Incident Overview
On July 18, 2020, at 12:47 UTC, a serious airprox incident was recorded at Grenchen Airfield (LSZG). The event involved two aircraft: a Pacific Aerospace LTD 750XL registered as HB-TCP and a Cessna Aircraft Company 182T registered as HB-TER. The minimum separation achieved between the two aircraft was approximately 0.1 nautical miles horizontally and 100 feet vertically, classified as an ICAO Category A airprox.
Operational Context
The HB-TCP, operated by Verein Skydive Grenchen, was conducting a skydiving operation under Visual Flight Rules (VFR). The pilot, holding a Private Pilot Licence (PPL(A)), had just dropped 13 skydivers and was on final approach to runway 06. The HB-TER, operated by Fliegergruppe Brunair, was on an Instrument Flight Rules (IFR) training flight from Speck-Fehraltorf (LSZK) to Grenchen. The crew consisted of a pilot and an examiner performing an instrument rating renewal.
Sequence of Events
The air traffic controller at Grenchen Tower activated the "Parabox," a local restricted airspace north of the airfield to protect skydiving operations. This activation prevented the standard missed approach procedure for runway 24 from being flown through that zone. The incoming controller, who had just taken over during a shift change, issued an instruction to the HB-TER crew to fly the missed approach visually along the runway axis due to parachute activity.
At 12:46:15 UTC, the HB-TER passed over runway 24 and began climbing westward to 4,000 feet as instructed. Simultaneously, the HB-TCP was on a steep, fast descent (approximately 175 knots) for runway 06. Due to high frequency congestion, the HB-TCP pilot could not transmit a position report. The controller did not provide traffic information to either aircraft regarding the other's presence.
Detection and Avoidance
Collision warning systems on both aircraft, including PowerFlarm and TAS, issued alerts. The HB-TER crew spotted the HB-TCP at 10 o'clock, while the HB-TCP pilot saw the HB-TER at 2 to 3 o'clock. Both pilots executed evasive maneuvers. The HB-TCP pilot extended his flight path along the crosswind approach, which helped mitigate the conflict. The closest point of approach occurred at 12:47:23 UTC. Both aircraft subsequently landed safely in Grenchen without further incident.
Findings and Analysis
The investigation found no technical defects contributing to the event. Key factors included the late visibility of the steeply descending skydiving aircraft to air traffic control, high radio frequency congestion preventing position reports, and the controller's lack of experience with this specific conflicting traffic scenario. The failure to issue mutual traffic information was a significant contributing factor.