What happened
At approximately 1200 Newfoundland standard time, two aircraft flying on nearly reciprocal tracks at the same altitude experienced a period where air traffic controllers could not establish radio contact. The first aircraft, a MD-11 operated by Lufthansa Airlines with registration DLH8ly61, was traveling eastbound from New York to Frankfurt. The second aircraft, a Boeing 757-224 operated by Continental Airlines with registration COA65, was traveling westbound from Lisbon to Newark.
While the aircraft were approximately 100 nautical miles apart, the controller attempted to issue a climb clearance to DLH8161 to ensure vertical separation. However, a failure in the landline connecting the Gander Area Control Centre to the peripheral station used for radio transmissions meant the instruction never reached the crew. For roughly eight minutes, the controller attempted to contact the aircraft nine times without success. During this period, the aircraft were closing at a rate of 16 nautical miles per minute.
While the controller was unable to reach the Lufthansa flight, a supervisor used a backup radio to contact COA65 and instructed the crew to descend to flight level 320. Eventually, radio contact was restored with DLH8161 via an alternate frequency, allowing the controller to issue the necessary climb clearance. The aircraft achieved the required vertical separation of 1000 feet approximately one minute after the climb began, but they had closed to within 18 nautical miles of each other during the communication blackout.
The investigation
Investigators examined the technical infrastructure at the Gander Area Control Centre and the communications equipment used by the controller. The investigation established that a fault in the lines connecting the ACC to the Trepasert PAL site had severed the connection to the primary radio frequency. This failure occurred without triggering any alarms for the technical staff or the service provider.
Furthermore, the investigation looked into the controller's ability to detect the failure. It was found that the Voice Switch Communications System (VSCS) display provided a misleading indication; a green light appeared on the control panel even when transmissions were not actually leaving the station. The investigation also reviewed the training and awareness of available backup resources, such as the multi-channel, tunable radio available in the operations room.
Findings
- A failure in the landline linking the Gander ACC to the Trepassey PAL site caused the loss of the primary communication frequency.
- The controller did not immediately realize the frequency was unserviceable, leading to a delay in establishing alternative contact methods.
- The VSCS control panel provided a deceptive green indication that suggested transmissions were successful when they were not.
- The safety of flight was not assured for approximately 10 minutes because no alternative radar separation minima were in place during the communications outage.
- There was a lack of awareness among some controllers regarding the coverage area and availability of the backup tunable radio.