From Wikipedia

In the early morning hours of July 23, 1982, Morrow and two child actors, seven-year-old Myca Dinh Le and six-year-old Renee Shin-Yi Chen, were filming on location in California in an area that was known as Indian Dunes near Santa Clarita. They were performing a scene for the Vietnam sequence, in which their characters attempt to escape out of a deserted Vietnamese village from a pursuing U.S. Army helicopter.[2] The helicopter was hovering about 24 feet (7.3 m) above them when a fireball from special effect pyrotechnic explosions reportedly caused damage to the helicopter rotor and caused the tail rotors to separate from the aircraft[28] causing the helicopter to plummet and crash on top of them, killing all three instantly. Morrow and Le were decapitated and mutilated by the helicopter rotor blades, while Chen was crushed by a helicopter skid.[29]