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Carbon monoxide exposure likely significantly degraded the ability of the pilot of a Beaver floatplane to safely operate the aircraft before it collided with water in Jerusalem Bay on the Hawkesbury north of Sydney in December 2017, fatally injuring all six people on board, the final report from Australian Transport Safety Bureau investigation into the accident has found.
As advised with the ATSB's publication of two safety advisory notices arising from the investigation in July 2020, several pre-existing cracks in the aircraft engine's exhaust collector ring very likely released exhaust gas into the engine/accessory bay, which then very likely entered the cabin through holes in the main firewall where three bolts were missing. The pilot also undertook a 27-minute taxi - to free the dock for another arriving and departing aircraft - before the passengers were boarded which likely exacerbated the pilot's elevated carboxyhaemoglobin level.
"Shortly after take-off for the return flight from Cottage Bay to Sydney Harbour's Rose Bay, the aircraft conducted a 270 degree right turn in Cowan Water and then entered Jerusalem Bay, below the height of the surrounding terrain," said ATSB Chief Commissioner Greg Hood.
"It stopped climbing, continued along the bay and then made a very steep right turn. The aircraft's nose then dropped and the aircraft collided with the water."
From detailed analysis of photos recovered from the camera of one of the passengers on board the aircraft, as well as witness accounts, the ATSB was able to establish the accident sequence of events, and found that some of the circumstances of the flight were unexpected, given the nature of the flight and the pilot's significant level of experience.
"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," Mr Hood noted.
"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. This was 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 Mr Hood.
"Consequently, the investigation found that it was likely that this significantly degraded the pilot's ability to safely operate the aircraft."
Mr Hood noted that at the time of releasing an interim report into the accident in December 2018, investigators were considering the possibility of pilot incapacitation due to the series of unexpected, and up to that point, unexplained events during the flight.
The ATSB engaged an aviation medical specialist, who, working with other medical specialists closely examined all aspects of the pilot's medical history including electrocardiogram traces and medical reports, with no pre-existing medical conditions evident.
The ATSB was of the understanding that testing for carbon monoxide exposure on the aircraft's occupants was conducted as part of initial toxicology examinations. However, in late 2019, the ATSB's aviation medical specialist recommended that this be confirmed, Mr Hood noted.
"Subsequent toxicological testing indicated that the pilot and all passengers had elevated levels of carboxyhaemoglobin."
Mr Hood thanked the families and colleagues of those lost in the accident for their understanding and support while the ATSB progressed this comprehensive investigation, culminating in the public release today of the 144-page final report.
"This investigation report release will bring the families of those lost in this tragic accident a greater sense of understanding of the circumstances of the loss of their loved ones."
Mr Hood said the investigation would have been considerably aided if the aircraft had been fitted with an on-board recording device. The accident Beaver aircraft's maximum take-off weight was less than 5,700 kilograms and so was below the regulatory threshold requiring the fitment of a flight recording device (such as a cockpit voice recorder and/or a flight data recorder).
"Recording devices have long been recognised as an invaluable tool for investigators in identifying the factors behind an accident, and their contribution to aviation safety is irrefutable," said Mr Hood.
Historically, due to cost considerations and technological limitations, the fitment of recording devices has only been mandated for larger aircraft.
"However, advancements in technology have made self-contained image, audio and flight data recording systems far more cost-effective and accessible to all aircraft.
"That is why we are today formally recommending that the International Civil Aviation Organization and the Civil Aviation Safety Authority consider mandating the fitment of lightweight recording devices to smaller passenger-carrying aircraft.
"There are a large number of commercial passenger-carrying operations conducted in aircraft that do not require the fitment of flight recorders. So there remains the potential for unresolved investigations into accidents involving smaller passenger carrying aircraft, which poses a significant limitation to bringing about safety improvements in this sector of aviation."
Mr Hood said the circumstances of the Jerusalem Bay accident highlight the insidious danger CO exposure poses to aircraft occupants.
"This investigation reinforces the importance of conducting a thorough inspection of piston-engine exhaust systems and the timely repair or replacement of deteriorated components," he said.
"In combination with maintaining the integrity of the firewall, this decreases the possibility of CO entering the cabin."
Further, the investigation also highlights that the use of an attention attracting CO detector provides pilots with the best opportunity to detect CO exposure before it adversely affects their ability to control the aircraft or become incapacitated.
"The ATSB strongly encourages operators and owners of piston-engine aircraft to install a CO detector with an active warning to alert pilots to the presence of elevated levels of CO in the cabin. Where one is not fitted, pilots are encouraged to carry a personal CO detector."
The ATSB has recommended that the Civil Aviation Safety Authority consider mandating the carriage of active warning CO detectors in piston-engine aircraft, particularly passenger carrying aircraft.
You can find here the final report: AO-2017-118 Collision with water involving de Havilland Canada DHC-2, VH-NOO Jerusalem Bay, NSW, 31 December 2017
