Dave S wrote in message ...
Umm...

Whats the difference between applying enough force by hand over the tail
spar to raise the nose (by pivoting on the main gear) and using enough
force in flight to push the tail down? How is one more likely to cause
cracks than the other?

Dave

The stabilizer is designed to counter the force couple generated by
the centre of lift being behind the C of G. This distance is only a
few inches, while the distance from the CG to the main gear contact
point on the ground is much more than that. Worse, the CG is normally
farther forward when the pilot moves the airplane that way, since the
aircraft is more or less unloaded. The forces required to lift the
nose on the ground are therefore higher than they are in flight, are
concentrated in a small area, and are assymmetrical. I have seen
pilots pushing down on the stab well out on it, not right against the
fuselage, and the loads can become much too high.
We have replaced stab nose ribs crushed by this maneuver, and have
replaced an entire stab due to centre-section cracking. In their
Service Bulletin that addresses this issue, Cessna specifically warns
against using the stab to move the airplane.
I once ferried a 172, and in its subsequent acceptance inspection
we found that the stab spar was broken all the way through the web and
both flanges. Only the skin was holding it together. Any abrupt or
high-G maneuver would have failed it. And this aircraft had just had
an annual "inspection!"

Dan

Dan Thomas wrote:

The gaskets we use on that pump are a black composition material; very
strong and solid. Haven't seen cork in a long time, and never a rubber
gasket.
I agree that any cork gasket there should be replaced with a new
composition gasket.
As far as ADs that aren't, there's another one that should be: the
horizontal stabilizer forward spar centre section cracks and could
fail. Cessna has a service bulletin on it, but sooner or later someone
will die because it wasn't mandated. Those who push down on their
stabs to turn the airplane are almost certainly going to have cracked
stab spars.
Dan