Depression after Washing

On Jul 4, 4:56 pm, "Mike" wrote:

Obviously, you haven't seen some of the things I have seen. One of the
more tender, and more invisible spots on some airframes is where the
horizontal stabilizer connects to the fuze. Many designs allow a
tremendous moment arm for any non-balanced load on the stabilizer to
stress the attachment points. This shows up as cracked spars on Cessnas,
and I have seen stressed and cracked fittings from another airframe. How
can this happen? Well on Cessnas it happens from folks using improper
procedures to back the plane into a parking spot. It can also happen from
innocent (but ignorant) bystanders, mowers, animal activity, or any of
thousands of other posibilities.

I always give each side a good heave up and down for this very reason, so
such can easily be checked on the preflight for impending failure.

A good heave up and down on the end of the stab of a 172
flexes the center of the forward spar, eventually cracking it. A
gentle bit of push-pull is all that's needed, to see if there's
unusual tip travel. I bet your spar is cracked now. Many are. Cessna
calls for stopdrilling the crack unless it has reached the spar
flange, in which case it has to be repaired. I once flew a 172 that I
subsequently found had a broken spar, busted clear through both
flanges so that the skin was all that was holding it. The thing could
have killed me if I gotten into turbulence or had to take violent
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.
172s suffer cracking at the bottom of the aft doorposts. Some
models crack inside the forward doorposts. Do I need to point out that
these doorposts are what the wing pulls on to lift the airplane, along
with the struts? No preflight will find those. The wing spar attach
lugs are known to crack at the bolt holes. In older 172s the forward
elevator bellcrack bracket would break loose, reducing elevator
control. In newer 172s (rod-style gear; 1973 or so and on) the landing
gear retaining bolt sometimes shears and totals the airplane on
landing. As the years go by, these older airplanes will become the
subjects of ADs addressing age-related airframe failure, probably
after a couple come apart in flight. Sooner or later.

Dan

On Jul 6, 4:45*am, wrote:
On Jul 4, 4:56 pm, "Mike" wrote:

*Obviously, you haven't seen some of the things I have seen. *One of the
more tender, and more invisible spots on some airframes is where the
horizontal stabilizer connects to the fuze. *Many designs allow a
tremendous moment arm for any non-balanced load on the stabilizer to
stress the attachment points. *This shows up as *cracked spars on Cessnas,
and I have seen stressed and cracked fittings from another airframe. *How
can this happen? *Well on Cessnas it happens from folks using improper
procedures to back the plane into a parking spot. *It can also happen from
innocent (but ignorant) bystanders, mowers, animal activity, or any of
thousands of other posibilities.

I always give each side a good heave up and down for this very reason, so
such can easily be checked on the preflight for impending failure.

* * * * *A good heave up and down on the end of the stab of a 172
flexes the center of the forward spar, eventually cracking it. A
gentle bit of push-pull is all that's needed, to see if there's
unusual tip travel. I bet your spar is cracked now. Many are. Cessna
calls for stopdrilling the crack unless it has reached the spar
flange, in which case it has to be repaired. I once flew a 172 that I
subsequently found had a broken spar, busted clear through both
flanges so that the skin was all that was holding it.


Why did your push-pull test not detect it? I prefer to give a shake
and feel the nature of surface response.

Cheers

On Jul 5, 3:58 pm, More_Flaps wrote:

Why did your push-pull test not detect it? I prefer to give a shake
and feel the nature of surface response.

That's why I have a push-pull test now. Didn't do it until
after I flew that airplane and took it apart after I got it here.
Found a lot of other stuff, too: lower strut attach bolts with no
nuts, and backing out of their holes; cracks in lots of places; scary
stuff galore. The owner trucked it away. In pieces.

Dan

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.

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

wrote in message
...
On Jul 4, 4:56 pm, "Mike" wrote:

Obviously, you haven't seen some of the things I have seen. One of
the
more tender, and more invisible spots on some airframes is where the
horizontal stabilizer connects to the fuze. Many designs allow a
tremendous moment arm for any non-balanced load on the stabilizer to
stress the attachment points. This shows up as cracked spars on
Cessnas,
and I have seen stressed and cracked fittings from another airframe.
How
can this happen? Well on Cessnas it happens from folks using improper
procedures to back the plane into a parking spot. It can also happen
from
innocent (but ignorant) bystanders, mowers, animal activity, or any of
thousands of other posibilities.

I always give each side a good heave up and down for this very reason, so
such can easily be checked on the preflight for impending failure.

A good heave up and down on the end of the stab of a 172
flexes the center of the forward spar, eventually cracking it. A
gentle bit of push-pull is all that's needed, to see if there's
unusual tip travel. I bet your spar is cracked now. Many are. Cessna
calls for stopdrilling the crack unless it has reached the spar
flange, in which case it has to be repaired. I once flew a 172 that I
subsequently found had a broken spar, busted clear through both
flanges so that the skin was all that was holding it. The thing could
have killed me if I gotten into turbulence or had to take violent
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.
172s suffer cracking at the bottom of the aft doorposts. Some
models crack inside the forward doorposts. Do I need to point out that
these doorposts are what the wing pulls on to lift the airplane, along
with the struts? No preflight will find those. The wing spar attach
lugs are known to crack at the bolt holes. In older 172s the forward
elevator bellcrack bracket would break loose, reducing elevator
control. In newer 172s (rod-style gear; 1973 or so and on) the landing
gear retaining bolt sometimes shears and totals the airplane on
landing. As the years go by, these older airplanes will become the
subjects of ADs addressing age-related airframe failure, probably
after a couple come apart in flight. Sooner or later.

1. By "good heave" I don't mean raising the nosewheel off the ground. I
just mean applying enough pressure both directions to listen for any
irregularities.

2. I don't have a 172.