Grob Twin III Must read!

This got me thinking about something. I recall when I was a teenager I got a
hold of my father's ratchet set and started using it on various bolts around
the house. I quickly learned why torque wrenches were invented as I sheared
the heads off of many of them. Since then I've been respectful of how much
force you can exert with a lever, even one only a foot long.

So I occurs to me that there may be a downside to repetitive PCCs if they
are not done properly. If the person applying the force on the stick (a
lever) applied a great amount of force and the individual opposing that
force on the control surface (also a lever) did likewise it might be
possible to repetitively stress and weaken the bellcrank and bolt since
that's where the forces would converge in opposite directions. I'm not an
engineer, but it seems to me that some care must be taken here so that the
forces do not exceed those that the system would be designed for, i.e.,
those that would be encountered in flight. The forces required to assure a
stable connection are far less than what might be accidentally applied by
individuals who are not aware of how to do this properly.

There was another Grob SB this summer about the shearing of the removable
rear control stick just below the attachement point. The picture in the SB
looked to me more like metal fatigue rather than corrosion. Whether this is
from PCCs or fighting students is not clear, but the force mechanisms might
be the same.



" wrote in message
...
Our club's Twin III had a major problem that was found in a Positive
Control Check, the bolt holding the belcranks for the airbrakes and
the elevator (located behind the rear seat in the control hookup area)
had sheared.

Grob will probably be doing an AD for instpection soon, our plane is
at Grob right now being inspected (they are only a 20 minute drive
from the club!).

Please inspect your Twin III before you fly in again.

And for those of you who don't feel the need to do a PCC .... This
wouldn't have shown up with a visual inspection nor probably on a
"hands on" inspection.

Bob

"Stephanie and Ed" wrote:

Ed or Stephanie ;`

The initial thoughts from our "Guru's" is that this problem came about
as a result of "heavy handed" PCC checks. Maybe we need someone to
develope an adjustable strain guage that can be used for PCC's. Set
the guage to the proper load and test away!

Bob