What happened
On November 20, 1998, at 19:19 local time, a serious loss of separation occurred near the GOGLA waypoint, approximately 90 km southeast of Helsinki-Vantaa Airport. The incident involved two aircraft flying on opposing tracks: a Finnair MD11, registration OH-LGB, operating a scheduled flight from Helsinki to Osaka, Japan, and a Jetflite HS 125-700B, registration OH-JET, operating a medical repatriation flight from Moscow to Helsinki.
The MD11 was climbing through flight level 150. At approximately 19:19, the aircraft's Traffic Collision Avoidance System (TCAS) issued a Traffic Advisory (TA), followed by a Resolution Advisory (RA) instructing the pilot to descend. The pilot transitioned to manual control, reducing altitude and turning right. At the moment of closest approach, the vertical separation between the two aircraft had decreased to approximately 400 feet, and the lateral distance was 1.4 nautical miles.
Simultaneously, the OH-JET was descending through flight level 160. While the crew had initially set the altitude alerter to 160, the aircraft began to descend below the assigned level. The cockpit voice recorder captured an altitude alert indicating a deviation of more than 300 feet. The aircraft continued to descend to flight level 137 before stabilizing.
The investigation
The investigation examined radar recordings, radio communications, and the flight data from both aircraft. Investigators analyzed the cockpit voice recorder (CVR) from the OH-JET and reviewed the integrated flight control system of the HS 125-700B. The investigation also reviewed the operational procedures and manuals of Jetflite Oy to determine if standard operating procedures were followed during the descent and altitude management.
Findings
- The primary cause of the near mid-air collision was the failure to maintain assigned altitude by the OH-JET crew, which led to the violation of vertical separation minima.
- The OH-JET crew experienced an uncontrolled descent from flight level 160 to flight level 137, during which the aircraft's altitude alerter triggered warnings for deviating more than 300 feet from the assigned level.
- The MD11 crew responded to a TCAS Resolution Advisory by descending, which contributed to the reduction in vertical separation during the encounter.
- While the MD11 crew identified the traffic via TCAS and visual contact, they did not explicitly report the specific TCAS avoidance maneuver to the Helsinki Approach controller during the initial follow-up communication.