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AS350 crash due to Jack Stall...

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AS350 crash due to Jack Stall...

Old 2nd Dec 2023, 13:20
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Originally Posted by 212man
Please explain. Force equals pressure x area. So if you have two pistons of the same surface area being acted on by the same pressure, you’ll have twice the force. It’s a simple way to keep the set is light and compact compared to having one large servo (plus redundancy). If you lose a hydraulic system there are normally RFM limits on speed and angles of bank - because you have less servo power.
that video explain it better than me:
its because both pressure have two different regulator, kind of the series versus parallel concept
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Old 2nd Dec 2023, 13:37
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Originally Posted by Agile
that video explain it better than me:
https://www.youtube.com/watch?v=lJZ-ml75dF0
its because both pressure have two different regulator, kind of the series versus parallel concept
Not able to watch right now, but I have been mulling it over and I suspect the answer is twice the power and not twice the force. With two actuators with the same force moving the same distance each in the same period, that’s double the work done in unit time. I’ll look later.

Last edited by 212man; 2nd Dec 2023 at 13:57.
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Old 2nd Dec 2023, 16:23
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Originally Posted by Agile
that video explain it better than me:
https://www.youtube.com/watch?v=lJZ-ml75dF0
its because both pressure have two different regulator, kind of the series versus parallel concept
I think Archimedes just rolled over in his grave!
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Old 2nd Dec 2023, 17:12
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Originally Posted by Agile
I like your logic, but I had understood that he got overenthusiastic in the dive, overspeed past VNE, got blade stall, and consequently jack stalled.

just my personal reminder note: its an error to think that dual hydraulic have twice the actuator thrust power, two piston on the same shaft (with pressure equaly regulated for the two pistons), does not double the thrust.
only higher hydraulic pressure can do that and actually the dual hydraulic version has a slightly inferior hydraulic pressure.
single hydraulic: 40 bars
dual hydraulic: 35 bars
That may well be but the jack stall would be symptomatic of exceeding the G-limit not the cause. You would still have control of the aircraft but may be in a position from which you have neither time nor space to recover.

As an aside the RFM of the Astar 350D stated that if you encountered it you had reached the G limit and how to recover from same.
Because of the dual hydraulics on the 355 this was indicated by the illumination of the limit light. This was also in the RFM. No jack stall would be present, Hence we only demonstrated it momentarily to point out that you had reached the G limit, the light was on, no Jack Stall was present but they should recover immediately by reducing bank angle, lowering collective ect. As jack stall had already been demonstrated in 350 training there was no reason to do anything more in the 355 than a steep turn to illuminate the light and recover from that.

I am a little mystified by the video, it is counterintuitive that the heavier, more powerful 355 would require only 35 Bar of Hyd. Pressure vs the 45 Bar required on lighter, less powerful 350D. That is not what I recall being taught on my original factory course on the 355 in the early 80s.
Don’t have the RFM to hand but wonder what the ECL for a single Hyd. failure on a 335 or 350 with dual hydraulics states.

. Never flew a 350 with dual hydraulics and have had the sore arm to prove it. I experienced 3 Hyd failures in the 350D.

Many, many moons since I flew either one. Freely sprinkle, therefore, all the grains of salt you may feel are required

Last edited by albatross; 2nd Dec 2023 at 17:33.
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Old 2nd Dec 2023, 17:57
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Originally Posted by Agile
that video explain it better than me:
https://www.youtube.com/watch?v=lJZ-ml75dF0
its because both pressure have two different regulator, kind of the series versus parallel concept
Nope. Servos are parallel, not in serie.
That's why 355 has limit light, otherwise you will ovestress rotor system before you overstress hydraulics. Even with lower pressure, individual servo might have less power than single system one, but together they exert more force.
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Old 2nd Dec 2023, 18:17
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The servos are in series. In parallel both cylinders would be side by side, while here they are facing each other.
35 / 40 bars says nothing about the available force without mentioning the surface on which that pressure applies. Same about the power if the flow provided by the pump(s) is unknown.
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Old 2nd Dec 2023, 19:35
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Most are trying to describe the same thing with confused terminology and reasoning. The resultant of 2 identical colinear forces is double the individual force.

However, the AS350 dual hydraulic system does not produce twice the force of the single system because they operate at different pressures (and presumably with different piston sizes).

The above video simply states that the two independent systems don’t result in double pressure.

The AS350 dual system is not a traditional series nor parallel hydraulic system. Customarily, a series hydraulic system is where the hydraulic outflow of the first piston is fed as input to the second piston. And typically, a parallel system has a single pump feeding separate pistons. The AS350 is a hybrid parallel system utilizing two pumps (operating at the same pressure) that feed separate pistons that happen to be connected (colinear) to one another via the same shaft.

Grain of salt: The 2 cylinders of the dual system are not identical due to the construction of the actuator which shares the same shaft. The upper cylinder incorporates a liner which serves to reduce that system’s piston area to be comparable with the lower system.


Last edited by JimEli; 2nd Dec 2023 at 21:12. Reason: spelling/clarified series system description
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Old 3rd Dec 2023, 12:19
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Found this:

As an aside: In the video it is mentioned that VNE of 155 KTS is to reduced 3 KTS per 1000 ft.
Therefore at 10,000 a VNE of 125 KTS would apply. A darn important thing.

I mention this because, and I kid you not, I have had to listen to pilots bitch, gripe, whine and complain about this on many helicopters. (especially on the 212 where some develop a nervous twitch at anything below 100 KTS IAS. (Fun to Watch). Strangely they sometime complain at the same time about how rough the aircraft is while they blithely exceed VNE as they seem unaware that the VNE placard is in Hd NOT Hp) providing they looked at it at all.

Just a reminder to some, a small group I am sure, that while you may think you are crawling over the landscape at such a speed that you will reach retirement age before your destination.

Let’s look at flight 1000 ft in a standard atmosphere. Temp 13c Vne would be 152 KTS
Using a lapse rate of 2 C/1000 ft 10000 ft Temp should be +- -5C VNE is 125 KTS

But lets look at our True Airspeed
Using the numbers above and your Handy Dandy all Doing, all Telling Computer. ( Old School )
Or looking at your “No thought required Glass Panel” ( New School )

You TAS at 1000 feet is 153 KTS VS the 152 KTS calculated VNE indicated on the gauge.
Your TAS at 10,000 is 144 KTS VS the snail like 125 KTS Calculated VNE on the gauge .
See you will make it home in time for Beer Call!
Everybody Happy Now?

In any case fly safe and avoid unplanned, inadvertent Airframe / Planet interface……the Planet always wins.



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Old 3rd Dec 2023, 18:32
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Originally Posted by albatross
Found this: https://youtu.be/U7sc2BodDHo?si=tarmuSmLTeGFKQYi

As an aside: In the video it is mentioned that VNE of 155 KTS is to reduced 3 KTS per 1000 ft.
Therefore at 10,000 a VNE of 125 KTS would apply. A darn important thing.

I mention this because, and I kid you not, I have had to listen to pilots bitch, gripe, whine and complain about this on many helicopters. (especially on the 212 where some develop a nervous twitch at anything below 100 KTS IAS. (Fun to Watch). Strangely they sometime complain at the same time about how rough the aircraft is while they blithely exceed VNE as they seem unaware that the VNE placard is in Hd NOT Hp) providing they looked at it at all.

Just a reminder to some, a small group I am sure, that while you may think you are crawling over the landscape at such a speed that you will reach retirement age before your destination.

Let’s look at flight 1000 ft in a standard atmosphere. Temp 13c Vne would be 152 KTS
Using a lapse rate of 2 C/1000 ft 10000 ft Temp should be +- -5C VNE is 125 KTS

But lets look at our True Airspeed
Using the numbers above and your Handy Dandy all Doing, all Telling Computer. ( Old School )
Or looking at your “No thought required Glass Panel” ( New School )

You TAS at 1000 feet is 153 KTS VS the 152 KTS calculated VNE indicated on the gauge.
Your TAS at 10,000 is 144 KTS VS the snail like 125 KTS Calculated VNE on the gauge .
See you will make it home in time for Beer Call!
Everybody Happy Now?

In any case fly safe and avoid unplanned, inadvertent Airframe / Planet interface……the Planet always wins.
Video shows a 355N roll left at about 70 knots? Wut?

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Old 3rd Dec 2023, 19:12
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Well I don’t know about the cause of that accident shown in the video above, however, since the discussion is about Jack Stall, Servo Transparency, ect.
It would be possible to enter that area of the Flight Envelope in a left as well as a right turn. Especially if you were at a high gross weight and maneuvering enthusiastically.
It is not necessary to be near, at or over VNE to encounter it.
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Old 3rd Dec 2023, 20:30
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Originally Posted by RVDT
Video shows a 355N roll left at about 70 knots? Wut?
Servo hardover? See NTSB report NYC97LA058 for a similar occurrence in a 355. Although, Airbus Helicopters makes much ado about the jam-proof double rotary distributor valves.

Last edited by JimEli; 3rd Dec 2023 at 21:34.
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Old 3rd Dec 2023, 20:35
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FWIW, older versions of the AS350 RFMs define the maneuver limitation as:

Do not exceed the load factor corresponding to the servocontrol reversibility limit.

Which is an odd way to put it, since pilots wouldn’t have a genuine way of knowing where the limit is until after exceeding it (Maybe it was a Frenglish artifact).

The latest versions state:

Continued operation in servo transparency (where load feedback is felt in the controls) is prohibited.

However, this version possibly reduces the respect servocontrol reversibility is due and gives the impression it's okay to tickle it now and then.
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Old 4th Dec 2023, 02:07
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Originally Posted by albatross
You TAS at 1000 feet is 153 KTS VS the 152 KTS calculated VNE indicated on the gauge.
Your TAS at 10,000 is 144 KTS VS the snail like 125 KTS Calculated VNE on the gauge .
The interesting thing about VNE in "level flight":
1/ at sea level there is not enough power to get to 152kts; at 10,000 ft there is not enough power to get to 125kts.
2/ at 10,000ft the vibration level that you get, the colective position, they all come in a way that 125kts feels like 152kts.
unless you are thick and only look at the indicated speed, its not such a pitfall.
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Old 4th Dec 2023, 16:53
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Originally Posted by Agile
The interesting thing about VNE in "level flight":
1/ at sea level there is not enough power to get to 152kts; at 10,000 ft there is not enough power to get to 125kts.
2/ at 10,000ft the vibration level that you get, the colective position, they all come in a way that 125kts feels like 152kts.
unless you are thick and only look at the indicated speed, its not such a pitfall.
Trust me there are some thick people around sometimes through no fault of their own but the hapless victims of substandard training.
Also some folks have been happily flying an airframe at sea level in a cool environment for years and thousands of hours with an altimeter that has never indicated over 2000 feet being thrust into a high altitude, high temp environment and expected to do wonderful things.

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Old 5th Dec 2023, 07:07
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When demonstrating jackstall on the Gazelle, it was shown in two different conditions - a left turn and a dive to VNE.

A Gazelle could normally tolerate a 2g sustained turn - 60 deg AoB - albeit sacrificing speed through the manoeuvre. So a harder pull was required to get it to jackstall, at which point it would pitch up and roll right - exactly why we did it in a left turn and not a right one.

The dive to VNE used the conditions of the day to calculate the DA and therefore the VNE at 1500' agl. From 2500' agl you dived to VNE with MPS set (14.5 deg collective pitch), usually somewhere between 160 and 168 Kts and then pulled aft cyclic - it usually went into jackstall fairly easily in that condition compared to the turning state.

In both scenarios, you could hold the aircraft in the jackstalled state to show how the aerodynamic back loads on the retreating blade (and therefore lateral jack) had to be reduced before the jack would un-stall.

The AS365 had a LIMIT light and audio warning which we also demonstrated in a left turn but never pulled past that - the two side of the hyd jacks had different cross sectioned pistons so the upper half would 'collapse' before the lower half, making a microswitch on the jack and illuminating the LIMIT light and giving the audio 'bong'.

It seems to have been a design philosophy in Aerospatiale/Airbus helicopters.
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Old 5th Dec 2023, 07:09
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I have only flown one aircraft that had a g meter and that was the Lynx for aerobatics - since the manufacturers don't quote g limits you can't be criticised for exceeding something you don't know and couldn't measure anyway.
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Old 5th Dec 2023, 07:12
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We used to refer to the max speed in level flight that could be obtained at max power as VMax since it was usually lower than VNE but that may be because military release to service limitations were usually lower than the manufacturers limits.
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Old 5th Dec 2023, 08:58
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Originally Posted by [email protected]
We used to refer to the max speed in level flight that could be obtained at max power as VMax since it was usually lower than VNE but that may be because military release to service limitations were usually lower than the manufacturers limits.
The civilian term is Vh - maximum speed in level flight using maximum continuous power.
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Old 5th Dec 2023, 14:12
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Originally Posted by 212man
The civilian term is Vh - maximum speed in level flight using maximum continuous power.
I knew there was a grown-up V something for it, just couldn't recall it and too lazy to google
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Old 6th Dec 2023, 10:22
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Originally Posted by [email protected]
When demonstrating jackstall on the Gazelle, it was shown in two different conditions - a left turn and a dive to VNE.

A Gazelle could normally tolerate a 2g sustained turn - 60 deg AoB - albeit sacrificing speed through the manoeuvre. So a harder pull was required to get it to jackstall, at which point it would pitch up and roll right - exactly why we did it in a left turn and not a right one.

The dive to VNE used the conditions of the day to calculate the DA and therefore the VNE at 1500' agl. From 2500' agl you dived to VNE with MPS set (14.5 deg collective pitch), usually somewhere between 160 and 168 Kts and then pulled aft cyclic - it usually went into jackstall fairly easily in that condition compared to the turning state.

In both scenarios, you could hold the aircraft in the jackstalled state to show how the aerodynamic back loads on the retreating blade (and therefore lateral jack) had to be reduced before the jack would un-stall.

The AS365 had a LIMIT light and audio warning which we also demonstrated in a left turn but never pulled past that - the two side of the hyd jacks had different cross sectioned pistons so the upper half would 'collapse' before the lower half, making a microswitch on the jack and illuminating the LIMIT light and giving the audio 'bong'.

It seems to have been a design philosophy in Aerospatiale/Airbus helicopters.
My instructor on my QHI course in 1976 decided to do the descending turn demo in a 60 degree RIHT HAND TURN.. The aircraft flicked though inverted and came out almost straight and level. I didn't fly with him again.
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