Sonex failed to transmit required signal per NTSB report

A preliminary report issued by the National Transportation Safety Board (NTSB) on October 8 sheds new light on the mid-air collision Sept. 17 of two airplanes near Niwot. Investigator in Charge Michael Folkerts wrote, “On September 17, 2022, about 0853 mountain daylight time, a Cessna … and a Sonex Aircraft Xenos, … were destroyed when they were involved in a mid-air collision.”

The flight instructor and student pilot aboard the Cessna, identified by the Boulder County Coroner as Daniel Wilmoth, 22 and Samuel Fisher, 23, respectively, and the pilot aboard the Sonex, identified as Henry Butler, 69, sustained fatal injuries. Wilmoth’s obituary indicated that he is survived by his girlfriend, Darby Berringer, whose father Todd Berringer is a Niwot High graduate.

The Cessna was operated as an instructional flight, while the Sonex was operated as a personal flight, according to the report. “A review of air traffic control flight track data revealed the Cessna departed Rocky Mountain Municipal Airport, Denver, Colorado, about 0843, and the Sonex departed Platte Valley Airpark Hudson, Colorado, about 0838,” the report notes.

The airplanes climbed to about 7,000-7,500 feet above sea level, or about 2,000 feet above the ground, and were operating under visual flight rules. The report states, “The Cessna flew northbound and the Sonex flew westbound toward the Longmont area. After the Cessna completed a left 360° turn, it turned eastbound. The flight track data of the two airplanes merged and subsequently showed both airplanes rapidly descending. Fragmented sections of the outboard left wings of both airplanes were found in a field below the point where the two flight tracks merged.”

The report indicates that at the time of the accident, both airplanes transmitted a transponder code of 1200, and neither airplane was in radio contact, nor was either aircraft required to be in contact with Air Traffic Control. But the report notes, “Both airplanes operated within the Mode-C veil of the Denver International Airport Class B airspace that required automatic dependent surveillance broadcast (ADS-B) “out” transmissions.”

The report said, “The Cessna was equipped with ADS-B “in/out” equipment and transmitted ADS-B data during the accident flight. The Sonex did not transmit ADS-B data during the accident flight; ATC records indicated the Sonex last transmitted ADS-B data on July 14, 2022.”

A local pilot for a commercial airline indicated that it was not unusual for the Cessna to be doing maneuvers such as a left 360° turn as part of the instruction. He also noted that both aircraft were required to transmit “out” data during the flight indicating the plane’s altitude, but that the requirement was relatively new, having been instituted a few years ago.

The Cessna had “in” reception capability, and would have seen “out” signals transmitted from the Sonex had they been transmitted. The “in” transmission reception capability, however, was not required by either aircraft. He said that ironically, if both aircraft had only the required “out” ADS-B transmission without the “in” reception capability, neither aircraft would have been alerted to the location of the other airplane.