An Australian Transport Safety Bureau preliminary report has detailed evidence established so far in the ongoing investigation of a fatal accident involving a twin turboprop Commander aircraft which was in transit to conduct fire surveillance in Queensland's remote northwest late last year.
The aircraft's three crew were fatally injured when it collided with terrain near Cloncurry on 4 November 2023.
"Today's preliminary report is released to provide timely, factual information," ATSB Chief Commissioner Angus Mitchell said.
"The evidence collected to date and areas of ongoing investigation detailed in the report will help us develop analysis and findings, which will be detailed in a final report at the conclusion of the investigation."
The preliminary report details that the aircraft, operated by AGAIR under contract with Queensland Fire and Emergency Services, had taken off from Toowoomba to photograph fire zones north of Mount Isa. On board were a pilot and two camera operators, employed by the operator.
About 20 minutes after reaching the planned cruise altitude of 28,000 ft, the aircraft descended to 15,000 ft, remaining there for about 6 minutes, before returning to 28,000 ft.
After another 25 minutes, the air traffic controller requested the pilot change radio frequency.
"The pilot acknowledged this request, but the controller was then unable to establish two-way communications with them for over an hour," Mr Mitchell said.
Once communications were eventually re-established, the pilot advised the controller they were to conduct airwork in an area near Mount Gordon. When they were provided clearance for this, the pilot, seeming uncertain, repeated the clearance four times over a four-minute period.
"Although a formal speech analysis has not been undertaken at this stage, radio recordings during this period indicate that the pilot's rate and volume of speech had substantially lowered from earlier communications and was worsening," Mr Mitchell noted.
Flight data indicates approximately 20 minutes after the pilot's last transmission, the aircraft's groundspeed decreased, before it departed controlled flight, and began to descend rapidly.
The aircraft impacted terrain in flat, open bushland, and it was destroyed by a significant post-impact fire.
"During their on-site examinations, investigators were able to account for all major aircraft components, and determined that both engines and propellers had indications that the engines were running at impact. However, it was not possible to determine the operability of the aircraft's pressurisation and oxygen systems."
Along with on-site work, ATSB investigators have also interviewed relevant parties, collected radio communication recordings, aircraft tracking and navigational application data, and gathered documentation relating to the aircraft, pilot, crew and operator.
"As the investigation proceeds, it will include further analysis of the pilot's speech during radio communications, including an examination of hypoxia indicators," Mr Mitchell said.
"Investigators will also analyse weather information, maintenance records, operational procedures, flight data, and pilot and crew training and medical records."
A final report will be released at the conclusion of the investigation.
"If at any time should a critical safety issue identified during the course of the investigation, the ATSB will immediately notify relevant parties so safety action can be taken," Mr Mitchell concluded.
You can find here the preliminary report: Collision with terrain involving Gulfstream 695A JetProp Commander, VH-HPY, 30 NM south-east of Cloncurry, Queensland on 4 November 2023.
