Crash investigators say that the Germanwings Airbus was probably crashed deliberately by co-pilot Andreas Lubitz. The way pilots are screened is now under intense scrutiny.

Lufthansa, the parent company of Germanwings, has said that there had previously been nothing to suggest that Lubitz was mentally unstable.

The pilot’s training in 2009 had been briefly interrupted, but was resumed after “his suitability as a candidate was re-established”. Carsten Spohr, the chief executive, said he was not allowed to reveal the reason for the interruption. When Lubitz returned, “his performance was without criticism” and “nothing was striking” about his behaviour.

The inexplicable nature of the pilot’s actions has put the focus on to how pilots are psychologically assessed. Most passengers probably assume that the person flying their plane has gone through rigorous mental assessments to check they have the right character and temperament to be responsible for hundreds of lives. But is that true?

Spohr seemed to admit that no special psychological test was mandatory across Europe.

Most pilots in the UK start at a flight training school, says a spokesman for the Civil Aviation Authority, the UK regulator. But these schools do not screen out candidates for psychological reasons. They are purely looking at someone’s ability to fly.

Once the pilot gets a job with an airline they will have a medical before they can fly passengers. Captain Mike Vivian, a former head of Flight Operations at the CAA, said this is “very intensive”.

This process does involve an element of psychological screening. Candidates are asked about their background, including their interests and family relationships, as well as whether they had suffered depression or had suicidal feelings, Vivian suggests.

But the screening processes appears to rely on the candidate’s answers and the examiner’s judgement of their answers. “There is an element of trust in it. We have to trust,” says Vivian.

This medical is carried out by a specially trained aviation medical examiner. It is repeated every year or six months depending on age.

But the CAA has said that all screening procedures should be reviewed. “Following the details that have emerged regarding the tragic Germanwings incident, we are co-ordinating closely with colleagues at the European Aviation Safety Agency (EASA) and have contacted all UK operators to require them to review all relevant procedures.”

Most of the medical relates to the pilot’s physiology – height, weight, blood and urine. The mental health aspect is small and general in tone. Just six lines out of a three and a half page guidance document set out what the “psychiatric” aspect should cover:

“During assessment of the applicant’s history, the doctor should make a general enquiry about mental health which may include mood, sleep and alcohol use. The doctor should observe the applicant during the process of the examination and assess the mental state of the applicant under the broad headings of appearance/ speech/ mood/ thinking/ perception/ cognition/ insight. The doctor should also be looking out for any signs of alcohol or drug misuse.”

Both BA and Virgin Atlantic declined to comment on how they psychologically assess pilots.

Tristan Loraine, a former BA captain who retired in 2006 after 20 years flying airliners says he had no psychological profiling during his career. “The medical was having an ECG, peeing in a bottle, having a blood test and all that.” It did not touch on mental health, he says.

Most of the testing related to technical ability. He had to do a simulator test twice a year and a Route Check in which he flew passengers as normal but was observed by a captain. At BA, unlike his previous airlines, he had to sit down with two human resources staff. But again it was hardly rigorous. “The HR people said tell us about your life. They were not giving you a proper neuropsychological test telling you if there were any [psychological] trends in there.”

The British Airline Pilots Association rejects this. Dr Rob Hunter, head of flight safety at Balpa, said in a statement: “The annual medical certificate application includes a legal requirement for a pilot to declare if they have had any psychological problems and the examiner is expected to pursue any signs of mental health issues apparent during the examination. Pilots operate a strict open culture of reporting any concerns whether related to technical matters, safety issues or any possible medical or mental issues with colleagues.”

Vivian said a major check on possible psychological vulnerability was “peer pressure”. Colleagues look out for signs that a fellow pilot is troubled over a bereavement or a relationship breakdown, Vivian said. “If I was really concerned I would either replace them or mention it to the management later on.”

Loraine, who campaigns on the issue of pilot welfare, says that while pilots and cabin staff are expected to report personal problems, often that didn’t happen. “I’ve flown with people who were in no state to be flying. They had domestic problems or financial problems and would put more pressure on me. You land and say ‘maybe you should take a few days off’. You remember those people.”

Jim McAuslan, Balpa general secretary, said that there should be no kneejerk reaction on more rigorous screening. But he added that while planes and air traffic control had been made safer “the point of weakness” might now be the pilots themselves.

Pilots deliberately crashing a plane can be hard to prove. Among the cases that are strongly believed to have been deliberate are the EgyptAir crash in October 1999 and a flight between Mozambique and Angola in November 2013.

A JetBlue flight bound for Las Vegas had to make an emergency landing in Texas in March 2012. The co-pilot locked the captain out of the cockpit as the latter was behaving erratically. The captain then had to be wrestled to the ground by passengers.

Clinical psychologist Prof Robert Bor, author of Aviation Mental Health, says incidents like that can never be prevented entirely. He reviewed the regulations on behalf of the Federal Aviation Administration after that incident. “We concluded nothing could have been done that would have prevented that incident from happening.”

Incidents of pilots trying to crash the plane are “utterly rare” happening every five to 10 years. Pilots are being observed all the time especially by their colleagues. “Every time they enter the flight deck they are being scrutinised because someone is sitting next to them.”

Prof Bor says that even psychometric testing will not tell you about the person who wakes up different one day. And it’s impossible to prevent 100% of cases where someone wants to abuse their position of authority. He points to the case of Harold Shipman, an apparently “avuncular” local GP who killed many of his patients.

Since then the medical profession has brought in more screening of doctors. But it’s hard to say whether these would have picked up Shipman’s evil intentions, he says.

Psychological screening might help, Loraine says. But it won’t solve everything – people’s mental health can change over their life. In the case of Lubitz there were no warning signs, according to Lufthansa.

Ultimately, Vivian said, it isn’t obvious how airlines would achieve perfect screening. “I don’t know about any tests whereby you can investigate a person’s state of mind comprehensively and objectively.”

The answer is to prevent a pilot ever being alone in the cockpit, Loraine says. “The most sensible solution is to put a cabin crew member in the cockpit.” This is the approach that the US Federal Aviation Authority has taken. Europe will now consider whether it needs to do the same.