What happened
On August 13, 2001, a serious loss of separation occurred in the Turku approach area involving two aircraft operated by Air Botnia Ab. The first aircraft, a SAAB 200 and registration KFB 422, was descending through the flight level of a second aircraft, a SAAB SF340A with registration KFB 411.
At the time of the incident, the Turku tower was staffed by a trainee air traffic controller under the supervision of a qualified controller. The trainee issued a descent clearance to KFB 422 that allowed the aircraft to descend through the altitude of KFB 411. The supervising controller failed to detect this error.
The pilots of KFB 411 observed the oncoming aircraft and, upon receiving a TCAS Traffic Advisory (TA), performed a left turn. This maneuver was contrary to standard flight rules. The pilot of KFB 422 also received a TCAS Traffic Advisory but, due to a language barrier regarding the Finnish radio communications, did not immediately realize the other aircraft was turning left. Eventually, the TCAS issued a Resolution Advisory (RA) of "CLIMB" to KFB 411 and "MAINTAIN VERTICAL SPEED" to KFB 422. The minimum horizontal separation between the two aircraft reached approximately 1.1 NM, with a vertical separation of 700 feet as they passed each other.
The investigation
The investigation examined the air traffic control handover process, the trainee's performance, and the use of radar and radio frequencies. Investigators found that the trainee was performing qualification training and that the supervising controller had not been formally designated in writing as a supervisor for such training.
Additionally, the investigation looked into the radar handling of KFB 422, which had been placed in a "HOLD" status by the Area Control Centre (ACC). Because the aircraft was in a HOLD state, the radar system did not trigger a Collision Avoidance (CA) warning. The investigation also noted that the primary radio frequency for Turku was being used outside its prescribed functional coverage area.
Findings
- The primary cause of the incident was an error made by the trainee air traffic controller, which went unnoticed by the supervising controller.
- The trainee issued a clearance to KFB 422 that permitted it to descend through the altitude of KFB 411.
- The pilot of KFB 411 executed a left turn that deviated from standard flight rules.
- The radar system failed to provide a collision warning because KFB 422 had been placed in a HOLD status prior to entering the unit's airspace.
- There were deficiencies in the formal designation and supervision of trainee controllers at the airport.
- The primary radio frequency (118.300 MHz) was being utilized beyond its intended operational range.