Pilots and flight attendants are aware of risks, but commercial aviation tragedies have been infrequent in the U.S. for years.

Investigators will spend months validating specifics that help explain the Jan. 29 midair collision of an American Airlines regional jet and a U.S. Army helicopter in crowded airspace near Washington.

Thank you for choosing Air France,” said Daniel Harding, the flight’s first officer. “And remember: Rehearsal is at 6 p.m.” ...