As United Airlines Flight 1193 touched down in Denver on Feb. 2, a snowplow blundered onto the runway in front of the speeding jet.
Up in the airport's tower, controllers were unaware of the danger. The safety system designed to track planes and vehicles on the ground never issued an alert, according to a report by the National Transportation Safety Board (NTSB). Luckily, the United pilots spotted the plow in time, narrowly averting a crash by jamming their brakes and coming to a full stop on the runway.
Because the runway safety system in use at large airports such as Denver has repeatedly failed to alert controllers to hazards, the Federal Aviation Administration is scrapping it and turning instead to a modernized digital version that does a better job of monitoring planes on the ground.
The new system, however, may be no better at preventing collisions between planes and ground vehicles.
Even though the system, Airport Surface Detection Equipment Model X (ASDE-X), is designed to use new technology to track plows and other ground vehicles with great accuracy, the FAA is not allowing ground vehicles to be equipped with the radio identification beacons necessary to optimize the system.
"The technology is there. It was developed for this, and it should be used," says Brad Rosenthal, who heads the controller's union chapter at Lambert-St. Louis International Airport. "We are trying to keep vehicles and equipment off of these ... runways."
The FAA acknowledges it has blocked radio beacons from being put on airport vehicles, but says it is doing so for a good reason: The beacons could interfere with the ability of radar to track aircraft around an airport. The agency is working on dozens of fronts to try to improve runway safety, spokeswoman Laura Brown said.
'Potential for catastrophe'
The deadliest plane accident in history occurred not in the sky but on the ground. On March 27, 1977, two Boeing 747s slammed into each other at an airport in the Canary Islands when the pilots on one jet mistakenly attempted to take off in fog while another jet was on the runway. The impact and fires killed 574 people.
"This is where the greater concentration of aircraft is at any time while passengers are on them," NTSB Chairman Mark Rosenker says. "This is where the potential for catastrophe is greatest at this point."
A review of the most serious FAA and NTSB runway incident records show that the dangers are not restricted to crashes between planes:
*On May 25, an American Eagle ATR-42 twin turboprop was approaching Miami International Airport when a tug operator pulled a Boeing 747 onto its runway. Only a quarter-mile away, the Eagle pilots saw the jet and aborted their landing, passing only 200 feet above the other plane.
*On Nov. 30, 2004, a Mitsubishi MU-2 turboprop attempting to take off at Philadelphia International Airport struck an airport tug towing a Boeing MD-80. The MU-2 suffered substantial damage, but no one was hurt.
*On Sept. 19, 2003, a dump truck that was part of an airport construction project drove across a runway at Houston Hobby Airport. At the same moment, a Southwest Airlines Boeing 737 arrived at the runway for landing. The Southwest jet flew directly over the truck. The FAA estimated the two were separated by 106 feet.
Best system available
ASDE-X is not a perfect solution, but it is superior to anything that currently exists, according to the FAA, NTSB and controllers. Unlike the system at Denver, known as Airport Movement Area Safety System, ASDE-X is capable of tracking vehicles and planes in all weather and it is far less prone to interference. Controllers at Atlanta's Hartsfield-Jackson International Airport have grown so reliant on ASDE-X that the number of landings and takeoffs drops when it is switched off for maintenance, said Gary Brittain, president of the controllers' local union.
Without radio beacons on all vehicles, however, the system has a blind spot.
Capt. Mitch Serber, the ground safety chairman of the Air Line Pilots Association, said his union supports swift installation of ASDE-X in as many airports as possible. He says vehicles should get beacons soon.
"If we're going to get 100% out of our investment, we are going to have to equip ground vehicles," he said. "The severity and consequences are something we can't afford to ignore."