What happened
On July 18, 1997, at approximately 20:50 local time, a serious loss of separation occurred near Jyväskylä Airport. A Cessna TU206G, registration OH-JLK, operating a skydiving mission under VFR flight rules, passed within approximately 200 feet of a Piper PA-28-181, registration OH-PKT, which was conducting an IFR transit flight from Oulu to Malmi.
The skydiving aircraft was operating within a pre-approved airspace reservation that allowed for jumps at various altitudes. To maintain separation from the passing IFR traffic, the air traffic controller decided to implement a 1,000-foot vertical buffer between the two aircraft. However, because the aircraft were flying with different altimeter settings—the skydiving plane using QFE and the transit flight using QNE—the controller had to manually calculate the appropriate altitude limit.
In the process of calculating this limit, the controller used an unofficial, handmade reference tool known as a "calculation stick." Due to the worn condition of this tool, the controller misread the required altitude, instructing the OH-JLK to climb to 9,600 feet QFE. In reality, this altitude placed the skydiving aircraft only 184 feet below the OH-PKT.
The investigation
The investigation examined the air traffic control procedures, the use of the altitude reference tool, and the communication between the aircraft and the tower. The investigation established that while the skydiving operation had proper airspace authorization, the controller's attempt to manually manage vertical separation led to the error. The investigation also noted that the radar display was not utilized effectively because the aircraft were occasionally in a radar blind spot directly above the tower. Furthermore, while the controller provided traffic information to the IFR flight, no traffic information regarding the skydiving aircraft was provided to the VFR flight, and neither pilot reported seeing the other aircraft during the encounter.
Findings
- The primary cause of the near-miss was the incorrect altitude restriction issued to the skydiving aircraft.
- The controller relied on a worn, unofficial "calculation stick" to determine the necessary altitude separation between different pressure settings.
- The controller failed to verify the calculated altitude through any other means.
- The skydiving aircraft was flying significantly closer to the IFR flight than the intended 1,000-foot separation, with a vertical gap of only 184 feet.