Incident
On Sunday, 27 May 2001 at 17:08 UTC, an air traffic incident (Airprox) occurred over Grenchen Airport between two aircraft: CRX667, a Crossair Saab2000 on a scheduled flight from Bilbao to Basel, and HB-FLK, a Pilatus Porter PC-6T operated by Para Club Grenchen for skydiving operations.
Sequence of Events
CRX667 was flying under instrument flight rules and had been cleared to descend to FL150 by Zurich Radar West Sector. At 17:07, the crew received traffic information about an aircraft on the opposite course at the same altitude and were instructed to turn left to heading 270°. The pilot acknowledged but did not execute the turn, fearing loss of visual contact. Instead, he descended approximately 300 ft to ensure safe crossing. The pilot reported receiving a Traffic Advisory (TA) from the Traffic Conflict Alert System (TCAS) and estimated the other aircraft at FL152. The commander later stated that in his assessment, there was no immediate collision risk due to good weather conditions.
HB-FLK had been conducting parachute jumps throughout the day. For its final flight, the pilot planned to climb to FL160 with 10 skydivers on board. At 16:51, the pilot contacted Bern Arrival and requested FL160. Bern Arrival coordinated with the Radar Planner West in Zurich for clearance above FL100, as per procedures. The coordination call at 16:58 included a request for FL160, but the Radar Planner (RP) was being relieved and subsequently miscommunicated the cleared altitude to his replacement, stating that HB-FLK was allowed only up to FL140.
ATC Coordination
The handover of the RP West position occurred during the coordination. The outgoing RP informed his successor that HB-FLK was cleared to FL140, and this information was relayed to the Radar Executive West (RE) trainee, who placed a control strip with the note "Para-Grenchen FL140". The trainee was supervised by a licensed coach. Meanwhile, Bern Arrival interpreted the coordination as approval for FL160 and instructed the HB-FLK pilot to climb to that level.
Conflict and Separation
At 17:05, HB-FLK passed FL143, climbing at approximately 500 ft/min. The new RP West noticed a Short Term Conflict Alert (STCA) showing both aircraft near FL150. He alerted the trainee RE, who was overwhelmed, and the coach took over after about 30 seconds. The coach issued traffic information to CRX667 and attempted a left turn, but the CRX667 pilot had already taken evasive action. The two aircraft crossed with a horizontal distance of 0.8 NM and vertical separation of 500–700 ft. The HB-FLK pilot did not see the other aircraft until after it had passed underneath, but was alerted by skydivers on board.
Findings
Both aircraft were in Class C controlled airspace. CRX667 was in contact with Zurich Radar West on 135.675 MHz, while HB-FLK communicated with Bern Arrival on 127.325 MHz. The Saab2000 pilot had visual contact with the Porter; the Porter pilot did not see the Saab2000 before the crossing. The RE West provided traffic information and instructed an avoidance maneuver, which the pilot did not execute. The pilot of CRX667 manually descended 300 ft. The Porter pilot had to obtain three separate clearances for the jump (from Grenchen Tower, Bern Arrival, and the jump dispatcher), all of which were correctly obtained. The coordination between ATC units contained an error in the transfer of altitude clearance information.