Malaysian plane mystery: Pilots' mental health a concern

"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.