommanded by the pilots. The apparent aggressive handling (based on the movement of the disc) may explain why the helicopter descended.

The number of scenarios that could explain this are limitless. Right pilot incapacitation (as simple as an impending sneeze) requiring a yaw turn for the left seat to gain reference could be as likely as the autopilot giving away or the tailrotor not providing enough thrust.

________

One other thing that was touched on that I'd like to reiterate is for those who have seen the video, what an awesome job the Sikorski did of saving the lives of the crew!!!! Having been to a number of apparently less severe crashes that resulted in immediate fatalities I think that video says a lot for the ~hawk line of helicopters.

____________

I'm still not sure if this is the same unit that we've visited in the past and they've come to see us for talking turkey and drinking beer. If they are, then I know that they train for this type of flying and have much experience on squadron. Of course, I'd expect most of the units in that area would do appropriate training.


My condolonces to the injured, the families and the unit.

[Flight Safety]

Could a sudden engine failure operating at that altitude have caused this accident? It would seem that power from both engines would have been required for hover OGE at that altitude. This might explain the suden loss of altitude, but I'm not sure about the yaw to the right.

[[email protected]]

Another factor to consider which again would require a knowledge of the local wind at the time is that the right turn may well have been completed to put the aircraft back into wind between winching lifts. As was stated on an earlier thread the pilot would tend to put the hover references (the mountain) on the right side (if he was flying RHS) along with the winch; with a steep cliff, it is easier to get to the overhead of the winching area that way. This would require a definite decision to operate downwind, if indeed this was the case, and a considerate pilot might elect to carry out the winching downwind, but any waiting into wind as an accurate overhead would not be required. Whether or not this was the case, it is very likely that the pilot lost good visual references during the turn (staring at the snow is not good as already mentioned) and may have moved closer than intended to the mountainside. If you introduce a downdraught to this situation, the most likely course of action for the pilot would be to try to fly his nominated escape route ie towards clear air - this manoeuvre would require a nose down attitude to accelerate the aircraft and could, if combined with a rate of descent from a downdraught, have caused the blades to strike the mountain.
The situation these guys found themselves in is every SAR pilots worst nightmare (apart from ditching at night in a big sea) and it is extremely fortunate that the whole crew were not killed.

[heedm]

Hadn't thought of that one, crab. If I was in that situation (ie forced to hover downwind) then rather than hover turn into wind to loiter, I'd go for forward flight. After dumping fuel, that might not have been an option for these guys.

I had one mission where we found some crash survivors just as night was falling. They were on the right side of a deep narrow canyon. We tried hoisting nose away from mountain but 255' cable wasn't enough so we had to turn around, nose into the mountain and it was still a very high hoist. By this time it was a black night. very black. Kinda scarey looking to the left knowing there are trees and rocks within 50' and not being able to see anything. But that's another story.

[[email protected]]

HEEDM, I was given to understand that the Canadian authorities were investigating the use of NVG in civil ops - has this not happened or is it taking them as long as the CAA to sort out?

[heedm]

As far as I know NVG are still being investigated. The RCMP are training (maybe flying) with them. I've heard rumours of Air Ambulance wanting them. I'm just not 'in the know'.

I'm in the military and we use NVG for these types of missions. The one above was when the use of NVG was in its infancy. We were still restricted to unaided flight for hoisting and landing. Also, we could not have mixed cockpits. When we did the hoist the second time I wasn't flying, so I ended up putting on my goggles, just so I could see that terrain I mentioned. At the time, that was against the rules, now we can use judgement in situations like that.

[Heliport]

I haven't seen / heard any reports whether the crewman who was critically injured survived? Or his condition?

Some reports said the Hawk rolled on him after he was thrown out. :(

Anyone know anything?

[What-ho Squiffy!]

This thread migh have died, but my five cents anyway...

I saw this accident and thought that LTE was responsible, arising from one of two causes:

1. If the helo was flying at max cont, the flying pilot may have overpitched (for whatever reason), resulting in NR decay. Tired engines may contribute to this situation (i.e. not producing advertised power).

2. The more likely scenario would be an engine failure:

- Resulting in rapid NR decay, which must be responded to by reducing the power requirement on the remaining engine to a sustainable level (i.e. to within OEI contingency limits).
- The only way to do this is to descend and attempt to fly out. If immediate descent is not possible (as was the case in this accident due to the helo orientation to the slope), the NR will decay, and the tail rotor will lose effectiveness - hence the uncommanded right yaw, wobbly hover followed by descent.

The above factors seem to suggest to me that if the helo was hovered left side to the slope (i.e. captain on downslope side), an engine failure could be reacted to by allowing the right yaw to occur, which points the helo downslope, allowing an escape route for descent and acceleration to flyaway speed.

I'd be very interested to get feedback on the above, because if I ever had to do a similar mission, that's what I would brief. Thanks.