Many aviation officials consider it the most dangerous part of a plane trip: moving across a runway just as another aircraft is taking off.
The U.S. averages almost one runway incursion a day, creating the potential for serious accidents. Ground collisions between commercial airliners have been among the deadliest plane disasters. Pilots and safety officials are watching a program at Dallas/Fort Worth International Airport, the primary test bed for preventing such runway incidents.
The system uses a series of computerized lights embedded along a runway to signal pilots. It's more useful than the way they now get information -- by looking out a cockpit window or relying on controllers.
A year into tests at D/FW, the Runway Status Lights program has won broad support from pilots and airport officials. But with funding constraints at the Federal Aviation Administration, the program may not expand fast enough to prevent another serious accident.
Incursions represent "a highly dangerous situation," said Capt. William Mino of the Allied Pilots Association, the union for American Airlines Inc. "The chance of loss of life is so great that any of them is too many."
Since 1990, the National Transportation Safety Board has included stopping runway incursions as one of its five "most wanted" aviation safety improvements. The independent agency, which makes safety recommendations to the FAA, wants pilots to get immediate warnings of possible ground collisions instead of waiting on air traffic controllers.
Airports in Boston, New York and Las Vegas have experienced high-profile near-collisions since last summer, prompting heightened attention from safety officials.
In the incident at Las Vegas' McCarran International Airport, a controller confused two departure aircraft and cleared an Air Canada jet just as an America West plane was taking off.
The America West pilot later said he was 100 feet above the Air Canada plane as he passed over it, according to the NTSB. (America West is now part of US Airways Group Inc.)
The worst runway incursion occurred in March 1977, when a KLM Boeing 747 attempting to take off from Tenerife in the Canary Islands collided with a Pan Am 747 coming from the other end of the runway. The crash, the deadliest in commercial aviation, killed 574 people.
How it works
At D/FW, Runway 18L/36R features the series of red lights embedded in the runway, flush with the pavement.
If another plane is crossing a runway, "takeoff hold" lights illuminate to warn pilots to stop their departure. If a runway is unsafe for entry or crossing because a plane is taking off, runway entrance lights illuminate to warn pilots to stay away.
The system operates for every plane moving across the runway, not just when someone has made an error, said Jonathan Bernays, assistant group leader for surveillance systems at the Massachusetts Institute of Technology's Lincoln Laboratory.
That means the system must be able to process as many as 3,000 light activation commands a day for a busy runway -- one that operates up to 20 hours a day, with 50 takeoffs or landings an hour, at several intersections.
"If all we had to do was turn on one light every few minutes, it would be trivial," Mr. Bernays said. "What's hard about the status lights is doing it right every time."
The takeoff hold lights have been in place for four months. They're the second phase of the safety lights program that launched in March 2005 after two years of engineering and software testing.
The runway entrance lights were initially intended for a three-month evaluation.
But after an "overwhelmingly positive" reaction during initial testing, the program has remained in place, said Jaime Figueroa, the FAA's surface systems manager in Washington.
"The response has been very positive, from pilots, air traffic controllers and airport operators alike," Mr. Figueroa said.
But the system hasn't been flawless. Some pilots have been reported to taxi over illuminated lights. Other concerns remain about pilots seeing the lights go off and moving ahead without clearance from a controller, though officials say that properly trained pilots have handled the system well.
The system's designers are quick to note that the lights were never meant to be a first-line defense but a tool that helps in case of human error.
"We have a very low tolerance for accidents," said Mr. Bernays. "You need a layered defense. The expectation of this program is it provides an independent backup to all of the procedures and training that currently give us a very safe system."
The program's cost for one runway at D/FW was $2 million, though other sites could be less. The San Diego airport is launching its own lights test program using a different surface radar.
When the program might expand beyond there is unclear. The FAA is revamping its funding structure to implement key technology upgrades and has fallen short on plans to expand other surveillance infrastructure.
Meanwhile, airport officials and pilots in Boston, Chicago and Los Angeles are interested. The FAA even hosted a Japanese delegation interested in the safety lights technology, Mr. Figueroa said.
Pilots and safety officials say they're hopeful that the program will attract the funding needed.
The U.S. had 324 incursions in the fiscal year that ended last fall, including three close calls between commercial jets that were deemed the most serious, according to FAA data. That figure has dropped from 424 incursions in 2000.
"We are working hard and making progress, but we are not there yet," FAA administrator Marion Blakey told a Senate panel last fall.
The D/FW test grew out of more than a decade of efforts to build an effective warning system for pilots.
MIT researchers, working with the FAA, sought to use marine radar technology for a pilot warning system in the early 1990s. The project, tested at Boston Logan International Airport, had too many false alarms and was discontinued until new surface surveillance systems could be deployed.
D/FW Airport has been the test bed for other aviation systems, including one to help pilots and controllers predict the weather planes would fly through. As the nation's third-busiest airport, D/FW is a top candidate for such projects.
The airport invested in the advanced technology at the beginning of this decade as part of a broader construction effort. The system, which gives controllers a comprehensive view of planes on the ground, also provides the foundation for the runway lights program.
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