Mystery Vibration - 74 Cherokee 140

Plugs might be clean but the resistor in them can fail or
conduct intermittently, weakening the spark. It won't usually show up
during a mag check, but in cruise, if the mags aren't timed exactly the
same, the plug that fires first causes a pressure rise in the cylinder
that the defective plug now has trouble sparking against. Plugs need to
be tested in a pressure-type spark tester, and even then they can act
up when hot.

Dan

The Visitor wrote:
: I remember finding some play in the stab on my Seneca. So the shop
: replaced the bushings. I am going to look in the tech logs to see if the
: bolts were also done. If not, I'll have it done. It's a 83 model.
: 125,000 lp tensile strength on those bolts though I believe.

Yeah... when I was replacing the bolts/bearings on mine, I did a quick
calculation just to make sure. I don't remember the exact numbers, but each one is in
double-shear (both sides of the bearing). That's 4 cross-sections of 1/4" to split
the load over. Not super-special bolts, either... just regular AN stuff. Anyway,
from what I remember, my computation on the shear strength of the bolts concluded that
I could pick up the airplane loaded at gross weight by just the tail, and then shake
it with 4G's before the bolts would shear....

Now, that doesn't say anything about excess material for corrosion resistance,
stress risers due to corrosion, or other strength-reduction due to wear. As long as
it's in good shape however, nothing to worry about... just seemed too small.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

Thank you for the comments. They are most reassuring.

John

wrote:
The Visitor wrote:
: I remember finding some play in the stab on my Seneca. So the shop
: replaced the bushings. I am going to look in the tech logs to see if the
: bolts were also done. If not, I'll have it done. It's a 83 model.
: 125,000 lp tensile strength on those bolts though I believe.

Yeah... when I was replacing the bolts/bearings on mine, I did a quick
calculation just to make sure. I don't remember the exact numbers, but each one is in
double-shear (both sides of the bearing). That's 4 cross-sections of 1/4" to split
the load over. Not super-special bolts, either... just regular AN stuff. Anyway,
from what I remember, my computation on the shear strength of the bolts concluded that
I could pick up the airplane loaded at gross weight by just the tail, and then shake
it with 4G's before the bolts would shear....

Now, that doesn't say anything about excess material for corrosion resistance,
stress risers due to corrosion, or other strength-reduction due to wear. As long as
it's in good shape however, nothing to worry about... just seemed too small.

-Cory

Thank you for the comments. They are most reassuring.

I'll "second" that -- thanks for that data, Cory!

I've always looked at those stabilator attach points and quickly averted my
eyes, cringing. They look soooo small!

Of course, take a look at how a 172s wing attaches, and you'll never fly one
of those again. Yet they've never had a wing failure.

All I can say is: Thank goodness for engineers who can figure this safety
stuff out.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"