History of Flight
On September 17, 2022, at approximately 0853 mountain daylight time, a Cessna 172S (N2117Y) and a Sonex Aircraft Xenos (N255BF) were destroyed when they collided midair near Longmont, Colorado. The Cessna was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight, carrying a flight instructor and a student pilot. The Sonex was operated as a Title 14 Code of Federal Regulations Part 91 personal flight, piloted by a single occupant.
The Cessna departed from Rocky Mountain Metropolitan Airport (BJC) in Broomfield, Colorado, at about 0843 and proceeded north to a practice area near Longmont to perform practice maneuvers. Air traffic control (ATC) flight track data showed that the Sonex departed Platte Valley Airpark (18V) in Hudson, Colorado, at about 0838 and proceeded westbound toward the Longmont area. Both airplanes climbed to approximately 7,000–7,500 feet mean sea level (msl) and were operating under visual flight rules (VFR).
Upon reaching the practice area, the Cessna executed a left 360° clearing turn, standard procedure according to the flight school. After completing the turn, the Cessna made a right turn just east of Niwot, Colorado, and proceeded eastbound. The Sonex approached from the east and tracked west-northwest, passing about 4 miles north of Erie, Colorado, and continued toward a point about 1 mile east of Niwot. Flight track data showed that the two airplanes merged at 08:53:01 at an altitude of 6,939 feet msl, after which both airplanes descended rapidly. Fragmented sections of the outboard left wings of both airplanes were found in a field below the merging point.
At the time of the accident, both airplanes transmitted a transponder code of 1200 (VFR) and were not in radio contact with ATC, nor were they required to be. Both operated within the Mode-C veil of Denver International Airport Class B airspace, which required ADS-B "out" transmissions.
Aircraft Information
The Cessna 172S was equipped with ADS-B "in/out" equipment and transmitted ADS-B data during the accident flight. The last three flight instructors to fly the Cessna reported no maintenance issues and that the ADS-B equipment functioned properly. The Sonex Xenos did not transmit ADS-B data during the accident flight; ATC records indicated it last transmitted on July 14, 2022.
Meteorological Information
At the time of the accident and relevant to the location, the sun was at an azimuth of 108.38° and an elevation above the horizon of 23.63°.
Wreckage and Impact Information
Both airplanes impacted in a rural area with a ground elevation of about 5,100 feet msl. The Cessna main wreckage was located in a field next to a fence and trees on the north side of an east–west road. It impacted the ground in a near-vertical, nose-down attitude. The main wreckage consisted of the fuselage, engine, propeller, right wing, left inboard wing, and empennage. The fuselage was crushed aft and broken open; wings were broken aft and separated; the propeller showed S-bending, chordwise scratches, and leading edge nicks and gouges. The left outboard wing, aileron, and wingtip were found about 1,300 feet northeast of the main wreckage.
The Sonex Xenos was located in a sparsely wooded area beside a cornfield south of the road, about 2,400 feet east of the Cessna main wreckage. It struck a tree and impacted terrain in a near-vertical, nose-down attitude, coming to rest inverted on the edge of a drainage ditch. The engine and propeller were embedded in soil at the base of a tree; forward fuselage and cockpit were crushed; instrument panel was crushed aft and fragmented. The aft fuselage separated from the forward fuselage. The V-tail remained attached with minor damage. Both propeller blades separated outboard of the hub with torsional fractures and chordwise rubs. The right wing sustained ground impact damage; the left wing was bent downward and fractured near mid-span. The left wing tip and about 2 feet of outboard skin showed separation and impact damage consistent with striking the Cessna.
A postaccident examination revealed no mechanical failures or malfunctions that would have precluded normal operations. A Garmin GNS-430W GPS receiver and a Garmin GTX-330 transponder were recovered from the Sonex. Examination showed the GPS receiver was not configured to provide position data for ADS-B to the transponder, and the transponder displayed a "NO ADSB" message when tested, indicating it was not transmitting position data.
Medical and Pathological Information
Autopsies of the three occupants were conducted by the Boulder County Coroner. The causes of death for the flight instructor and student pilot were multiple blunt force injuries; manner of death was accident due to involvement in a plane crash. The Sonex pilot's cause of death was also multiple blunt force injuries; manner of death was accident due to involvement in a plane crash.
Toxicology testing by the FAA Civil Aerospace Medical Institute was negative for all tested substances in the flight instructor. The student pilot's specimens revealed volatile concentrations of Cetirizine, an over-the-counter antihistamine used to treat allergy symptoms, which carries a warning about possible drowsiness and a requirement to wait 48 hours before flying per FAA regulations. The Sonex pilot's specimens detected Tamsulosin, a prescription medication for enlarged prostate, generally considered not impairing.
Tests and Research
The NTSB Office of Research and Engineering conducted an aircraft performance and cockpit visibility study. Using ADS-B data from the Cessna and Traffic Information Service-Broadcast (TIS-B) data for the Sonex, it was determined that the Sonex would have been visible to the Cessna about 60 seconds before the collision, initially in the right half of the windscreen, then moving to the left side. The right edge of the instrument panel would have obscured it for 4 seconds; the compass for about 15 seconds. The Sonex would have appeared as a small object just below the horizon, moving slowly against complex background terrain, potentially making detection more difficult given the sun's position. For the Sonex pilot, the Cessna would have appeared in the windscreen as a small object moving left to right, obscured by the canopy frame for about 10 seconds. In the three seconds before collision, the Cessna would have been below the horizon against the terrain.