AS350 crash due to Jack Stall...
 

Interesting video...
Apologies if posted before.

Speed, weight, and vigorous cyclic movement = soiled underwear at the minimum.

The cyclic movement/pushover didn't look that vigorous to me? Is this something that the AS350 is particularly prone to?

you forgot "Altitude" into the mix, VNE goes down 3 knots for every 1000ft, if they are at 6000ft (judging from the tree line) VNE is close to 130Kts.

I’m not really familiar with the 350 but know it’s susceptible to Jack stall. I don’t know enough about that to comment.
When watching my initial thought was it’s retreating blade stall, he’s obviously high, he’s gets a lot of speed in the dive then pitches up and rolls right at the moment he aggressively pulls the cyclic back.
Lucky to be alive looks like the deep snow save them

All I can see is the deep snow.

Either that, or the page is blank.

edit: The link appeared when I clicked on “quote”.

A similar scenario to the Colin McCrae accident, by the looks of it.

Looking again at the video he seems to push the cyclic abruptly forward at 1:05, (speed increases, passengers woop for joy) then cyclic back and right roll at 1:15

I think the symptom are similar

If I was going to put money on it, RBS would be my choice. The occupants (assuming that they all survived), are very lucky.! One of my early jobs on S&R was recovering the personal effects of my predecessor from the remains of an RBS induced crash on the South coast which killed three. - deep snow is useful !

If I have got all the informations right, that accident happened in 2016 and the accident report is already out. It was servo transparency aka. jack stall. The Company was TRK helicopters. Everybody walked away.

This appears to be the accident in question:

"Summary
On 16 March 2016, at 1618 Pacific Daylight Time, the TRK Helicopters Ltd. Airbus Helicopters AS 350 FX2 helicopter (registration C-FBLW, serial number 2955) departed from the base of a ski run approximately 82 nautical miles northwest of Smithers, British Columbia, on a day visual flight rules flight to the base camp (located approximately 7 nautical miles south-southeast of the ski run), with the pilot and 6 passengers on board. Approximately 1 minute after takeoff, while operating at low altitude, the pilot initiated a descent into a ravine. During the descent, the helicopter's airspeed increased rapidly. Moments later, the helicopter abruptly rolled to the right, pitched up, and collided with terrain on a steep snow-covered slope. There were no injuries, and all 7 occupants egressed the aircraft. The helicopter was substantially damaged. There was no post-impact fire and the emergency locator transmitter did not activate."

https://www.tsb.gc.ca/eng/rapports-r.../a16p0045.html

What doesn't gel with the RBS/jackstall theory (they are so similar it's difficult to tell them apart) is the low Nr horn - that sort of disc loading produces high Nr not low.

Shy - great tip about hitting quote to see the link - I have been having this problem with my browser for ages.:ok:

So, primary cause, servo transparency, secondary cause, lack of imagination.

​​​​​​​Have another listen - sounds like it worked as advertised.

RTFM!

The reason for it - elastomeric bearings - i.e. the spherical bearing.

https://cimg1.ibsrv.net/gimg/pprune....a4b75c8245.jpg
This bearing is multiple steel spherical cups separated by thin layers of rubber material. Not unique to the AS350.
It can bend and twist but not move so much in compression. Distorting the bearing takes some effort. The maximum effort is naturally at maximum pitch i.e. the retreating side and is always there.
In forward flight some of the disk is constantly stalled and even going backwards with regard to flow. Stall creates a shift in centre of lift aft on the blade and creates an additional feedback which is cumulative with speed and loading further outboard on the disk where it matters.
The forces are quite high and affect fatigue life and ultimate strength, I think the weak point is the PCL's which will possibly bend. Hence the HYD system was designed to be not strong enough to let this happen.
As it is at the edge or slightly outside of the certified flight envelope and marginally before RBS it seems to be acceptable.
Dual HYD aircraft in the same family which will not yield to feedback have a load cell (flat spring and microswitch) on one servo which will warn with a LIMIT caption to avoid overloading the control system.

There are warnings and limitations in the RFM. "Forewarned is forearmed" but maybe not communicated readily.

"Grey" 355 style wide chord blades may be more susceptible than the old "Blue" B model blades but even a B model is susceptible.

This discussion has been had before....probably a couple of times.

Refresh my fading memory as to why just making stronger PCL's and marginally stronger hydraulic jacks would prevent the problem although allowing honest to goodness RBS?

I recall one wonderful old Sikorsky and a venerable Bell product that with youthful indiscretion could both demonstrate RGS for the young and dumb. (Hand raised!).

In those aircraft with correctly turning rotor heads, the nose would pitch up and roll left....and the airspeed would bleed off till the RBS abated or ceased....and assuming one was wise enough to reduce Collective to a reasonable cruise setting....normal flight returned.

Surely some Whirlwind experienced Pilots had some exposure to that.


In the early CH-47....A Models being grossly over weight could provide occurrences of reactions very similar to RBS but in both heads....and with contra rotating rotors....;think about how those symptoms were manifested!

Good point RVDT, I didn't realise it had a high Nr warner as well.

This is a well known pprune problem and fix (reply). I seemed to fix mine by resetting the pprune cookies.

It went something like:-

Turning down/off ad blocker
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Closing pprune
Clearing pprune cookies (will delete saved password)
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