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Safety Advisory Notice issued to: Civil Aviation Safety Authority

Recommendation details
Output No: SAN19970071
Date issued: 15 July 1997
Safety action status: Closed
Background:

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.

Output text

The Civil Aviation Safety Authority should note the safety deficiency detailed in this report and take appropriate action.

Initial response
Date issued:
Response from: Civil Aviation Safety Authority
Action status: Not Required
Response text:
 
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Last update 01 April 2011