What happened
On February 9, 2023, two aircraft experienced two separate close approaches while performing circuit patterns at the Bonn-Hangelar aerodrome. The first aircraft, a Cessna 172R operated by a flight school, was conducting training maneuvers. The crew, consisting of a flight instructor and a student pilot, had decided to extend their downwind leg due to existing traffic in the circuit.
During the transition from the downwind leg to the final approach for runway 29, the CessNA 172R encountered a Piper PA-28-181 that was also turning onto final. Radar data indicates that the two aircraft had two distinct close encounters. The first approach occurred at 12:01:55, with a horizontal separation of approximately 0.12 NM and a vertical separation of 200 feet, with the Piper positioned below the Cessna. The second approach occurred at 12:02:14, with the horizontal distance remaining at 0.12 NM but the vertical separation decreasing to 100 feet, with the Piper positioned above the Cessna.
During the second encounter, the pilot of the Piper noted the Cessna's presence and, upon being queried by the tower, confirmed visual contact. The air traffic controller subsequently instructed the Piper to go around. Simultaneously, the crew of the Cessna initiated a right-hand avoidance maneuver before continuing their approach and landing safely. There were no injuries and no damage to either aircraft or third parties.
The investigation
The BFU examined radar data from the Bundeswehr and air traffic control, as well as statements from the pilots and the air traffic controller. The investigation focused on the flight paths and the effectiveness of onboard collision warning systems. Radar analysis revealed that neither aircraft was following the published circuit procedures exactly; the Cessna was flying a wider turn than prescribed, while the Piper abbreviated its downwind leg, turning onto final much earlier than standard.
Findings
- The primary cause of the close approach was the Piper PA-28-181 turning onto final too early, which caused the flight paths of the two aircraft to intersect.
- The collision warning system (Air Traffic AT-1) installed in the Piper failed to display the Cessna 172R, despite the Cessna transmitting a transponder signal.
- The failure of the warning system was attributed to potential signal shadowing, where the aircraft's fuselage blocked the transponder signal from the Cessna's underside-mounted antenna from reaching the Piper's receiver.
- The air traffic controller observed that the Cessna had extended its downwind leg, while the Piper's early turn into final created the conflict.