I now strongly feel that this area should have
a bushing of some sort installed into the stabilizer spar that
will prevent the spar from flexing (crushing) when the bolt is
being brought to the proper torque and to prevent the sharp edge
of the bolt hole in the steel spar tube from contacting the bolt.
There are a number of ways to address this. One builder removed
the bolts and then sprayed expandable foam sealant (as used for
sealing gaps in household construction) into the spar tube through
the bolt hole. He waited 24 hours for the foam to set. He then
used a pick to clean any foam away from the bolt holes and then
filled the voids around the bolt holes with 2 part epoxy(the stabilizer
was flat on his work table which allowed him to pour the epoxy
into the up facing holes while the down facing holes were sealed
with some tape to prevent the epoxy from oozing out). The expandable
foam formed a dam that prevented the epoxy from flowing away from
the area of the hole.
Once the epoxy was hardened he drilled the bolt
holes through the spar holes and through the epoxy. This method
seems to be a good way to achieve a bushing inside of the spar
without removing the skin from the spar. I have talked with others
who have taken the skin off and welded steel inserts into the
spar for the bolts to pass through and others have inserted aluminum
plugs in from the ends and pushed them up and into place and held
in place with epoxy applied into the spar through the bolt holes
just before the aluminum plug was slid into position. The aluminum
plugs were then drilled through the existing bolt holes.
I feel that the builder or subsequent owner should
make some sort of bushing inside of the spar tube holes at the
front of the stabilizer. There is a good amount of force applied
to both the vertical and horizontal stabilizers during flight
with up to 110mph air passing around them and the downward air
pulses from the main rotors continually assaulting them.
I give each of my students a Pre-boarding
check list that I have developed over years of observing
those areas that can get a pilot into trouble during a flight.
This simple check list is performed prior to boarding the helicopter
every time. One of the items on the
check list is to check the vertical stabilizer. This is done by
grabbing the bottom of the tail stinger and applying pressure
sideways while viewing the brackets and attachment bolts for movement.
Jim was returning home from a breakfast meeting
of the local flying club when the vertical stabilizer departed
the helicopter. The stabilizer was found some distance behind
the wreckage and a witness stated that the helicopter blades stopped
turning while the helicopter fell vertically to the ground. The
rotors showed no sign of rotational damage supporting the witness
statement that the blades had stopped in flight.
From the photos of the wreckage, it is evident
that the bracket had broken allowing the stabilizer to move excessively,
fatiguing and finally fracturing the other end of the front attachment
bracket (part of which was still attached to the stabilizer).
The stabilizer then blew back in the relative air flow ripping
out the rear attachment bolt as it departed the helicopter. The
pilot was most likely so distracted by vibrations associated with
the breaking of the support bracket and the departure of the stabilizer
that he allowed the helicopter rotor RPM to decay to the point
of main rotor stall, the results of which are never positive.
The NTSB report stated that the cause of the
crash was that the builder had drilled additional holes in his
vertical stabilizer bracket (not per the factory drawings and
instructions) and then did not de-burr the holes. A crack began
at one of the burrs and progressed along the weakened bracket.
The loss of the bracket was a survivable event. The pilot failing
to maintain main rotor RPM was not.