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ASN Wikibase Occurrence # 214066
Last updated: 13 April 2020
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Date:30-APR-1979
Time:16:20
Type:Silhouette image of generic P32R model; specific model in this crash may look slightly different
Piper PA-32R-300
Owner/operator:
Registration: VH-WKJ
C/n / msn:
Fatalities:Fatalities: 2 / Occupants: 2
Other fatalities:0
Aircraft damage: Written off (damaged beyond repair)
Location:3.5km SW of Marulan, NSW -   Australia
Phase: En route
Nature:Private
Departure airport:YSCN
Destination airport:Narooma, NSW
Investigating agency: BASI
Narrative:
The attention of persons on the ground, just to the south of Marulan township, was attracted by hearing abnormal aircraft noises. They then heard a loud sound as the main wreckage of VH-WKJ struck the ground. Those persons in the immediate vicinity of the accident site observed pieces of the aircraft continue to fall from the low cloud base for some time after the main impact.

Examination of the wreckage found evidence of three pre-existing defects. Two of these, a dry rudder hinge bearing and cracks in the plastic stabilator tips, were minor in nature and had not significantly affected the aircraft's airworthiness. The third defect was a spanwise fatigue crack, approximately 400mm long, in the lower skin of the right stabilator, just aft of the rivetted joint between the skin and the main spar.

The crack had multiple origins and had slowly grown over a significant period of time, as indicated by fretting products and corrosion in association with the crack. As the crack was on the lower surface of the stabilator and the stabilator was mounted low on the tail of the aircraft, it would not have been visible at normal eye-level.

In association with the crack, there was a small crease or 'joggle' in the skin panel. This had permitted abnormal flexing of the skin under in-flight aerodynamic loads, which had caused the initiation and growth of the fatigue crack. Laboratory testing of an identical stabilator in which a cut was made to duplicate the fatigue crack, indicated that the crack would not have significantly affected the structural characteristics of the stabilator. Nevertheless, reverse bending failures of the skin at the ends of the fatigue crack were consistent with rapid vertical movement of the skin panel behind the crack, probably under aerodynamic loads.

It is probable that fatigue cracking had propagated to a critical length at which the skin panel, lacking forward support, had moved sufficiently to disrupt the airflow around the right stabilator. The increased speed of the aircraft as it was descending from 6000 feet and the increased download that would have to be applied to the stabilator to level the aircraft at 5500 feet possibly contributed to the panel movement. As the panel moved up and down it would have caused rapid changes in aircraft pitch forces and pitch control behaviour. Control of the aircraft would have been lost and was probably not recoverable because of the rapidly increasing skin failure. Eventually, the right stabilator partially failed in first upward and then downward bending. The resultant nose-down pitch of the aircraft had caused a sequence of failures and separations of both wings, both stabilators and the rudder and fin.

Because aerodynamic forces are essentially symmetrical, it is improbable that they caused the isolated joggle in the lower right stabilator skin. An inspection of the stabilator and the aircraft maintenance records found no evidence of structural re-work having been carried out on the component since manufacture. Tests conducted
on an identical stabilator in an attempt to induce a joggle similar to that found on VH-WKJ indicated that it was not possible to do so on an assembled component without causing significant damage to other sections of the stabilator.

Sources:

https://www.atsb.gov.au/publications/investigation_reports/1979/aair/197901378/
https://www.atsb.gov.au/media/5226406/197901378.pdf

Accident investigation:
cover
  
Investigating agency: BASI
Status: Investigation completed
Duration:
Download report: Final report


Revision history:

Date/timeContributorUpdates
05-Aug-2018 01:52 Pineapple Added

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