The reasons why an Aero Commander 500S crashed off Horn Island last year remain a mystery after the ATSB was unable to reach any firm conclusions.
VH-WZU was holding over the Torres Strait north of Horn Island on Feburary 24 2011 due to low visibility, when the pilot failed to make an "ops normal" call around 0800.
The aircraft had been on an early-morning cray run from Cairns, flown by the company Chief Pilot (CP) after the rostered pilot was reluctant to make the trip because of the weather forecast.
Despite a search, the aircraft was never seen nor heard of again until the following October, when the wreckage was located on the sea bed 26 km NNW of the airport.
The ATSB investigation report suggests the aircraft was not out of control when it struck the water.
"The ATSB found that the aircraft had not broken up in flight and that it impacted the water at a relatively low speed and a near wings-level attitude, consistent with it being under control at impact," the report states.
"It is likely that the pilot encountered rain and reduced visibility when manoeuvring to commence a visual approach. However, there was insufficient evidence available to determine why the aircraft impacted the water."
Because the CP had to make the run at the last minute, there was evidence to suggest he had had only four hours sleep the night before, leading the ATSB to think fatigue may have been a factor.
"The pilot had less than 4 hours sleep during the night before the flight and the operator did not have any procedures or guidance in place to minimise the fatigue risk associated with early starts.
"In addition, the pilot, who was also the operator’s chief pilot, had either not met the recency requirements or did not have an endorsement to conduct the types of instrument approaches available at Horn Island and several other locations frequently used by the operator."
The full report into the crash is available on the ATSB website here.