Recommendation

SUBJECT: BEECH 200 CABIN DOOR LATCH HOOK FAILURE





OCCURRENCE SUMMARY



The Beech 200 King Air was was passing through Flight Level 230 on
descent from FL270 when the pilot heard a loud bang. He advised Air
Traffic Services that both he and the sole passenger were now on
oxygen due to an emergency decompression. The aircraft subsequently
made a normal approach and landing. A post-flight inspection found
that the upper door latch had failed. The latch is an item that
needs to be replaced at intervals of 12,000 hours. The time in
service of the latch was approximately 4,700 hours.





ANALYSIS



The Civil Aviation Safety Authority Failure Analysis Group
conducted a metallurgical examination of the failed door latching
hook (see attachment 1). Several factors were considered to have
contributed to the failure of the hook at a time well before the
expiry of its designated safe life:



(a) a surface irregularity created during the machining of the
hook;



(b) a fatigue crack initiation at this machining
irregularity;



(c) lack of corrosion protection of the door latch hook; and



(d) stress corrosion cracking from in-service exposure to a
corrosive environment.



The failure of the cabin door upper hook was the subject of Major
Defect Report (MDR 96/1678) submitted by the operator to CASA. The
operator submitted a further Major Defect Report (MDR 96/1716)
suggesting that "the previous hook failure was a result of
misrigging rather than fatigue or age as originally thought". This
was prior to detailed analysis of the hook failure being
available.



While misrigging may have contributed to circumstances surrounding
the failure, it was not the cause of the ultimate failure. It was
clearly identified by the Failure Analysis Group that the cabin
door latch hook failed due to fatigue/stress corrosion cracking
initiating from a manufacturing irregularity. Incorrect rigging of
the door latching mechanism leads to overload failure (as opposed
to fatigue) of the door hooks.



A data search has found further supporting evidence of cabin door
latch hook failures. A 1987 occurrence (MDR 87/6028) involved a
cabin door upper latch hook broken through the centre of the jaw. A
crack had initiated through stress corrosion cracking on the
bearing surface. In a 1992 occurrence (BASI Occ. No. 9201773), a
Beech 200 cabin door separated in flight resulting in
depressurisation and an emergency situation. A factor in this
accident was considered to be incorrect rigging of the cabin door
locking mechanism, although this could not be confirmed because the
door was not recovered.



As a result of the 1992 accident,the Civil Aviation Safety
Authority issued Airworthiness Directive BEECH 200/53 Amdt 1 (see
attachment 2). This required an inspection of the cabin door
locking mechanism and alignment. Airworthiness Advisory Circular
No. 1-79 ("Beechcraft King Air in-flight door loss", see attachment
3) was also issued. This stated that Beech considered the accident
to be the result of inadequate maintenance and also that attention
should be paid to the latching mechanism during the 150-hour
inspection as stated in the Maintenance Manual.



Airworthiness Advisory Circular No. 1-83 ("In-flight door
unlatching", see attachment 4), was also issued to advise of the
need to check the security of the door and its latching prior to
departure on all Beech models.



The issue of fatigue/ stress corrosion cracking of these hooks has
not yet been addressed.



SAFETY DEFICIENCY



The cabin door upper latch hook on this Beech 200 aircraft failed
due to fatigue/stress corrosion cracking initiating from a
manufacturing irregularity. This occurred at a considerable time
prior to expiry of the hook service life.