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The incident happened on Saturday, March 22, 2008 aboard an Airbus A-319 aircraft, operating as US Airways Flight USA1536, en route from Denver to Charlotte, NC. The flight, which the TSA claims "was never in danger," landed safely in Charlotte.
According to the TSA, the pilot was authorized to carry the gun aboard the aircraft under provisions of the FFDO program. The FFDO program trains civilian flight deck crew on the use of firearms, use of force, legal issues, defensive tactics, the psychology of survival and program standard operating procedures. The pilot whose gun discharged in flight was said to have last re-qualified as an FFDO on Nov. 7, 2007.
Airline pilots probably can expect to be hearing about the details of this incident during recurrent training. Pilots who also are FFDOs will most certainly see the incident addressed during their next re-qualification.
Here is a link to information about the Federal Flight Deck Officer program on the TSA website.
UPDATE March 29, 2008:
Video: Possible scenario for pilot's accidental discharge of his firearm.