Applies to:
All Cessna 150, 152 and A152 series aircraft.
Purpose: The purpose of this bulletin is to
advise operators and maintenance organisations that cracks may
exist in the vertical fin attachment bracket P/N 0432004-9,
horizontal stabiliser spar P/N 0432001-56 and/or its reinforcement
P/N 0432001-15 and to provide a method for a visual inspection to
detect these cracks if they exist.
Background: During inspections for the cracking
in the vertical fin attachment bracket nut plates in accordance
with AD/CESSNA 150/31 Amendment 3, several operators have noticed
cracking along the edge of the plate welded to the forward face of
the fin attach bracket. On very rare occasions, this cracking may
extend into the horizontal stabiliser spar and its
reinforcement.
The vertical fin attach bracket is attached to
this spar by twelve fasteners (6 each side) and when the bracket
cracks, the additional loads are transferred directly into the
spar, causing cracking. The cracking in the vertical fin attachment
bracket is a slow, progressive failure during which time the
tailplane assembly is still capable of withstanding the original
design loads. Once the crack has extended into the
stabiliser spar, the tailplane structural strength may be reduced
below the original design strength.
AD/CESSNA 150/31 Amendment 3 provides for an inspection for
cracks in the vertical fin attach bracket, nut plates and fittings.
The visual inspection specified in this AD will not normally detect
cracks the stabiliser spar and reinforcement, because these areas
are hidden from view and not accessible without disassembly of the
structure.
Previous experience has shown that the cracking in the spar is a
fatigue phenomenon, driven by in-flight cyclic loads on the
tailplane structure. There have been no reported cases of in-
flight horizontal stabiliser failure due to these cracks and only
seven reported incidents of cracking in the type's history.
Recommendations: While inspecting the vertical
fin attachment bracket in accordance with AD/CESSNA 150/31
Amendment 3, the following actions are recommended:
If any cracks are found in the fin attach bracket itself (not
the nut plates) during inspection per CASA AD/CESSNA 150/31
Amendment 3, the vertical fin attach bracket should be removed so
that the horizontal stabiliser spar and reinforcement can cab be
inspected for cracks. Particular attention should be paid to the
weld at the top, forward corner of the fin attachment bracket and
the corresponding sections of the spar and reinforcement
doubler.
Visually inspect the horizontal stabiliser spar and reinforcement
for cracks using a bright light source, removing protective coating
where necessary. See figure 1 for inspection location.
Re-protect the subject area with an approved protective treatment
and return the aircraft to its original condition per the Cessna
maintenance manual.
Report any cracks found in the stabiliser spar or reinforcement
to CASA via a Defect Report form.
NTSB Final Report: Stinson 108
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