Thousands of airline passengers are being flown by depressed and suicidal pilots, a study suggests.
Findings from an anonymous global survey of 1,848 pilots showed more than 12.6% met the criteria for clinical depression.
Of this group, 4.1% - a total of 75 pilots - admitted having suicidal thoughts within the previous two weeks. Ten pilots felt as if they would be "better off dead" almost every day.
Suicide is thought to have been behind Germanwings co-pilot Andreas Lubitz's deliberate decision to crash his plane into the French Alps, killing all 150 passengers and crew on board.
Lubitz, who locked the captain out of the cockpit, had been treated for depression and contacted dozens of doctors in the weeks before the tragedy in March last year.
Renewed concerns about the psychological state of airline pilots were raised by the US study authors, who claimed the problem was hidden behind a "veil of secrecy".
Commercial airline pilots from more than 50 countries took part in the survey between April and December 2015.
A total of 233 pilots (12.6%) provided answers that showed they could clinically be described as depressed. Of these, 75 reported experiencing suicidal thoughts in the past two weeks.
Of 1,430 pilots who had flown within the previous seven days, 193 (13.5%) met the criteria for depression.
Each of these pilots routinely sits at the controls of aircraft that may be carrying hundreds of passengers.
Lead researcher Dr Joseph Allen, from the Harvard T.H. Chan School of Public Health in Boston, said: "We found that many pilots currently flying are managing depressive symptoms, and it may be that they are not seeking treatment due to the fear of negative career impacts.
"There is a veil of secrecy around mental health issues in the cockpit. By using an anonymous survey, we were able to guard against people's fears of reporting due to stigma and job discrimination."
Of the male pilots surveyed, 10 said that "nearly every day" they had thoughts of self-harm or felt they would be "better off dead".
The survey indicated male pilots were more likely to experience daily problems than their female colleagues.
Twenty seven men and four women felt depressed on a daily basis. In addition 34 men had feelings of failure and trouble concentrating nearly every day.
The authors concluded: "Hundreds of pilots currently flying are managing depressive symptoms perhaps without the possibility of treatment due to the fear of negative career impacts..
"Although results have limited generalisability, there are a significant number of active pilots suffering from depressive symptoms. We recommend airline organisations increase support for preventative mental health treatment."
A total of of 3,485 pilots took part in the survey which addressed a range of general health topics, but not all completed it. Of the total, 1,848 answered questions about mental health.
Sixty five British pilots were among the participants. The largest numbers were from the US (1,586), Canada (438) and Australia (387).
The study is the first to investigate the mental health of pilots without relying on data from aircraft accident investigations, regulated health check-ups or identifiable self-reports.
Pilots providing information through such official channels were not likely to speak openly about suffering from depression, said the researchers.
Dr Rob Hunter, head of flight safety at the British Airline Pilots' Association (Balpa), said: "We welcome any research into pilot mental health and will study these results in detail.
"In general, pilots have very good physical and mental health. But, as in all walks of life, some pilots suffer from depression and other mental health issues and the problem applies across many, if not all, safety-critical occupations.
"The lessons we need to learn from Germanwings, and from this study, are that everyone needs to work to remove the stigma attached to mental health, and that no one should have any fear from discussing it or reporting it. And the official Germanwings accident investigation recommendation that proper insurances must be in place for the small number of pilots who lose their licenses as a result must be enforced - currently it is not."
Professor Sir Simon Wessely, president of the Royal College of Psychiatrists, said: "In the UK, those suffering from acute depression are not allowed to fly a plane - largely because of the impairments in matters such as memory and concentration rather than suicide.
"When pilots recover from depression - and people can recover - they are fully able to return to work and should be allowed to do so whilst continuing to be monitored. Incidents such as the Germanwings air crash are extremely rare and it is still unclear if this tragedy was related to depression.
"We do know that in this country doctors are able to breach medical confidentiality if they feel there is a serious risk to others, which was not the case in Germany."