The Defence department is being accused of being trying to blame an Army pilot’s “human error” for the deaths of four soldiers in a Taipan helicopter crash off Queensland three years ago.

A senate inquiry on Monday again examined the handling of the fatal accident involving an MRH-90 helicopter which came down in poor weather over water during a 2023 military exercise in the Whitsundays.

Captain Danniel Lyon and co-pilot Lieutenant Maxwell Nugent were killed along with Warrant Officer Class Two Joseph Laycock and Corporal Alexander Naggs in an incident which prompted the Army to retire its Taipan helicopter fleet early.

In a written submission to the inquiry Sue Lyon, the stepmother of Danniel Lyon, claimed: “there appears to have been a determination from the outset to attribute responsibility for this accident to Danniel through a finding of human error”.

“Danniel was my stepson for the last 30 years of his life. From the time of Danniel’s crash, the Department of Defence has shown little empathy or consideration towards the families of the pilots involved.”

“As a grieving stepmother, I have personally witnessed and experienced behaviour from Defence personnel that I consider unacceptable, insensitive and dismissive,” Mrs Lyon says in her submission to the “MRH-90 Taipan Helicopter incident” inquiry.

Mrs Lyon has hit out at the “lack of transparency, accountability and honesty” during the investigation process and accused the Army and Defence investigators of being determined to blame “human error” by her stepson Danniel for the fatal accident.

“Rather than providing families with confidence that all relevant issues have been thoroughly and independently examined, Defence has left me with serious concerns about the way this matter has been handled.”

“Having watched the Senate Inquiry proceedings, those concerns have only been reinforced. I have no confidence in the transparency, honesty or reliability of the investigative processes that have been followed,” Mrs Lyon writes.

“From my perspective, there appears to have been a determination from the outset to attribute responsibility for this accident to Danniel through a finding of human error and this has been their focus from the very beginning of the inquiry.”

In June former Army test pilot Ian Wilson told the Senate committee that an investigation into the Taipan crash was “corrupted” and there was a well-known conflict of interest inside the inquiry team.

Senior military officers appeared before the senate inquiry on Monday where they denied the Defence department covered-up parts of the investigation into the 2023 fatal Taipan helicopter crash.

Air Vice-Marshal Tim Innes, representing the Defence Aviation Authority, told the committee there was a full aviation investigation safety report publicly available.

“I’m not aware of any cover-up of any sort,” he told the public hearing in Canberra.

The Defence Flight Safety Bureau’s investigation found the primary cause of the accident was “spatial disorientation”, which occurs when a pilot misperceives the position of their aircraft in relation to the surrounding environment.

Greens Senator David Shoebridge grilled the officials about the “significant fatigue” the pilots were experiencing and how Army Aviation managed the safety issue.

In response, Commander Army Aviation Command Major-General David Hafner said the ADF was constantly looking at the schedule of activities to make sure personnel were able to take time off.

“Really looking through a lens of cumulative fatigue and making sure that personnel are afforded activities through the annual schedule of events to take leave as part of fatigue management as well,” he said.

The parliamentary inquiry’s report is due in early September.

Family members of the four soldiers killed in the crash last week lodged a class action against the Commonwealth and helicopter manufacturer Airbus, with the Defence department saying it is currently considering the claim.