Flight 409 left Denver, Colorado at 6:33 a.m. on October 6, 1955. This was 83 minutes after its scheduled departure time. The assigned path the airliner was expected to fly was along airways V-4 Denver to Laramie, Wyoming V-118 to Rock River, Wyoming radio, V-6 to Fort Bridger, Wyoming, and V-32 to a landing in Salt Lake City. The flight was operating under Visual Flight Rules and was assigned a cruising altitude of 10,000 feet. Because the aircraft was not pressurized, the altitude was chosen to keep the passengers and crew from experiencing the discomfort that flying higher could cause. The route assigned to the airliner was specifically designed to allow safe passage at 10,000 feet over the continental divide in the Rocky Mountains.
An expected position report from the United crew, scheduled for 8:11 a.m. while over Rock Springs, was not received, and repeated attempts to make radio contact with flight 409 were met with no response. With the airliner's status unknown, the Civil Aeronautics Authority was alerted to the missing aircraft. No radar was in place for civil aviation in this region in 1955. With no radar traces, manual searches were required to find the aircraft.