The Australian Transport Safety Bureau is advising owners, operators, pilots and maintainers of piston-engine aircraft to take measures to detect the presence of, and prevent the entry of, carbon monoxide in aircraft cabins.
The national transport safety investigator is today issuing two Safety Advisory Notices and releasing an update to its on-going investigation into the collision with water of a DHC-2 Beaver floatplane at Jerusalem Bay, on the Hawkesbury River north of Sydney, on 31 December 2017, in which the pilot and five passengers lost their lives.
"During the draft review process for the investigation's final report, the aviation medical specialist engaged by the ATSB recommended that carbon monoxide toxicology testing be undertaken on blood samples of the aircraft occupants," said ATSB Chief Commissioner Greg Hood.
The results of that testing, provided to the ATSB in March 2020, indicated that the pilot and two of the passengers, whose post-mortem examinations established received fatal injuries sustained as a result of the impact sequence, had elevated levels of carbon monoxide.
Accident aircraft's engine exhaust crack
Note pre-existing crack spread and widened during the impact. Source: ATSB
Subsequent to receiving those results the ATSB consulted widely with medical experts to fully understand those results.
"From that consultation with medical experts, and research into the effects of carbon monoxide on aircraft operations, the ATSB considers the levels of carbon monoxide were likely to have adversely affected the pilot's ability to control the aircraft," Mr Hood said.
The ATSB then re-examined the accident aircraft and undertook testing on an exemplar Beaver aircraft to replicate the potential source of carbon monoxide and ingress into the aircraft cabin.
"Having discounted other potential sources of carbon monoxide exposure, the ATSB considers it likely that the pilot and passengers were exposed to carbon monoxide inside the aircraft cabin," Mr Hood said.
"The ATSB found pre‑existing cracking of the engine exhaust collector-ring, which could lead to exhaust leakage into the engine bay. Further, the ATSB found a breach in the firewall from missing bolts used to secure magneto access panels in the firewall under the instrument panel in the cabin. Any breach in the firewall can allow the ingress of gases from the engine bay into the cabin."
The aircraft had departed from Cottage Point and taxied for about seven minutes before taking off on its planned return trip to Rose Bay. Shortly after take-off, the aircraft deviated from the operator's standard flight path, stopped climbing, and entered the confines of Jerusalem Bay below the height of surrounding terrain. The aircraft then continued along the bay, made a very steep right turn, and collided with the water.
The confirmation that there were elevated levels of carbon monoxide in the pilot's blood, and the potential for engine exhaust gases to exit the exhaust system in the engine bay and enter the aircraft's cabin has prompted the ATSB to issue the two Safety Advisory Notices to industry.
"This investigation is on-going, and our final report, which will contain specific findings, is anticipated to be released in coming months, so we are limited in discussing specific details. However, if at any time during an investigation, should the ATSB identify issues that are critical to safety, we will immediately notify relevant stakeholders so proactive safety action can be taken to help prevent similar occurrences," said Mr Hood.
Accident aircraft's engine firewall showing the location of missing bolts
Source: ATSB
"That is why today the ATSB is publishing two Safety Advisory Notices focused on the prevention and detection of carbon monoxide in piston-engine aircraft."
Although the accident aircraft involved a DHC-2 Beaver, these issues are relevant to piston-engine aircraft in general, Mr Hood noted.
"The ATSB is reminding aircraft maintainers that the primary mechanism for the prevention of carbon monoxide exposure to aircraft occupants is to carry out regular inspections of aircraft exhaust systems to identify and repair holes and cracks, and to detect breaches in the firewall," he said.
The ATSB is also highlighting the limitations of disposable carbon monoxide chemical spot detectors, as used commonly in general aviation, and was fitted to the accident aircraft.
Spot detectors have a limited shelf-life, can be affected by factors such as direct sunlight and cleaning chemicals, and are passive, relying on pilots to regularly monitor them.
"In contrast, electronic active carbon monoxide detectors are designed to attract the pilot's attention through auditory and/or visual alerts when carbon monoxide levels are elevated," Mr Hood said.
"These detectors are now inexpensive and widely available. Had there been an alert of the presence of carbon monoxide, the pilot would have been able to take measures to reduce the risk to those on board."
Mr Hood noted that the ATSB has kept the Civil Aviation Safety Authority (CASA) informed as to the investigation's progress. To date, CASA has contacted all operators and owners of the 20 DHC-2 Beaver aircraft registered in Australia to emphasise the importance of inspections of the exhaust system, to confirm that the scheduled inspections were being conducted, and to seek information pertaining to the number of exhaust ring segments requiring repair or replacement.
In addition, CASA has published an Airworthiness Bulletin today to highlight the risks and dangers of carbon monoxide poisoning to all piston-engine owners, operators and aircraft engineers, and advising of the fitment of active carbon monoxide detectors.
You can find here the investigation update AO-2017-118
You can find here the Safety Advisory Notices AO-2017-118-SAN-001: Inspection of exhaust systems and engine firewalls
You can find here the Safety Advisory Notices AO-2017-118-SAN-002: Are you protected from carbon monoxide poisoning?
