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Thread: 77 Sierra MC-460 - Gear Problems, AOG

  1. #1

    77 Sierra MC-460 - Gear Problems, AOG

    Hi guys,

    I am hoping someone out there can help me, I had a problem with the Gear in Transition during flight. The gear motor seemed to be cutting in to try and raise the gear as it was loosing pressure. My Mechanic removed the gear motor and said it was burned out. He replaced the gear pump motor along with some O-Rings in the nose gear actuator. Now he tells me there is a problem with the pressure switch. At this rate it will be 10K before we get this bird back in the air...HELP!

    Here is what my Mechanic is telling me:

    When the new motor was installed, Pat found where the leak was in the gear hydraulic pressure system. The O-rings (3) in the nose gear actuator had worn down pretty badly and were actually the wrong thickness. Anyway, they were replaced and the gear now extends and retracts much faster than before. This is the good news. The bad news is when the gear is retracted, the pressure switch that should deactivate the gear motor when the required 1500 PSI is reached remains closed, meaning that the motor remains on. Eventually, the motor will burn out from this. This is apparently what happened to the previous motor. The bottom line is that you need a working pressure switch to resolve the problem. This is where it gets ugly. The switch assembly, now only available through Ratheyon, is $6K US. It would take about 10-14 days to get here.
    I told them to hold off on ordering such a part until I had a chance to check with some other Beech Owners.

    The Beechcraft part number 169-380099-1.
    The standard part number is PH8P-94.

    The switch by itself is part # A4403 but apparently Raytheon will not sell it that way, just the whole assembly. I have the schematic for the complete assembly and it is made by United Electric Controls, Precision Sensors Division in Milford CT. My Mechanic said he called them today directly but they said they could not sell it to him directly and we had to deal through RAPID. Another parts supplier said he sold one in October to someone and they paid approx $1300 through Rapid at that time. Now it seems the price is 5 times as much. The worst part is RAPID tells us they actually have 2 on the shelf.
    If anyone can offer a suggestion of another source for this part (certified or not) or even a used one it would be much appreciated. Basically I am not even convinced that this is the answer to all of our problems and am hesitant at this price to "try it and see". If I could get any kind of hydraulic switch that is rated 1200 - 1500 lbs then at least I would know that this is the answer.
    All I want for Christmas is a 1500lb pressure switch....
    Thanks,
    Bruce Dwyer
    77 Sierra, MC-460
    C-GMTT


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  2. #2
    The quick check to test the pressure switch is to disconnect the high pressure line at the powerpack and plug it, then put the gear switch in the up position. The pump should quickly come up to pressure and shut off. If the pump keeps running, either the pressure switch is bad, or the pump is not producing enough pressure. If the pump only continues to run only when plumbed to the gear, then you most likely have more o-rings to replace. On my '78 Sierra, the main uplock actuators and the nose downlock actuator were all leaking from the high pressure side to the low pressure side (so no fluid escaped). After replacing all the seals, no more running pump or flashing actuation light.

    If you do indeed need the pressure switch, do a google search on the part numbers, both the Raytheon and the manjuufacturer's numbers. I found one (but I can't remember where) when I was working on my gear last year, but I didn't need the switch, just the o-rings.

  3. #3

    77 Sierra MC-460 - Gear Problems, AOG

    > Hi guys,
    >
    ,
    > the pressure switch that should deactivate the gear motor when the
    > required 1500 PSI is reached remains closed, meaning that the motor
    > remains on. Eventually, the motor will burn out from this. This is
    > apparently what happened to the previous motor. The bottom line is that
    > you need a working pressure switch to resolve the problem. This is where
    > it gets ugly. The switch assembly, now only available through Ratheyon, is
    > $6K US.

    *** Another killer part! It's not quite up to the $17,000 gascolator, but
    it's up there.....

    I wish that when Raytheon decides they're not going to sell any more of
    something, they'd just take it off the @#$# parts list. That way, one
    could go to the FSDO with a clear heart with plans for a substitute.

    A quick web search for "Pressure Switch" revealed that such are a
    standard industrial item. A 1500-PSI switch can be bought off the
    shelf.

    Compatibility worries would include

    * Vibration: can it take the guff?
    * Is the diaphragm compatible with aircraft hydraulic fluid?
    * adapting the fluid fitting to the military-style stuff used in
    aircraft...

    - Jerry Kaidor ( jerry@tr2.com )


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