What happened
On February 4, 2021, a near-collision occurred in Class E airspace near Bochum, Germany, involving a Cessna F 172H and a TL-96 Sting ultralight aircraft. Both aircraft were operating under Visual Flight Rules (VFR) on private recreational flights, with Loemühle airfield as their destination.
The Cessna pilot, flying at approximately 1,800 ft AMSL, received an alert from an onboard traffic warning system indicating an approaching aircraft at the 11 o'clock position. In response, the pilot executed a 30-degree right turn. However, the pilot did not establish visual contact with the other aircraft. Radar data indicated the TL-92 Sting was actually positioned at the 1 o'clock position relative to the Cessna.
The TL-96 Sting was traveling at approximately 90 knots on a course of 240 degrees. The pilot of the ultralight reported seeing the Cessna only one to two seconds before the encounter, noting that the Cessna passed just meters above the ultralight. The minimum separation during the encounter was measured at approximately 43 meters horizontally and 141 feet vertically, with a closing speed of roughly 180 knots.
The investigation
The BFU investigation focused on the effectiveness of the collision warning systems and the pilots' ability to maintain visual separation. The investigation examined the functionality of the Avidyne TAS600 and FLARM systems in the Cessna F 172H, as well as the AIR Avionics AT-01 system in the TL-96 Sting.
Investigators analyzed the visual physics of the encounter, noting that the small dimensions of the aircraft and the high closing speed left the pilots with only 6 to 10 seconds of visibility, which is insufficient for an effective evasive maneuver. The investigation also reviewed the use of Flight Information Services (FIS) and the configuration of the onboard electronic flight bags.
Findings
- The primary cause of the encounter was that both aircraft were approaching each other at a sharp angle at the same altitude, making visual detection nearly impossible given their size and speed.
- The pilot of the Cessna F 172H increased the collision risk by performing a lateral maneuver rather than a vertical one, potentially searching for traffic in the wrong direction due to a potentially inaccurate direction indicator on the traffic warning system.
- The collision warning system in the TL-96 Sting failed to provide a warning, likely due to outdated FLARM firmware and/or incorrect software configuration (specifically the 'Bearingless Targets' setting).
- The pilot of the TL-96 Sting did not utilize the Flight Information Service (FIS), which could have provided radar-based traffic information.
- The Cessna's traffic warning system may have provided misleading directional information, leading the pilot to turn into the path of the approaching aircraft.