On Jul 5, 4:26 pm, Peter Clark
wrote:
On Sat, 5 Jul 2008 09:45:11 -0700 (PDT),
wrote:

evasive action. We run several 172s and have seen cracks, up to four
of them, in a spar; we now forbid students to push down on the things.
No more cracks.

Just out of curiosity, is this before or after Cessna put the
reinforced spars (R and S models?) in at the factory? I don't believe
the newer models are prone to this kind of issue, but wouldn't do it
in practice anyway - using the towbar is always better for the
aircraft, and done by hand I don't think it's possible to damage a
172/182 nosewheel.

There's a Service Bulletin dealing with this, and it's dated to
before the R/S models went into production, so it's safe to figure
that Cessna fixed the problem in these models. The SB calls for
stopdrilling and monitoring every 100 hours, and fixing it by
installing a doubler either right away or when the cracks go too far.
The problem with waiting is that the centre section needs to be
replaced if the cracks reach the flanges.
I'll know more later. We're getting a new or newer one, maybe
two, soon. Interesting to see what they've improved, and what they
haven't but should have.
Which reminds me: there are other places that crack, not
readily visible. The rudder hinge brackets (on the rudder itself, not
the fin) will break in the bend radii. The top hinge is the worst, as
it has the loads of the lead balance weight wobbling around in the
turbulence to deal with. Got to use a tiny mirror and lots of light to
see the break. We spin these airplanes all the time, and in a spin
the tail wiggles around a lot, so maybe the non-spinning pilot won't
have a problem with these. Spins are also hard on gyros, whacking the
internal gyro cases against their stops and Brinelling the bearings.
But that's all part of flight training, and we charge enough to cover
things like that. We go through more propellers, too, operating off
rougher strips and picking up small stones.
These rudder brackets also wear against the fin lugs, since the
spacing at the factory was often screwed up so that the bottom hinge's
top bracket doesn't ride on its bearing flange like it's supposed to,
letting the middle and/or upper hinges take the thrust loads so they
wear thin. No thrust flanges on those bearings; just aluminum against
aluminum, and sitting outside in the wind those hinges get full of
grit and the wind wiggles the rudder and the brackets eat themselves.
I've asked Cessna to issue some teflon washers to shim the bottom
bearing and get the load off the others, but they pay no attention to
a hick from rural Canada. The law here says we have to use only the
stuff from the parts manuals, and they don't list any such washers.
The rudder bar springs put tension on the rudder cables. Those
cables aren't pulling perpendicular to the hinge line becease the
rudder is tilted back, so the rudder is pulled down, increasing the
load on the hinge thrust faces.

Dan