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A DHC-1 Chipmunk light aircraft that crashed in Western Australia earlier this year may have been compromised by weak rivets, the Australian Transport Safety Bureau (ATSB) says.
The transport safety watchdog has issued a safety advisory noting that “non-specification rivets” had been installed on the plane, which went down at Jandakot Airport in April, killing its pilot. The ATSB previously said the plane’s engine cowl door may not have been properly fastened.
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The rivets in question attached the upper structure between the front and rear cockpits to the fuselage, providing the attach point for the front cockpit shoulder harness, the ATSB said.
“Two sets of rivets – 12 in total – attaching the structure to the fuselage sheared during the accident,” ATSB director of transport safety Dr Stuart Godley said.
“The ATSB found that the rear row of rivets in each set – that is, three of the six rivets on each side – were pure or near-pure aluminium.
“Testing indicated a significant reduction in strength, estimated to be about one-third of the specification strength.”
The rivets would have been originally replaced during the embodiment of modification H.268, issued in 1966 by the aircraft’s type certificate holder at the time, Hawker Siddeley, to replace alloy structure elements with steel.
The aircraft may have modified in the 1960s, however, the ATSB has not determined precisely when, or if, the rivets had been replaced since the modification.
It is important to note that while the crashworthiness of the aircraft had been compromised by the presence of non-specification rivets, the ATSB has yet to establish whether it contributed to the outcome of this accident.
However, as there is the potential for other Chipmunks to have incorrect rivets installed in this location, the ATSB determined it was important to bring the issue to the wider attention of Chipmunk operators.
“The use of upper torso restraints such as a shoulder harness can prevent the occupant from striking the surrounding structure during an accident. It is crucial that all components forming part of that restraint system and the structures to which they are attached are maintained to defined specifications,” Godley said.
“The notice therefore advises DHC-1 Chipmunk maintainers and owners to be aware that fitment of incorrect specification rivets where the upper structure between the front and rear cockpits attaches to the gussets on either side could significantly compromise the crashworthiness of the aircraft.
“Those conducting work on aircraft must ensure modifications are carried out to the required specification, or during maintenance returned to that specification.”
The notice therefore advises DHC-1 Chipmunk maintainers and owners to be aware that fitment of incorrect specification rivets where the upper structure between the front and rear cockpits attaches to the gussets on either side could significantly compromise the crashworthiness of the aircraft.
“Those conducting work on aircraft must ensure modifications are carried out to the required specification, or during maintenance returned to that specification,” said Godley.
The investigation is continuing and the ATSB will issue a final report, which will detail findings and analysis, at the conclusion of the investigation.
