Two Northwest Airlines jumbo jets carrying a total of 500 people narrowly avoided colliding on a slushy runway in Minneapolis Sunday night partly because the pilot of one plane had recently participated in special tests on low-speed flying, federal investigators said yesterday.
The National Transportation Safety Board is investigating an apparent communications breakdown in the air traffic control tower that allowed the two McDonnell Douglas DC10s to be on the same runway at the same time. One of the planes was taking off and the other was taxiing across the runway.
The plane taking off cleared the plane on the ground by 50 to 200 feet, depending on which crew member made the estimate, safety board Chairman Jim Burnett said.
D.F. Nelson, the captain of the plane taking off, is Northwest's top management pilot and recently participated in special simulator and actual flight tests on how to recover from low or stall speed brought on by dangerous low-altitude wind shifts, Burnett said.
"The truly astonishing thing about this is that he saw this aircraft in front of him when he was between 100 and 120 knots," Burnett said. "He rotated pulled the nose off the ground shy of V1."
V1 is the speed at which a pilot decides to continue the takeoff or to abort, and was calculated at 139 knots for this flight. The next critical speed is VR, the speed of rotation. VR for this flight was 145 knots. Thus Nelson pulled the nose and the plane off the ground several knots below what would be considered safe takeoff speed.
The other plane was crossing from left to right. "As he overflew the other plane, he dipped his left wing and raised his right wing to get more clearance over the tail," Burnett said.
The flight continued to Seattle, with the passengers apparently unaware of the incident, Burnett said. The plane that was taxiing subsequently flew to Phoenix.
The situation was reminiscent of the world's worst aviation disaster, when 577 people were killed in Tenerife, Canary Islands, in 1977 when a plane taking off collided with one taxiing. Then, too, the pilot of the plane taking off attempted to leave the ground early to avoid a collision, but did not have enough speed and succeeded only in dragging his plane's tail along the runway.
In that case, the pilot started his takeoff without clearance from the tower. In Minneapolis, "both crews were executing the air traffic control instructions they were provided, no question," Michael O'Rourke, investigator in charge for the safety board, said.
Northwest Flight 51 to Seattle was given permission to take off by the "local controller" in the Federal Aviation Administration's tower. At about the same time, 9:04 p.m. Sunday (CST), Northwest Flight 65, taxiing out for the flight to Phoenix, was given permission by the "ground controller" to cross the runway. The crossing point was about 6,000 feet from where Flight 51 started its takeoff roll.
Local controllers handle planes taking off and landing; ground controllers handle the planes as they move around the terminals and taxiways in preparation for taking off and after landing.
The two controllers stand side by side in the tower. They use different radio frequencies in talking to the planes they are controlling and normally talk to each other without radio or telephone links. "You sometimes don't hear what you thought you heard," one controller expert said in speculating about what might have happened.
The board is studying tapes of radio transmissions by both controllers, but there is no recording of the conversation between them.
Visibility at the time of the incident was estimated by the controllers at 10 miles; the official weather service visibility listing was 20 miles.
Burnett said crew members of the plane taking off told him and other investigators that the only option they had other than to try and fly over the crossing airplane was to go off the runway, but that there were airplanes on both sides.
The airport was congested after a heavy snowstorm earlier in the day caused it to close for an hour. Several taxiways and runways were clogged with snow, contributing to the congestion and delays. The safety board is looking at snow removal plans and operations as part of its investigation, O'Rourke said.
The airport was closed after another airplane aborted its takeoff when one of its engines stopped because of slush on the runway, Burnett said. Another aircraft landed behind it "and they may have been on the runway at the same time," he said. Staffing levels and traffic flow at the airport will be an important part of the investigation, he added.
The local controller, board officials said, has 24 years' experience; the ground controller, a military veteran, only recently completed training to become fully qualified, and then transferred from Los Angeles to Minneapolis in September. Both controllers were relieved of duties after the incident.