Tuesday, June 7, 2011
Getting A Few Winks
There was an outcry after a controller fell asleep, but concerns about tired controllers aren’t new
A study done by the Battelle Institute for the FAA noted that as fatigue becomes more acute, so do increased errors of omission, followed by errors of commission and microsleeps. When you're affected by microsleeps, you involuntary fall asleep for anywhere from a few seconds to a few minutes, and may not be aware of it.
Controller fatigue and sleep loss figured in an incident that had the potential for becoming a tragedy investigated by the NTSB. On August 19, 2004, a Boeing 747-400 came dangerously close to flying into a Boeing 737 at Los Angeles International Airport. Asiana Airlines flight 204, the Boeing 747-400, had been cleared to land on runway 24L. Southwest Airlines flight 440, the Boeing 737, had been cleared into position for takeoff on runway 24L. The crew of flight 204 recognized a problem, and executed a go-around. Radar reconstruction of the event found that flight 204 passed over flight 440 by 200 feet. At the time of the incident, a controller change had just occurred, and the relief controller was responsible for handling both airplanes.
Just after 2:51 p.m., the controller who was being relieved cleared flight 204 to land on runway 24L. At this point, the Boeing 747 was 9.3 miles from the runway. About two minutes later, the controller provided a briefing to a controller just coming on duty. After taking over, the first transmission from the relief controller was to flight 440 instructing the pilots of the Boeing 737 to taxi into position, and hold on runway 24L. At that time, flight 204 was 1.81 miles from the runway at 700 feet. According to flight 440's captain, he saw the Asiana Boeing 747 on final approach, but believed that the aircraft was landing on runway 24R. Twelve seconds later, the relief controller cleared Southwest 440 for takeoff. Radar data indicated the 747 was 1.26 miles from the runway. Calculations showed it would reach the runway threshold in about 35 seconds. Data retrieved from flight 440's flight-data recorder indicated the airplane was on taxiway V approaching runway 24L, when the controller gave it the takeoff clearance. This meant that the pilots had less than 35 seconds to taxi onto runway 24L, begin a departure roll, gain speed and lift off before the 747 crossed the landing threshold, which would be impossible. According to the Asiana captain's statement, he observed the Southwest Boeing 737 approaching runway 24L, but believed the airplane would hold short of the runway. Once he recognized the aircraft was entering the runway, he initiated a go-around.
The relief controller told investigators that he believed the 747 was landing runway 24R. When the Airport Movement Area Safety System (AMASS) generated an alarm, the 747 was only about 12 seconds from colliding with the 737, according to the NTSB. When the relief controller recognized the problem, he canceled flight 440's takeoff clearance and flight 204's landing clearance. However, the 747 had already overflown the 737.
The Safety Board found that the relief controller was briefed on the location and clearances for seven aircraft, and noted that seven pieces of information is about the limit that can be effectively retained in short-term memory. Five certified professional controllers and one operations supervisor were working in the tower cab when there would normally be 10 people. Injuries and illness had reduced the available shift staff to five.
The investigation determined that the relief controller had only eight hours off between the end of his August 18 evening shift at 11:30 p.m., and the beginning of his morning shift at 7:30 a.m. on the day of the accident. As a result, the relief controller reported sleeping just "five or six hours" the night before the incursion, and described his shift leading up to the incursion as a "hard day." The Safety Board concluded that his acute sleep loss hurt his cognitive performance on tasks involving working memory and reaction time.
Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.
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