Jackstall: What is it? Any experiences?

[Ioan]

Hi,
I had an instructor mention something calles Jackstall to me earlier today. He's an ex Gazelle pilot and apparently you could get it to jackstall with difficulty, but he couldn't explain what it was. I've had a look in the bible (Wagtendonk) but can't find any reference to it in there either.

Anyone know? Is it something that occurs just on the Gazelle, or a military term, or something else?

Ioan

[Flying Lawyer]

Ioan

The pros here will be able to explain this much better than me but, as I understand it .....


'Jack stall' occurs when the blades overpower the hydraulic servos - the jacks stall - and the controls lock solid.
If you're training in a Robinson that may not mean much because it doesn't have hydraulic controls.

It's most likely to be caused by extreme manoeuvring.
eg In the Gazelle, if you're in a descending right turn and pull really hard, you might experience jackstall.

It's very unlikely to happen in 'normal' flying.
However, if you were flying straight and level at high speed, and suddenly pulled up and to the right (eg to avoid a bird), it could happen.

Recovery from jackstall:
Release the pressure and the controls will unlock.

If it happened near the ground you'd have a problem because you wouldn't have much time/space to correct it - and possibly exacerbate the problem by instinctively trying to pull away from the ground, instead of releasing the pressure. However, as a PPL, you're unlikely to be doing extreme manoeuvres near the ground.

Is it something that occurs just on the Gazelle?
No.
It can happen in the Squirrel - certainly in the older models. I don't know if the newer models have more powerful servos.
And possibly in some other types.
I associate it with French design helicopters, but that may be wrong.

Military term?
I don't think it is, but the FI who did my Gazelle conversion was a Mil pilot so possibly.

Or something else?
It's called 'servo stall' and 'servo transparency' in America.
I don't know what it's called in France - it’s like those French have a different word for everything. :) (Credit: Steve Martin)


FL

[NickLappos]

FL,
You did very well!

One tidbit to add - the force that locks the hydraulics comes from the blades as they are stalled, and is the same as the pitch down that the wing of an airplane performs when the wing stalls. This "moment shift" occurs on every airfoil when it stalls, because the orderly lift is no longer being produced, and instead, the wing behaves more like a barn door, with the wind pressure acting about the midpoint of the wing area, instead of at the 1/4 chord point when the airflow is sweet and smooth.

As the rotor blade stalls more and more (the stalled area moves outward toward the tips) the blade presses down more on the controls and can overpower the pilot or the hydraulics. In a non-hydraulic helo, the controls stiffen up, and can throb and vibrate as the stall gets worse and worse.

Jackstall is not experienced on most helos because the hydraulics are powerful enough to hold the controls against whatever the blades can dish out. Some french helos are actually designed with weaker hydraulic controls, and thus their pilots get the thrill of experiencing jack stall if they maneuver excessively.

[Gordy]

Have nothing to add other than yes you can get it in newer models---had it a few times in a B2 Astar.

[Semi Rigid]

had a fatality @ needle rock in the coromandel, nz, bak n '94 i think.low time as350 pilot giving some locals a free ride high speed descending turn to the right and flew straight into the water.killed a female pax.fed's proved he was below 500' because the pax said they could see the mainland through the hole in needle rock & the only way that was possible was to be below a certain height at a specific seaward area.
apparently his type rating never covered hydraulic jack stall.

[212bushman]

Gents, I believe this topic was covered around Dec 04'.
Thread then called 'A-Star "Jack stall".
If searched for, it could help aswell.

[Ioan]

Thanks everyone, that was really useful. I did do a search for it but nothing came up. Seems strange that more people don't know about it - none of the rotary guys I talked to yesterday did - maybe it is just because we've all spent most of our time on robbies. Never heard of it in the 206 course either though.

Very helpful - thank everyone

Ioan

[bladepitch]

there is a video if you want to see it. its a US coast guard experiencing a jack stall while doing a low pass in front of a ground camera.

its pretty nasty and he does a great job of recovering it.

pm me if you want the address.

cheers BP

[sprocket]

Question:

1. With jackstall on the AS350, has it been definately been determined whether it is the hydraulics/servo not being powerful enough, or does the hyd pump drive belt slip under high demand?

2. How is the Gazell hyd pump driven?

[oldbeefer]

Sprocket. The pump produces constant pressure. Any excess not required when not moving the controls is vented through a pressure regulating valve. So, belt slip doesn't come into it. Gazelle pump gear driven, if memory serves me right. Known in France as 'Jaques S'tall'.

[ShyTorque]

Yes, the feedback forces overcome the pump output so there is a sudden "jolt" and temporary loss of control. We used to be required to demo it to studes going through the basic rotary course at Shawbury. Goodness knows what it did to the airframe, as it didn't feel pleasant! We used to go into a steep high speed descent and pull up quite hard, whereupon the aircraft "kicked" more nose up and rolled a little, which actually helped the recovery. It would be quite difficult to see this in normal ops, as it was really at the "extreme of manoeuvre".

Apologies for thread drift - I'm certain it was also possible to get close to main rotor jackstall in the military Puma. I displayed that aircraft for a while and sometimes the old girl registered her displeasure by suddenly vibrating hard, again in fairly extreme circumstances (high speed pull up, etc). The recovery was just to back off the collective pitch and/or reduce the extremity of manoeuvere, just as in the Gazelle. However, a pilot flying on normal squadron ops would have to be very hamfisted to reach those limits.

As for the two display pilots who achieved the impossible by hitting the tail rotor blades against the tail pylon...... "LSH", I think, might remember one of those occasions quite well :E

[Aser]

http://www.pprune.org/forums/showthread.php?t=225261&highlight=jack

[What Limits]

I ended up with about 2000 on the Gazelle, and IIRC we did the 'Dive to Destruction (VNE) and Jack Stall' quite often. We also did it in a turn to the left I think because the aircraft rolled hard right (or the other way around !) In fact I do recall that this manoeuvre was banned after an inexperienced pilot went off to do part of the flight test schedule which called for jack stall and had to recover from 'an inverted position' !

It also reminds me of sending off solo students in the Firefly: 'My aerobatics were poor, Sir, but my spin recoveries were spotless' !

[Ioan]

Thanks for the link - very insightful! bet those dauphin pilots had some washing to do after that! :)
Apologies too by the way. I just tried the advanced search on this page for jack stall and came up with two pages or results that didn't show in the basic search. Life's a learning curve!

[rotorboy]

Jack stall = Being stuipid and flying the aricraft in ways it was not ment to be flown ( i.e. cowboying the thing around)


Anyone who says they have experienced jackstall, has done so being an idot (IMO).....


rb

[oldbeefer]

What Limits - remember it well. The CFS course used to teach recovery from descending 60 degree bank turn to the left. My instructor decided to try it in a descending turn to the right. He pulled hard and the next thing I knew was Newport and my chinagraphs were on the roof. I did ask for an instructor change after that.

[Sailor Vee]

Part of the post 100 hour service test flight on the UH1 was to move the cyclic forward and right to aft and left in three seconds, and vice versa to check for any propensity for jack stall. This exercised only one jack at a time. The observer used to cover the hydraulic boost switch, to turn it off and then on if a stall occured!!
You could induce it fairly well by pulling up and rolling from a high speed pass!

[Devil 49]

Two recent occurrences come to mind, AS350B2-
First, After a "dog and pony show" at an elementary school, clear ground all around- High speed (125 knots, yes I know...) low level pass- 300' AGL- and attempt climbing turn to the left- No left cyclic, it feels like I hit a control stop left of cyclic center- released pressure, and cyclic normal. (NO MORE HOTDOGGING, EVER) I'm very glad I wasn't NOE.
Second, max NG cruise at 2000 ft, DA approx 4000, vulture spirals up approx 200 feet dead on nose, attempted to roll right, not "agressive"- no right cyclic- a little aft cyclic and a lesser movement right. In 13 years on the airframe, that's the only "normal" control input I've ever done that was limited by hydraulics, and I couldn't repeat it.

[Broadcast Control]

Rotorboy, I think that you are probably right that in most cases the aircraft must be flown outside the normal envelope for jack stall to occur. However, I have been been told of one instance where extreme mechanical turbulence (at high DA) caused jack stall.

What Limits - sorry for grabbing a user name so similar to yours. I'll go get another one.

[B Sousa]

I have to say Rotorboy is right in many cases. However we have a little situation in the Southwest called the "Grand Canyon" For those who have flown there on shall we say Breezy days. It will seriously kick your a$$. Many pilots while trying to keep the world level there have had the sweet occasion of getting into jack stall. From their descriptions it happened while making many manuevers to stay upright.
I can say that I have not encountered it myself, and really it would scare the sh1t out of me anyway. So after thirty six years of stick time, heres hoping I never do find out what fun it can be.

[ichris]

The current term used by Eurocopter is Servo Transparency (jack stall). The following is part of the research from 2001 AS350 accident. The report is a good outline of the effect.
http://www.ntsb.gov/NTSB/brief.asp?ev_id=20011025X02148&key=1

According to American Eurocopter, the main function of the hydraulic actuators in the main rotor flight control system is to reduce the force required to fly the aircraft, and to isolate the pilot from these forces. The aerodynamic forces are constantly changing and result in rotor blade stress, which increase as a function of speed, gross weight, density altitude, angle of attack, and positive maneuvering (g-loading). Since the hydraulic output (pressure) is essentially constant, the maximum force the actuators can isolate from the pilot remains constant, and under certain circumstances may be exceeded. In level flight, and at airspeeds less than Vne (never exceed velocity), hydraulic systems have the capability to isolate the pilot from the main rotor forces. But, as airspeed and/or g-loading increase, the forces can increase to the point that they exceed the opposing force generated by the flight control hydraulic actuators, and servo transparency or "jack stall" occurs. Jack stall results in uncommanded aft and right cyclic and down collective motion accompanied by pitch-up and right roll of the helicopter. The maneuver, often abrupt and a surprise to the pilot, tends to be self-correcting since the rapid loss of airspeed due to the pitch-up and down collective causes an equally quick reduction in feedback forces. Pilots rarely fly at speeds beyond Vne, but do occasionally induce jack stall as a result of excessive maneuvering. If jack stall is unexpectedly encountered during maneuvering, the pilot should decrease the severity of the maneuver and reduce collective.

Also see FAA bulletin:
http://www.faa.gov/aircraft/safety/alerts/saib/media/SW-04-35.pdf

[NickLappos]

ichris,
You republished, "Since the hydraulic output (pressure) is essentially constant, the maximum force the actuators can isolate from the pilot remains constant, and under certain circumstances may be exceeded."

That circular logic is simply amazing. Let me restate it:

"Since the servos are too small to take maximum blade forces, there are circumstances where the controls stop being controls, and the pilot might die if he maneuvers too much. By the way, we provide no way to know that maximum, so hold on tight if you blunder across the boundary."

It is a fact that the maximum blade forces during stall are measurable and part of the designer's lexicon - in other words, you don't get more and more force from the blade as you maneuver, because there is a maximum moment the blade can produce, just as there is a maximum lift. The plain facts are that some helicopter designers simply don't bother to build the servos large enough to withstand this maximum force, in fact, they fall so short of the maximum that the controls lock up during relatively gentle maneuvers (at high altitude and at high weight).

Like LTE, where a tail rotor that is too small is allowed, and instead the manufacturer starts to blame the pilot, Jack stall is not a sign of pilot error in all cases, it is a sign of a poorly designed helo that does not have the basic control capability to protect its occupants.

I firmly believe these jack stall designs would never be certified in the US or CAA, they are only allowed because of the bilateral agreements. Bell, Boeing, Sikorsky, Westland and Agusta all have design rules that do not permit jack stall, because they all design the servos to be large enough to withstand the rotor forces, and protect the occupants.

[diethelm]

I know this is a dumb question but here it goes. If you design a servo actuator strong enough to withstand the maximum force, would it then be logical to assume the other end of the force fails. Put another way, can the servo actuator be strong enough to withstand the maximum force from the blade but the result of such a strong actuator is that the pilot is allowed to put enough force into the controls such that the blade can fail, snap off and scatter but the servo actuator withstood the force? I understand a pilot can apply inputs into a robinson or a 500 which can result in catastrophic failure but in the larger hydraulic aircraft, where should the balance be? The control acutator, the part it is controlling, or the pilot?:confused:

[NickLappos]

Good question! In fact, EC uses that excuse as a justification for the weak servos, instead of also strengthening the blade horn and pitch links. Most helos are designed so those components cannot be harmed by anything that is aerodynamically generated, so the general answer to your question is no, but for some EC designs, the answer is yes, those components are also weak enough to be damaged, and have been bent in flight during maneuvers.

For US military helos, there exists a qualification test that proves the case against allowing jack stall. Known as a Structural Demo test flight, the pilot goes out at al weights and CG's and maneuvers to the maximum, to fully stall the system, and shows that there is sufficient strength margin so that no components can fail or "jack stall" in the process. These flight tests are the most critical, and most skill dependant of any except perhaps the H-V curve hard landing tests. I know for a fact that the argument made for allowing jack stall would get not 1 mm past a US Army/Navy/AF test engineer.

[diethelm]

Got it, the phrase "aerodynamically generated" makes sense.

[HillerBee]

I agree fully with Nick, I would only put it a bit harder being a design flaw.

[topendtorque]

Probaly explains the pucker tucker associated with the feed back forces of trying to go right way up after rolling a '47 way too quickly - either way.

Either the light weight or heavy weight blades will do it especially when fitted with the Texas No-bar kit, and very especially on the heavy weight blades '47 3B1 etc. with the Texas kit. I.E. stab bar removed, makes them almost identical to the 206 in feel.

letting off pressure when it is trying to roll itself upside down at thirty feet is NOT an option.

Never tried it in a 206 and don't intend to, but probably would not be as bad as shorter blades would make for a lot less leverage, me thinks.

[IFMU]

Put another way, can the servo actuator be strong enough to withstand the maximum force from the blade but the result of such a strong actuator is that the pilot is allowed to put enough force into the controls such that the blade can fail, snap off and scatter but the servo actuator withstood the force?
diethelm,

That's why they design the parts to be strong enough. New machines are subject to a proof & ops test, they jam the head and stall the servos. Since the big SA makes dual stage hydraulic machines, one stage has to fly the aircraft through the whole envelope, but the parts have strength to take full tilt boogie with both stages operating. Not only statically, but all those parts are good for fatigue as well.

-- IFMU

[Milt]

Aileron Jack Stall

Flight testing of the RAAF's F-86 Sabre included finding the corners of the manoeuvre envelope where the aileron hydraulic actuators/jacks stalled. This occurred at speeds in excess of Mach 1.0 during rapid aileron actuation. The stick would lock up at about half deflection and could have been most embarrassing in a combat situation.

The hydraulic system was designed with a pressure of 3,000 psi and there was no way to increase the pressure without major changes. Higher power jacks were considered but discarded.

The manoeuvre limitation was accepted as insignificant considering the infrequent excursions into the situations where it occurred.

[IFMU]

The manoeuvre limitation was accepted as insignificant considering the infrequent excursions into the situations where it occurred.
Maybe if we had ejection seats it might be less significant! :)
-- IFMU

[Ioan]

Actually that reminds me; I was asked the other day if any helicopters do have ejection seats! I couldn't think of any, apart from the one in one James Bond film. Know of any more? And if not, why not? Surely it wouldn't be too difficult to design into a cockpit, especially into high risk aircraft such as the Apache. Either charges on the rotor blades to lose them before the seat fires vertically, or maybe sensors on the aircraft to sense its level of pitch and roll before firing the seats one way or another horizontally. I guess with that solution there could be a problem with the G-forces involved, but I'm sure they could be solved?

[What Limits]

There has been some research into ejector seats and it all hinges on the guarantee of getting all of the blades off in sequence so that the out of balance forces don't get you before one of the blades left attached will.

Horizontal ejector seats are again not possible because the human body would not survive the acceleration.

IIRC the body can survive up to 45g in the vertical plane but only 5 to 10g in the horizontal. Most current ejector seats go out at something like 40g.

Even with the correct seating position and everything working properly you are going to lose around 2 - 5 cm in height in a successful ejection!

[Ioan]

Yeah I thought the G forces might be an issue. On the other hand though, are such large forces really necessary? You'd want a decent amount to get you away from an aircraft that was perhaps in a spin (loss of tail rotor, etc), but I would have thought rather than having high forces for a short length of time, moderate forces for a longer time could also be effective. Especially bearing in mind that helicopters are likely to be travelling much much slower than your average fast jet. Would also solve the problem of having rotor blades flying in all directions at high speed - prob not a good idea if you were flying formation beforehand!

Then again though, I'm sure people have looked into the idea. This is just me thinking aloud!

[NickLappos]

The two helos that I know of that have (had) ejection seats are the Kamov KA-50 coaxial and the Sikorsky experimental S-72 RSRA (which was also the DARPA X-Wing).

They both have elaborate mechanisms to blow off the rotor blades and they use extractor rockets with thether ropes so the G forces on ejection are low.

Frankly, the weight, cost and maintenance burden of blade removal/ejection (as well as the depression it will cause in your passengers) makes them poor bets in any helicopter that I want to fly.

[Matthew Parsons]

Weight is definitely a huge concern with helicopter ejection seats, as is cost, but operational use is also important. Generally the helicopters that could use ejection seats are flown aggressively and close to the ground. By the time you decide ejection is a good idea, then initiate the sequence, then wait for the blades to clear, then fire the seat out...you've already crashed.

Sure if you're at 500' and pull handles early you'll be alright, but under what emergencies would you choose to trust the seat and parachute rather than flying it yourself?

I'm not saying it would never be used, just that its value diminishes quickly when you realize the limitations.

Back to the original topic:

Interesting that Jack Stall is not allowed in US Military helicopters, but that doesn't mean a pilot cannot fly those machines to the breaking point, just that the breaking point doesn't include Jack Stall. There are limitations to follow on all machines, if high speed, high 'g' flight isn't allowed and is the only condition where Jack Stall occurs then that shouldn't be a problem.

To call it a design flaw is like saying any helicopter that can intentionally be oversped, overtemped, etc. is flawed.

Designers have to meet certification requirements and prove safety of flight, but they do so knowing there is a pilot in the seat and knowing that they can expect the pilot to follow rules. If that weren't the case then there would be no limitations section in your AFM/AOI/PH because the helicopter would handle that for you (maybe we'll build like that one day, but not yet).

I've seen Jack Stall and I don't want it to happen in the wrong conditions (low level), but I continued to fly that same design and occassionally had to fly it close to Jack Stall conditions.

It's not a flaw.

[andTompkins]

Nick:

Do you know if the LUH acceptance criteria includes a structural demonstration flight?

Thanks!

Tompkins

[Agony]

Hi all,

Jack stall is certainly as a result of an abrupt control input, intentional or otherwise, but as can be seen it can have a bad outcome if close to the deck.

Turbulence and low level manoevring bring it on, especially with rolls close to ridge lines and strong wind. The old bird avoidance has brought it on to.

Yes I have been there, and no I am not an idiot...........the reason, show the student so he can recognise, recover and avoid being there in the first place. Ignorance is no excuse for a death if you can prevent it.

IMHO opion if you know what where and when it makes you more appreciative of it's dangers. It is benign to recover as the load is what induced it, remove the load and the problem goes away.

To add fuel to fire of "doing things that are stupid" it is similar to demonstrating (incipient) vortex ring really. Another danger to be fully cognisant of..............

I'll watch the flak after that comment.....:ok:

[helmet fire]

I guess we have said most of this on the 5,000 other jack stall threads :} but I think the arguement is always worth going over.

One consistent position is those who believe an aircraft that suffers jack stall should not be certified.

The second is that jack stall is a poor design that can be compensated for by handling skills, knowledge and training in the same way teetering heads, LTE, and critical wind azimuths, etc, can be avoided. The big issue with this position is training and knowledge!!

The consistency with which these relatively well known "flaws" kill is an indictment on our training as much as it is the aircraft we fly. In modern error management the first principle is to engineer the problem out of the system so we dont have to train and prepare for it - and that is the position adopted by Nick and the others who contend the aircraft should not be certified.

Those adhering to the second position hold that the design flaw is too expensive to retrofit versus the ease with which training and knowledge can be used to overcome it. Each of us will change between these two positions (design out problem V train to avoid it) based on our own subjective assesment of the risks V training V design cost ratios.

Personally, I think jack stall is easy to train and avoid, but should not be tolerated in new design aircraft. The flaw in my arguement is of course training! And given that the opening post of these threads has inevitably been by pilots asking for more information on the phenomena - perhaps my position has been undermined.

The question then is - what is so wrong with our endorsement or type rating training system that pilots have to ask about such basic type limitations on pprune rather than seek the answers from their instructor? Should these "instructor limitations" be designed out of our system first?????

[Ioan]

That's pretty much the conclusion that I came to; you can design faults out of a system, and if possible you should (and in this case that seems completely possible), but you also need training for any that remain.

Maybe I should point out though that while I asked about this topic, I'm not actually rated on the Gazelle, so maybe it is covered in the type training. Was just it included the word 'stall' - something not generally considered to be a good thing in aviation - I just wondered if it would be applicable to any of the types I fly.

[NickLappos]

The issue is actually how we learn to compensate for the limitations of our machine, and the consistency and clarity of the warning that the machine gives.

Every pilot believes that he is better than the others, and that a pilot error accident is really just how bad the other guy is....until his machine bites him. Read the "Right Stuff" to see how we are rationalizing wen we scoff at making our machines better to make our mistakes less costly.

The reason why jack stall is a stupid thing to allow is that it is very preventable, as is LTE. The fact iis that we log accidents in them so they are more dangerous and less trustworthy than they should be. Yes we can learn to compensate for these flaws, hairy chested pilots like crappy machines, they make us feel so successful when we get them down in one piece. Our passengers expect more.

[Matthew Parsons]

Nick, nothing wrong with your design philosophy except that many can't afford it. If you were writing international certification standards I'd be very much on your side. Lets only make good product, from now on. Unfortunately, people don't want to spend $10 million on a training helicopter. Hence we have training helicopters that have noticeable accident rates in areas that designers could have prevented.

For the foreseeable future, helicopters will continue to carry a pilot. It is not a legacy item, it is a necessary piece of equipment that can override the errors of all the other pieces. Until helicopters are reliably designed without pilots, the pilot will be responsible to follow limitations. Whether a limitation is the result of an obvious efficiency in the design stage or if it is a compensation for a deficiency will be up for debate.

It's funny how helicopters seem to rely on piloting skill so much more so than fixed wing aircraft, yet the limitations to prevent Jack Stall are akin to the most accepted limitations of fixed wing aircraft: g limits. Yes, modern aircraft prevent the pilot from overstressing the machine, but almost every fixed wing aircraft I've flown had g limits that were easily exceeded by inciting the conditions that lead to Jack Stall.

[NickLappos]

Matthew, I have to disagree. You seem to think that designing a helo properl;y is more expensive or heavier. The ONLY helos that can experience jack stall are those mentioned in these threads, a small handfull (I want to avoid mentioning brands, this is not a commercial rant). It is not necessary to design substandard servos to be cheap enough or competitive enough, a thousand successful helos prove otherwise.

Don't give in so easily to the demon of crappy helicopters!

[Matthew Parsons]

Nick, I have to admit that the business side isn't my thing, but the thinking that brought me to my previous point went something like this:

Weren't the helicopters that we aren't talking about very commercially viable at one point, and remain relatively so today? Didn't they offer a significant increase in performance over the types that they were replacing, despite very similiar overall operating costs? If they were beefed up, would they have increased costs and decreased performance?

I don't think we disagree on what we want, and I know we both admit that throughout helicopter design there are compromises, I just think that jack stall was a compromise.

I'm not sure if there is any new product that suffers jack stall with systems operating properly, so the designers apparently have taken care of this.

Unfortunately I suffer the "demon of crappy helicopters" in other areas. When will we get the perfect machine?

Cheers,
Matthew.

[blodwyn]

Jack Stall used to be a requirement that had to be achieved as part of the Gazele MTF (5M Checks), until one day my colleague nearly chopped the tail off !
Requirment was removed rabidly from the 5M and his cheeks slowly reurnedto their normal colour !

Fling overalls subjected to a quick launder and assessed 'S'

[dakkg651]

.

Don't give in so easily to the demon of crappy helicopters!

Nick

As a pretty inexperienced helicopter chap I have had the pleasure of learning a lot of good stuff from your contributions to this forum. I was, therefore very surprised by your comments regarding EC products. I am British and therefore not overly disposed towards our neighbours across the channel. However, having been part of a military school which has operated the Squirrel for over 75,000hrs with no recorded cases of Jackstall (intentional or otherwise) I think I might br forgiven for suspecting a case of 'American is best' here. I really hope I am wrong. Have you personally encountered Jackstall in the Squirrel? If so then please describe how easy it is to encounter. 130 instructors wait with bated breath to hear how close to disaster they come every day flying this deathtrap.[/I]

[NickLappos]

dak,
Please do not generalize my words, you then wouldn't be so surprised. I have avoided putting manufacturer's labels on this discussion because of exactly what you have done, put your nationalistic/manufacturistic (is there such a word?) ethos first and your safety judgement second.

My contention is that we apologize for crappy systems that fail to do their job by bragging "any pilot worth his salt should be able to handle the fact that the controls lock up or the tail rotor fails to control the machine!"

Meanwhile, the accident reports prove that assertion is simply bull, since accidents can and do happen all too frequently BECAUSE of jack stall and LTE. If accidents do not happen because of Jack Stall, I would not take this point of view. Since they do, your contention that pilots ought to be better is simply arrogant bull, since it implies that pilots and passengers get what they deserve, since they don't fly with perfect pilots like you. Such a position has no place when we speak of how to make our industry better.

Our passengers do not want to ride in substandard machines built with flaws that were engineered out decades ago just because our pilots are so proud of their testosterone driven skills that they do not fear a helicopter that can go out of control unless babied. What do we tell your widow when you try to avoid a bird at altitude and get jack stall and hit the dirt? That you were a bad pilot and deserved it?

Have I experienced jack stall? Yes, in several helo types, as far back as the AH-1G in 1969, but I have also flown an S-76 with one servo system turned off at maximum gross weight to Vne, and pulled 2.0 G's without feeling any difference in the controls. Why did I do that? Because that is how we test to be sure you and your passengers did not have to experience jack stall.

I contend that it is time for us to seek the best for our machines, and stop impuning the skills of our pilot brothers when an accident happens.

[Tourist]

I think the Jack stall demo is no longer taught officially on the UK Gazelle, but I was certainly demo'd one and tried one for myself when I went through.
Vne dive and pull up hard till it tried to flick itself on its back. Lots of fun.

Got to say though, having flown a large number of Helicopters from Seakings to Seahawks Squirrels to Hips, you can't beat the Gazelle. The French do make very fine Helicopters, (even if they always do have a flaw or two to keep you on your toes!)

A true gentlemans sportscar.
You dont fly it, you strap it on.

[dakkg651]

Wow. Light the blue touch paper and stand back.

dak,
Please do not generalize my words, you then wouldn't be so surprised. I have avoided putting manufacturer's labels on this discussion because of exactly what you have done, put your nationalistic/manufacturistic (is there such a word?) ethos first and your safety judgement second.

Have I? I thought I was asking a simple question or two.

My contention is that we apologize for crappy systems that fail to do their job by bragging "any pilot worth his salt should be able to handle the fact that the controls lock up or the tail rotor fails to control the machine!"

Meanwhile, the accident reports prove that assertion is simply bull, since accidents can and do happen all too frequently BECAUSE of jack stall and LTE. If accidents do not happen because of Jack Stall, I would not take this point of view. Since they do, your contention that pilots ought to be better is simply arrogant bull, since it implies that pilots and passengers get what they deserve, since they don't fly with perfect pilots like you. Such a position has no place when we speak of how to make our industry better.

Did I say that? I have re-read my post a dozen times but can't find it.


Have I experienced jack stall? Yes, in several helo types, as far back as the AH-1G in 1969, but I have also flown an S-76 with one servo system turned off at maximum gross weight to Vne, and pulled 2.0 G's without feeling any difference in the controls. Why did I do that? Because that is how we test to be sure you and your passengers did not have to experience jack stall.

This doesn't answer my question ref the Squirrel. From all the comments in this thread, we all know that the Gazelle could be jackstalled in extreme manouevring. Does the Squirrel, a later design, have the same characteristics? If no improvement in jackstall threshold has been achieved then I agree with you that this is unacceptable.

I do not have figures on the number of accidents caused by jackstall, but if they are occuring with the frequency that you imply, then surely the offending helicopter types should have their certification withdrawn!

I realise that this is an issue which you feel very strongly about. That is why I am looking for reasoned answers from someone I consider an expert - not abuse.

Dak

[Spurlash2]

My only experience of jack stall was in a single engine helo at MAUW, which occured over 25 years ago. We were asked to carry out a single pass over a military parade ground having just disembarked from 'mother' a few miles away. Approached the target downwind in a slight descent for maximum speed (must have been approaching 95 kts, at least), as we on topped the target, the aircraft was pulled up and right and instantly the most violent vibration was experienced.
The aircraft then continued on the original approach hdg in a slight descent, as opposed to the control input of up and right, (probably because the controls were relaxed!) the speed washed off and the vibration stopped. Ht loss was maybe 50-100 feet, which was OK because our escape route was over ground that dropped away. We made our excuses and left.
I have experienced many flight regimes in helicopters with nothing scary happening, but the factors on this occasion for onset of 'Jack stall' were (I believe), very heavy aircraft, in a descent for max speed with collective applied and forward cyclic, and the application of more collective and cyclic to pull up and right which resulted in a lot of 'G'.
Certainly caught us out, and it never happened again!

[NickLappos]

dak,
Lets chalk this up to the difference between verbal banter and written miscommunication. I read your "america is best" "bated breath" and "deathtrap" different than you meant them, I guess.

Yes, there have been several jack stall accidents mostly involving out-of-control helos smacking the ground. My guess is that your demo does a nice job of showing a nibble, and those poor fellows swallowed the whole whale. I know of one Pax River TP whho flipped upside down while having all the control of a passenger. He actually enjoyed it!

[petitfromage]

So...we are all agreed then? :}

a. Jack Stall occurs when the limits of the designers chosen hydraulics jacks are exceeded by the aerodynamic forces of stalled rotor blades.

b. Both roll &/or pitch control (or both) are lost for the period of exceedence.

c. It is violent, potentially fatal & probably reduces the airframe & component fatigue life &....

d. Can, like LTE, be significantly reduced (even eliminated) with more robust design efforts.

**

My 2cents:

Risk management is the very essence of being a pilot (esp a helo pilot).
I agree with Nick entirely but the manufacturers also have valid points. Eg: Mr Robinson designed a the R22 to commute from A to B. If you're throwing it around mustering or deer hunting then you must bear some responsibility if things break.

RBS and Jack Stall are not always the result of deliberately being a cowboy. Scooting around a ridge into a string wind is a common cause.
(other factors are weight, blade angle, rate of control inputs, temperature/humidity/density altitude and TAS)

Perhaps the compromise is that if you're NOE at 8000ft, ISA+20 and dodging gunfire in the Afgan mountains......you surely deserve powerful hyd jacks.
If you're helo was designed for flying tourists on city tours of NYC, then I you should be flying within the most limiting factor (be that mechanical or crew experience).

So where does that leave us?
Should manufacturers be providing a comprehensive list of tasks, locations or environments that void the helos warranty?

btw: The Huey jackstalls with carefree abandon. (Euro vs USA isn't a rivalry worth anyones time/effort).

Nick...I can only agree with your points but following that same route:

a. We'd all be flying twin engined helos that can hover at MAUW on 1 at ISA+15 'just in case' the engineers failed us and,
b. We'd all have anti-icing and full IFR avionics just in case, in poor weather with poor airmanship, we failed ourselves?

[dakkg651]

Nick

We don't demonstrate jackstall in the 350 as was done years ago on the Gazelle. We do a lot of low flying, however, and your bird avoidance scenario is what makes me wonder where that jackstall threshold lies. I shall certainly be seeking information from the people who ought to know about the more murky corners of the envelope.

Cheers

Dak

PS American was certainly best when it came to producing a certain twin engined fixed wing beauty - you called it the Skytrain. We named it Dakota.

[ShyTorque]

Watch this linked video, it shows what can happen.

http://www.youtube.com/watch?v=9mzT13Hd5Gw&mode=related&search=

[nigelh]

having flown the squirrel for a few years, sometimes pretty heavy, sometimes a little enthusiasticly, with no sign of this jack stall, i would love to see what you need to do to induce it.:eek: Also flown regularly in hills with 40 - 50 kt with severe turbulence again with no problem . Am i just lucky ?:rolleyes:

[paco]

dak - Re: the Squirrel, one pilot reported the problem in Canada coming off a mountain at 8000 feet, and another reported it flying at 40 kts around a fire - "in a 15 degree banked LH turn around a fire at 80 Kts, 4500' +30 C" Yet another got it about once a month in the Rockies. There have also been two reported incidents of a hardover on the ground to my knowledge, one at Remote helicopters (to a friend of mine) and one at Canadian, which wasn't originally reported but subsequently discovered.

I certainly think more pilot understanding of the hydraulic system is required before the average type rating is given.

Phil

[NickLappos]

The fact is that the controls loads that are produced by stall are those that cause jack stall, and it does NOT take a wild maneuver to cause them, all it takes is some degree of blade stall. At high weight and high density altitude, the maneuver required can be shockingly small, and there is little or no warning.

I repeat what I have posted earlier: A control system is supposed to control, not lock up. Jack stall is a design problem that is only experienced in ONE manufacturer's production helicopters, because all the other manufacturers make their servos powerful enough.

Like LTE, jack stall makes pilots need to fear their machines, and like LTE, jack stall is not a condition that need be suffered by pilots anymore.

[nigelh]

How do you get jack stall on the ground ? surely the forces are not enough with no disc loading ?

[[email protected]]

Dak, in the days of the military Gazelle we used to low fly at IPS or MPS to give 120 kts or 135-140. At those high pitch settings, the risk of jackstall in a bird avoidance manoeuvre was real. We also demoed the dive to VNE and recovery (including jackstall if you were harsh in the manoeuvre).
Nowadays the DHFS Squirrel is artificially torque limited and so high pitch settings at low level are uncommon and the dive to VNE is seen as unneccessary. I suspect that if the Squirrel was actually owned by the military and used on the front line as the Gaz used to be, then you proably would demo the limits of its flight envelope but because it is owned by a civilian company and operated in a training environment to their limits, you don't.
That doesn't mean that the Squirrel won't jackstall (or suffer servo transparency if you prefer that term) - the only way to find out is to calculate the VNE (if it has a graph for it) and then dive at max pitch/Torque and pull - if it doesn't do it then great - write and tell us.:)

[paco]

nigelh - this wasn't jackstall on the ground, but a hardover, which nobody has yet really come up with an explanation for, as far as I am aware, despite taking a machine into the deepest Arctic to see if cold was affecting the servos. Transport Canada banned all hydraulics-off training for a while, which upset all the type conversions, since it's on the syllabus!

My friend was sitting there at the run up and the cyclic just motored over to the left and he couldn't pull it back, which is why there is now an extra hydraulics off check at low RPM before you wind the machine up. I believe the Canadian Helicopters problem was similar.

I only mentioned it in my previous post because it is a problem with those machines that is hydraulics related and some people might not have heard about it.

Phil

[tacr2man]

Talking of gazelle Jack stall In the early eighties flew into a small airshow out Bedford way as i was in a helo they parked me with a display gazelle that was RN but being flown by RAf pilot . He did his display, when he landed back I asked him did he normally leave his pull out so late, it was then I first heard the term jackstall, he said he had been lucky it bounced off the ground air cushion as it was jack stalled at the time . :mad: lucky

[NickLappos]

nigelh,

The blades produce a downward pitching moment as they get near stall, so they must be producing a high percentage of their maximum lift to produce jack stall. In other words, jack stall is NOT a hydraulic situation, it is when the blades produce enough control force to stall the hydraulics because the hydraulics are too small. This stall force is caused by the same airfoil pitching moment that makes an airplane dip nose down at stall. Remember that stall can occur at low maneuver rates, if the DA is high or the MGW is high, so don't think cowboying is how you get jack stall.

[Helicopterist]

Has anyone ever experienced Jackstall in the as355 or other dual hydraulics equipped Squirrel?