A system of in-pavement stoplights, designed to prevent runway incursions, has received endorsement from Transportation Department Inspector General Calvin Scovel. Known as runway status lights (RWSL), the system has been under test at Dallas/Fort Worth International Airport (DFW) since 2003 and at San Diego International Airport since 2005.

The technology uses automated surveillance-driven lights that work as an independent, direct warning system to alert pilots of aircraft departing or crossing a runway that it is already occupied and active. The lights are installed at runway/taxiway intersections and at departure points along the runways.

The DOT IG determined that RWSL is a viable technology for preventing runway incursions. But the report cautioned that while the FAA has made progress in developing RWSL, the technology is still in the early stages of implementation and much work remains for the agency to achieve full deployment.

At DFW, RWSL uses input from the Airport Surface Detection Equipment-Model X (ASDE-X) prototype system, which in turn uses three sources of surveillance: Airport Surveillance Radar (ASR); ASDE-Model 3; and a transponder multilateration system.

Clear Message to Pilots
The external surveillance information provides position and other information for aircraft and vehicles on or near the airport surface. The RWSL safety logic processes the surveillance information and commands the field lighting system to turn the runway status lights on and off in accordance with the motion of the detected traffic.

The RWSL automatically indicates promptly and clearly to pilots and vehicle operators when it is unsafe to enter or cross a runway or to begin takeoff. It does not interfere with controller workload or the normal flow of airport traffic. The lights indicate runway status only and do not convey an ATC clearance.

Illuminated lights warn pilots of potential runway conflicts and prompt them to notify the tower before proceeding if a contradicting clearance has been issued. Therefore the system might also help to identify potential controller operational errors.

The IG said that during operational evaluations and subsequent modifications at DFW for runway entrance lights and takeoff hold lights, RWSL met or exceeded all performance criteria specified. In addition, all system users at DFW agreed that it works as intended and has no known negative impact on capacity, communication or safety.

“Further, runway incursions on the test runway at DFW (Runway 18L/36R) have decreased by 70 percent,” the report said. “During the 29 months before testing (Oct. 1, 2002, through Feb. 28, 2005), 10 runway incursions occurred at DFW. During the 29 months after testing (March 1, 2005, through July 31, 2007), only three occurred.”

The RWSL illuminate red when it is unsafe to cross or depart from a runway, thus increasing the crew’s situational awareness and decreasing the potential for a runway incursion. It consists of both runway entrance lights and takeoff hold lights. Runway entrance lights, which illuminate red when a runway is unsafe to enter or cross, are visible to aircraft from taxiways holding short of runway intersections.

Takeoff hold lights illuminate red to indicate an unsafe condition when an aircraft is in position for takeoff and another aircraft or vehicle is either on or about to enter the runway in front of it. Takeoff hold lights are visible from the takeoff hold position.
Most runway incursions are caused by a lack of situational awareness, and more than half of all runway incursions are the result of pilot deviations. But there is currently no automated technology in place to directly warn pilots of potential runway conflicts.

Reducing runway incursions has been on the NTSB Most Wanted List of Safety Improvements since the list’s inception in 1990. But the Safety Board has characterized the FAA’s response to this safety improvement to be unacceptable because the agency has not yet implemented a technology that gives immediate warnings of probable runway incursions directly to flight crews in the cockpit.

RWSL was first developed and demonstrated at Boston Logan International Airport in the early 1990s, but the surveillance systems available at that time needed improvements before the technology could be viable.

The FAA is also developing a third type of runway status lights–runway intersection lights. These lights are designed to warn pilots on a runway when another aircraft is departing from or landing on an intersecting runway. The FAA plans to begin testing these lights at Chicago O’Hare International Airport this year.

The IG’s report also cautioned that before RWSL can be deployed system-wide, differences between the ASDE-X prototype system at DFW and the ASDE-X system it is deploying at other airports will have to be addressed. The ASDE-X system used to test RWSL at DFW was purchased by the Dallas/Fort Worth Airport Authority in 2001 and is not part of the FAA’s current, national ASDE-X program.

The initial investment decision for RWSL identifies 19 airports for planned RWSL implementation, between November 2009 and March 2014. In the FY2008 House transportation appropriations bill, the House Appropriations Committee provided $20 million for RWSL, an increase of $14.7 million over the budget request.