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AS350 crash due to Jack Stall...

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AS350 crash due to Jack Stall...

Old 25th Nov 2023, 09:11
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Surely some Whirlwind experienced Pilots had some exposure to that.
Yes. Almost 45 years ago, when I was a basic rotary student, my RAF QHI decided to carry out a “spirited arrival” at the unmanned Chetwynd airfield in Shropshire, first thing one fine morning. He descended to very low level at what was very close to Vne (not fast in a Whirlwind 10) and pulled the aircraft round in a hard turn. All I remember is being at close to 90 degrees to the ground, the aircraft wobbling slightly, then us being suddenly placed straight and level at a much reduced speed, alongside some 100’ high cables near the airfield. I was shaken rather than scared (the innocence of youth) but more intrigued to know how he’d done it. There was a nervous laugh from the other seat and he explained we’d just inadvertently experienced retreating blade stall. The aircraft did it, rather than himself.

So, very similar to the situation under discussion, although as far as I know, the Whirlwind reached RBS rather than jack stall/servo transparency, whatever we’d prefer to call it. It makes little difference to the symptoms. Had we been in a left turn, the aircraft might have put us inverted and I might not be here.

Sadly, said QHI was killed in a terrible motorcycle accident a short while afterwards whilst travelling on RAF duty.
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Old 25th Nov 2023, 09:27
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My memories of my own flick roll in the Culdrose SAR Whirlwind 7. It was not a good day for flying, a howling north westerly wind, so strong that we had to start up and engage in the hangar as we were out of limits to engage outside. Probably frowned upon today but it did get the dust out of the rafters!

We taxied out and took off to look for a fisherman who was missing from below the cliffs near St Agnes, up to the North West. The only way to progress was to fly very low indeed to try to keep out of the gale as our ground speed otherwise would have been even more pathetic than normal. After what seemed like an eternity we arrived close to the scene at very low level and balls out.

As we crossed over the cliff edge those of you with even a rudimentary knowledge of aerodynamics will know what happened next. The upflow of the gale over the cliff edge suddenly and dramatically increased the angle of attack on the blades – the retreating blade stalled – the aircraft flicked rolled to the left – reared enormously nose up – and hurled itself with little control from me across the ocean. I sat there bemused for a very short while, as the unamused SAR crew in the back were flung across the cabin to the port side along with their kit.

After what seemed an age as I was jostling with cyclic, rotor rpm, collective and so on this hunk of aluminium sorted itself out and flew on as though nothing had happened.

We never did find the fisherman and the trip back to Culdrose was at supersonic speed – well for a Whirlwind 7 it was.

Happy days.
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Old 25th Nov 2023, 10:52
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The summary in #11 and Shy Torque's Chetwynd 'event' are,essentially pointing at the same sequence - the direction of divergence being opposed, down to the preferred 'National' rotor rotation direction.
Shy - your QHI was my guide and mentor on my first Whirlwind ride ... which included a Chetwynd visit for sloping ground ... but no RBS demo ! His accident was a huge shock.
I only experienced jack stall as a demo on my first trip at Boscombe (in a Gazelle) and (from memory) it was something of a non-event.
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Old 25th Nov 2023, 14:15
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It is also worth noting that the W-Wind Mk7 RRPM was normally 207,whereas the Mk9/10/12 was normally 219-220,and also had the `beefed-up` dinner-plate` rotating swashplate,and Vne95 kts..!

In fact ,I think that the WW Mk3 we had at B-D, RRPM was normally 203...

During my early Gazelle flights at Marignane ,as Project pilot,we flew early `development` Gazelles,which had stiffer fuselages,only 2doors,and no /small roof transparencies,solid u/c,low skids,and short-backed seats.During `tactical ALAT `Spectrum` flying(flight envelope) ,at `jack-stall` in both left and right turns,the aircraft would roll back towards level flight,in a `self-righting manner.
Sometime later,myself and my CEV ALAT Pilot were requested to fly new mods,as these had to be `Joint` agreements.The hyd system had been modified,and different `jacks` fitted to a different airframe,also ,more plastic roof panels and side doors. So,`orf we went to `explore the envelope,and soon found that hard rolling and pulling produced the expected roll towards level in left hand turns at the jack-stall,to an initial `eye-watering` further roll in right hand turns,The cockpit was full of the usual expletives,both in French ,and English,until control was returned to normal.!! Further investigations into why the jacks had been changed was never fully explained by SNIAS/A`spatiael, just a `c`est la vie`...However,it was fully agreed that,whilst not `dangerous`,handling at the point of jack-stall,had to be fully explained in the Flight Manual ,and fully demonstrated on aircraft conversion,...at a safe height...!
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Old 25th Nov 2023, 17:22
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I remember carrying out Range Autos in the Whirlwind 10. The parameters were 180 RRPM & 80 kts (reduce Nr and increase the IAS from a normal one).
You could actually see the individual blades going round, which was a bit disconcerting, but the old lady picked up her skirts and really glid well!
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Old 25th Nov 2023, 22:26
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"It is also worth noting that the W-Wind Mk7 RRPM was normally 207"
Actually I think it was 210 normally and 213 in the hover. The crewmen in the back would look up between ones legs and make a very loud noise if the rrpm dropped below 213 in the hover over the sea. It seems amazing now that just the loss of a few rrpm could cause one to ditch. Ah - the manual throttle - didn't we just love it! And cartridge starters - clutch pumps - high energy button for starting and so on. If you had to do a manual start, and yes it had a starting handle! it was 82 turns of the handle for one revolution of the engine. Had to do that once during the P1127 trials on HMS Bulwark. D
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Old 26th Nov 2023, 07:28
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Sycamore - very interesting background on the Gazelle jacks and jackstall response - never heard that before

Georg1na - a very similar experience happened to a Valley Wessex SAR pilot in about 1989/90 - he came into our crewroom next door looking for a QHI to ask if it was possible to lose control of a helicopter in the hover.

He described an almost identical winching scenario on Tryfan in very strong winds and updraughts and fortunately they hadn't deployed the winchman when the aerobatics started - I told him I was pretty convinced it was RBS
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Old 26th Nov 2023, 10:47
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Shy - re the 180 RRPM - Bill McE, , when he was 'boss' at Valley training, took a 10 out to South Stack on a very on-shore windy day, went manual and took the revs back to 190-ish and slope-soared ! Produced some ridiculous consumption figures !
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Old 26th Nov 2023, 11:31
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CJ, I once experienced a similar thing in a SK-76. I was tasked as a favour by the RAF to pick up some British Army soldiers from a rocky offshore outcrop in the South China Sea (obviously, there's a bit of a story behind why they were there and why the RAF couldn't do it themselves), in the teeth of an incoming typhoon. The headwind on the way out and on arrival was somewhere over 70 kts, even at low level. The only place to pick the pax up was from an into wind, steeply sloping rock platform. The slope caused a strong updraught and every time I tried to fully lower the lever, the aircraft autorotated well above the surface and the Nr kept rapidly going to the top limit (doesn't take much with the 76's slippery blades). It took quite a few attempts to get even one wheel on the ground. Thankfully, the soldiers were very keen to leave their precarious perch as they were almost being blown off into the sea and beginning to get rather wet. As soon as the one wheel touched, they leapt on the aircraft like rats up a drainpipe. On our way back to base the indicated ground speed showed 223 kts.
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Old 26th Nov 2023, 13:22
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ST, was that the same typhoon when the drilling rig under tow was evacuated and then sank? Sadly, taking the hapless divers still in saturation with it!
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Old 26th Nov 2023, 16:41
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Georg1na,you forgot to carry `sixpences`,in case the safety disc `blew`...anyway ,the Mk3 had a `Cyclone`,so it was an electric starter...not so many `prayers and incantations,burning of `joss-sticks`,etc,to get it going...
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Old 26th Nov 2023, 18:35
  #32 (permalink)  

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Originally Posted by 212man
ST, was that the same typhoon when the drilling rig under tow was evacuated and then sank? Sadly, taking the hapless divers still in saturation with it!
No, different typhoon!
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Old 27th Nov 2023, 11:59
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Crab...I remember at Valley it was possible on certain wind direction and w/v (around 20-25kt) days to take a Wessex out to Snowdowia and find a near vertical cliff face with updraughting air,

Sit in the hover along side the cliff face at just over half power, in the hover, zero airspeed showing.....then over a period of about a minute or less, slowly have to pull to max power to maintain position, before having to dive away lowering the lever and gaining fwd airspeed to recover. It was repeatable over and over again.

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Old 29th Nov 2023, 11:13
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This accident in Norway in 2011 claimed 5 lives
https://aviation-safety.net/wikibase/137259
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Old 30th Nov 2023, 09:42
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Onehunglow - yes plenty of opportunities in Snowdonia to do that - sometimes when you didn't want to
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Old 30th Nov 2023, 11:18
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There you go. One accident in Switzerland with a lot of details and explanations. Even pictures.
https://www.sust.admin.ch/inhalte/AV...hte/2265_e.pdf
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Old 30th Nov 2023, 20:38
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Originally Posted by onehunglow
Crab...I remember at Valley it was possible on certain wind direction and w/v (around 20-25kt) days to take a Wessex out to Snowdowia and find a near vertical cliff face with updraughting air,

Sit in the hover along side the cliff face at just over half power, in the hover, zero airspeed showing.....then over a period of about a minute or less, slowly have to pull to max power to maintain position, before having to dive away lowering the lever and gaining fwd airspeed to recover. It was repeatable over and over again.
I remember one occasion doing similar in the Wessex, sitting in the hover over a ridge in Snowdonia with lever full down. For a few seconds anyway, before the simultaneous realisation from the two of us up front that this probably wasn’t a great idea, and escaping quickly before the wind suddenly dropped.
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Old 1st Dec 2023, 21:57
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What a positive Whirlwind of posts.
Anyhoo as I have posted previously a few years ago.

IMO if you encounter Jack Stall you have, , with the exception of actual servo failure, already exceeded the G limits of the helicopter for some reason be it overloading, over torque or overenthusiastic maneuvering.

Many seem to think that when you encounter Jack Stall the controls lock. This is NOT the case.

The controls will move and work as designed but you will feel feedback and need to apply force to move the controls.
Manipulate the controls as required to lower the G Load back within limits by reducing bank angle, lowering the collective ect.
If you don’t have space to do this before ground contact bad things will happen.

We used to demonstrate this in training on the AS350D and the 355F in a very controlled manner. Note: with the Twinstar we just pulled to the Limit Light not to actual Jack Stall which, due to the dual hydraulics, have been very bad practice. Because all the pilots getting checked on the 355 already had lots of 350 time that was all that was required on the 355.

When the 350 first came into service a lot of pilots gained knowledge by pushing the button on the tip of the collective, thereby bypassing the accumulators and turning off the hydraulic system suddenly and completely. They sometimes even did this at high-speed cruise. The ensuing exhilarating rapid, high G, nose up roll to starboard really got their attention. Fortunately they usually did this when going somewhere solo and doing as pilots new to an aircraft sometimes do “Humm What happens when I push this?” After things were returned to normal that stunt usually went into the “Not Going To Do That Again” file.
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Old 2nd Dec 2023, 08:37
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Originally Posted by albatross
IMO if you encounter Jack Stall you have, , with the exception of actual servo failure, already exceeded the G limits of the helicopter for some reason be it overloading, over torque or overenthusiastic maneuvering.
I like your logic, but I had understood that he got overenthusiastic in the dive, overspeed past VNE, got blade stall, and consequently jack stalled.
Originally Posted by albatross
Note: with the Twinstar we just pulled to the Limit Light not to actual Jack Stall which, due to the dual hydraulics, have been very bad practice. Because all the pilots getting checked on the 355 already had lots of 350 time that was all that was required on the 355.
just my personal reminder note: its an error to think that dual hydraulic have twice the actuator thrust power, two piston on the same shaft (with pressure equaly regulated for the two pistons), does not double the thrust.
only higher hydraulic pressure can do that and actually the dual hydraulic version has a slightly inferior hydraulic pressure.
single hydraulic: 40 bars
dual hydraulic: 35 bars

Last edited by Agile; 2nd Dec 2023 at 10:39. Reason: added respective hydraulic pressures
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Old 2nd Dec 2023, 11:53
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just my personal reminder note: its an error to think that dual hydraulic have twice the actuator thrust power, two piston on the same shaft (with pressure equaly regulated for the two pistons), does not double the thrust.
only higher hydraulic pressure can do that and actually the dual hydraulic version has a slightly inferior hydraulic pressure.
single hydraulic: 40 bars
dual hydraulic: 35 bars
Please explain. Force equals pressure x area. So if you have two pistons of the same surface area being acted on by the same pressure, you’ll have twice the force. It’s a simple way to keep the set is light and compact compared to having one large servo (plus redundancy). If you lose a hydraulic system there are normally RFM limits on speed and angles of bank - because you have less servo power.
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