On Mar 13, 6:49 am, "John" wrote:
On Mar 10, 10:00 pm, wrote:





Our 172 has two screws missing from under the wing where
it connects to the fuselage above passenger door.

The size of those screws are quite big. My question is whether
they are important to the stuctural integrity of the wing ? They
may have come off during flight due to the vibrations. But it
is still some hours before the next 100 hours.

Are those screws important for holding the wing to the fuselage ?
or are they only there to cover the holes, and there are some
other bigger screws/nuts inside the wing that are doing the real
work ?

A speedy reply will be appreciated.

P S

Those screws are for the fairing that covers the joint between the
fuselage and the wing. There is a long narrow fairing panel secured
with a 10 or 12 screws. The holes in the panel are quite large ( to
allow for variation in space between the wing and fuselage when first
installed) the screws are put in with countersunk washers to cover the
holes. The holes in the wing have #8 size Tinnerman nuts. These are
sheet metal type fasteners and take a type B sheet metal screw. A
type B sheet metal screw has a blunt point and a different thread
pitch than the more common Type A screw which has a sharp point.
These are not self locking nuts so the screws can back out over time
if not properly torqued. Type B screws will torque up properly many
times Type A screws will eventually damage the nut and lose torque
more easily. Some times a loose or damaged #8 tinnerman nut will take
a #10 screw for a while. Sometimes when the Tinnerman nuts are
completely stripped out they will be replaced with a Rivnut which is
an aluminum nut designed to be installed in a blind area. These take
machine screws so you might have a mix of both styles.- Hide quoted text -

- Show quoted text -

The Tinnermans often lose one side of their "thread." The
steel is brittle and the thing will break off. There's the possibility
of a Rivnut but the Tin nut has to be removed first, and doing so will
leave loose bits rattling around under the fuel tank. Those bits have
a way of wedging into places they shouldn't be and will wear holes in
the tank. The better solution is to replace the nut, but the tank has
to come out. To do that, the tank has to be completely drained,
another pain.
Still, it's a better system than Citabria uses. They have
really short sheet metal screws into the root rib, and the compression
rib is just inside that root rib and gets in the way. The rib holes
strip out and won't hold the screws anymore.

Dan