What happened
On August 27, 1998, a mid-air separation loss occurred in airway G5 involving two aircraft flying in opposite directions. The first aircraft, a Cessna 402B with registration OH-CGA, was operating an IFR flight from Helsinki-Malmi to Turku. The second aircraft, a Dorn/Dornier 228-213 with registration OH-MVO, was flying from Turku to Helsinki-Vantaa.
During the flight, the pilot of OH-CGA requested a descent. The Helsinki Approach controller provided instructions that included traffic information for an opposing aircraft at flight level 70. The pilot of OH-CGA performed a readback, specifically using the word "confirm" to verify the instruction. However, the controller did not closely monitor the readback and acknowledged it with "Roger." Consequently, the pilot of OH-CGA descended to flight level 70, believing the clearance was valid.
As the two aircraft passed each other, they were separated by approximately 1.6 NM horizontally and only 100 feet vertically. The pilot of OH-CGA was in a descent while the OH-MVO was maintaining flight level 70.
The investigation
The investigation examined radio communications, radar recordings from Tampere, and noise monitoring data from Helsinki-Vantaa. Investigators noted significant timing discrepancies between various recording systems, which complicated the precise reconstruction of the timeline. The investigation also reviewed the initial departure procedures from Helsinki-Malmi, where ambiguous phraseology led to a discrepancy in whether the flight was being handled under VFR or IFR procedures.
Findings
- The primary cause of the loss of separation was a communication breakdown where the Helsinki Approach controller failed to notice that the pilot's readback contained an incorrect altitude intention.
- The pilot of OH-CGA only partially understood the controller's complex instruction due to high frequency workload and used "confirm" to seek clarification, which was not properly addressed.
- At the Tampere Area Control Center, the controller in charge was preoccupied with a separate coordination task via telephone and was not monitoring the radio or radar for this specific traffic.
- The assistant controller at the Tampere station noticed the altitude discrepancy but did not intervene to prevent the conflict.
- Ambiguous phraseology used by the Malmi tower during departure contributed to initial confusion regarding the flight's regulatory status.