While airline pilots submit to yearly medical tests by an FAA doctor, psychiatric screening is not a required element of testing.
"If there's no suspicion on the part of the doctor, they will sign that they're clear to fly," Kevin Hiatt, a veteran commercial captain and now safety consultant said.
Hiatt also adds that there is no specific training for what to do if your co-pilot loses it during flight either.
"The pilots are trained on what to do if a partner becomes incapacitated and can't land the aircraft," Hiatt said.
Tuesday's incident is not the first time airline crews have alarmed or even killed passengers. In October 1999 on Egypt Air, flight 990, a 767 from New York to Cairo with 217 aboard, disappeared when the co-pilot deliberately crashed the plane into the Atlantic.
In 1997, a Silk Air 737 flying over Indonesia nose-dived, killing 104 passengers. Investigators say the pilot committed suicide, taking all of his passengers with him.
Earlier this month, an American Airlines flight attendant had to be restrained after threatening impending doom. That plane returned to the gate and the flight attendant was taken away, complaining of psychiatric problems.
ABC News' Josh Haskell and Kevin Dolak contributed to this report.