In March 2015, Germanwings co-pilot Andreas Lubitz deliberately crashed a passenger jet into the French Alps, killing all 150 people on board.

The investigation that followed revealed the 27-year-old had been undergoing treatment for clinical depression. A relationship breakdown was the final straw.

When pilot Patrick Sonderheimer got up to use the toilet, Mr Lubitz locked the door and turned the Airbus A320 toward the mountains, accelerating until he hit the rocks.

Mr Sonderheimer could be heard on the black boxes screaming at Mr Lubitz to "open the damn door" before trying to beat his way through with an axe.

The tragedy prompted Harvard University's school of public health to launch a study into the sensitive subject of airline pilot mental health.

Researchers surveyed 3485 professional pilots, and the results are shocking.

Nearly 14 percent were considered depressed, while four per cent admitted to flying during a period when they thought they would be "better off dead".

The paper concluded hundreds of pilots currently flying are managing depressive symptoms without treatment due to the fear of negative career impacts.

Karen Phillip is an Australian counsellor who has worked with a number of pilots.

She confirmed to news.com.au there is a stigma around mental health medication and treatment, and that pilots fear it will be viewed detrimentally.

"They think it means they're unable to cope with the pressure of their job. That's not the truth at all, they could be having relationship problems, or problems with their children," Dr Phillip said.

"Their work hours are crazy, and they're constantly sleep-deprived. They often have relationship difficulties simply because they're not around."

She said the best way to manage depression is with regular, confidential check-ups.

"Our society is very reactive. We wait for someone to go down, then get help. With a pilot that's the worst thing to do, especially with hundreds of lives in their hands."

On the face of things, Mr Lubitz was a fit, happy and healthy professional.

He enjoyed long-distance running, and competed regularly. He lived with his parents in a small town in Germany, and neighbours described him as "quiet" but "fun".

According to the BBC, he was accepted as a trainee for Lufthansa almost immediately after finishing high school, but his training was interrupted by psychiatric problems.

German newspaper Bild reported he had a serious depressive episode at the end of his training in 2009, and went on to receive treatment for a year and a half.

He moved across to Lufthansa's budget arm, Germanwings, and worked as a flight attendant before taking a job as a co-pilot.

Mr Lubitz passed a medical assessment with 100 percent just two months earlier, but investigators found torn-up medical certificates at his home - including one declaring him unfit for work on the date of the tragedy.

However, while depression can be managed, Dr Phillip said under no circumstances should a pilot take the controls in a suicidal state.

"Suicidal thoughts happen only because the person is so deep in their dark hole, so lost, they have no way of knowing how to get out of it," she told news.com.au.

"The first step is to recognise it, then go to a good therapist, who can help set steps to move them in a good direction. It's treatable, but no - they shouldn't fly while they're working through that phase."

Australia's Civil Aviation Safety Authority said mental health is a high priority, adding they've just launched a series of new web pages about health.

"All pilots have to undergo regular medical checks - more often for commercial pilots and also more often for older private pilots," a spokesman told news.com.au.

"Part of this medical assessment are psychological questions by the doctor."

The Australian Federation of Air Pilots offers its members a substantial list of services, including psychological counselling and a welfare representative program.

"We just see this as good practice as a champion for pilots and the industry," president David Booth told news.com.au.

"Overall, we prioritise the health and wellbeing of our pilots, and believe the best path to good mental health is one that provided non-stigmatised, accessible and professional support whenever and wherever our members might need it."

However, Dr Phillip said in her experience, these services aren't always used.

She said the problem is two sided. First, pilots are afraid of being stigmatised and damaging their careers. Second, airlines are frightened to say "our pilots need help" due to the risk of negative public perception.

In a similar vein, the Harvard study expressed concerns over under reporting of "adverse" symptoms and incomplete records.

Researchers concluded that although current policies are improving screening and evaluation, the industry should increase support for preventive treatment.