Douglas DC-10 accident at Carleton, Michigan, 26 Nov 1975

Accident narrative

On November 26, 1975, an American Airlines Douglas DC-10 and a Trans World Airlines Lockheed L-1011 nearly collided head-on at 35,000 feet near Carleton, Michigan. Both aircraft were operating in instrument meteorological conditions under the control of the Cleveland Air Route Traffic Control Center. The DC-10 captain executed an abrupt evasive maneuver that injured 24 occupants, three seriously, and extensively damaged the cabin interior. The L-1011 occupants were uninjured.

### The flights

American Airlines Flight 182, a Douglas DC-10-10, was a scheduled passenger flight from San Francisco, California, to Newark, New Jersey, with a stop at Chicago, Illinois. It departed Chicago at 1839 e.s.t. with 13 crewmembers and 192 passengers aboard.

Trans World Airlines Flight 37, a Lockheed L-1011, was a scheduled flight from Philadelphia, Pennsylvania, to Los Angeles, California. It departed Philadelphia at 1815 with 11 crewmembers and 103 passengers aboard.

### The sequence of events

Both flights were operating in positive control airspace handled by the Wayne sector of the Cleveland Center, which was responsible for traffic at or above 35,000 feet (FL 350). TWA 37 was cruising westbound on jet route 584 (J-584) at its assigned altitude of FL 350. The Wayne sector radar controller accepted the handoff of the flight at 1903:53.

American 182 was climbing eastbound on the same airway, J-584. The Wayne radar controller accepted the handoff of American 182 from Chicago Center at 1914:40. The flight was climbing to an assigned altitude of FL 370. At 1921:19, American 182 was about 40 nautical miles west of TWA 37, reporting at FL 330. The two aircraft were on reciprocal courses, closing at a speed of about 850 knots.

At about 1922, the radar controller was relieved by a third controller who had returned from lunch. During the transfer of duties, several aircraft were mentioned, but American 182 was not.

At 1922:52, the relieving radar controller queried American 182 about its altitude. The flight reported passing through FL 347, noting they could see stars above but were still in the clouds. At 1923:03, the controller instructed American 182 to "descend immediately to 330."

The captain of American 182 started an immediate descent using the autopilot vertical speed control. Simultaneously, the crew sighted the lights of another aircraft at the 12 o'clock position, and the captain applied forward pressure on the control wheel. He estimated the aircraft passed with 100 feet of vertical separation. On TWA 37, the flight engineer saw what appeared to be position lights pass under the right side of the aircraft.

The evasive maneuver on American 182 produced negative G forces that threw unrestrained persons and service carts against the cabin ceiling, followed by positive G forces that brought them down heavily to the floor. The seatbelt sign had been on throughout the flight. Ten flight attendants and 14 passengers were injured; three passengers who did not have their seatbelts properly fastened sustained serious injuries. The flight diverted to Detroit, Michigan.

### What the investigation found

The Wayne sector was normally manned by a radar controller, a manual controller, and a handoff controller. Because the handoff controller was at lunch, the radar controller was working combined positions.

The radar controller stated that when he accepted the handoff of American 182, he realized there might be a traffic conflict with TWA 37. However, based on previous experience that day, he assumed the eastbound flight would level off at FL 370 well before the aircraft met. He intended to keep an eye on the situation and turn the aircraft if necessary.

Around the time American 182 reported at FL 280, Chicago Center called with a manual handoff of a Learjet. The radar controller accepted it and spent about five minutes attempting to insert a routing change for the Learjet into the computer. He stated he handled this task, normally done by the manual controller, because he felt the manual controller was busy.

The Board concluded that the radar controller became preoccupied with secondary duties to the extent that he failed to see the impending conflict clearly displayed on his radarscope. The Board noted that his intent to separate traffic was not a substitute for positive action at the first opportunity.

During the position relief briefing, neither controller reviewed the actual situation depicted on the radar display, resulting in an incomplete briefing. The relieving controller noticed the conflict about 50 seconds after taking over. He queried American 182's altitude because he considered the radar data block readout might be lagging. When he issued the immediate descent clearance, the aircraft were about a mile apart.

Flight data and radar records showed that American 182's pitch attitude decreased to a minimum of -10.9 degrees, creating vertical forces as low as -.86G, followed by +2.07G. The aircraft reached a maximum pressure altitude of 34,953 feet. At that time, TWA 37's pressure altitude was 34,965 feet. Radar interpolation showed the targets converged to within 0.108 nautical miles.

The Board found that the automated radar system was functioning as programmed. However, the Board noted that automation technology can lead to complacency, and the automatic altitude readouts induced the controller to rely solely on his own observation rather than taking positive steps to ensure separation.

### Probable cause

The National Transportation Safety Board determined that the probable cause of the near-collision was "the failure of the radar controller to apply prescribed separation criteria when he first became aware of a potential traffic conflict which necessitated an abrupt collision avoidance maneuver." The Board found that he allowed secondary duties to interfere with the timely detection of the impending traffic conflict when it was displayed clearly on his radarscope. Contributing to the accident was an incomplete sector briefing during the change of controller personnel about one minute before the accident.