What happened
On November 29, 2004, two separate air traffic incidents occurred at Bergen Flesland Airport involving a Boeing 737-800 (registration LN-WFR) operated by Widerøes Flyveselskap and an Eurocypria Airlines flight.
The first incident occurred when the air traffic controller at Flesland Approach mistakenly cleared the LN-WFR flight on a path that caused it to turn onto a southwesterly heading, deviating from its intended track. While attempting to re-sequence the aircraft, the controller cleared the Boeing 737-800 for a descent that resulted in a loss of vertical separation with the Eurocypria aircraft. For approximately 30 seconds, the vertical distance between the two aircraft dropped to 700 ft, violating the required 1,000 ft minimum. The flight crew of the Boeing 737-800 reported they could not increase their descent rate because they were already at idle power and maximum speed. Both aircraft received Traffic Advisory alerts from their TCAS.
A second loss of separation occurred minutes later during the approach sequence. As the aircraft were being vectored for an ILS approach, the lateral distance between the two aircraft fell below the 5 NM minimum, reaching a minimum of 4.6 NM.
The investigation
The Norwegian Safety Investigation Authority (NSIA) analyzed radar recordings from the Norwegian Air Traffic Control System (NATCON) and communication logs. The investigation faced challenges because certain radar recording files had been deleted during routine maintenance, and the investigation noted that different recording facilities at the airport were not synchronized to a single time standard, making precise event plotting difficult.
Findings
- The primary cause of the first separation loss was descent clearances provided without rate restrictions, combined with a descent profile that differed from the controller's expectations.
- The air traffic controller did not utilize the available radar function that displays the rate of climb or descent, citing concerns that it would clutter the radar display.
- The second separation loss was caused by the geometry of the aircraft tracks; specifically, a 90-degree turn by the lead aircraft reduced the lateral distance between the two planes.
- The controller's assumption that separation would be maintained did not account for the speed changes typically occurring as aircraft transition to landing configuration.
Safety action
- The NSIA recommended that Avinor evaluate the use of climb/descent-rate functions on radar displays and establish operational guidelines for their use.
- Recommendations were made to improve the storage and safeguarding of radar recordings to prevent accidental deletion.
- The authority called for improvements to the NATCON playback software to allow better analysis of radar data.
- A recommendation was issued to ensure all recording equipment at Bergen Flesland uses Coordinated Universal Time (UTC) for synchronized time-stamping.