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Thread: How do you tell if your donuts need to be replaced?

  1. #1

    How do you tell if your donuts need to be replaced?

    Hello,

    I have a 1970 A24 with fixed gear (S/N MA 367). During the annual that is
    being done as I type this email, the plane was jacked up and the mechanic
    reports that there is 1/2 inch of play when he lifts the wheel up until it
    hits the donut stack.

    How does one know if you need to replace the donuts? These have never been
    replaced.

    Thank you,

    Scott Flood (SFlood@iBallot.Com)
    N6146N



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  2. #2

    How do you tell if your donuts need to be replaced?

    You can download the Service Manual from BAC. While the print quality in
    the current download isn't very good, it can be read. If you go to the
    section on the fixed gear cushion disks, it specifies the nut adjustment.
    It also essentially tells you to disregard the free play after the cushions
    have taken a "set". And in fact, it warns you to not to do anything to
    readjust that play. I may be misremembering, as it has been ages since I
    read that part, but I think it refers to as much as two inches of play being
    acceptable on in-service aged disks.



    The same section also specifies the minimum distance from the disk
    compressor pin to the main casting/housing, which defines the status of the
    cushion disks. I'm thinking that the minimum clearance is one inch. A
    reasonable rule of thumb is that if you can see the entire end of the pin,
    the disks (donuts) are probably still OK. If you can just see the center
    below the casting (the flush grease fitting location, if yours has the
    fittings), the disks are borderline. Once most or all of the pin has
    disappeared into the upper casting recess, you are no longer getting any
    cushioning from the disks. They are basically hockey pucks at that point.
    Your tires and your wing spars are doing all the heavy hauling, with the
    resulting added wear-and-tear (and risk) longer term.



    Your mechanic should be answering his own question; he isn't supposed to be
    working on your plane without having the "approved data" in the form of the
    service and parts manuals. That's where most of the official data resides.
    His lack of that data can have a lot of repercussions, not least of which is
    to your wallet. If nothing else, print out a copy of the BAC download and
    give it to him. I don't mean to be "getting on my high horse", but you can
    tell that this is a point of irritation to me. If he is willing to work on
    your plane, and take your money, he should at least spend what he needs to
    spend in order to obtain the required reference material to do things right.
    Part of what you pay him is supposed to be financing his tools, materials,
    continuing education, and reference materials. Otherwise you should
    consider finding someone else to handle your aircraft maintenance for you.



    And finally, it is a virtual certainty that 35 year old disks aren't doing
    any cushioning anymore, no matter what the free-play and clearance specs
    say. How many pieces of 35-year old rubber have you seen in your life,
    which still had apparent flexibility? Planes that are usually "parked
    light", as opposed to being "parked heavy" (i.e. full fuel, tool bags and
    ballast bags, etc.), will develop hardened disks at a point where the
    compressor plate is further extended than on a heavier plane. The disks do
    not have to be over-compressed, to develop age hardening. You can look up
    into the housing and try to view some disk surface, ideally with the plane
    jacked on that side and the gear hanging free. If you see badly bulged
    rubber disks that have numerous vertical cracks around their perimeter,
    that's your first clue. Then have someone rock your aircraft nose up and
    down, while you watch the fork flex up into the housing. Have them do the
    same thing on the outer wing section on each side, using their back under
    the outer spar. Watch the main gear forks. They won't flex like the nose
    does, but you should be able to see some "give and take" in the form of
    travel. If all you see is the tire sidewall and wing flex, you have no main
    gear cushioning anymore, regardless of the measurements and clearances.



    Best of luck with the Annual and your related projects. All our sweeties
    need more TLC at this point in their lives. Airplanes too..







    _____

    From: musketeermail@yahoogroups.com [mailto:musketeermail@yahoogroups.com]
    On Behalf Of Scott Flood
    Sent: Friday, November 18, 2005 7:50 PM
    To: musketeermail@yahoogroups.com
    Subject: [musketeermail] How do you tell if your donuts need to be replaced?



    Hello,

    I have a 1970 A24 with fixed gear (S/N MA 367). During the annual that is
    being done as I type this email, the plane was jacked up and the mechanic
    reports that there is 1/2 inch of play when he lifts the wheel up until it
    hits the donut stack.

    How does one know if you need to replace the donuts? These have never been
    replaced.

    Thank you,

    Scott Flood (SFlood@iBallot.Com)
    N6146N

    _____



    [Non-text portions of this message have been removed]



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    www.beechaeroclub.org


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