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Hydraulic Lock /Jack Stall

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Hydraulic Lock /Jack Stall

Old 25th Oct 2007, 19:11
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K48
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Hydraulic Lock

Hi all.. In the ole ATPL(H) material Hydraulic Lock is described as different to Jack Stall- which if I remember correctly is a hydraulic force overload.
Hydraulic Lock as I understood it from the ATPL(H) studies is a failure (design or mechanical) in which the hydraulic pressure gets around the back of the relevant actuator(s) and the controls become hydraulically 'locked'. Turning off your HYD may make no difference..? Does anybody know of this?
While training in NewZealand I was flown over the site of an AS350 accident where the pilot had pulled a hard right turn (fully loaded with 4 Pax) to show a view of something. He had (I was told) got 'Hydraulic Lock' and continued into the ground at high speed. I was told it was well known with As350s, in NZ at least, that high positive G and especially right turns in the As350 was to be avoided.. (If I recall correctly). I asked which type of hydraulic failure he meant and was not given a definite response either way..'Is there a difference?'
For interest in this Dauphin clip... he seems to be straight and level, but this looks more likely to be just Jack Stall from an aggressive pull back on the stick. see here
Can anybody shed light on this issue of Hyd Lock...? (not a big discussion about Jack Stall that has been talked about in depth in other threads).
(Note: My previous refs to specific recent accidents have been removed out of respect and sensitivity to those affected. I ask to keep the thread on the technical issue proposed, Hydraulic Lock)

Last edited by K48; 26th Oct 2007 at 06:16. Reason: Accident references removed for Sensitivity
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Old 25th Oct 2007, 19:18
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Ahhh, the power of 'search'.

Edited to add: Sorry, I linked to the wrong thread. Try this one:

http://www.pprune.org/forums/showthread.php?t=254165




PS A quick word of advice - be very careful about playing 'accident investigator'. Jack stall probably didn't have an influence on the Morecombe Bay accident. And nobody really knows what happened to Colin McRae yet. Be careful about adding 2+2. It's all too easy to come up with 5.

Last edited by Bravo73; 26th Oct 2007 at 00:30.
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Old 25th Oct 2007, 19:32
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Do you realy know WTF you are talking about ?????

Dont bring up old and ongoing investigations to explore 'YOUR' mind, !!!
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Old 25th Oct 2007, 23:59
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edited

CHC. Very sorry to offend.
Killabeez. The issue is what I am looking to answer?

Bravo, I did search.. thanks.. but no answers on Hyd Lock as described above. Thread continues with orginal references removed due sensitivity/respect CHC.
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Old 26th Oct 2007, 00:28
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From how you describe it (I've never come across the term but I'm by no means an expert), Hydraulic lock sounds like what would happen to any actuator if it failed (ie hydraulic fluid in both chambers.)

Jack stall occurs when the actuators are still working but they can't apply enough force to overcome the aerodynamic forces. So they 'lock' momentarily.

So, 'is there a difference?' Yes. One is a permanent failure of a component. And the other is a temporary exceedance of a design limitation (to put it kindly.)
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Old 26th Oct 2007, 01:12
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Tut-tut, K48! You should know (or should have known) that on PPRuNe there is at least a two-year moratorium before *any* accident can be discussed, other than the obligatory, lengthy thread in which condolences are wished to the pilot and crew. When an accident occurs, no matter how big or small, we must always wait until the "official" report comes out (for as long as that may take) before the event can even be spoken about out loud. There must be nothing mentioned, analyzed, postulated, theorized, hinted, discussed, surmised or even educated-guessed at because...umm...because...uhh...because...oh yeah, because the lords of PPRuNe say so, that's why. And if you don't like it, just take your curious kiester over to any of the other aviation discussion boards in which accidents are discussed in a manner that might bring up some useful, timely, or worthwhile information.

I do apologize that you weren't fully briefed on this procedure. However, if you'll be so good as to put a tick-mark in the appropriate box on the form, initial it and then fax it to me, I'll sign you off so you'll know better for next time.

Remember: Accident talk here = Verboten!

Happy to help.
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Old 26th Oct 2007, 02:16
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Got jack stall.....reduce collective..no more jack stall, now that wasn't to hard was it
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Old 26th Oct 2007, 03:18
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Oh oh! The dreaded "jack stall" question....did we not have a detailed hashing of this topic not so long ago? Perhaps we should revive that thread and carry on with it.
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Old 26th Oct 2007, 03:27
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SASless,

This thread here, or this one maybe?


FH1100,

Harsh words: I know of no edict from PPRuNe that forbids healthy discussion, but you always run the risk of criticism from other Rotorheads should the discussion become inappropriate.

Senior Pilot
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Old 26th Oct 2007, 09:34
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FH1100 Says 'There must be nothing mentioned, analyzed, postulated, theorized, hinted, discussed, surmised or even educated-guessed at because...umm...because...uhh...because...oh yeah, because the lords of PPRuNe say so,'
ERM NO, the reason is and always has been because why try to educate-guess when WE ALL dont know the full facts !!
If you want that sort of information you can always just pick up a daily paper and read the full 'bullshit and missinformation' stories to satisfy anybodies appetite for postulation, theorization, hints, or NON educated guesses,
How would your grieving family and relations feel if we was discussing an accident you had been involved in without the facts !
Good luck,

Said my lot, thanks to the original poster for editing the post, cheers chap ,
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Old 26th Oct 2007, 09:51
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K48 - jackstall is where the aerodynamic backloads from the blades (specifically on the retreating side during manoeuvering) overpower the hydraulic jacks because they are not man enough for the job. Aerospatiale (as they were then) claimed this was designed into the system for Gazelle but most people are of the opinion that they just under-engineered the hyd system.

I have heard of, but not experienced, similar problems in Squirrels and another label that was given to the condition was 'control transparency'.

However, a hydraulic lock is a hydraulic lock with pressure trapped on both sides of the actuator, usually due to a blocked port or failed pilot valve. Only a detailed stripdown of the components would reveal the source of the problem and even then might not be 100% conclusive.
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Old 26th Oct 2007, 10:26
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Hydraulic lock is used to hold down undercarriage..

Crab and Bravo thanks for reply on subject...
I just read that Hydraulic Lock is actually used to hold down undercarriage in some light fixed wing a/c, perhaps in some helis too. Anyone know of their type's u/c design?
I have just dug up the ole notes.. (the power of search that Bravo taught me):
It seems most Power Control Units consist of "a balanced double acting actuator where the body of the actuator is attached to the flying control surface input and the ram is attached to the structure of the a/c".
The pilots selector is linked to a valve, moving the selector will open one side of the actuator to pressure, the other to return, thus assisting the movement. Things equalise in a follow up action (not sure exactly how)and the control therefore assists only.
However in the event of fluid being lost (I assume after accumulator exhaustion) an interconnecting valve drops to break the 'hydraulic lock', allowing the control to be moved manually.
Of course most bigger helis are not really controllable manually. But it is one thing to be dumped with manual control and it is another for the interconnecting valve or equivalent to fail and hold the controls in place = hydraulic lock.
Now of course this is just me digging out exam material of old which was mostly fixed wing related and says no more... So I wonder could people familiar with their type/hyd systems e.g AS350/365 dual system and As332s etc please volunteer more detail? For example how does a dual system work in practice. Thanks Crab for that...
Cheers
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Old 26th Oct 2007, 10:48
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From B73:
Hydraulic lock sounds like what would happen to any actuator if it failed (ie hydraulic fluid in both chambers.)
Don't the jacks always have fluid in both chambers?

Or do you mean pressure to both chambers?

CG
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Old 26th Oct 2007, 10:56
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On the 332L & L2 the flight control servos are in hydraulic lock when there is no control input - ie the fluid is trapped both sides of the piston and the piston cannot move. Operating the flight controls (with or without hydraulic pressure) operates the distributor valve thus breaking the hydraulic lock and letting the hydraulic pressure motor the piston (or in the case of loss of a hydraulic system allows the other piston in the jack to motor the piston, circulating the unpressurised fluid in the duff system).

The main undercarriage legs are held both up and down by hydraulic lock (nosegear has mechanical lock however).

Jackstall can occur on the 332L2 following failure of 1 hydraulic system. Remembering that force = pressure x area (area is fixed and is the area of the piston head) if the pressure from the hydraulic system results in a force less than the feedback forces from the rotor head, the controls will appear to lock or even perhaps motor the wrong way. This can happen during high speed right-banked flight (with 1 hyd system out) and so there is an accumulator on the RH jack to allow level flight to be obtained should the hyd system fail in this flight condition, after which there is a bank limitation imposed by the EOPs.

The EC225 also has the possibility of jack stall when on 1 hyd system though new servos make this less likely than on the L2. As well as the EOPs specifying a bank limit following a hyd failure, there is also an aural warning calculated depending on mass and load factor - if the audio sounds you are approaching the jack stall limit and its time to back off (again, only following failure of 1 hyd system).

HC

Last edited by HeliComparator; 26th Oct 2007 at 13:52. Reason: Technical correction
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Old 26th Oct 2007, 11:46
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Usefull thread hijack, I think...

There's very good reason why one should discuss an accident without the illusion of perfect knowledge- survival. There's lots to be learned in the discussion of what is known of any accident circumstances. It's my belief that almost all accidents start with a a reasonable pilot.
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Old 26th Oct 2007, 12:46
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I hate to play devil's advocate here (heh-heh-heh, I kill me) but I completely agree with him.

On the other hand, Killabeez asks: "How would your grieving family and relations feel if we was discussing an accident you had been involved in without the facts?"

They'd feel fine because we was doing exactly what *they* was doing. See, I'm pretty sure *they* would want to know what happened too - the known facts and circumstances, the possible causes along with those things that might be immediately ruled-out (e.g. "Could Bob have been drunk?" "NO FRIGGIN' WAY!!! Uhh, well maybe...yeah, possibly...okay, probably."). *They* wouldn't want to just sit around, shrug their shoulders and patiently say, "Ahh, no point in asking questions - even non-expert ones, let's just wait a couple of years for the formal report to come out."

No. That's not human nature. Human nature makes us curious and inquisitive. If a pilot like myself were to be involved in an accident, at least some people on this planet would go, "How did THAT happen?! ...And WHAT happened?" That's called being normal. I know some of you won't understand that.
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Old 26th Oct 2007, 13:05
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K48 might have helped us uncover another myth! The distinction that someone told him about a mysterious hydraulic lock condition that runs around waiting to infect your servos, and that somehow explains the cases where "straight and level" aircraft lose control (because someone thinks you can't get jack stall in an aircraft that is not at weird attitudes).

For the record, you can make an aircraft that is prone to jack stall get the condition in every flight attitude known to man. The stall of the rotor is the key. Rotor stall causes the servos to be loaded up by the changing pitching moment from the blades. In a helo that gets jack stall, the servos are too weak to fight this condition, and the forces overwhelm the servo, which then can lock the controls or even cause them to whip around the cockpit. Jack stall is a euphemism for "out of control because the servos are too small for the rotor". Jack stall is preventable by designing proper controls for the helicopter (servos big enough to always tell the blades to keep their forces out of the cockpit.)

The new item that K48 throws out (he calls it hydraulic lock) has nothing to do with a maneuvering helo, it is a condition that old worn out servos might get, or servos with worn seals, and it is rare enough indeed. When it does show up, it can be seen during the controls check during runup, when each servo is felt while the other is turned off, and weak servos usually tell themselves by producing jumps as the systems are switched, and odd forces when the bad servo is manipulated.

Last edited by NickLappos; 26th Oct 2007 at 17:08.
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Old 26th Oct 2007, 13:25
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Devil and FH1100,

I'm with both of you on this - silent speculation is worthless; best that we discuss these issues and improve our understanding. Long gone are the days when we used to stay behind after flying and discuss all current issues with our peers (I really did learn about flying from that).

We should not tread on the sensitivities of relatives or friends but, waiting for the accident report - which from experience takes years to emerge, is a vow of silence that is way too long.

Jim
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Old 26th Oct 2007, 15:55
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Originally Posted by charliegolf View Post
From B73:

Don't the jacks always have fluid in both chambers?

Or do you mean pressure to both chambers?

CG
Erm, yes I probably do. But I refer the learn'd gentleman to my previous comment:

Originally Posted by Bravo73 View Post
(I've never come across the term but I'm by no means an expert)
Either way, the actuator is knackered. And it's not really related to jack stall.



With regard to speculation about accidents, it is my view that informed speculation (before the accident report comes out) might well have some merit. And what I mean by informed is speculation or input from people who have experience of either the aircraft, the operation or the environment. What K48 did was particularly ill-informed and as such is really no better than what one might consider as 'tabloid speculation'. ie 2+2=5
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Old 26th Oct 2007, 17:31
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Actually, Jack Stall is a character, played by Ashton Holmes, in the 2005 movie “A History of Violence”. So, can we drop the colloquialism and refer to the phenomenon as “Servo-transparency?” Furthermore, servo-transparency is discussed under the normal operating procedures section of the POH, and described as “smooth” and “presents no danger.” I draw your attention to the tagline of the aforementioned movie, “Everyone has something to hide.”
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