What happened
On August 9, 2020, a Cessna 208B used for parachute operations was descending through Class G airspace near Marl-Polsum. After dropping 18 skydivers at FL140, the pilot was cleared by Langen Radar to descend and exit Class C airspace. During this descent, the pilot maintained a very high sink rate of approximately 4,000 ft/min and a ground speed of 180 kt.
Simultaneously, a Cessna 172P was conducting a private flight in the same area. The pilot of the Cessna 172P reported seeing a fast-moving aircraft appearing from above and behind the cockpit A-pillar. The two aircraft passed within approximately 18 meters horizontally and 30 feet vertically. The pilot of the Cessna 172P performed an immediate evasive maneuver, pulling the control column fully back and banking hard left, but noted that a collision would have likely occurred within two seconds had he not reacted.
The investigation
The BFU investigation examined radar data, radio communications, and the technical status of the aircraft and air traffic services. The investigation focused on why neither pilot detected the other earlier and why air traffic services did not provide traffic information.
Investigators found that the Cessna 208B pilot was performing a steep descent and a turn, which significantly restricted his ability to maintain effective visual lookout. Furthermore, the collision warning system (PowerFLARM) on the Cessna 208B was non-functional because the firmware had expired, a fact unknown to the pilot.
Regarding air traffic services, the Langen Radar controller was managing a high and complex workload, with several sectors merged. The investigation revealed that the Short Term Conflict Alert (STCA) system was configured to suppress warnings for certain transponder code combinations, meaning the system failed to alert the controller to the impending conflict between the two aircraft.
Findings
- The primary cause was the high sink rate and speed of the Cessna 208B, which severely compromised the effectiveness of the "see and avoid" principle.
- The collision warning system on the Cessna 208B was inoperative due to an expired firmware version that had not been updated during the last maintenance interval.
- The air traffic service's STCA system failed to trigger an alert because specific collision warnings were intentionally suppressed by configuration to reduce controller workload.
- No traffic information was provided by the air traffic services to either pilot, despite both aircraft being under radar and radio contact.
- The pilot of the Cessna 172P only detected the approaching aircraft at the moment of passing, leaving insufficient time for a successful avoidance maneuver.