What happened
On May 11, 2018, a serious flight disturbance occurred in Class E airspace near Mönchengladbach Airport. A Cirrus SR 22 was operating under Instrument Flight Rules (IFR) on an ILS approach to runway 13. Simultaneously, a Remos GXNXT ultralight aircraft was conducting a VFR cross-country flight from Grefrath-Niershorst to Aachen-Merzbrück.
During the approach, the air traffic controller instructed the pilot of the Cirrus SR 22 to descend to 2,000 ft AMSL and turn right to a heading of 100°. This descent placed the aircraft on a collision course with the Remos GXNXT, which was cruising at 2,500 ft AMSL. The ground-based Short Term Conflict Alert (STCA) triggered a predicted alert when the aircraft were 4.1 NM apart, transitioning to a current alert when they were 2/2 NM apart.
The controller provided the first traffic information regarding the ultralight aircraft at 15:46:50, but the aircraft were already only 1.2 NM apart. The pilot of the Cirrus SR 22 reported that his onboard collision warning system also indicated an aircraft 100 ft below him, prompting him to disconnect the autopilot and initiate a climb. The two aircraft passed each other at 15:47:10 with a lateral separation of approximately 120 m and a vertical separation of only 25 ft. There were no fatalities and no damage to the aircraft or third parties.
The investigation
The BFU examined radar recordings, radio communications, and the actions of the flight crew and air traffic controller. The investigation focused on the timing of the STCA alerts, the controller's prioritization of tasks, and the effectiveness of the traffic information provided. The investigation also reviewed the operational procedures of the air traffic service provider regarding traffic avoidance advice.
Findings
- The primary cause was an ATC instruction for the Cirrus SR 22 to descend into the altitude occupied by the Remos GXNXT.
- The air traffic controller prioritized a routine instruction to another aircraft over the urgent need to provide collision prevention traffic information.
- The controller failed to issue a lateral or vertical avoidance instruction or an avoidance recommendation, despite the STCA alert.
- The first traffic information was issued too late to allow for an effective maneuver.
- The pilot of the Cirast SR 22 was unable to establish visual contact in time to avoid the encounter.
- Contributing factors included the lack of clear instructions in the air traffic service provider's manual regarding the obligation to issue avoidance recommendations, and the fact that the pilot of the Remos GXNXT was not in contact with Flight Information Service (FIS), relying solely on the "see and avoid" principle.