Longmont, CO, USA
N2117Y
CESSNA 172S
N255BF
SONEX AIRCRAFT XENOS
The flight instructor and student pilot took off in a Cessna airplane from their home airport and proceeded north to a practice area to perform practice maneuvers. The Sonex pilot departed from an airport east of the practice area about 5 minutes earlier and proceeded westbound toward the vicinity of the Cessna. Both airplanes climbed to 7,000 - 7,500 ft mean sea level (msl) and operated under visual flight rules (VFR). When the Cessna reached the practice area, it made a left 360° turn consistent with a clearing turn, which according to the flight school was standard procedure. On completing the turn, the Cessna proceeded eastbound. The Sonex approached the same area from the east and tracked west-northwest toward the vicinity of the Cessna. The flight track data showed the two airplanes come together at an altitude of about 6,939 ft msl and subsequently showed both airplanes rapidly descend. Fragmented sections of the outboard left wings of both airplanes were found in a field below the point where the two flight tracks merged. Both airplanes came down in fields about 2,400 ft apart and were destroyed on impact with terrain. Postaccident examination of both airplanes showed no preaccident failures or malfunctions that would have precluded their normal operations. The Cessna was equipped with automatic dependent surveillance - broadcast (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. An examination of the GPS and transponder from the Sonex determined that the units were configured to not transmit position data via ADS-B. Investigators were unable to determine the exact configuration of the cockpit traffic displays for the Cessna. The sun was in the east (108o) about 24° above the horizon. A visibility study showed that the respective airplanes would have traversed the respective windscreens from right to left and left to right over the 60 seconds before the collision. However, the instrument panel, canopy frame, compass, and sun glare would have obscured the view of each pilot presenting challenges to detecting the respective airplanes. Toxicology specimens from the student pilot revealed Cetirizine in liver and muscle tissue. Cetirizine is an antihistamine available over the counter and is used to treat allergy symptoms. It may cause drowsiness. However, it is uncertain whether the student pilot was experiencing any sedating effects of this medication at the time of the accident. Given the circumstances of the accident, including the presence of a qualified flight instructor, it was unlikely that the student pilot’s Cetirizine use contributed to the collision.
HISTORY OF FLIGHTOn September 17, 2022, about 0853 mountain daylight time, a Cessna 172S, N2117Y, and a Sonex Aircraft Xenos, N255BF, were destroyed when they were involved in a mid-air collision near Longmont, Colorado. The flight instructor and student pilot aboard the Cessna and the pilot aboard the Sonex sustained fatal injuries. The Cessna was operated as Title 14 Code of Federal Regulations Part 91 instructional flight. The Sonex was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. Per the flight school, the flight instructor and student pilot took off from the Rocky Mountain Metropolitan Airport (BJC), Broomfield, Colorado, and proceeded north to a practice area near Longmont to perform practice maneuvers. A review of air traffic control (ATC) flight track data showed that the Cessna departed BJC about 0843 and proceeded north toward Longmont. The Sonex departed Platte Valley Airpark (18V), Hudson, Colorado, about 0838 and proceeded westbound toward the Longmont area. Both airplanes climbed to about 7,000-7,500 ft msl and were operating under VFR. On reaching the practice area, the Cessna made a left 360° clearing turn, which according to the flight school was standard procedure. On 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. The flight track data of the two airplanes merged at 08:53:01 at an altitude of 6,939 ft msl and subsequently showed both airplanes rapidly descend. Fragmented sections of the outboard left wings of both airplanes were found in a field below the point where the two flight tracks merged. Figure 1. Sonex and Cessna Airplanes’ Flight Tracks. At the time of the accident, both airplanes transmitted a transponder code of 1200 (VFR) and neither airplane was in radio contact, nor were they required to be in contact, with ATC. Both airplanes operated within the Mode-C veil of the Denver International Airport Class B airspace that required ADS-B “out” transmissions. AIRCRAFT INFORMATIONThe 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. The last three flight instructors to fly the Cessna reported no maintenance issues with the airplane and that the ADS-B equipment functioned properly. METEOROLOGICAL INFORMATIONAt the time of the accident and relevant to the accident location, the sun was at an azimuth of 108.38o and an elevation to the horizon of 23.63o. AIRPORT INFORMATIONThe 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. The last three flight instructors to fly the Cessna reported no maintenance issues with the airplane and that the ADS-B equipment functioned properly. WRECKAGE AND IMPACT INFORMATIONBoth airplanes each impacted in a rural area with a ground elevation of about 5,100 ft msl. Figure 2. Cessna Main Wreckage, Cessna Left Wing Pieces, and Sonex Main Wreckage Locations. The Cessna main wreckage was located in a field next to a fence and trees on the north side of an east-west running road. The airplane 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 airplane’s fuselage was crushed aft and broken open. The wings were broken aft and separated. The propeller was buried in the ground; on removal it showed S-bending, chordwise scratches, and leading edge nicks and gouges. The airplane’s left outboard wing, aileron, and wingtip were located in the field about 1,300 ft northeast of the main wreckage. Figure 3. Cessna 172S Main Wreckage. The Sonex Xenos was located in a sparsely wooded area beside a cornfield south of the east-west running road and about 2,400 ft east of the Cessna main wreckage. It had struck a tree and impacted terrain in a near-vertical, nose-down attitude and came to rest on the ground inverted on the edge of a drainage ditch that paralleled the road. The airplane’s engine and propeller were embedded in the soil at the base of a tree. The forward fuselage and cockpit were crushed. The instrument panel was crushed aft and fragmented. The aft fuselage was separated from the forward fuselage. The lower portion of the fuselage showed tree impact damage and was torn in multiple locations. The V-tail remained attached to the aft fuselage and showed minor damage. Both propeller blades were separated outboard of the hub and showed torsional fractures, chordwise rubs, and trailing edge chips and gouges. The right wing remained attached to the fuselage and sustained ground impact damage to the leading edge skin. The right-wing tip and aileron remained attached. The left wing remained attached to the fuselage and was bent downward and fractured near mid-span. The left-wing tip and about 2 ft of the outboard left-wing skin were located near the leading edge of the left wing. It showed separation and impact damage consistent with it striking the Cessna. The left aileron remained attached to the wing and showed no damage at the outboard end. Figure 4. Sonex Xenos Wreckage. A postaccident examination of both airplanes showed no mechanical failures or malfunctions that would have precluded their normal operations. A Garmin GNS-430W panel-mounted GPS receiver and a Garmin GTX-330 Panel-mounted Mode S digital transponder were recovered from the Sonex. They were examined at the NTSB Vehicle Recorders Division. Power was supplied to the GPS receiver and it powered up normally. No anomalies were found. The unit showed that all ARINC 429 channels were configured to “Off” for input and output data. In addition, all four RS232 channels were configured to “Off” for input and output data, indicating that the unit was not configured to provide any GPS position data for ADS-B to the transponder. The Garmin GTX-330 panel-mounted Mode S digital transponder was powered up normally. The initial screen showed the altitude mode as “ALT” and transponder code as “1200.” The channel 1 and 2 ARINC 429 input configuration screen showed channel 1 data as “GPS/FMS” and channel 2 as “Off.” The following screen showed channel 3 as configured to “Off.” The RS-232 was configured to “remote” for channels 1 and 2 input, which would have allowed the unit to accept RS-232 data from a position source as the GPS had the device been configured properly. The ADS-B transmission configuration was set to “Enable.” With the unit configured this way, with no position source information, a “NO ADSB” value would show in the upper left corner of the screen when the unit was set to transmit in “ALT” mode. To confirm, the unit was repowered to simulate normal operation and set to transmit data. After 150 seconds, the unit displayed a “NO ADSB” message in the upper left corner of the display as expected. MEDICAL AND PATHOLOGICAL INFORMATIONAutopsies of the flight instructor, student pilot, and pilot of the Sonex were conducted by the Boulder County, Colorado, Coroner. The reported causes of death of the flight instructor and student pilot were multiple blunt force injuries, and the manner of the deaths was accident due to involvement in a plane crash. The reported cause of death of the Sonex pilot was multiple blunt force injuries and the manner of death was accident due to involvement in a plane crash. Toxicology testing of the specimens from the flight instructor, student pilot, and the Sonex pilot was conducted by the FAA Civil Aerospace Medical Institute, Bioaeronautical Sciences Research Branch, Forensic Sciences. Specimens from the flight instructor were negative for all tests performed. Specimens from the student pilot revealed volatile concentrations of Cetirizine; 294 (ng/mL, ng/g) in liver, and 69 (ng/mL, ng/g) in muscle. Cetirizine is an antihistamine available over the counter and is used to treat allergy symptoms. It typically carries a warning that users may experience drowsiness and should use caution when driving a motor vehicle or operating machinery. The FAA required that pilots wait 48 hours after using Cetirizine before flying. Specimens from the Sonex pilot detected Tamsulosin in cavity blood and liver tissue. Tamsulosin is a prescription medication commonly used to treat symptoms of an enlarged prostate and is generally considered not impairing. TESTS AND RESEARCHThe NTSB Office of Research and Engineering conducted an aircraft performance and cockpit visibility study of the two airplanes. Using ADS-B data on the Cessna and Traffic Information Service-Broadcast (TIS-B) data on the Sonex, it was determined that the Sonex would have been visible to the Cessna about 60 seconds before the collision. It would have first appeared in the right half of the windscreen and would have moved across to the left side of the windscreen. The right edge of the Cessna’s instrument panel would have obscured the Sonex for 4 seconds and the compass in the center of the windscreen would have obscured it for about 15 seconds. The Sonex would have first appeared as a small object just below the horizon moving slowly against the background terrain. This complex background, compared to a sky background, might have made detecting the Sonex more difficult, particularly with the position of the sun and the Sonex, which were similar relative to the Cessna at the time of the collision. During the same minute, the Cessna would have appeared in the Sonex’s windshield as a small object moving slowly from left to right. The Cessna would have been obscured by the Sonex canopy frame for about 10 seconds. At times, the Cessna would have been below the horizon presented by the mountains and at other times it would have appeared above the horizon against the background of the sky. In the three seconds before the collision, as the Cessna descended to the collision altitude, it would have appeared below the horizon and against the terrain.
The pilots’ inability to see and avoid each other, resulting in a midair collision.
Source: NTSB Aviation Accident Database
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