Boeing 777 - 236 N773AN incident near Shemya, Alaska

Probable cause

The air traffic controller’s failure to identify a conflict between two aircraft and to issue an adequate clearance, which resulted in a loss of required separation. Contributing to the incident was the Federal Aviation Administration’s failure to provide controllers adequate automated conflict detection assistance, which removed a layer of backup to controller judgment and reduced the overall level of safety.

NTSB analysis

A loss of required separation occurred near Shemya, Alaska, between a Boeing 777-200 en route to Shanghai Pudong International Airport, Shanghai, China, and a Boeing 747-400 en route to Ted Stevens Anchorage International Airport, Anchorage, Alaska. There was no damage to either aircraft, and no injuries were sustained by the passengers or crewmembers.

The flight crew of the Boeing 777 requested to climb from flight level (FL) 360 to FL 370. Given the Boeing 777's position at the time, the requested climb would have conflicted with two other airplanes that were traveling at FL 370 but in the opposite direction. The air traffic controller identified the potential conflict between the Boeing 777 and the first airplane, but he did not identify the conflict between the Boeing 777 and the second airplane, the Boeing 747. The controller instructed the flight crew of the Boeing 777 to remain at FL 360 until they passed the first airplane and then to climb to FL 370. The flight crew of the Boeing 777 acknowledged and complied with the clearance, which resulted in a loss of required separation with the Boeing 747. Both flight crews received and responded to resolution advisories from their onboard traffic alert and collision avoidance systems after the controller issued the climb clearance. The Federal Aviation Administration (FAA) estimated that the minimum separation between the two aircraft was 1.04 miles laterally and 900 ft vertically.

Typically, two types of automated conflict detection were available to controllers in the Shemya radar coverage area: a "conflict probe" that provided controllers with extended warning time for future conflicts and a short-term conflict alert system that looked about 2 minutes ahead to detect conflicts between radar-observed aircraft. However, the FAA had intentionally disabled the extended conflict probe in the Shemya radar coverage area because of unresolved performance issues, and the only automated conflict detection assistance available to the controllers in this area was the short-term conflict alert system; this removed a layer of backup to controller judgment and reduced the overall safety of flight operations in this area. Postincident corrective actions resulted in the extended conflict probe being returned to service. If the extended conflict probe had been available at the time of the incident, it is likely that it would have detected the conflict and alerted the controller about it before he issued the incorrect clearance and that no loss of separation would have occurred.

Text from the NTSB. Open the NTSB docket for OPS14IA011 for the full record.