What happened
On April 27, 2003, at 17:33 local time, a loss of separation occurred in the Helsinki Terminal Control Area between two aircraft. The first aircraft, a Finnair Boeing 757-20 and (call sign FIN 2710), was on radar vectors for a landing on runway 22L at Helsinki-Vantaa. Simultaneously, a Golden Air Flyg AB Saab 340 A (call sign GAO 338) was departing runway 15 on a standard instrument departure toward Savonlinna.
During the approach, the air traffic controller (APP) cleared FIN 2710 to descend to 4,000 feet. However, the first officer misheard the instruction and read back an altitude of 3,000 feet. The controller failed to notice the error and confirmed the incorrect read-back. Consequently, the aircraft continued its descent through the 4,000-foot level.
As the aircraft's altitude dropped, the controller eventually ordered FIN 2710 to climb back to 4,000 feet. Although the crew initiated the climb, the maneuver was delayed because the air brakes remained extended for 18 seconds after the climb was initiated. This delay caused the aircraft to penetrate the required separation minima. The two aircraft were without required separation for 40 seconds, reaching a minimum horizontal distance of 1.2 NM and a vertical distance of only 157 feet.
The investigation
The investigation examined the radio communications, radar data, and flight recorder information from both aircraft. Investigators analyzed the air traffic control procedures at the Helsinki approach workstation and the cockpit actions of both crews. The investigation also reviewed the functionality of the Traffic Alert and Collision Avoidance System (TCAS) on both aircraft and the impact of the extended air brakes on the climb performance of FIN 2710.