United Airlines N7431 was a United Airlines training flight. On December 4, 1951, it departed Denver, CO at 06:55 to provide training to two first officer applicants of the company. 30 minutes into the flight, the DC-3 aircraft stalled at an altitude of around 8,200 feet MSL, which was approximately 3,200 feet above the ground. The aircraft subsequently entered a spin, which could not be recovered from before the plane crashed. According to the official investigation, the probable cause of the accident was determined to be "an inadvertent spin at an altitude too low for recovery." It is not uncommon for this type of accident to occur during training flights as pilots may be experimenting with different maneuvers or training exercises. Casualties There were three crew members onboard who died in the accident. References Aviation accidents and incidents in the United States in 1951 Aviation accidents and incidents in Colorado