Ok, the wiki article is not that great on a second glance. But in any case, my point was that an uncommanded yaw (yes, technically not necessarily LTE) can be caused by effects which are independent of TR/NOTAR, like the wathervane effect caused by shape of the fuselage or wind/gusts countering the thrust direction of the TR/NOTAR.

The full video
 

In better resolution
https://www.liveleak.com/view?i=50c_1501851597

Watching the full video brings up more questions.
What was with Crew coordination?
The sliding door was opend and pax started to get in immediately.
Did the pilot even had a chance to put the bird down before additional weight was added?
Might have contributed to the accident!

We don't know from this video whether this was his first approach to the location. If it was his first approach then perhaps being close to an edge beyond which there were unexplored air conditions was not a good plan.

My thoughts exactly.

Also wasn't this a medevac flight? What business had the guy (?) to jump in as he did!?

When I said "running out of power" I referred to not having "control power", or directional authority, or basically running out of left pedal.

Yes, 3 years ago after 45 years and 15,000 hrs. Not much, in many circles, but enough to say "been there, done some of that."

I looked at the footage a few times, and the pilot plus crew as well as the person at the landing site, had more than just an ounce of luck. Also, and that is just sheer speculation, the power margin seemed narrow, the surface of the landing site prompted the pilot not to put it down fully on the ground, and when he possibly decided to turn the aircraft around, he ran out of directional control. It is scary to look at this scenario. But then again, this is my view. I am certainly interested on the findings of the accident investigation team.

Well the dustball at the end, highlights the "backlash rotor" quite nicely at the landing spot, no question as to the relative winds. Makes we wonder if the ol Coanda effect was a little disturbed by the windward upflow of the face.

Brain must have been screaming "mechanical" to miss the chance to dump into wind down the valley.

I am just guessing now (like the rest of us):

The pilot was well aware of being "on the edge" of power there, so he wanted to keep it short and quick....
He was not able to put full skids on the ground, as the spot he picked did not allow this, so he was trying to balance it on the skids, which at that altitude and which the wind (?) required some power changes in pedal and pitch...
He ran out of left pedal, and the helicopter started to turn.....instead of lowering the pitch (and risking to have the helicopter start sliding on the ground), he increased it, making it worse...

When the helicopter started coming around he probably had the pitch already all the way up under his arm..........easy to say what he should have done now, but once you´re in the cockpit, we all know that a decision has to be made in a split of a second.....this is where experience kicks in.........

"Brain must have been screaming "mechanical" to miss the chance to dump into wind down the valley.[/QUOTE]


That's an very good advice!
We had the same in our company a few years ago! Simple LTE in 1000ft ground, Heli starts spinning, pilots was shure that he lost the tailrotor!
So he startet autorotation and crashed in a village!
Nothing mechanical was found! If he speeds up with nose down nothing happend.....

A question for those with NOTAR experience - how good is the directional control at lowish power ie less than hover power?

If he was sitting in updraughting air - he was at the forward edge of the ridge - he may have found that he was struggling to keep the aircraft down when he landed - a combination of the surface and gusting updraught.

In those conditions, is the directional bucket capable of producing enough thrust, quickly, to cope with rapidly changing demands?

It seems that once he gets airborne, he should have been OK but if he was already struggling with yaw control, he might have been reluctant to apply full power pedal to arrest the yaw.

Crab

Never flown a 902 but a lot on a 520, the biggest thing is the slow response of the anti torque system. Once you are used to it not a problem but does give a problem when there is an unexpected gust ! Well that was my opinion compared to a tail rotor. In some respects it is comparable to a fenestron on a 341 in a strong wind doing spot turns if you want a comparison you would understand. Before everyone shoots me down here I know the comparison is not hugely accurate but it is to give a feel for Crab as he would have more than a few hours on a 341

Agree with you, I doubt the pilot was ready or had given the ok

Hughes 500 - thanks, I know exactly what you mean:ok:

While I like the NOTAR system a lot, the 902 anti-torque is less immediately responsive than most conventional tail rotor systems I've flown. When at MAUW and hot, even at lower altitudes you can find yourself applying a surprising amount of left pedal when flying helipad profiles or HOGE with a wind from the right, or arresting a clockwise yaw. It really wouldn't surprise me at all if at 11'000 ft PA on a +ISA day you might easily run out of left pedal. When you pass around three quarters left pedal there is a noticeable audible change in the aircraft as the NOTAR fan blades' pitch increase towards max - something that personally I think is apparent in this video whilst the aircrwaft is yawing to the right. My feel is that he's sat at full left pedal in efforts to stop the yaw, but it's not working.

Crab, generally no problem with NOTAR at low power settings but I have no first hand experience of the impact of a strong/gusty updraught on the effect of coanda in the hover. It's an interesting question - the coanda effect alone is providing approximately 60-70% of the anti-torque in the hover with just 30-40% provided by the thruster can (or so they say). If the updraugh were to significantly disrupt the required flow around the tailboom, enough to reduce the coanda, then it would put significantly more demand on the thruster drum which was probably already at full chat. That said I still think it was more likely a case of WAT and running out of pedal.

Thanks for the info Aucky - it's probably difficult to differentiate exactly what caused the loss of yaw control but operating at high DA in gusty mountain conditions will always be challenging and it will expose any weakness in man or machine.

That's an very good advice!
[/QUOTE]

Yes, it' a heck of a plan provided you are sure you haven't rapped the tail rotor in to a tree/rock outcropping. Also wonder if in the midst of all the excitement if he pardonably forgets he's flying a NOTAR machine from years hard wired in a conventional machine. Rough day.

Many years ago I did my introduction to mountain flying in a Hughes269 in the on a hot breezy day in the Cascades.

The in instructor picked out several ridge lines for me to attempt a light on the skids landing. In each case, he asked me to set it up with an exit strategy in case we ran out of power or started to get LTE before we set the skids down. Wouldn't you know it, in the highest 3 attempts, we did get one or both! and we had to bail out down the mountain.

Perhaps he was not properly instructed, but I cant imagine any pilot with the mountain flying basics who wouldn't begin that approach with the expectation that he was at the limits of the aircraft and he may have to have to bail out over that ridgeline at the first sign of trouble.

500guy - the problem is that your escape route generally disappears once you get the skids on the ground.

This guy's approach was fine and he makes it to the forward edge of the ridge line to try and stay in clear air - just as I expect you were taught - the last few feet to the ground may well have taken him below the demarcation line and into the gusty turbulent air.

According to the factory data-sheet - in this case it is possible to do the work on this site with the conditions as they were - but depending on the actual weights he might have been past the limit! ISA+20 is possible but at a way reduced AUW....

Curious what the report will say....