The final ATSB investigation into the crash of a Beaver floatplane that killed six has reaffirmed carbon monoxide exposure was the predominate cause of the accident.
The exhaustive 144-page report significantly reveals several pre-existing cracks in the aircraft engine’s exhaust collector ring likely released exhaust gas that eventually found its way into the cabin.
This exposure “significantly degraded” the ability of pilot Gareth Morgan, 44, to operate the De Havilland Canada DHC-2 Beaver that collided into water in Jerusalem Bay on the Hawkesbury north of Sydney in December 2017.
Others killed included British businessman Richard Cousins, 58, his 48-year-old fiancée, magazine editor Emma Bowden, her 11-year-old daughter Heather, and his sons, Edward, 23, and William, 25.
The findings, first outlined as part of an earlier ATSB release in July 2020, revealed 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 established 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,” Hood said.
“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 Hood. “Consequently, the investigation found that it was likely that this significantly degraded the pilot’s ability to safely operate the aircraft.”
The ATSB 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 then 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 said it 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.
Significantly, the ATSB also 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 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.
“This investigation reinforces the importance of conducting a thorough inspection of piston-engine exhaust systems and the timely repair or replacement of deteriorated components.
“In combination with maintaining the integrity of the firewall, this decreases the possibility of CO entering the cabin.”
The ATSB said 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 CASA consider mandating the carriage of active warning CO detectors in piston-engine aircraft, particularly passenger carrying aircraft.