The Federal Aviation Administration's failure to properly maintain communications equipment led to Tuesday's breakdown of radio contact between air controllers and pilots over parts of the western US, according to officials.
The disruption, which was not resolved for nearly five hours, forced airlines to cancel hundreds of flights into and out of Los Angeles, San Diego, other California airports and Las Vegas.
The FAA said late on Wednesday that it was investigating five instances in which planes had flown too close to other aircraft. The 10 planes involved included two passenger aircraft, cargo aircraft and business jets.
Radar data showed that at least two planes had changed course, apparently after collision avoidance systems in the cockpit had alerted the pilots to potential danger, the agency said.
Vertical and horizontal aircraft separation standards vary, but the agency said at no time were any planes about to collide.
The FAA said preliminary results of an investigation found that a 30 day maintenance check on the primary radio and voice communications system for the control center handling high altitude traffic for the southern California region was never made.
The radio system automatically shuts down after a period of time if the maintenance work is not performed. Also, the back-up radio communications system was not configured properly and did not work.
No explanation was given for why the work was not completed. Regulators said the investigation is continuing.
The air traffic control facility, northeast of Los Angeles, was able to maintain radar contact with affected aircraft. Other control centers in the West handled radio communications.
All checklists and protocols for proper maintenance procedure are being reviewed, the FAA said. The agency also plans to adjust the radio system to prevent automatic service disruptions if maintenance is not performed.
