No amount of personality tests could necessarily have prevented co-pilot Andreas Lubitz from crashing a passenger plane in the French Alps, an expert has told Sky News.
Such "incredibly rare" events could not always be predicted, said aviation psychologist Professor Robert Bor.
He suggested in the Lubitz case, the 27-year-old's problems had gone "below the radar" and he was able to conceal his mental illness.
And this appears to be borne out following the revelation by German prosecutors that Lubitz had a sick note for the day of the crash, in which 150 died.
This was found torn-up along with medical documents during searches of Lubitz's flat in Dusseldorf and the home he shared with his parents in the town of Montabaur.
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While it was not routine for pilots to undertake regular psychometric testing, Prof Bor said: "When we are looking at incredibly rare events such as this no amount of that kind of scrutiny will necessarily pick up every single person who is at risk or susceptible."
He also cautioned that tighter rules and tougher tests likely to be introduced in the wake of the disaster, will not necessarily improve safety.
"It will probably improve our sense of safety," he said.
"With very rare events we can't always predict them with 100% accuracy."
Prof Bor told Sky News: "It's incredibly rare and in fact the number of cases in the last 25 years involving commercial jets is probably no more than half a dozen."
The fact he took time off with depression while in pilot training back in 2009 did not necessarily indicate his future behaviour, he added.
"Many people in the general population, pilots as well, occasionally are going to have periods of low moods and anxiety and this may be temporary it may not be something that is lasting," Prof Bor said.
"On the other hand all of us can change.
"Within a matter of days, weeks, there could be a bad trigger event in our life."
A relationship break-up or a dispute at work may "have just tipped him over and turned somebody who might have a depressive illness into somebody who was very despairing and very angry".
"Because that's usually what can lead to or trigger a suicidal act in an individual," he said.
Prof Bor said: "Pilots as an occupational group are very closely scrutinised really from the time they enter flight school to really the last flight that they are on.
"At any moment they are being closely observed, their actions, their behaviour, their communication patterns and so on are being monitored in some kind of way.
"On top of that they undergo rigorous medical tests at least once a year, sometimes twice a year depending on the kind of aircraft they are flying.
"And of course each time they are flying they are being watched by their co-pilots, by the dispatcher when they leave the stand.
"And they are also subject to random drug and alcohol testing as well.
"So the chances of these sort of things happening are very rare.
"And one can only think in this particular case his problem went literally below the radar. He was able to conceal it in some kind of way.
"Unless it was, as we might come to discover, apparent to the airline that he was having some kind of difficulties but for whatever reason they were not implementing the standard cut-offs which are when pilots have significant psychological stress they definitely shouldn't be flying."
