The Australian Transport Safety Bureau (ATSB) has determined that a carbon monoxide (CO) leak into the cabin of a de Havilland Beaver floatplane contributed to a fatal crash on the Hawkesbury River, killing all five on board.
Beaver VH-NOO had just taken off from Cottage Point with four passengers on board on New Year's Eve 2017 when it circled back from Cowan Water into the narrow Jerusalem Bay. During an attempt to perform a high bank-angle low-level 180-degree turn, the pilot stalled the aircraft into the water.
In the investigation report released today, the ATSB found that cracks in the exhaust collector ring on the engine allowed CO gas to escape into the cabin via holes in the main firewall where three bolts were missing.
The ATSB believes that the elevated levels of CO in the pilot's blood degraded his judgement and ability to fly the aeroplane. There was no operational reason for the aircraft to have entered Jerusalem Bay.
“The aircraft entered a known confined area, Jerusalem Bay, below the height of the surrounding terrain, when there was no operational need to enter the bay,” ATSB Chief Commissioner Greg Hood remarked.
“Further, the aircraft did not continue to climb despite being in the climb configuration, and a steep turn was performed at low‑level and at a bank angle in excess of what was required.
“The aircraft likely aerodynamically stalled, with insufficient height to recover before colliding with the water.”
Toxicology results identified that the pilot and passengers had higher than normal levels of carboxyhaemoglobin in their blood, almost certainly due to elevated levels of carbon monoxide in the aircraft cabin.
“The pilot would have almost certainly experienced effects such as confusion, visual disturbance and disorientation,” said Hood.
“Consequently, the investigation found that it was likely that this significantly degraded the pilot's ability to safely operate the aircraft.”
The ATSB report focuses on maintenance issues that allowed the CO to leak into the cabin. It found that the gas very likely entered the cabin through holes in the main firewall where bolts were missing from the magneto access panels.
"In addition, the examination also found that the in situ bolts used by the operator’s external maintenance provider to secure the panels were worn, and were a combination of modified AN3-3A bolts and non-specific bolts," the ATSB report states.
It is also likely the pilot's CO levels were already elevated after a 27-minute water taxi with the pilot’s door ajar, before the passengers boarded. As a result, the ATSB believes the pilot would have almost certainly experienced effects such as confusion, visual disturbance and disorientation.
“This investigation reinforces the importance of conducting a thorough inspection of piston-engine exhaust systems and the timely repair or replacement of deteriorated components,”Hood said.
“In combination with maintaining the integrity of the firewall, this decreases the possibility of CO entering the cabin.”
The ATSB has also issued a statement encouraging GA pilots to consider installing CO detectors in aircraft cabins or carry personal detectors. A CO detector was fitted to VH-NOO, but it was not considered reliable.
The full report is on the ATSB website.