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Hydraulic system issues

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hello C-130 World,

1st During flight, crew noticed an issue with the elevator, needing more force input to stay level.

2nd During flight, crew noticed an issue with the aileron, needing more force input to roll out (they were in a turn).  After aborting the mission and turning back, they also noticed more force needed to move the rudder. Controllability check was performed and crew landed normally.

  • history tells us that the technicians mixed H515 with H537 hydraulic oil, the percentages can not be defined.
  • the whole mechanical system is checked, cable tension ect...
  • motion was checked and free of any rubbing
  • hydraulic system will be flushed and all filters will be replaced
  • oil sampling done, no abnormalities found, no debris found....

any other suggestions?

should we look into the mix of both hydraulic oils? viscosity ==> pressure issues per inch²


thx4coop, BOB

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Not sure I have ever heard of a problem like this where all flight control surfaces are affected and a "Feel" issue is noted as a discrepancy. There have been instances where a single viscus damper has caused a feel issue but to have all surfaces involved may be more the feel"er" than the feel itself. If there were extremely cold conditions present each of the dampers may have been affected increasing the viscosity of the fluid which could have increase a sense of more force required "feel".    

The fluid being mixed should not be an issue as they are fully compatible. 


will be interested to see what transpires with this one... Good luck.


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Only other thing I can think of to help isolate your issue is to start only the left or right engines or use the hyd pump switches or boost shutoff switches to "turn offf" one of the hyd systems and measure the force "feel" required to move each flight control. You may find a difference in required force, maybe test a few other aircraft to get a benchmark but feel is not too scientific... maybe a fish scale etc to get specific readings... 

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Mechanically, make sure that the rudder pedals and set in a midway position (not fully aft).  Look for this condition:

"If control wheel is held fully counterclockwise with a force of 40 pounds or more and rudder pedals are adjusted to full aft, there may be contact between aileron tension regulator lever and rudder pedal adjustment lever. This contact is not a binding condition and is a result of stretching the aileron control cables. A problem does not exist unless full aileron travel cannot be achieved or the affected parts become dam-aged and need to be replaced."

Hydraulically look for this:

You need to look into your return system.  The only way all flight controls are linked is through pressure and return.

The fact that pilots/and yourself did not mention issues with pressure, it is safe to assume the issue lies in the return.

First, change all return/case drain filters and vent filters in booster and utility.

Second, if problem is not solved, check the return line check valves for the packs.  If you have bad return check valve(s), the pressure leaving packs through the return can flow back into the other packs.  For example, with a failed aft boostpack return check valve, the aileron return pressure can back flow into the rudder/elevator.  

Third, change the filters in the packs themselves (aileron,rudder, elevator).

If you still have issues, start isolating packs.  It's possible that you have failed pack(s) that are dumping too much return pressure back into the return line.  For example, an elevator boostpack bypass that is dumping too much back to return can effectively turn the return line into a pressure line.  This overpowers the normal return fluid that wants to come out of the packs when they are moved and will essentially bind them up.  Similiar to how the pack would feel if you tried moving it with the return lines capped off.  Start by turning off elevator.  The fact that the elevator required more input to move, makes me think you have a bypassing actuator or an incorrectly setup bypass that is dumping off too much pressure back to return.  

Hope this helps.  

1C-130H-1 224.pdf

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I would have to agree this seems to be a placebo discrepancy.  A "feeling" is not a discrepancy, especially when it spans 3 different systems.  Unless they have indication such as low system pressure on the gauges, or flickering low pressure lights, I would have to say this is a CND, especially if there are no issues in the rigging or hydraulic system.

I understand the word of pilots is the same as god, twice I have had to deal with issues on flight controls that didn't feel "right".  Went up on a red ball and had the pilot tell me that "when I move the rudder normally, there is no issue...but when I try to move it fast I get a lot of resistance"  After me trying to explain it is not meant to be moved "Fast" and it is only #6 lines running the pack and the whole diverter system...he refused the acft.

Second was a write up that stated "2 hours into flight moving ailerons felt like loss of power steering." After 8 hours of checking rigging, pressures, even hanging off the ailerons while someone moved them, it struck me that most likely the pilot was fighting the autopilot servo motor and not fighting it long enough for it to disconnect.  Production agreed but still made me change the pack to keep "crew confidence" intact.

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