What happened
On June 25, 1998, a serious loss of separation occurred in the Helsinki approach area between an Airdeal Oy Learjet 24 D, registration OH-GLB, and a Finnair DC-9-87, registration OH-LMB.
The Learjet 24 D was operating a repositioning flight from Hannover to Helsinki. While under radar vectors, the aircraft was cleared to descend to a flight level of 80. However, during its descent, the aircraft passed through the assigned altitude of 80, reaching as low as 75 feet. This descent brought the aircraft into the path of Finnair flight FIN 269, a DC-9-87 climbing on a standard departure route from Helsinki.
At the closest point, the two aircraft were separated by only 0.3 NM horizontally and 500 feet vertically. The loss of required separation lasted for 32 seconds, during which time the aircraft had no vertical or horizontal separation. The crew of the DC-9-87 received a TCAS traffic advisory indicating the Learjet was approximately 600–700 feet above them.
The investigation
The investigation examined radar recordings, radio communications, and the flight operations manual of Airdeal Oy. Investigators analyzed the descent profile of OH-GLB, noting a descent rate of approximately 1,500 to 2,000 feet per minute.
The inquiry also looked into the cockpit environment of the Learjet 24 D. It was noted that the aircraft's altitude pre-selection warning system—which provides an audible signal via a speaker on the right side of the cockpit and a visual light—was not effectively utilized. The investigation found that the crew's attention was diverted to the center console, likely while managing GPS waypoints.
Findings
- The primary cause of the incident was that neither pilot of the Learjet 24 D was monitoring the altimeter for altitude changes, as both pilots had focused their attention on the center console.
- The crew failed to follow established cockpit procedures, specifically the requirement for the monitoring pilot to call out "Thousand to go."
- The audible altitude warning was difficult to perceive because the pilots were wearing headsets with active radio traffic, and the physical location of the warning speaker (on the right cockpit wall) was unknown to the crew. It was also noted that the monitoring pilot's elbow may have partially obstructed the sound.
- The aircraft's descent through the assigned altitude of 80 was not corrected until the 300-foot warning triggered, at which point the pilot manually disconnected the autopilot to initiate a climb.
- Poor visibility due to cloud layers prevented the flight crews from maintaining visual contact with each other.