(A)B 412 with collective stuck down....

[Leeuwarden AFB-Ed]

Hope to receive some response on our latest problem with one AB412SP.
After 25hrs inspection (only inspection) we encountered a collective problem during 100% HYDR check (first flight of the day): with only one hydr system on (no matter #1 or #2 !) not able to move collective fully down, stuck at about 30% Torque. When we switched on both systems: collective went down easily. Checked everything from sleeve assy to hub, even replaced the HYDR actuator. Last test flight: max rate-of-descent with one HYDR system off: 1500ft/pm!
This can not be good....
Maybe somebody else encountered this? Hope to hear some reactions
Many thanks from The Netherlands/ 303 SAR Sqn!

[maeroda]

Hello.

What about the ciclic stick when you switch off one hydraulic sistem AND cannot pull down collective for MPOG?

Maeroda

[Matthew Parsons]

I wouldn't fly a 412 that fails either hydraulics check.

Lots of work, but check the rigging completely (from collective to the head).

[Leeuwarden AFB-Ed]

Many thanks for the reply!
It only happens with collective down. Not able to come down fully, stays up with about 30% torque. Cyclic feels good on both systems.
We tried different OAT for elastomeric bearings, no difference. It occurs a few weeks ago. Aircraft still grounded.
Hope that somebody heard about a simular problem.
Greetz from NL

[Maverick Laddie]

Which machine is it R01/02/03, Regards to Bart PM me Colin.

[Leeuwarden AFB-Ed]

Thanks, will send regards to Bart (small world).
Checking rigging again including measurements. When we check it static (hydr test unit in hangar then evrything works great.....)
And we did not found any foreign objects so far.................
Greetings, Ed.

[oldbeefer]

You talk about the first flight of the day, but what happened during the post start hydraulic functional checks - both at idle and at 100% - did the collective stay down or jump up then?

[TRC]

Ed.

You say this occurred after a 25hr inspection - presumably it didn't happen before the insp. What was done during the inspection that could have affected the control system?

I'm not familiar with the 412, but I encountered a similar problem with a 206.

100%Nr, flat pitch on the ground, switch Hyd Boost off with minimal collective friction and collective lever jumps instantly to about half travel.

Couldn't reproduce the fault with hydraulic rig with blades stopped - i.e. in the hangar - main rotor had to be turning at at least idle rpm

Fault was a missing O ring/seal inside centre hyd servo.

Don't know if this helps, but it's a good story anyway.

[Leeuwarden AFB-Ed]

Thanks!

First, the hydr test in idle is ok. 100% fails (collective will not be able to move fully down from approx 80% RRPM and up).
We were thinking about servo, and replaced actuator but re/check!
Work during inspection only on tailrotor. It seems to becomming worse every week....
We checked and-or replaced
Inspection revealed minor leakage on Collective Servo Actuator (212-076-005-111). New (o´hauled) actuator was installed.
Collective Control Rigging performed. Minor adjustment needed.
Replaced Balance Spring. Checked and measured, within limits.
Performed Groundrun with Mast Boot removed.
Checked Hub and Sleeve Assy.
Replaced Shim between Hub and Drive Plate set. New Shim showed 0,4 mm thicker.
Replaced Drive Plate set.
Checked Collective Lever for axial play.
Checked Cyclic Control rigging.
Checked all Elastomeric Bearings.
Checked Droop Compensator Cable for smootheness of operation. Replaced Nut and Bolt of Droop Compensator to Cable.
Checked Collective Engine Throttle System for binding.
Checked all Collective Control System for binding.

Many thanks for all the kind reply!
Best regards,

Ed

[chopper_doctor]

Ed, sounds like you are going at it the right way. Now, if it only happens with rotors turning and not with the ground test rig, i think you are starting to narrow your problem down. What is in the system that operates differently with the test rig as compared to rotors turning? Im going to have to go back and check my 212/412 stuff, but is there not some sort of 3 way valve in the system, or a check valve that prevents ground test rig from driving the pumps/rotor system?
Also, as was asked before............with collective at flat pitch and engines at idle, what happens to collective when RPM increases to 100%? Does collective "jump up"?

[oldbeefer]

I think reading Ed's last post that there is no stick jump at idle - only when the Nr is above 80%. That's really got me beat. Also, a 25hr involves nothing on the control runs or hyd systems. Spoke to my engineers this afternoon and they can only think servo, but for the problem not to show below 80% is very strange.

[TRC]

Looks like it could be a PMC issue to me.......

The only difference between rotors stopped/low rpm and 100%Nr is the dynamic effect of the main rotor itself - throwing on pitch.

What is the Pitching Moment Compensation arrangement on a 412?

On a 47 it's done by the PMC weights, on a 206 it's a combination of built-in twist in the TT Straps and irreversibles in the servos.

[Shawn Coyle]

What does Bell helicopter say about this???

[Encyclo]

Bell does not support AB412... I guess you get what you pay for :E

Back to the problem; what you are seeing is the infamous "tennis racket" effect, the tendancy of the blades to feather, which in some OAT and DA conditions, can be greater than what a single hydraulic system can take. Easy way to proove this is to roll the throttle down to 95% Nr and see if you can bottom the collective.

Another issue can be out of wack autorotation RPM adjustment. On a 412, you should see around 22 to 24% Tq, flat pitch ground, 100% Nr.

Let us know how it turns out & good luck :ok:

[yukonbrown]

Have you replaced you collective check valve?

[Torquelimited]

"what you are seeing is the infamous "tennis racket" effect,"

Agreed,
This is a response which is normally observed on the 412 in a low density altitude enviroment and is related to the "tennis racket effect".
The amount of resistance occuring at the last bit of collective full down travel during single hydraulic operations will vary from one ship to the next dependant on the sum of the 4 main rotor blade's variance in built in blade-twist. The effect is also a product of the loads created by centrifugal force so, as rotor rpm reduces below 100% so does the resistance.

Hope this helps.

[Leeuwarden AFB-Ed]

What we did too reduce the feedback load from the rotor to the collective system is:
Lenghten all four pitch links with two full turns followed by the 100% full hydr.test during ground run.(we're not going to fly)
Up to 92% rrpm it is possible to bottum the collective with only one hydr.sys.so it defenitly has to do with the rotorsystem. With 100% rrpm its about 10-15 mm from full down with a small difference between the hydr.systems.
We're gone change a set rotor blades and hopefully this will work out.

[Shawn Coyle]

There's another part in the rotor head that can cause problems - unfortunately I forget the name of it! Look carefully for cracks. This has been a problem before.

[victor papa]

You say they worked on the tail rotor only during the 25hr. I do not know the 412 at all, but on a EC product we have a colective/yaw coupling. The S76 has similiar couplings between her control channels affecting each other. If the 412 has a collective/yaw coupling I would check the tail rigging and/or assembly as it might be inhibiting the collective travel at higher rpm due to it's pitch setting adapts as the torque increases.

If no collective/yaw coupling-guess it can't be the tail then.

[Maverick Laddie]

ED :

Didn't I hear you had a problem with the first machine you sent up north for a major in Dec 2007 not being able to get the blades to fly right and had to replace the set to cure the problem was that not 02.:confused:
ps Love to Rechelle

[knob strangler]

:eek:I have had a different but similar problem solved (eventually) by replacing one of the intergrated valves. I think you have done all the other possibilities already.

[spinwing]

Mmmmm ....

Time to change the Hydraulic System Interlock control block ....

seems like the Hyd pressure when a/c is run above the 80% is a clue ... but snag not being there with the Hyd test rig running is the confuser.

There must be a leak-by somewhere causing the Hydraulic lock my guess is the interconnect block.


As usual I stand to be corrected as required!

Good Luck :confused:

[Leeuwarden AFB-Ed]

We installed a overhauled set of main rotor blades and that solved the problem. Still we've a questionmark how this problem occured from one day to the other. For now we are operational again and thats all whats matter.
Many thanks for all the reply.