What happened
On January 25, 1997, shortly after midday, two aircraft operating in the Helsinki approach area experienced a situation that led the crew of an Air Botnia Jetstream, registration OH-LYX, to file a safety report. The Jetstream was conducting a scheduled flight from Helsinki to Jyväselkä, climbing through FL 130.
Approximately six minutes after the OH-LYX departure, a Finnair DC-9-51, registration FIN-3439, departed Helsinki for Oulu. Due to a higher airspeed, the DC-9-51 began to close the distance with the preceding aircraft. During the climb, the air traffic controller, observing that horizontal separation was at risk, issued a new instruction to the DC-9-51 to maintain FL 120, rather than its previously assigned higher altitude.
As the DC-9-51 climbed, its pilot noticed the Air Botnia aircraft passing below. The sudden change in altitude instructions caused momentary urgency in the cockpit of the DC-9-51, as the crew had to manually adjust the autopilot to prevent overshooting the new assigned level.