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


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

Thank you,

Scott Flood (SFlood@iBallot.Com)


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