It is the term to describe when the blades overpower the hydraulic servos, causing the controls to whip around, and usually causing control loss.

It is the product of having servo hydraulics that are too small for the helicopter in question, because the servos should be able to muscle the blade without feedback. No modern US Military aircraft is allowed to experience servo stall or jack stall, as the hydraulics are designed to have enough force to overcome the most extreme blade forces without being back driven.

The forces produced by the blade are generally the stall forces due to the strong pitching moment changes produced when the stall occurs, usually due to extreme maneuvering.

It doesn't need to be particularly aggressive. If you are travelling at high speed and a bird appears in front of you, you tend to pull the stick back and perhaps to the right.

When i did this, the controls moved a bit, but the cyclic then locked solid, and the aircraft, which had started to turn right and nose up, suddenly flipped back to a level attitude. When I stopped trying to fight the controls, and the aircraft stopped reacting, all returned to normal, though my heart rate took a little longer.

Lucky it flipped back to the left and not further to the right, as I would have been upside down.

Agrees with DD, it does not have to be that violent. On a straight B model at max gross and MCP, a dive up to 135 kts and just a bit aft stick will give this symptom. Lessen the collective or aft stick and it goes away. Not any diffrent load than servo off in cruise. IMHO I think it is vital to have felt this during initial training, to recognize it and act accordingly.

I think the idea with this is to limit the load being fed into the swashplate, the pilot will feel the "jack stall" and lessen the load. On the first 355 with dual servos they did not have any limit switch, and to much load was feed into the swashplate with permanet deformation seen as a result. After that the limit switch was installed on top of one of the servos.




CB

Disguise,

What altitude, OAT, Indicated Air Speed and gross weight were you when this occurred?

Collective Bias,

I think your description is quite correct, but let me translate the logic just a bit:

The swashplate is too weak for the rotor blades, because the rotor forces generated can create high enough stresses to bend the swash plate. As a result, the manufacturer reduced the strength of the hydraulic servos, so that the servos would back-drive before these high stresses could be generated. This safety feature prevents over stressing the helicopter, but also reduces the pilot\'s ability to control the aircraft during relatively mild maneuvers within the flight envelope.

Is this reasonably correct?

Errrr...Nick...guys....let me get this straight.....the 350 is designed and certified with this "capability" or as I see it....built in lack of control or questionable handling ability?

What you guys are describing are somewhat "normal" situations that easily result in the loss of control of the helicopter....frozen controls....feedback during cruise flight maneuvering?

Am I getting timid in my old age or is this a desirable situation? Has this a bearing on some recent crashes due to hydraulic failures....feedback forces that preclude safe landing of the helicopter?

For Nick...have you encountered...or know of other aircraft that have similar characteristics....or is this like unique to Euro-choppers? (of the French variety)?

I couldn't possibly comment but I'll have a go anyway.

Any aircraft will have an envelope which it is dangerous to be outside (even if you feel you can fly well beyond it in terms of your own abilities)

Jack stall is easy to encounter during manouvering such as quick stops where the blades decellerate the aircraft rapidly against the air mass, jack stall in this situation will cause loss of tail rotor authority,

In the same way you can over torque, you can jack-stall.

good pilots know the limits of their aircraft and will fly accordingly.

That said I have no problem with manf's fitting more powerful actuators.

I have rarely heard a discussion such as this that does not relate to EC350/355

'hydraulic jack stall' as everyone refers to it is something that can be avoided.:E

I agree with what everyone else says,but here's some more info.

When this accurs, the aircraft will normaly roll to the left with slight back pitch. The correct action to take is to not fight the controlls and the aircraft will fly itself out of this situation.
All of the accidents I have read about have been low level with high speeds and aggresive control inputs.:}

We used to demonstrate (nearly said 'teach' then!) Jackstall to ab initio students in the gazelle during basic trng.

If memory serves me right:

Dive the helo to close to Vne (160 in Gaz piece),
pull like f**k on the cyclic (aft) and follow me through bloggs :uhoh:

A/c then reared up and rolled (I think) towards the retreating blade side, quite violently.
In so doing it 'recovered itself' because the helo slowed down and the forces on the blades eased somewhat.
But during that 3-5 seconds she was in JS, the controls locked solid and we instantly became - passengers :ooh:

What stresses this imposed on the a/c, God only knows, but each a/c went through this about 50 times/year and not one bent pitch change rod was ever found..........

The students cra**ed themselves:\

A10

I really don't know how you would get jackstall during a quickstop, as the aerodynamic surfaces are fairly lightly loaded during this manoeuvre. The only way I've ever been able to demonstrate it is by fairly harsh rearward movement of the cyclic while flying at high speed, normally in a dive and approaching VL.

Dynamic C

The recovery action is to 'reduce the severity of the manoeuvre'. That is, whatever you've just done to cause it - stop doing it.

Jack stall?
 

Jack stall can occur due to several reasons. From a design point of view the hydraulic pump is not properly sized to provide the necessary fluid flow when all three servos are actuated. There is still a charge in the servo accumulators but when this bleeds off the pilot is mechanically connected to the blade feedback forces. What is needed in this case is a constant pressure variable delivery pump,which is installed in most helicopters. With this type of pump you do not require accumulator(s).

Another reason for jack stall is the slippage of the belt drive that turns the pump. I believe this is the reason given by Aerospatiale.


:E :E

I've experienced the famous Astar "Servo Transparency" in a 350 B model.

The flight conditions were:

AUW 4200 lbs (GW 4300)
80 Kts
4,000'
+30c
Light winds
15 degree LH bank

No aggressive flight, no sudden flight control movements, the controls just simply froze when circling a spot fire.

Pub user

Apologies, perhaps I was using the term "quick stop" too loosely , I defer to your greater knowledge.

What I actually meant was going at max chat then stopping as quickly as one could, I would describe this as extreme handling (as described by Thomas Coupling) rather than gentle handling associated with slowing an aircraft under normal circumstances.

It all depends on how quick one's quick stop is, don'cha find?

A10, in a normal quick stop you lower the collective to stay level, that in itself is going to stop any chance of a problem, plus I think the g loading required is normally well above 2-3 g.

Lu, are you saying AS350s have belt driven hydraulic pumps? Not disputing it as have no knowledge either way, just curious that they are not MGB driven directly.

212 Man, yes the AS-350 has belt driven hyd pump. Looks like something that came from a Citroen or Peugeot (French cars!)

I have never really heard of problems with the belt or hyd pump in the AS-350 other than the aforementioned hydraulic jack stall. Never experinced it myself flying the series but maybe that´s because I fly them very gently.

I always heard that the controls really "freeze" on you rather than it feeling like a hydraulic boost failure.

However I have from a Dauphin AS-365 pilot that he has also experienced momentary stuck controls in extreme handling situations. I guess that you really have stuck controls if you loose dual hyd boost in the AS-365 and just really heavy controls in the AS-350.

This is why I prefer to: "FLY SMART, FLY BELL" :)

Jack Stall was taught to AAC students so that they would know in which situations JS could occur. The severity and onset of JS differed with each a/c and one could experiment to discover the limiting point, at and from a safe recovery situation.
Later flying the AS350 series I discovered that if the cyclic inputs were violent enough the cyclic would baulk and go into a sort of JS situation. My thoughts are that if one moves the cyclic fast enough you can feel that the fluid is not moving as fast as your input. OR it could well be that the pitch changing just cannot keep abreast of the inputs. The hyd pump is designed to provide adequate pressure and flow for all normal requirements and the system is so designed to protect the a/c from extreems of G.

QHI course, 1976. Staff used to demo jackstall on the Gazelle in a straight descent and with 60 degrees AOB to the left. My instructor, bless him, decided to do this to the right. He pulled hard from 150 odd knots. The next thing I knew was that Newport and my chinagraphs were above my head. Aircraft flipped right under. Next day, asked for a change of instructor! Jack stall on Gazelles has certainly written off a few at low level - usually when pulling hard in a descending right hand turn (but you have to pull very hard). Never had a problem in 2000hrs with the 350.

my ignorance
 

please excuse my ignorance here, but I have always wondered this:

Are the cyclic & collective in a 350/355 purely a hydraulic linkage (like the brakes in a car) rather than an assisted mechanical linkage (say like power steering) as in a B206?

In the latter case jack-stall couldn't occur could it?

thanks!

The symptoms of jack stall are caused by blade stall pitch change link loads pushing through the servo, not by fluid supply limits. The hydraulic pressure warning lights would illuminate if the pump capacity were reached, since the pump could not keep up the system pressure if the fluid demand were too high.

The blades are always fighting the servos, and the pitch link loads get progressively more severe as the rotor is progressively stalled more and more (blade stall is not a flip of a switch, stalled or not-stalled).

as A10 Thundybox said, "Any aircraft will have an envelope which it is dangerous to be outside (even if you feel you can fly well beyond it in terms of your own abilities)" The real question is not that helos have envelope limits, but rather if those limits occur in normal required maneuvers. Military regulations forbid jack stall anywhere near the operational envelope, and most companies strictly avoid allowing it to occur, even with one failed hydraulic system (down to a single servo).

The consensus here seems to be that the situation in jack stall of the 350 is manageable, the propensity is predictible, and the outcome is not catastrophic. Without doubt, the French authorities examined the jack stall thoroughly in the certification tests and deemed it acceptable. Were any accident to be blamed on jack stall, I would recommend re-thinking that attitude!

Frankly, I know that Sikorsky would deem that behavior unacceptable, and require stronger servos (and swashplates, if the parts could be bent in maneuver!) I believe it is a governing philosophy of US manufacturers (certianly Sikorsky) that the controls must not lock up and the rotating control system not be damaged by virtually any maneuver the pilot can conceive, and it is a strong requirement in US Military regulations. I am also quite certain that the FAA would not approve such behavior, absent the bi-lateral agreements that make it necessary to overlook some things from foreign certifications.

Having reviewed my retirement portfolio last night....and considering my advanced age...seems to me....knowing the 350/355/Gazelle/365 probably the 155 can engage in recalcitrant control behaviour (....that is locking up the cyclic at a minimum....or doing its very own thing.....) which might be counter to what I (the Pilot-in-Command) desires at that moment and phase of flight.....maybe sticking to non-French gear is the answer. There's been times that I would have been much better off leaving things alone....but I think I want that to be my choice rather than some Vin swilling Gaul version of Lu.

Nick's post certainly convinced me of the advantages of riding Igor Iron.

If what you guys are talking about is "servo-transperency" in the 350 series, I have some info. I see little bits and pieces in some of the posts above that are involved. The easiest way that I've seen to get into this phenomenon is abrupt control inputs. The higher the gross weight/airspeed, the less aggressive the inputs have to be in order to cause it. I've never had the controls actually "lock-up" on me. You can tell when you are getting close to the onset. The controls start to feel a little heavier. If you stop/decrease the input you were doing at the time, it won't progress any further. The only time I've ever felt it was when performing a hard turn at high airspeed close to max gross.

Now as far as the system itself goes, from what Eurocopter told me, its isn't a matter of the pump lacking the capacity to overcome the high control loads. The pump has the capability. It is intentionally limited to a certain boost pressure (I can't remember the exact figure in bar) as to not damage any of the components in the head.

(212man It is belt driven off of the input shaft to the m/r transmission) The belt driven pump doesn't inspire a hell of a lot of confidence in people (especially the older style green belt) but I've never really heard about as many problems with the belts. I've heard about more failures of the splines inside the pump than actual belt failures.

At high airspeed the contol loads are very high, probobly too high to be manageble at close to Vne. For this reason the aircraft has a servo accumulators is that they allow supply sufficient hydraulic pressure for inputs to be made to bring the aircraft back to about 60 kts in the event of a hydraulic failure. Other than that, it isn't much different in operation from the Bells.
Hope this helps. BTW alll this information is based on the 350 B2/B3, I don' t have any experience with the earlier models.

Great thread!

One thing about the FAA and US manufacturers...

NickLappos said
Isn't a U.S. manufacturer together with FAA responsible for introducing us to the term "LTE" to cover a design-compromise, or am I completely wrong? -That happens alot :O
I know that Nick is partially referring to the military but everything flying on the civilian market is a compromise, and those are beeing made by every manufacturer, no matter where they're based.

Well, I'm learning alot anyway.

Cheers!
/2beers

If servo transparency and jack stall are being used interchangeably and incorrectly, could somebody explain the difference?

As to servo transparency in the 350/355-
In more foolish days, I'd fly these like ordinary state-side helos.
Flown vigorously- Say, at better than cruise speed- load a little "G," like a zoom and control RPM with collective, add a turn- and the normally impressively agile aircraft's cyclic will stop moving in one direction( or you'll get a "limit" light in a 355). A little less turn (left, last time), or a little less collective and you get some back. Load it again, even with less pitch- and there's the wall, once more. It ruins your plans for hotting up the dog. If there's something that you were turning or zooming to avoid, well- too bad- you're not going to push the stick in that direction, right now. I hope it's a forgiving obstruction, or you left an "out."

I've never encountered this in any other mode besides playing around. And never in any other helo.

The terms can confuse you. We call it servo stalling, or jack stalling but "'Transparency" is a good ephimistic way to describe the same thing - the servo reaches its maximum force and then passes the forces to the crew (becomes "transparent").

The idea that this is used to protect the rotor components from flight loads in certainly novel! We always thought it was a good idea to make them strong enough to not bend!

Frankly, the FAA "Proof and Operations" tests on the helicopter controls forces the rotors to be strong enough.

Regarding LTE as an example of the pot and kettle calling each other black, I don't think you would find me supporting that either, much to Sultan's ire!

I wish I had had the benefit of this thread some years ago, cos it would have saved me putting some embarasing photos in my flying album.

Having sadly been on the receipt of "jack Stall" in a French made heli, there is one aspect to this topic that has not been considered. If you are taught that if you place the aircraft in a 160kt dive and pull back harshly on the cyclic it will 'js'. Why bother with all that when it does the same thing at 30deg AOB/5deg nose up and 60kts in a RH decending turn.

If I could offer any advice it would be that the 'envelope' is something of a vague line that exists in the grey areas only probed by the BOI. Dont be under the illusion that your handling has to be harsh for the aircraft to Jack stall, and the feeling through the sticks can be minimal when you are in full blown 'js'.

All the above is naturally 'IMHO'

Since the question was if any other aircraft have this kind of behaivor, yes sort of the same, the Sikorsky S-55T (sorry Nick). It might not really be a jack stall I think, more like a very veak servo system to very heavy blades, or it might be same thing but just feel diffrent to the pilot.

The S-55T is VERY easy to get in to a bladestall situation if not adhering to the VNE table and follow the WAT reduction closely, especially in mountain flying with gusts. But before it bladestalls on you it starts letting control forces through the servos to warn you that you are getting close. And on the S-55 you can't miss if you loose the servos.

I have never had the questionable "honour" to experience a blade stall in any helicopter, and I hope I never will, but I have for sure had some frightning cases of control forces through the servos on the S-55. I know of at least 3 cases of bladestall on the S-55 at our company in the past, and all went well.

The T version is flying with roughly 20 rpm higher MR rpm than with piston engine (199 to 219), so i wonder how bad the original was. But I guess it did not have the power to reach VNE with max gross as the T version does. In the RFM on the original S-55 it is actually a great deal of text regarding how to avoid and how to cope with bladestall.

Good ol days ;)


CB:{

quote

Nick's post certainly convinced me of the advantages of riding Igor Iron.

Did you really need that post to be convinced ?

I know of some people who would have been more than happy to ride a jackass Star when a blade of the Igor Iron they were on decided to part.

Don't want to start another Airbus vs Boeing or Bell / SK vs EC, though.

Cheers

ATN

ATN,

If you are referring to the Bristow S-76 that shed the blade after it was put back into service after being struck by lightning....I would suggest one might look towards the operator that trashed almost every moving part on the Lightning struck aircraft and after having done that for safety sakes....then sent the blades off for inspection and then put it back into service. The 76 had two other blade failures in its very earliest hours....and knew one of the guys killed in the second accident (a man I respected highly in all regards I might add.)

The same company lost Bell aircraft to blade issues, Wessex aircraft to unknown causes....but these are all catastrophic failures.

My point is I have concerns about knowingly flying a machine that in "normal" flight can have control problems. That does give reason for pause or should anyway. Logically, it does not follow that one would wish to fly a helicopter whose controls "lock" up in the cruise....or while maneuvering near the ground.

Having only flown in the back & front of helicopters for nearly 50 years, I seem to have missed out on all these jacking problems you guys seem to have had.

The closest to it I can recall is in a Bell 47 & 206 when flying in mountainous areas parallel to a cliff face in very windy conditions, maybe 30 + kts, a gust has hit me towards the cliff face and I could not move the cyclic in the opposite direction. The cure was to move the cyclic towards the cliff to unjam the cyclic and then put in opposite cyclic. Is this what you mean by jack stall? I just considered it as running out of hydraulic control.

I can't imagine this in normal cruise unless you are flying through a massive thunderstorm when anything can happen.

ATN,
I certainly don't want this to become an us and them thing. I believe that the servo strength of virtually every helicopter I know, civil and military, does not show any jack stall in regions anywhere close to the operating envelope. This 350 discussion is teaching me plenty. I do believe that many/most/all of the helicopter world (British, US, Russian, etc) has grown past the need for the pilot to worry about having his helo take control away from him in most maneuvers.

Collective Bias, that S-55 condition you mention is most certainly stall related servo load, but happens at or beyond Vne, and to an aircraft that is arguably three generations old. If jack stall was only experienced at Vne in 1949 (S-55 first flight Nov, 1949!), one would have hoped we stayed the same or got better in 55 years!

I did the servo adequacy tests on the S-76, and took a loaded S-76 to Vne, turned off one servo system, then maneuvered to 1.7 g's to clear the aircraft for certification. Not a hint of jack stall, believe me.

The hydraulic belt on the AS350 has caused some problems. One was lost in the GOM a few years back when the belt broke and the pilot tried to land on an offshore platform. Sadly, there were no survivors. I have around a thousand hours in the AS350D, and always had mixed feelings - lots of fuel, plenty of speed and comfort, but if things went wrong, they went badly wrong.

Two tales, but both concerning SA/AS/EC type helicopters and servo/hydraulics:

1. SA-341 Gazelle (MIL version) suffered a fatal accident at Farnborough around 1973. The helicopter performed a low level, right-hand turn at high speed. The roll to the right continued unchecked and the helicopter crashed inverted. The conclusion was that by increasing the angle of attack on the advancing blade, which provides most of the lift, the forces on the blade at high forward speed were sufficient to overcome the hydraulic servo and jack stall occurred, and the pilot was unable to counter the roll.

This occurs mainly when turning to the right - French helicopter = advancing blade on the left side of the helicopter. When turning left the angle of attack on the advancing blade decreases, and the forces on the servo decrease accordingly.

Even though this was a known phenomenon, and demonstrated under training, it was the presumed cause of another Gazelle fatal accident in 1975, during a low level sortie over Dartmoor, killing the instructor and student.

2. AS-350B2 – When this type was introduced it had new spherical bearings, new single hydraulic system (both taken from the 355 dual hydraulic system), a new power plant, beefed up rotorhead, & etc. Better, more powerful version of the B & B1……?

The hydraulic pump is driven by a belt (“rubber band” - see previous postings on this). In cold weather (under - 25oC) the hydraulic fluid increases in viscosity and the belt starts to slip. As it cannot drive the pump at normal speeds, the belt deforms, due to the friction from the accessory drive, and eventually stops turning the hydraulic pump, or slips off the drive wheel. OK you think, small helicopter, no hydraulics, what’s the problem ?
Well it turns out that the new spherical bearings (“rubber balls” from the 355) freeze solid at temperatures under - 25oC, and without a hydraulic servo the whole control system locks up !

The pilot then becomes a passenger in the helicopter, and only by using extreme force can the controls be moved.

AS/Eurocopter say they have fixed this problem with new types of spherical bearings, but the fact remains that if you lose the hydraulic drive belt on a 350, it becomes extremely difficult to control – see the news gathering AS-350 that plunged to a rooftop in New York earlier this year

Reading Collective Bias's story about the S55T reminds me of an incident I had some years ago when a leak in the cockpit roof caused a drip resulting in a bridging of the hydraulics Primary off, Secondary off test switch.

Both sets of hydraulics failed on me leaving only the emergency system - driven I think by transmission oil pressure on the lateral jacks. All could have been restored by switching off the electrical master switch (fail safe on) but I lacked the courage and free hand to reach for the switch.

From that incident on I made a special effort to learn the aircraft systems and not just skim over the text to pass the type rating exams.

Yes I did manage to get the aircraft down and land it after 20 minutes flying in that condition - pure luck.

Jim

Reading about the accident involving a hard right turn near the ground....and being unable to stop the roll and for sure probably not wanting to reduce collective at that point....a question arises.

The BO-105 and BK-117 have a similar trait in that situation....one can run out of cyclic authority if a high roll rate occurs.....the recovery is to apply "full" opposite pedal.....fully and quickly...."stomp" was the word used but something slightly short of that seemed to work during demos.

Adequate control is quickly gained and as the nose pitches up to level or more....then the collective can be lowered and cyclic authority is regained.

Just a thought!

This is a very intresting subject and I have to send my 2 cents again.


BB I think the problem you refer to was from an accident on Greenland in the mid or late 80 ies. This was due to (if I remember right) a broken drive belt to the hydraulic pump in very cold conditions and with a new style of Spherical Bearing (load carrying rubber bearing) fitted. Not to the fact of slipping drive belt.

The fix was a new style of bearing, and a new lower temp limit of the previous one.

The RFM also calls out for warming up the spherical bearings before takeoff in cold conditions by moving the cyclic 3-4 cm fwd for 2 min and to check the force to move the cyclic without servo.

We have been flying B, B1, B2, B3 in very cold conditions (down to lower limit) for a long time without problems, we started with B in 1979. We even tested a cog belt driven pump for EC, but it never went into production. Even if the belt drive looks very simple, it has just failed on us once in all these years, and it was a successful landing afterwards.

I have always as a training captain had the opinion on single hyd system helicopters that if there is a procedure in the emergency checklist that calls for turning off the hyd system ASAP, then it should also be trained at cruise speed. Normal procedure is to bring down the speed to around 60 KIAS and then turn it off (even at ECF they do not train this at high speed if they have not changed there way of training). But if you get some sort of hardover you need to get it off immidiately, no matter the airspeed, without having tried it before it could be intresting

:ooh:

Therefore when I train someone on the 350 I demonstrate this and then let the pilot try it with increasing airspeed up to cruise. At cruise it is hard work, but since the cyclic want to go right and aft, it is very much controllable. But since many 350 pilots from time to time hold the cyclic "very light" to get good feel, I think it is important to know what they are up against if you turn off the servo at cruise speed.

:ok:

CB

I taught on AS 350Bs for a few years. One of the course demonstrations was"jack stall". The point of this demo was to show the trainee that the phenomenon was possible and could be dangerous close to the ground.

It was a while ago but there was very rapid rolling and pulling to achieve it. Once it occured, it felt much like the aircraft did hydraulics off. Although the controls were heavier they could still be moved and the aircraft recovered. You really had to work hard to make it happen.

It's a limitation to operations, like left crosswinds and LTE are to Bell 206s, left pedal stops and crosswinds to Bell 205s, vortex ring is to any helicopter and the size of the fuel tank is to any aircraft. No big deal.

As for the "rubber band", the fleet of about 20 to my knowledge never had a failure, even in a temperature range from -5 deg C to 40 deg C. The biggest problem with them was getting them on and off. Undoing drive train etc. They were lifed for 10 years so when the fleet hit 10 years old, not enough rubber bands to go round.

I've seen a few cases of sudden belt slippage when arriving in the hover in loose dry snow (read - - big nasty white snowball ! )

This not the time you need control issues, horns, etc as you are searching for the ground in a white-out ! I have also seen a few broken belts, plus a lot of burned - glazed ones.

I work with a 100% Bell company now( :ok: ), but my past employer used to have a spare belt fastened around the drive-shaft of each 350, so the complete dis-assembly of the drive system was not required to change the belt -- once anyway.

I think a lot of the hydraulic problems mentioned above during high speed turns are not really hydraulic problems but retreating blade stall which causes a euro machine to flick right and a US machine to the left. For this reason when teaching steep turns, you get the student to turn the opposite way to the flick, so that if it goes wrong, you flick upright.
I well remember being number 5 in a tail chase in a S55 when the leader sped up close to vne and wrapped on lots of right bank. A few seconds later, trying to follow number 4, I was doing a steep turn to the left. Very worrying but not really a hydraulic problem.
Maybe the Gazelle had this happen? :confused:

The Gazelle's controls give a definite feedback "jolt" when jackstall occurs. I saw retreating blade stall in a Whirlwind 10 (at 150 feet agl) and if I recall correctly, it didn't give the same effect.

Both have a similar effect on the pilot though...... :ooh:

407 Driver's Comment
 

This is a very good example of how prone the ASTAR is to jack stall. It doesn't take very much. At high DA's in combination with a High gross weight, light to meduim turbulance can result in jack stall. Anyone who has flow an ASTAR in the mountains has probably experienced this.

Mr Osborne, my experience is contrary tp your speculation regarding blade stall. I've had it happen in turns of both directions, and vigorous but not terribly rapid roll rate or exceptionally high speed- well within the green arc and low DA. The surest way I've induced it is cruise or higher, pull some g, and roll. The stick just stops, lateral. I'd hate to force it, as the onset is sudden and unpredicted- it could well cease with me leaning on it- That would introduce new and exciting issues. Pitch reduction (both senses of the word), slow, and the aircraft's normal again.

As to belts- An Arizona 350 had an air conditioner compressor drive belt break and take out the hydraulic drive belt, too. It's a chintzy system in an otherwise very satisfactory helo.