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.