With no answers yet in the disappearance of Malaysia Airlines Flight 370; investigators have said they're considering many options: hijacking, sabotage, terrorism or catastrophic equipment failure. Nobody knows if the pilots are heroes who tried to save a crippled airliner or if one collaborated with hijackers or was on a suicide mission.
Whatever the outcome, the mystery has raised concerns about whether airlines and governments do enough to make sure that pilots are mentally fit to fly.
"One of the most dangerous things that can happen is the rogue captain," said John Gadzinski, a Boeing 737 captain and aviation-safety consultant. "If you get somebody who, for whatever reason, turns cancerous and starts going on their own agenda, it can be a really bad situation."
Malaysia Airlines said this week that its pilots take psychological tests during the hiring process.
"We will obviously look into all these and see whether we can strengthen, tighten all the various entry requirements and examinations," CEO Ahmad Jauhari Yahya said. He did not describe the tests.
Many US airlines also perform mental health screenings when pilots and crew apply for jobs.
"The airlines have a lot of data on what a successful pilot looks like, and the mental aspect is a big part of that," says Brad Tate, a pilot for a leading US airline. He said he's known applicants who were rejected because of their performance on a standardised mental test.
"I have never once flown with somebody who I questioned their mental health," Tate says.
Once a pilot is hired, however, US airlines rarely if ever test a pilot again for mental health, say several experienced pilots. According to Federal Aviation Administration rules, US pilots must pass a physical exam annually or every six months,
depending on their age, but there is no specific requirement for a mental-health test. Buried in 333 pages of instructions, the FAA tells doctors that they should "form a general impression of the emotional stability and mental state" of the pilot.