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Astar Hydraulic Problems

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I have heard from a number of people that the AS350 family have a generic problem with their hydraulics. Even though Eurocopter have said there is no problem, they have recently redesigned the hydraulic belts on the aircraft from what I understand.


Can anyone shed their thoughts or comments on this topic.


Heli Ops

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I believe it to be a case of supply and demand.  Under certain conditions the pilot can move his controls at a rate where the pump can’t keep up with the servo movement and the pressure will drop to the point that the servo accumulators will be depleted and then there is no hydraulic boost.  Under this condition the servo becomes a link in the push pull system and the feedback forces are reflected in the cyclic and the collective.  The condition is known as jack stall.


Changing the belts may help in a way but the thing that will solve the problem is to change the pump to a Constant pressure, variable delivery pump.





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In spite of what the manufacturor says, look at the accidents after hydraulic failures-lots! In training situations, the 350 is harder to fly unboosted than 206's, but not uncontrollable. Worlds of difference in situations-training is all light gross and pilot expects failures...

My guesss is belt slippage, marginal capacity, poor pilot technique (gotta know the different functions of Hyd test and hydraulic cutoff switches) and preflight actions contribute. You can fly a 350 away with the hydraulic warnings all disabled/indiscernable or just plain inop-if you don't check them. Heavy and fast (high control loads) I suspect these things are a handful.

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  • 1 month later...



Indeed a 350 is harder to fly unboosted than a 206, but this is IMHO only really a factor if the pilot decides to ignore certain parts of the flight manual.  Specifically, the parts where it says to make a running landing after a hydraulic failure.  Not that hard in a 206 on level ground, quite difficult in a 350, not recommended in either case.


I know of 3 accidents in Astars after hydraulic failures, and in every case the pilot flew the aircraft to an open place where a running landing could be made, then proceeded to try and hover and lost control.  I do know of other cases where a running landing was made, all without mishap.


And yes, in every case the culprit was that @#$% little belt breaking...

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I've had 4 hyd failures in Astars. Every landing was made to a hover without incident. My training was to hover the Astar around and fly full patterns without hyd. I think the outcome of a hyd failure is dependent on pilot training and common sense (which a friend of mine says isn't too common).


I have trained for both running landings and hover after loss of hydraulics in the AS350 series. I prefer hovering it to a landing but I think it depends on how you were trained. The flight manual says do a running landing. Your terrain and how tired your arms will get are other factors to consider. Before on this forum, a wise individual pointed out that you can do a combination of both: do a slow running landing. That seems like the best method.


Another question relating to this: anyone notice a connection between the Dunlop or Samms servos and the number of failures? Most of my failures were due to the hyd. pump. The belt fails first but that is just a symptom of the hyd pump starting to seize.



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Just something to think about. Skiding on is different because of the right skid low syndrome on the Astar.  Also the SAMM accumulators often get over pressurized (218 psi) from the more is better philosophy (I can't believe I tried to spell that).

If you you reingage your hyd. system after the test and you only get a very short beep from the horn it means there's too much presure in the accumulators. Not good when you deplete the system. My problem with the pump from a mtnc. point of view is that the mount is very French and it takes special care to align the belt and tension it to the right strech.

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  • 3 weeks later...

I can't recall that we have had any problems with the hydraulics on our AS350's. We operate 7 of them and fly approx. 3500 hrs anually.

We have Dunlop servos on all of them except the B3 which has SAMM.

Checking belt tension every 100 hrs and replace the belt every 500.

Received a reccomended service bulletin from Eurocopter regarding a change to the V-belt but we are going to use the "green flat one" still.

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  • 3 months later...

Now to revive a long dormant topic. AS-350 Hydraulic problems.


From a design/mechanic point of view:


The basic problem with the AS-350 is that the belts are a P.I.T.A to change, so too many mechanics are tempted to just let it go a little bit longer. Once they show signs of damage you had better be planning to get it replaced.

You have to remove/disconnect way too much stuff to change out a belt. While the AS-350 is generally a very well thought out aircraft, this part is just badly designed. Most outfits will stash a spare belt tiewrapped up out of the way but by the time its needed it is probably in pretty crummy shape from the heat and exposure. The bearing that the belt pulley rides on (pump side) has also had some problems in the past (including with the splines) but the pump itself is usually not the problem.


That little bitty green belt is not too confidence inspiring but it is perfectly capable of handling the load provided its replaced regularly. BTW, the new belt design may LOOK better but its going to be just as difficult to change out.


Its also important for mechanics to understand the importance of maintaining proper pressure in the accumulators, and for pilots to know the symptoms of a low accumulator. Pilots should review the flight manual and the accompanying factory training manual. They clearly spell out what you are looking for when testing the hydraulics.


As to the pilot side of things:


NEVER PUSH BOTH THE HYDRAULIC SWITCH (on the collective) AND THE HYDRAULIC BUTTON (on the console)AT THE SAME TIME! Find me somewhere in the manual where it says to mess with both during an actual emergency. Far too many so-called "trainers" don't understand the system as well as they should. The emergency procedure says NOTHING about pushing any buttons. USE the COLLECTIVE SWITCH in a real emergency, and ONLY the collective switch. The console button will do nothing but depressurize the tail rotor accumulator, thereby adding to your problems (B2 and B2 versions). Why give yourself a stuck pedal when you already have problems with the hydraulics? Know your emergency procedures, they are not that hard on an AStar.


Keep in mind that both the outside air temp and the way the aircraft is loaded will affect how it flies hydraulics off. The elastomeric blade bushings/feather bearings get a LOT stiffer when its cold and thats reflected in the effort involved at the cyclic. While it CAN be flown to a hover, its a LOT more work than a slow run-on, so why take a chance if you don't have to? Slow it down as soon as the horn sounds, because its a lot harder to fly it at cruise than at the recommended airspeed.


How about this one, its good advice not just for AStar pilots but for anyone that wants to make this a long-term profession....




Especially in hard right turns for the AS-350 series. Once you see how much the control loads increase at speed without hydraulics and how fast the load builds once the accumulators are depleted, think of what would happen if that occurred when you are racked over in a hard bank at 50' agl when that little bitty green belt calls it quits.


Besides, you are always taking a chance on catching a wire or a cell phone antenna or something else down there. It may be fun but is it really worth it?


If you spend a lot of time down trolling for powerlines, you will almost certainly catch one someday whether you want to or not.


BTW, if you are manuvering hard enough to encounter "control transparency" or the 'jack stall' the previous poster referred to...you are already beyond the normal manuevering limits.


BACK OFF, and fly like a mature adult instead of like a 16 year old with daddys corvette.  


Do you really like cleaning your passengers vomit off the interior? Try it once and you may find youself a more conservative pilot. Make a habit of it and you may find yourself a former pilot.


The AS-350 hydraulic system when its maintained as per the maint. manuals is about as trouble free as they get. It does operate at a fairly low pressure. It does have a dubious looking little green rubberband impersonating  a hydraulic belt. It does have more than one control in the cockpit that says 'hydraulic' on it.


As simple as it is, it really shouldn't be the issue that it is.

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Good post, Rotor.head. It could stand saying again-


Besides the on the ground check, that button's there for only one reason-loss of tail rotor control (jam).


Final bit- takes power to engage these switches. Can't think of why one would want to do so, but dumping all the electrical energy inactivates all the switches-sorta like pulling the breaker on a 206.

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