What happened
On November 13, 1996, a serious air traffic safety incident occurred in the airspace west of the Turku approach area. Two Finnair ATR-72-201 aircraft, registered OH-KRF (flight FIN511 and FIN519) and OH-KRA (flight FIN608), were placed in a potentially dangerous situation due to conflicting altitude clearances.
Flight FIN608, arriving from Stockholm, had been cleared to descend from FL150 to FL70, but was subsequently re-cleared to FL90. Simultaneously, flight FIN519, departing from Turku, was intended to fly at a lower altitude before climbing to FL120. However, due to a failure in the transmission of information, FIN519 climbed through FL90 and reached FL91, placing it on a collision course with FIN608, which was maintaining FL90. The aircraft eventually passed each other with a lateral separation of approximately 4 nautical miles, avoiding a direct impact.
The investigation
The investigation examined the communication links between the Tampere Area Control Center (ACC) and the Turku Tower (TWR). Investigators analyzed telephone recordings, which revealed that the direct telephone line between the two centers had been experiencing intermittent malfunctions, including "ghost calls" and instances where the line would fail to connect.
Furthermore, the investigation reviewed the use of the Pommery system for flight clearances and the phraseology used by air traffic controllers. Forensic phonetic analysis from the University of Helsinki was even utilized to attempt to interpret ambiguous recordings of controller communications.
Findings
- The primary cause of the incident was that the intermediate altitude clearance communicated by the Tampere ACC was not successfully relayed to the Turku Tower for transmission to the aircraft.
- Telecommunication malfunctions on the direct line between the two control centers prevented the reliable transfer of critical flight data.
- The controller at the Tampere ACC used inadequate phraseology, failing to ensure the message was properly acknowledged through a full read-back.
- The Turku Tower controller failed to adequately investigate a "ghost call" on the line during a period of high telephone traffic.
- While the local communication technology in Turku appeared functional during testing, it failed to capture the intermittent connection errors reported by the Tampere center.