What happened
On December 15, 2000, a serious loss of separation occurred near Espoo, Finland, involving two commercial aircraft. The first aircraft, an Aeroflot IL62M (registration RA 86532), departed Helsinki-Vantaa Airport on a standard departure route. Shortly after takeoff, the aircraft's position and label information failed to appear on the air traffic control radar screens.
Simultaneously, a Finnair MD83 (OH-LMG), operating as flight FIN2162, was approaching Helsinki-Vantaa. Due to the lack of radar contact with the departing Aeroflot flight, controllers coordinated a new traffic solution, directing the Finnair aircraft to a different arrival path to avoid the departing plane.
As the Finnair aircraft reached its assigned flight level of 80, its TCAS (Traffic Collision Avoidance System) issued a "Climb" command, indicating traffic 500 feet below it. At that exact moment, the Aeroflot aircraft finally appeared on the radar, showing it was at flight level 76, significantly lower than its cleared altitude. The two aircraft's flight paths intersected at nearly a 90-degree angle. The conflict was resolved seconds later when the TCAS issued a "Clear of conflict" message, and the controllers were unable to issue manual warnings before the aircraft passed each other.
The investigation
The investigation examined radio communications, radar system performance, and the operational actions of both flight crews and air traffic controllers. Investigators analyzed the Helsinki-Vantaa radar system and discovered a significant pattern of radar visibility issues; a list provided during the investigation revealed that 11 Russian-built aircraft had disappeared from radar screens in the months preceding the event. Furthermore, the investigation identified 63 total instances of aircraft visibility problems at the facility within a one-year period.
Findings
- The primary cause of the altitude deviation was the incorrect altimeter pressure setting on the Aeroflot IL62M.
- A major contributing factor was the failure of the Aeroflot aircraft's transponder to provide a signal to the radar system, which rendered the aircraft invisible to controllers from takeoff until seconds before the near-collision.
- The departure controller did not notify the Aeroflot crew that they were not visible on radar or instruct them to check their transponder settings.
- The flight crews and controllers did not have sufficient time to implement manual separation measures once the aircraft became visible.
- There was a lack of effective reporting of radar visibility issues through the official safety reporting system, as only a small fraction of known visibility incidents had been formally documented.