The ATSB advises DHC-1 Chipmunk maintainers and owners that crashworthiness could be significantly compromised if incorrect specification rivets are used.
The safety advisory notice has been issued as part of the ATSB's continuing investigation into a fatal accident involving a Chipmunk at Jandakot Airport, WA, on 26 April 2024. As detailed in a preliminary report in July, shortly after take-off the aircraft was observed turning to the left at a low height before colliding with the ground, fatally injuring the pilot.
During the ongoing investigation the ATSB has identified that non-specification rivets had been installed on the aircraft, attaching the upper structure between the front and rear cockpits to the fuselage. This structure provides the attach point for the front cockpit shoulder harness.
"Two sets of rivets - 12 in total - attaching the structure to the fuselage sheared during the accident," Director 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."
This meant the rivets did not meet the specification of the relevant modification.
"Testing indicated a significant reduction in strength, estimated to be about one-third of the specification strength," Dr Godley said.
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 ATSB's safety advisory notice highlights the importance of maintaining aircraft crashworthiness design elements, including its restraint system, to keep the occupant within an aircraft's 'living space' during an accident sequence," Dr Godley said.
"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."
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," Dr Godley concluded.
The investigation is continuing, and the ATSB will issue a final report, which will detail findings and analysis, at the conclusion of the investigation.
You can find here the Safety Advisory Notice: DHC-1 Chipmunks may have incorrect rivets fitted
