American Airlines flight 951 was en route to Sao Paulo, Brazil, about 10:30 p.m. on Jan. 20 when a warning system alerted pilots to an impending collision with the two U.S. Air Force C-17s, the National Transportation Safety Board said in a statement.
The airline pilot, responding to the collision warning, took evasive action, the board said. Alarms also went off at the Federal Aviation Administration's air traffic control center near Islip, Long Island, and showed up on controllers' radar screens. The controllers ordered the planes to change course, the board said.
"The US Air Force is committed to safe flight operations and mishap prevention," said Todd Vician, an Air Force spokesman. "We are cooperating with the NTSB in its investigation."
The planes were flying at an altitude of about 22,000 feet over the Atlantic Ocean, board spokeswoman Bridget Serchak said. The C-17s, flying in formation, appeared as a single blip on air traffic control radar screens, she said.
It's not clear yet exactly how close the airliner came to the C-17s, but it was within a mile of the transport planes, the board said. Airliners typically travel at hundreds of miles per hour and can close that distance within seconds. Planes are usually required to maintain a minimum distance from other aircraft of about five miles at higher altitudes.
All airliners are required to be equipped with Traffic Collision Avoidance Systems, or TCAS, that not only warn of an impending collision, but tell the pilot what direction to turn or whether dive or climb in order to avoid a crash. The warning systems have nearly eliminated midair collisions involving passenger jets.
Serchak said she didn't know if the C-17s were equipped with TCAS. Air Force officials didn't respond to a request for information.
American Airlines spokeswoman Andrea Huguely said the Boeing 777-200 was carrying 247 passengers and 12 crew members.
FAA spokeswoman Sasha Brown said that as a result of the FAA's preliminary investigation of the incident, all air traffic controllers at the Long Island center "are already reviewing a variety of procedures including the handling of formation flights, aircraft near sector boundaries and TCAS requirements."
Each controller is responsible for a "sector" of airspace and all the planes that pass through that sector. As a plane travels from one sector to another, controllers are supposed to handoff responsibility for the plane to the controller in the next sector.
When a controller makes a mistake that causes planes to come too close together for safety, FAA counts the incident as an "operational error." There has been a surge in operational errors recently.
FAA officials attribute the surge to a new error reporting program that encourages controllers to voluntarily report their own errors without fear of reprisal. The advantage to the program, officials have said, is that it provides a more complete picture of the occurrence of errors. It also makes it easier to spot trends and correct problems.
The rate for the most serious violations of FAA aircraft separation standards rose to 3.28 per million flight operations in the nine months ending June 30, 2010, up from 2.44 in the full year ending Sept. 30, 2009. A flight operation is every time a plane takes off or lands or is handed off from one controller to another.
Overall, there were 1,889 operation errors in the 12 months ending on Sept. 30, 2010, according to FAA. Of those, 44 required evasive actions to avoid collisions and another 401 involved the serious risk of collision.