On June 30, 1951, United Air Lines Flight 610, a US transcontinental San Francisco–Oakland–Salt Lake City–Denver–Chicago service flown by the Douglas DC-6 aircraft Mainliner Overland Trail (registration N37543) crashed in Larimer County, northwest of Denver.

After completing its first two segments, Flight 610 departed Salt Lake City at 12:11 a.m. en route to Denver.

At around 1:47 a.m, Flight 610 reported to air traffic control that it had passed the Cheyenne radio range station, and requested a lower altitude, which was granted down to 8,500 feet.

[2] At that point, Flight 610 was scheduled to make a right turn to intercept the 168° course line of the DEN low frequency range, and then to proceed to the WONT intersection, its next clearance limit.

If the pilot had configured his audio selector switches properly, he would have been able to hear the aural Morse code identifier of "A", for the north side of that low frequency range.