AS350 Astar / AS355 Twinstar [Archive Copy]
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Two tales, but both concerning SA/AS/EC type helicopters and servo/hydraulics:
1. SA-341 Gazelle (MIL version) suffered a fatal accident at Farnborough around 1973. The helicopter performed a low level, right-hand turn at high speed. The roll to the right continued unchecked and the helicopter crashed inverted. The conclusion was that by increasing the angle of attack on the advancing blade, which provides most of the lift, the forces on the blade at high forward speed were sufficient to overcome the hydraulic servo and jack stall occurred, and the pilot was unable to counter the roll.
This occurs mainly when turning to the right - French helicopter = advancing blade on the left side of the helicopter. When turning left the angle of attack on the advancing blade decreases, and the forces on the servo decrease accordingly.
Even though this was a known phenomenon, and demonstrated under training, it was the presumed cause of another Gazelle fatal accident in 1975, during a low level sortie over Dartmoor, killing the instructor and student.
2. AS-350B2 – When this type was introduced it had new spherical bearings, new single hydraulic system (both taken from the 355 dual hydraulic system), a new power plant, beefed up rotorhead, & etc. Better, more powerful version of the B & B1……?
The hydraulic pump is driven by a belt (“rubber band” - see previous postings on this). In cold weather (under - 25oC) the hydraulic fluid increases in viscosity and the belt starts to slip. As it cannot drive the pump at normal speeds, the belt deforms, due to the friction from the accessory drive, and eventually stops turning the hydraulic pump, or slips off the drive wheel. OK you think, small helicopter, no hydraulics, what’s the problem ?
Well it turns out that the new spherical bearings (“rubber balls” from the 355) freeze solid at temperatures under - 25oC, and without a hydraulic servo the whole control system locks up !
The pilot then becomes a passenger in the helicopter, and only by using extreme force can the controls be moved.
AS/Eurocopter say they have fixed this problem with new types of spherical bearings, but the fact remains that if you lose the hydraulic drive belt on a 350, it becomes extremely difficult to control – see the news gathering AS-350 that plunged to a rooftop in New York earlier this year
1. SA-341 Gazelle (MIL version) suffered a fatal accident at Farnborough around 1973. The helicopter performed a low level, right-hand turn at high speed. The roll to the right continued unchecked and the helicopter crashed inverted. The conclusion was that by increasing the angle of attack on the advancing blade, which provides most of the lift, the forces on the blade at high forward speed were sufficient to overcome the hydraulic servo and jack stall occurred, and the pilot was unable to counter the roll.
This occurs mainly when turning to the right - French helicopter = advancing blade on the left side of the helicopter. When turning left the angle of attack on the advancing blade decreases, and the forces on the servo decrease accordingly.
Even though this was a known phenomenon, and demonstrated under training, it was the presumed cause of another Gazelle fatal accident in 1975, during a low level sortie over Dartmoor, killing the instructor and student.
2. AS-350B2 – When this type was introduced it had new spherical bearings, new single hydraulic system (both taken from the 355 dual hydraulic system), a new power plant, beefed up rotorhead, & etc. Better, more powerful version of the B & B1……?
The hydraulic pump is driven by a belt (“rubber band” - see previous postings on this). In cold weather (under - 25oC) the hydraulic fluid increases in viscosity and the belt starts to slip. As it cannot drive the pump at normal speeds, the belt deforms, due to the friction from the accessory drive, and eventually stops turning the hydraulic pump, or slips off the drive wheel. OK you think, small helicopter, no hydraulics, what’s the problem ?
Well it turns out that the new spherical bearings (“rubber balls” from the 355) freeze solid at temperatures under - 25oC, and without a hydraulic servo the whole control system locks up !
The pilot then becomes a passenger in the helicopter, and only by using extreme force can the controls be moved.
AS/Eurocopter say they have fixed this problem with new types of spherical bearings, but the fact remains that if you lose the hydraulic drive belt on a 350, it becomes extremely difficult to control – see the news gathering AS-350 that plunged to a rooftop in New York earlier this year
Reading Collective Bias's story about the S55T reminds me of an incident I had some years ago when a leak in the cockpit roof caused a drip resulting in a bridging of the hydraulics Primary off, Secondary off test switch.
Both sets of hydraulics failed on me leaving only the emergency system - driven I think by transmission oil pressure on the lateral jacks. All could have been restored by switching off the electrical master switch (fail safe on) but I lacked the courage and free hand to reach for the switch.
From that incident on I made a special effort to learn the aircraft systems and not just skim over the text to pass the type rating exams.
Yes I did manage to get the aircraft down and land it after 20 minutes flying in that condition - pure luck.
Jim
Both sets of hydraulics failed on me leaving only the emergency system - driven I think by transmission oil pressure on the lateral jacks. All could have been restored by switching off the electrical master switch (fail safe on) but I lacked the courage and free hand to reach for the switch.
From that incident on I made a special effort to learn the aircraft systems and not just skim over the text to pass the type rating exams.
Yes I did manage to get the aircraft down and land it after 20 minutes flying in that condition - pure luck.
Jim
Reading about the accident involving a hard right turn near the ground....and being unable to stop the roll and for sure probably not wanting to reduce collective at that point....a question arises.
The BO-105 and BK-117 have a similar trait in that situation....one can run out of cyclic authority if a high roll rate occurs.....the recovery is to apply "full" opposite pedal.....fully and quickly...."stomp" was the word used but something slightly short of that seemed to work during demos.
Adequate control is quickly gained and as the nose pitches up to level or more....then the collective can be lowered and cyclic authority is regained.
Just a thought!
The BO-105 and BK-117 have a similar trait in that situation....one can run out of cyclic authority if a high roll rate occurs.....the recovery is to apply "full" opposite pedal.....fully and quickly...."stomp" was the word used but something slightly short of that seemed to work during demos.
Adequate control is quickly gained and as the nose pitches up to level or more....then the collective can be lowered and cyclic authority is regained.
Just a thought!
This is a very intresting subject and I have to send my 2 cents again.
BB I think the problem you refer to was from an accident on Greenland in the mid or late 80 ies. This was due to (if I remember right) a broken drive belt to the hydraulic pump in very cold conditions and with a new style of Spherical Bearing (load carrying rubber bearing) fitted. Not to the fact of slipping drive belt.
The fix was a new style of bearing, and a new lower temp limit of the previous one.
The RFM also calls out for warming up the spherical bearings before takeoff in cold conditions by moving the cyclic 3-4 cm fwd for 2 min and to check the force to move the cyclic without servo.
We have been flying B, B1, B2, B3 in very cold conditions (down to lower limit) for a long time without problems, we started with B in 1979. We even tested a cog belt driven pump for EC, but it never went into production. Even if the belt drive looks very simple, it has just failed on us once in all these years, and it was a successful landing afterwards.
I have always as a training captain had the opinion on single hyd system helicopters that if there is a procedure in the emergency checklist that calls for turning off the hyd system ASAP, then it should also be trained at cruise speed. Normal procedure is to bring down the speed to around 60 KIAS and then turn it off (even at ECF they do not train this at high speed if they have not changed there way of training). But if you get some sort of hardover you need to get it off immidiately, no matter the airspeed, without having tried it before it could be intresting
Therefore when I train someone on the 350 I demonstrate this and then let the pilot try it with increasing airspeed up to cruise. At cruise it is hard work, but since the cyclic want to go right and aft, it is very much controllable. But since many 350 pilots from time to time hold the cyclic "very light" to get good feel, I think it is important to know what they are up against if you turn off the servo at cruise speed.
CB
BB I think the problem you refer to was from an accident on Greenland in the mid or late 80 ies. This was due to (if I remember right) a broken drive belt to the hydraulic pump in very cold conditions and with a new style of Spherical Bearing (load carrying rubber bearing) fitted. Not to the fact of slipping drive belt.
The fix was a new style of bearing, and a new lower temp limit of the previous one.
The RFM also calls out for warming up the spherical bearings before takeoff in cold conditions by moving the cyclic 3-4 cm fwd for 2 min and to check the force to move the cyclic without servo.
We have been flying B, B1, B2, B3 in very cold conditions (down to lower limit) for a long time without problems, we started with B in 1979. We even tested a cog belt driven pump for EC, but it never went into production. Even if the belt drive looks very simple, it has just failed on us once in all these years, and it was a successful landing afterwards.
I have always as a training captain had the opinion on single hyd system helicopters that if there is a procedure in the emergency checklist that calls for turning off the hyd system ASAP, then it should also be trained at cruise speed. Normal procedure is to bring down the speed to around 60 KIAS and then turn it off (even at ECF they do not train this at high speed if they have not changed there way of training). But if you get some sort of hardover you need to get it off immidiately, no matter the airspeed, without having tried it before it could be intresting
Therefore when I train someone on the 350 I demonstrate this and then let the pilot try it with increasing airspeed up to cruise. At cruise it is hard work, but since the cyclic want to go right and aft, it is very much controllable. But since many 350 pilots from time to time hold the cyclic "very light" to get good feel, I think it is important to know what they are up against if you turn off the servo at cruise speed.
CB
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I taught on AS 350Bs for a few years. One of the course demonstrations was"jack stall". The point of this demo was to show the trainee that the phenomenon was possible and could be dangerous close to the ground.
It was a while ago but there was very rapid rolling and pulling to achieve it. Once it occured, it felt much like the aircraft did hydraulics off. Although the controls were heavier they could still be moved and the aircraft recovered. You really had to work hard to make it happen.
It's a limitation to operations, like left crosswinds and LTE are to Bell 206s, left pedal stops and crosswinds to Bell 205s, vortex ring is to any helicopter and the size of the fuel tank is to any aircraft. No big deal.
As for the "rubber band", the fleet of about 20 to my knowledge never had a failure, even in a temperature range from -5 deg C to 40 deg C. The biggest problem with them was getting them on and off. Undoing drive train etc. They were lifed for 10 years so when the fleet hit 10 years old, not enough rubber bands to go round.
It was a while ago but there was very rapid rolling and pulling to achieve it. Once it occured, it felt much like the aircraft did hydraulics off. Although the controls were heavier they could still be moved and the aircraft recovered. You really had to work hard to make it happen.
It's a limitation to operations, like left crosswinds and LTE are to Bell 206s, left pedal stops and crosswinds to Bell 205s, vortex ring is to any helicopter and the size of the fuel tank is to any aircraft. No big deal.
As for the "rubber band", the fleet of about 20 to my knowledge never had a failure, even in a temperature range from -5 deg C to 40 deg C. The biggest problem with them was getting them on and off. Undoing drive train etc. They were lifed for 10 years so when the fleet hit 10 years old, not enough rubber bands to go round.
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I've seen a few cases of sudden belt slippage when arriving in the hover in loose dry snow (read - - big nasty white snowball ! )
This not the time you need control issues, horns, etc as you are searching for the ground in a white-out ! I have also seen a few broken belts, plus a lot of burned - glazed ones.
I work with a 100% Bell company now( ), but my past employer used to have a spare belt fastened around the drive-shaft of each 350, so the complete dis-assembly of the drive system was not required to change the belt -- once anyway.
This not the time you need control issues, horns, etc as you are searching for the ground in a white-out ! I have also seen a few broken belts, plus a lot of burned - glazed ones.
I work with a 100% Bell company now(
), but my past employer used to have a spare belt fastened around the drive-shaft of each 350, so the complete dis-assembly of the drive system was not required to change the belt -- once anyway.
Last edited by 407 Driver; 3rd Dec 2004 at 16:44.
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I think a lot of the hydraulic problems mentioned above during high speed turns are not really hydraulic problems but retreating blade stall which causes a euro machine to flick right and a US machine to the left. For this reason when teaching steep turns, you get the student to turn the opposite way to the flick, so that if it goes wrong, you flick upright.
I well remember being number 5 in a tail chase in a S55 when the leader sped up close to vne and wrapped on lots of right bank. A few seconds later, trying to follow number 4, I was doing a steep turn to the left. Very worrying but not really a hydraulic problem.
Maybe the Gazelle had this happen?
I well remember being number 5 in a tail chase in a S55 when the leader sped up close to vne and wrapped on lots of right bank. A few seconds later, trying to follow number 4, I was doing a steep turn to the left. Very worrying but not really a hydraulic problem.
Maybe the Gazelle had this happen?
Avoid imitations
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The Gazelle's controls give a definite feedback "jolt" when jackstall occurs. I saw retreating blade stall in a Whirlwind 10 (at 150 feet agl) and if I recall correctly, it didn't give the same effect.
Both have a similar effect on the pilot though......
Both have a similar effect on the pilot though......
407 Driver's Comment
This is a very good example of how prone the ASTAR is to jack stall. It doesn't take very much. At high DA's in combination with a High gross weight, light to meduim turbulance can result in jack stall. Anyone who has flow an ASTAR in the mountains has probably experienced this.
"Just a pilot"
Mr Osborne, my experience is contrary tp your speculation regarding blade stall. I've had it happen in turns of both directions, and vigorous but not terribly rapid roll rate or exceptionally high speed- well within the green arc and low DA. The surest way I've induced it is cruise or higher, pull some g, and roll. The stick just stops, lateral. I'd hate to force it, as the onset is sudden and unpredicted- it could well cease with me leaning on it- That would introduce new and exciting issues. Pitch reduction (both senses of the word), slow, and the aircraft's normal again.
As to belts- An Arizona 350 had an air conditioner compressor drive belt break and take out the hydraulic drive belt, too. It's a chintzy system in an otherwise very satisfactory helo.
As to belts- An Arizona 350 had an air conditioner compressor drive belt break and take out the hydraulic drive belt, too. It's a chintzy system in an otherwise very satisfactory helo.
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devil 49,
The reason for the jack stall is that the blades are stalling, and when they do, they progressively add more and more force to the swashplate. This is due to the pitching moment of the airfoil shifting with the stall. When the swashplate gets heavily loaded, it pushes the servos backwards despite the hydraulic pressure in them. this is called "transparency" or "stall" of the servo.
here is a web site that talks about the dynamic stall of rotor blades. Scroll down to where they talk about the pitching moment (the Cm plot) to see how the moment of the blade takes a big turn at stall. This is the reason why the nose drops on an airplane when the wing stalls - the wings pitching moment shifts sharply downward.
http://aerodyn.org/Dstall/dstall.html
The reason for the jack stall is that the blades are stalling, and when they do, they progressively add more and more force to the swashplate. This is due to the pitching moment of the airfoil shifting with the stall. When the swashplate gets heavily loaded, it pushes the servos backwards despite the hydraulic pressure in them. this is called "transparency" or "stall" of the servo.
here is a web site that talks about the dynamic stall of rotor blades. Scroll down to where they talk about the pitching moment (the Cm plot) to see how the moment of the blade takes a big turn at stall. This is the reason why the nose drops on an airplane when the wing stalls - the wings pitching moment shifts sharply downward.
http://aerodyn.org/Dstall/dstall.html
I am glad there was a very simple answer to my original question. I would hate to think I raised a complicated issue like why some places in the world still use the QFE altimeter setting vice the World-wide accepted QNH method.
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Dear all,
A search for "jack stall" shows the amount of previous discussion. I, like 'scrubba',
http://www.pprune.org/forums/showthr...ght=jack+stall
did all sorts of things in the AS350 that deliberately generated jack stall. I found it to be predictable, progressive and at the far edges of the envelope compared to normal operations.
While I note Nick's view in 2001 and now, I never felt frightened by the phenomena because I knew it existed and I knew the extremes to which I had to go to induce it. The other "mid-envelope" experiences reported just did not gel with my experience.
Stay Alive,
A search for "jack stall" shows the amount of previous discussion. I, like 'scrubba',
http://www.pprune.org/forums/showthr...ght=jack+stall
did all sorts of things in the AS350 that deliberately generated jack stall. I found it to be predictable, progressive and at the far edges of the envelope compared to normal operations.
While I note Nick's view in 2001 and now, I never felt frightened by the phenomena because I knew it existed and I knew the extremes to which I had to go to induce it. The other "mid-envelope" experiences reported just did not gel with my experience.
Stay Alive,
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4dogs,
Thanks for the link to the past! It is interesting to see that old topics don't just go away!
I have to note one issue with your comment,"did all sorts of things in the AS350 that deliberately generated jack stall. I found it to be predictable, progressive and at the far edges of the envelope compared to normal operations"
Could I ask you to add the note, "so far" to the end of your comment? One thing I have found in testing is that when a problem surfaces, it is not likely to just fix itself. When other pilots have loss of control events, and lose their aircraft in an accident, your evaluation has just been trumped by theirs. We all lose if we decide that something which hurt a buddy is really his fault, so that we apologize for the aircraft, blame the pilot and march on to the next accident.
I coined an aphorism to describe the loop we get into when we are trying to decide if a problem we have found really needs to be fixed. I would write this on the chalk board before we started the meetings to decide if it was a "feature" or a problem:
The size of your problem is proportional to the efforts needed to convince yourself that it is not a problem.
IMHO, Jack Stall is like LTE, a sign of a marginal helicopter that has bitten some of us, but others want to simply blame the pilot. Safety of helicopter flight operations will only be achieved when I, and my counterparts at the other manufacturers, face a raft of indignant pilots constantly trying to make their aircraft better, not a group of apologists who want to blame each other when an aircraft fails them.
Regardless of our collective opinion about how much out of control we will accept, the authorities should not let us debate it. In good military aircraft, there is no debate, and I can promise you, it would be a cold day in Hell before the FAA pilots I fly with would accept jack stall at 1.5 g.
Thanks for the link to the past! It is interesting to see that old topics don't just go away!
I have to note one issue with your comment,"did all sorts of things in the AS350 that deliberately generated jack stall. I found it to be predictable, progressive and at the far edges of the envelope compared to normal operations"
Could I ask you to add the note, "so far" to the end of your comment? One thing I have found in testing is that when a problem surfaces, it is not likely to just fix itself. When other pilots have loss of control events, and lose their aircraft in an accident, your evaluation has just been trumped by theirs. We all lose if we decide that something which hurt a buddy is really his fault, so that we apologize for the aircraft, blame the pilot and march on to the next accident.
I coined an aphorism to describe the loop we get into when we are trying to decide if a problem we have found really needs to be fixed. I would write this on the chalk board before we started the meetings to decide if it was a "feature" or a problem:
The size of your problem is proportional to the efforts needed to convince yourself that it is not a problem.
IMHO, Jack Stall is like LTE, a sign of a marginal helicopter that has bitten some of us, but others want to simply blame the pilot. Safety of helicopter flight operations will only be achieved when I, and my counterparts at the other manufacturers, face a raft of indignant pilots constantly trying to make their aircraft better, not a group of apologists who want to blame each other when an aircraft fails them.
Regardless of our collective opinion about how much out of control we will accept, the authorities should not let us debate it. In good military aircraft, there is no debate, and I can promise you, it would be a cold day in Hell before the FAA pilots I fly with would accept jack stall at 1.5 g.
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4Dogs,
My personal experiences have shown a complete disagreement to your post. I have unknowingly entered the "servo transparency" in a flight condition I feel was very routine and quite gentle.
I say to you, be careful of this situation, don't approach it with such casual disregard.
Given a higher gross weight ( Did you practice this with a full load ? ) a different flight altitude, a different 350 model, a different Outside air temperature, and most importantly, a flight situation where you MUST turn to aviod terrain, you may find that the aircraft may surprise you in how quick it bites.
You end your last post with "Stay Alive", good advice, I say ...practice it !
Fly the type smoothly....
Always leave yourself an "out".....
Anticipate that the control transparency may occur at the worst possible time.....
My personal experiences have shown a complete disagreement to your post. I have unknowingly entered the "servo transparency" in a flight condition I feel was very routine and quite gentle.
I say to you, be careful of this situation, don't approach it with such casual disregard.
Given a higher gross weight ( Did you practice this with a full load ? ) a different flight altitude, a different 350 model, a different Outside air temperature, and most importantly, a flight situation where you MUST turn to aviod terrain, you may find that the aircraft may surprise you in how quick it bites.
You end your last post with "Stay Alive", good advice, I say ...practice it !
Fly the type smoothly....
Always leave yourself an "out".....
Anticipate that the control transparency may occur at the worst possible time.....
Again Nick hits the nail squarely upon the head!
i believe that would be because the manufacturer originally called it the "ecureil" which i believe is french for squirrel...
AStar was a north american name attached to it...
AStar was a north american name attached to it...
Aerospecial called it Ecureuil - Squirrel - cause it was supposed to be a money saver for the operators just like the animal is provident and thrifty.
That was changed to A Star for North America since the name does not mean the same there.
Cheers
ATN
That was changed to A Star for North America since the name does not mean the same there.
Cheers
ATN