Runway status light (RWSL) systems rolling out at many large U.S. airports are meeting expectations as an added safety layer, but the FAA is stepping up educational efforts to address confusion about how ATC uses the information they provide. In a nutshell: ATC does not use it.

The agency is aware of several incidents in which pilots saw the lights, which signify that a runway is occupied and an aircraft or vehicle should not proceed, but nonetheless followed ATC instruction to use the runway anyway, believing that ATC’s directives supersede the RWSL system.

“Not all pilots know that ATC has no idea if these lights are on or off during operation,” said Jim Fee, manager of the FAA’s runway safety group, at the Air Charter Safety Symposium on March 7. “Some pilots feel that, ‘ATC must know these lights are on, but they are giving me these instructions anyway.’”  He clarified, “It’s not that the tower controller knows better. It’s the exact opposite.”

Designed to meet an NTSB recommendation to provide pilots with direct, real-time warnings that runways are in use, RWSLs are slated to be installed at 17 large-hub airports by next year. Red lights positioned at runway ends and entrances, RWSLs light up when aircraft or vehicles are detected on the runways. The fully automated system relies on surveillance data from several sources, among them Airport Surface Detection Equipment-X, to track surface movement.

RWSLs are meant to be a last line of surface-movement defense in the rare instances that ATC clearances create conflicts, or an aircraft or surface vehicle does not follow instructions. The system is designed to minimize additional controller workload, so its status, which changes constantly as aircraft enter and exit runways, is not indicated in the tower. Because RWSLs are installed to be visible from flight decks—embedded in the airport surface, angled up slightly—most lights cannot be seen by controllers.

The system has prevented several incidents, Fee said, including two saves that required high-speed aborts. In one case, a ground support worker drove onto an active runway, triggering the system as an aircraft was early in its takeoff roll.

“The majority of events that we see are complete saves,” Fee said.

Unsure? Stop and Check

But other incidents have caused concern, such as one last July at Las Vegas McCarran International Airport (LAS). A mainline-carrier narrowbody and a business jet were lined up for takeoff on intersecting runways. As a result of a miscommunication between controllers, both aircraft were cleared for takeoff almost simultaneously.

The airliner rolled first, and the RWSLs on the business jet’s runway illuminated. The business jet began its takeoff, however, and both flight crews executed an ATC-coordinated emergency procedure in which one aircraft climbed rapidly and the other stayed low to create needed separation at the intersection.

Communication between the business jet crew and ATC immediately after the incident made it clear that the pilots saw the warning lights. But they assumed the controller knew the lights were on when he issued the takeoff clearance. “[LAS] is not the only event we’ve had like this,” Fee said, noting that the agency stepped up its outreach after the incident. “It’s not a perfect system, but the data we’re getting shows it’s an effective one.”

What should pilots do when RWSLs contradict the tower’s instruction? “The best action is to stop and question ATC,” Fee said.