The apparent ignorance of Lubitz's employers of his health situation points to the difficulty that airlines face keeping tabs on their pilots. And even if an airline has been pre-authorised by a pilot to have access to their medical records, this could be circumvented.

One pilot who has worked for Cathay Pacific for more than a decade said that if a pilot wanted to keep medical treatment secret from Hong Kong's flagship carrier, they only needed to consult a different private doctor without the airline's knowledge.

Nevertheless, the pilot - speaking to the Post on condition of anonymity - said he believed the Hong Kong Civil Aviation Department's mandatory annual body check, conducted by aviation medical specialists, served as an adequate balance.

In Lubitz's case, he simply tore up doctors' notes that declared him unfit to work, including on the day of the crash, suggesting he sought to hide his diagnosis from his employer and colleagues.

"He seemed completely normal," said Frank Woiton, a Germanwings captain who flew with Lubitz in recent weeks.

Woiton said Lubitz told him he was happy to finally fly for the group, and that he wanted to pilot long-haul routes and become a captain on the Boeing 747 or the Airbus A380, the two biggest commercial aircraft and both part of parent company Deutsche Lufthansa's long-haul fleet.

The investigation of the worst disaster in German aviation history points to a potentially disturbed junior pilot whose dream of one day flying the most imposing planes may have been unraveling as he battled a mental condition. Lubitz also suffered from a detached retina, blurring his vision - potentially a career-ending diagnosis for a pilot, Bild Zeitung said.

But investigators have discovered few obvious signs that Lubitz had pre-planned a suicide or a deliberate crashing of a plane, and one working theory is that he altered Flight 9525's course on a deadly whim.

When the flight's more experienced pilot left the cockpit to go to the bathroom, Lubitz may have seized the opportunity to crash the plane.

Lubitz, who lived in the same town as Woiton, was being treated by several neurologists and psychiatrists for an unspecified psychosomatic illness, according to a person close to the probe. The authorities haven't yet recovered the data recorder that may conclusively depict Lubitz's final actions before the fatal crash.

Lufthansa, the parent of Germanwings, leaves aspiring pilots under no illusions about the demands of a career commanding the skies.

"Flying can be a tough, rigorous job, demanding mental resilience and peak physical performance," the company tells would-be pilots on its website.

"Pilots are subject to constant stress levels; I think that changes personalities," said Bryan Ware, chief technology officer at Haystax Technology Inc in Los Angeles, which helps companies rank employees by the likelihood that they may pose a threat to the organisation.

"Someone who has significant financial, or family, or psychological issues will likely not be able to handle that kind of stress in the same way that someone who doesn't."

In the case of Cathay Pacific, according to the pilot who spoke to the Post, the airline did not require pilots to give explanations if they declared themselves "unfit" to fly for periods of up to seven days.

"In my past 10-odd years of flying there was once or twice I couldn't fly because of emotional fluctuation due to family matters," he said. "It was a flexible policy, no question was asked and I never faced any consequence."

A Cathay Pacific spokesman said the airline respects the medical confidentiality of its employees. He said it was a requirement of both the Civil Aviation Department (CAD) and the International Civil Aviation Organisation that all pilots complete a periodic assessment for both physical and mental health with medical examiners approved by the CAD.

The airline also offers staff confidential and free mental health counselling services with qualified psychologists.

"To ensure our flight crew are fit to fly, in addition to the annual medical assessment, they are also required to secure medical clearance after prolonged sickness/injuries from CAD and the company," he said.

At Lufthansa, student pilots seeking employment have to undergo rigorous assessment centres, with participants estimating that fewer than 10 per cent pass muster.

Lubitz qualified in 2008 after taking several months leave for reasons unknown to Lufthansa before completing his training.

For those who make it through, the job prospects have become gloomier as Lufthansa capped its fleet and laid out a strategy to create Europe's third-largest low-cost carrier under the Eurowings brand.

Until 2012, Lufthansa had started training classes for about 200 students a year, suspending the programme at its flight school in Bremen in northern Germany in 2013 after capping its fleet, meaning it required about 1,000 fewer pilots. Some candidates have been waiting as long as three years to land a job, Handwerg said. With licences tied to Lufthansa, they can't work for other airlines in the meantime.

Training can set candidates back ¤70,000 (HK$588,845) in fees, an unusually costly education in a country where even university tuition fees are low by international standards.

Some candidates work as flight attendants to bridge the gap, as Lubitz did for 11 months, or in other positions.

"The student pilots are completely dependent on Lufthansa," said Handwerg. "Many go to university or educate themselves further, but that's hard to do as they constantly have to be on standby. Under such circumstances there's no way you can plan your future life. It's an extremely unpleasant situation."

Additional reporting by Phila Siu and Stuart Lau

The final minutes of the ill-fated Germanwings flight

The Germanwings flight started like any other, the conversation in the cockpit normal, with co-pilot Andreas Lubitz offering no indication of the horror he would allegedly inflict. This is how Flight 9525, which investigators believe Lubitz deliberately steered into a French mountainside, unfolded according to French prosecutors and Germany's Bild newspaper. Both accounts are based on information from the cockpit voice recorder captured by one of the black boxes.

• Captain Patrick S. apologises to passengers for a 26-minute delay in take-off and says they will try to make up the time during the flight from Barcelona to Duesseldorf.
• The plane took off at around 10am local time and for the first 20 minutes the pilots "spoke in a normal fashion, courteous, like normal pilots. There was nothing abnormal", Prosecutor Brice Robin said.
• During the conversation the captain said he didn't have time to go to the toilet before leaving Barcelona and Lubitz offered to take over at any time.
• At 10.27am local time, the plane reaches its cruising altitude of 11,600 metres and the captain asks Lubitz to begin preparations for landing.
• The responses from Lubitz remained normal, but "very short ... not a real dialogue", Robin said.
• The co-pilot is heard saying, among other things, "hopefully" and "let's see". After checks for the landing, he is heard telling the pilot he can go now. Two minutes pass and the pilot then says to Lubitz, "you can take over".
• The sound of a seat being moved back is heard and a door closing. "We can assume he left to answer nature's call," Robin said.
• At 10.29am, the plane begins to descend.
• The French prosecutor believes that, once left alone, Lubitz turned a button on the flight monitoring system that began the plane's descent. "This action can only be deliberate," Robin said. "It would be impossible to turn the button by mistake. If you passed out and leaned over on it, it would only go a quarter-way and do nothing."
• The Airbus A320 jet descended rapidly for eight minutes, according to the low-cost carrier Germanwings.
• At 10.32am, air traffic controllers try to make contact with the plane but receive no reply and an automatic alarm is heard going off at almost the same time.
• The newspaper said that shortly afterwards there was a loud bang, like someone trying to open the door and re-enter the cockpit. The captain can be heard shouting, "For God's sake, open the door", as passengers' screams begin to be heard in the background.
• Robin had indicated last week that the black box recorded increasingly frantic attempts by the pilot to break down the door, heavily reinforced as per international standards, to which Lubitz made no response.
• At 10.35am, Bild said "loud metallic blows" against the cockpit door could be heard, with another alarm sounding about 90 seconds later. At around 5,000 metres altitude, the pilot is heard screaming, "Open the damn door".
• At 10.38am, Bild's account indicates the sound of Lubitz's breathing but him saying nothing. "He does not say a single word. Total silence," Robin said of the co-pilot during the descent.
• Around 10.40am, the plane hits a mountain and passengers' screams are heard - the last sounds on the recording, according to Bild.
• The plane dropped gradually from around 10,000-12,000 metres to 2,000 slowly enough, the French prosecutor said, that passengers would have been unaware anything was wrong.

"I think the victims were only aware at the very last moment. The screams are heard only in the last instants before the impact," said Robin.

Agence France-Presse