Pilots killed in an army helicopter crash in Queensland were wearing night vision helmets known to cause fatigue, an inquiry has been told.
Pilots involved in the MRH-90 Taipan helicopter incident off Lindeman Island in July 2023 that killed four servicemen used a night vision device called TopOwl which offered enhanced sight capabilities.
Expert witness Maria Gavrilescu on Wednesday told an independent inquiry into the crash the device could become misaligned from manufacturing errors, rough handling, an accident or hot temperatures.
She said misalignment made the eyes do unnatural movements which could result in vision fatigue.
The longer minor effects of misalignment go undetected, the greater the risk of a pilot seeing double, the hearing in Sydney heard.
"Pilots might experience headaches, red eyes or tired eyes, but maybe they don't feel like reporting that because they expect this sort of condition," Dr Gavrilsecu said.
She did a test with pilots in Darwin a decade ago, with army personnel asked to fill out a questionnaire to identify if they felt fatigued after using the night vision helmet.
None of the pilots said they had symptoms but Dr Gavrilescu said she saw otherwise.
"Despite having observed obvious signs of visual fatigue, we were getting responses that there were no symptoms at all from their point of view," she said.
Dr Gavrilescu said pilots might become used to the level of fatigue they are experiencing and as a result could feel "normal".
When a simulation was done with civilians on the impacts of misalignment and eye fatigue, the group made more errors flying a helicopter and took longer to respond.
The study noted significant changes in physiological parameters associated with fatigue such as pupil diameter, heart rate variability and skin temperatures.
"In this obviously limited simulation ... we proved that misalignment at the level that is not perceived does have a negative impact on task performance," Dr Gavrilescu said.
Captain Danniel Lyon, Lieutenant Maxwell Nugent, Warrant Officer Class Two Joseph Laycock and Corporal Alexander Naggs died in the July 2023 crash.
The inquiry has previously heard fatigue was a major concern for air crews in the days and months before the fatal incident.
A sleep scientist gave evidence on Tuesday suggesting defence pilots often struggle to identify when they are fatigued but opt to push on regardless through a "get the job done" mentality.
The inquiry continues.
