Background
On July 2, 2008, at 2236 Pacific daylight time, a runway incursion occurred between Air Tran Airways flight 227, a Boeing 737-700 (N339AT), and Northwest Airlines flight 106, an Airbus 330-200 (N857NW) at Seattle-Tacoma International Airport (SEA), Seattle, Washington. The SEA tower controller had cleared the A330 for takeoff, and it subsequently overflew the B737 that had taxied onto the runway after landing on a parallel runway. Both flights were scheduled passenger operations under 14 Code of Federal Regulations Part 121 and operated under instrument flight rules. No injuries and no damage were reported. Night visual meteorological conditions prevailed. Data indicated the aircraft missed by 425 feet vertically.
Sequence of Events
Review of air traffic control voice communications revealed that at 2235:39, NWA106 was cleared for takeoff on runway 34R. At 2235:53, the local controller instructed the B737 pilots to exit runway 34C via high-speed taxiway F, hold short of runway 34R, and remain on tower frequency. The B737 captain, who was the pilot not flying, acknowledged. Airport Surface Detection Equipment, Model X (ASDE-X) replay showed NWA106 began its takeoff roll just before TRS227 exited runway 34R.
At 2236:13, the tower controller asked TRS227 to verify their hold short clearance; the first officer acknowledged. About 18 seconds later, the local controller received an aural and visual ASDE-X alert stating "Warning, Runway 3-4-Right, Occupied" when TRS227 crossed the taxiway hold short lines and proceeded across the runway edge line onto runway 34R. NWA106 became airborne between taxiways M and N and overflew TRS227.
When the alert occurred, the B737 was moving at 21 feet per second (12.4 knots) and was approximately 130 feet from the edge of runway 34R. The A330 was approaching taxiway L about 6,700 feet down the runway at 262 feet per second (155 knots). The exact altitude of the A330 at alert initiation could not be determined from Mode-C transponder data, but when it overflew the B737 at taxiway F, Mode-C data indicated 425 feet above the runway surface.
Crew Statements and Actions
After the event, both B737 crew members stated they looked for but could not see the taxiway F hold short markings or lights (located 932 feet beyond the 34C right runway edge markings and 252 feet short of the 34R runway edge markings). The first officer said the brightness of green taxiway centerline lights and developing haze obscured his view. The captain stated he expected the hold short lines to be perpendicular to runway 34R. Examination of the SEA airport diagram prepared by Jeppesen showed that the locations of runway hold short lines were not indicated.
The first officer (FO) was the pilot flying during landing and began exiting runway 34C using rudder pedal steering. Flight data recorder (FDR) data showed aircraft speed at about 50 knots. After the captain acknowledged the hold short clearance, he took control and called for the "After Landing" checklist, requiring the FO to direct attention inside the cockpit. As he slowed, the captain looked down at his airport diagram. The FO acknowledged the second hold short clearance. The crew then shut down the right engine. FDR data indicated 40 seconds elapsed from touchdown to right engine shutdown. The captain stated he made an idle reverse thrust landing per company policy for fuel savings.
During the visual approach to runway 34C, the FO had a clear view of runway 34R but stated he was not aware of another airplane on 34R and missed a tower transmission because he was concentrating on flying. The captain remembered hearing the NWA call sign on short final to 34C but did not think it significant and did not recall hearing the takeoff clearance.
Personnel Information
The Air Tran captain, aged 55, held an airline transport pilot certificate with 17,500 total hours (12,000 PIC, 7,000 in B737). He was type rated in the B737 and three other types. He flew 24 hours in the previous 7 days and 70 in the previous 30 days. His duty cycles were normal, and he stated fatigue did not contribute.
The first officer, aged 39, held an airline transport pilot certificate with 7,300 total hours (5,300 PIC, 1,100 on B737). He was type rated in two other types. He flew 21 hours in the previous 7 days and 83 in the previous 30 days. He stated he felt normal and alert at the time. He had flown with the captain before and believed crew coordination was good.
Post-incident statements from NWA106 pilots indicated they were unaware of the other aircraft or that a runway incursion had occurred.
Meteorological Information
The ASOS observation 53 minutes before the incident, at 2143 PDT, reported: wind 200 degrees at 4 knots, visibility 10 statute miles, scattered clouds at 12,000 feet AGL, ceiling broken 20,000 feet AGL, temperature 23°C, dew point 16°C, altimeter 29.87 inHg. Lightning was observed distant east through south.
Airport Information
SEA is located 10 statute miles south of Seattle, elevation 433 feet MSL. Annual operations average 347,000. It is a hub for Alaska and Horizon Airlines and an international gateway for NWA. The airport has two parallel north-south runways with two full-length parallel taxiways and another parallel runway under construction. Runway 16L/34R is 11,901 ft × 150 ft, asphalt/grooved, good condition. Runway 16C/34C is 9,426 ft × 150 ft, concrete/grooved, good condition. Runways 16L/16C are Category III, and 34C/34R are Category II certified, with appropriate lights and markings.
Taxiway F had enhanced hold short lines 252 feet from runway 34R left edge markings. At the time, the enhanced hold short line had in-pavement and elevated runway guard lights, as well as taxiway centerline lights operating on step one (lowest setting). About 10 minutes after the incident, tower requested and received verification that all lights, signage, and markings for taxiway F and runway 34R were present and operational. The verifying supervisor also indicated runways and taxiway F were dry.
Air Traffic Control Information
At the time, SEA ATCT was a level 9 facility providing ATC services within 4.3 NM of Seattle. It was equipped with ASDE-X.