What happened
On March 3, 2013, a serious near-collision occurred in the Zurich TMA, approximately 5 NM east-northeast of the Willisau radio beacon. The incident involved a Pilatus PC-6/B2-H4, registration HB-FKC, operating under visual flight rules (VFR) for parachute operations, and a Tecnam P2006T, registration HB-LBU, flying an instrument flight rules (IFR) training route.
After completing a jump sequence, the pilot of the HB-FKC initiated a steep descent. Although air traffic controllers instructed the pilot to level off at FL 90 to avoid oncoming traffic, the aircraft continued to descend. The HB-LBU was cruising at FL 80. The two aircraft crossed paths with a horizontal separation of only 0.1 NM and a vertical separation of approximately 150 ft.
The investigation
The investigation by SUST examined radar data, radio communications, and statements from the flight crews and air traffic controllers. Investigators found that the pilot of the HB-FKC had placed the propeller in a feathered position—a configuration the manufacturer recommends only for engine failure emergencies—to facilitate a high sink rate of up to 6,000 feet per minute. This configuration, combined with high speeds, significantly limited the aircraft's maneuverability.
Furthermore, the investigation revealed that while the controller issued a level-off instruction, the HB-FKC was already below the target altitude (passing FL 96) at the time the instruction was received. The pilot of the HB-FKC also incorrectly confirmed to the controller that they were maintaining FL 90, while actually descending to FL 81.
Findings
- The primary cause was the parachute aircraft descending below its assigned altitude, creating a high-risk collision scenario.
- A contributing factor was the atypical aircraft configuration, specifically using the feathered propeller setting during a routine descent, which hindered the pilot's ability to respond to altitude changes.
- An unsuitable separation concept used by air traffic control was also identified as a contributing factor.
- Neither aircraft was equipped with a collision warning system at the time of the event.
Safety action
Following the incident, the operator of the HB-FKC installed a collision warning system in August 2014, which utilizes both FLARM and transponder signals to assist in conflict detection and avoidance.