Lab Notes

Runway Status Lights
Posted October 2012
Protecting aircraft when they are most vulnerable—
during takeoff and landing

Jim Eggert and Eric Shank of the Surveillance Systems Group are happy to have assisted in the development and deployment of a system that aids in preventing runway incursions at several airports. Much work has focused on improving safety during flight, but at the point where aircraft come closest at high relative velocities—on takeoff and landing—more can be done to improve the safety of aircraft and passengers. Runway incursions, when an aircraft or vehicle is on a runway without permission from air traffic control, are a daily occurrence in the United States. Preventing runway incursions that lead to accidents has been on the National Transportation Safety Board's "Most Wanted List" for over two decades.

Although the domestic aviation system in the United States is one of the safest man-made transportation systems devised, accidents still occur. One of the aviation industry's most urgent safety concerns is that of high-speed collisions involving aircraft on runways. ("There are fender-benders [low speed interactions], but they usually don’t endanger the flying public," according to Shank.) Such collisions may be between two aircraft or one aircraft and a service vehicle.

Takeoff hold lights on a runwayTakeoff Hold Lights indicate that there is some obstruction (in this case crossing traffic), so the pilot should hold position until the runway is cleared.

Takeoffs and landings at airports are critical points in an aircraft's flight. An airplane accelerates down a runway on takeoff, trying to gain enough speed to achieve lift off, and it would be very difficult to stop it if something appeared on the runway in front of it. Similarly, landing aircraft are slowing down, trying to reduce their lift, and at some point, they cannot accelerate again to regain altitude. In either case, an object on the active runway would certainly be problematic. Eggert and Shank and their associates are working with the Federal Aviation Administration (FAA) to eliminate runway incursions during takeoffs and landings.

Runway accidents often develop so quickly that mediation by air traffic control (ATC) personnel is often impossible or ineffective. Timely communication of runway-safety information directly to pilots is often required to avoid a runway incursion or collision. The concept of notification relies on the ability to alert at least one of the aircraft or vehicles in the conflicting scenario. In some cases, for increased safety, redundant indications are provided to everyone involved.

Currently, the most effective direct notification system at towered airports is the runway status lights (RWSL) system, developed for the FAA by Lincoln Laboratory. RWSL indicates to a pilot when a runway is unsafe by turning on special red lights embedded in the pavement in full view of the pilot and other nearby personnel, or by flashing lights to pilots on approach to the airport.

It's easily understood. Red means stop. People understand that.The RWSL system operates independently from the clearances issued by ATC and thus serves as an independent layer of safety. RWSL meets a long-standing, well-defined safety need to help prevent runway incursions and accidents by combining current ground-based radar and multilateration technology with advanced processing to control in-pavement lights that directly alert pilots to runway collision hazards. Of the technologies specifically addressing runway incursions, RWSL provides the most timely, most effective, and most highly automated technology to notify pilots and vehicle operators on the airport surface of potential incursions.

Eggert recalls that their initial work was a pair of prototype simulations at Boston's Logan International Airport. The first one involved simulated surveillance, pseudopilots (a computer display that allowed a technician to control several simulated aircraft), and a controller. This simulation allowed the concept to be tested with realistic controller-pilot communications and aircraft motions. The second prototype used real, live surveillance data and showed runway status light operation as it would occur, but only on a computer display and on a model board with fiber-optic lights, not with real lights on the airfield.

The second simulation involved real data, a simulated controller, "pseudopilots," and a model board with light-emitting diodes. "We were given a room in the tower so we could watch the same data that the controllers saw," Eggert says. The conclusion of the initial work was that the radar technology at the time was insufficient to maintain the necessary high degree of proper signaling with a minimum of false alarms (which would reduce runway capacity). "If something is happening that would make it dangerous to continue what they are going to do, we want them to know," Shank says. "Otherwise, we don't want to interfere with operations because delays cost money." However, as technology improved, RWSL were installed at several airports. The first fully functioning operational prototype was installed at Dallas/Fort Worth International Airport (DFW), Texas, where a prototype Airport Surface Detection Equipment (ASDE-X) radar was installed. "It reduced runway incursions by 70% when tested at DFW," according to Eggert.

The four components of RWSL (as shown in the sketch of a typical airport) are Runway Entrance Lights (RELs), Takeoff Hold Lights (THLs), Runway Intersection Lights (RILs), and Final Approach Runway Occupancy Signal (FAROS). Each of the first three types of light has only two states: "On" (lights illuminate red) and "Off" (lights not illuminated). No third state exists; RWSL never, for example, displays green lights. The fourth component, FAROS, also has two states: "On" (lights illuminated white over red) and "Flashing" (lights flashing on and off).

Illustration of how RWSL workA typical airport environment supplied with an RWSL system will have four types of status indicators. The REL, THL, RIL, and FAROS components of RWSL are shown in their relative locations on and near active runways. (Click image to enlarge.)

RELs are placed at runway/taxiway intersections and are visible to a pilot taxiing toward a runway. They indicate to the pilot if it is unsafe to enter or cross a runway because it is currently or will soon be occupied by high-speed traffic, such as an aircraft taking off or landing, and the pilot should stop immediately. (For simplicity of description, the discussion in this note emphasizes aircraft-to-aircraft encounters, but it should be kept in mind that RWSL has also been shown to be effective in averting aircraft collisions with surface vehicles.)

THLs are placed on the runways to be visible to the pilot in position for takeoff. They indicate to the pilot that it is unsafe to take off because the runway ahead is occupied by another aircraft. If a pilot is holding on a runway when THLs illuminate red, the aircraft should remain in position. If a takeoff roll has begun when a pilot observes illuminated THLs, the pilot should stop the aircraft and notify ATC that the plane has stopped because of red THLs.

RILs are placed on runways approaching an intersection with another runway to indicate to a pilot in a takeoff or landing roll that the intersection ahead is unsafe to enter or cross because there is a potential conflict at the intersection. When RILs illuminate red, the pilot or vehicle operator should stop before the intersecting runway.

FAROS is a flashing signal imposed on the already-existing precision approach path indicator (PAPI) lights, visible to aircraft on final approach to a runway. They indicate to pilots that the runway is occupied, and the pilot should visually acquire the other traffic and may have to contact the tower to verify clearance to land or, absent that verification, go around instead of land.

RWSL works seamlessly with existing and planned ATC procedures. The RWSL system is effective because an indication of a conflict is

  • Transmitted directly to the pilot(s) involved.
  • Generated by computer logic.
  • Is not dependent on the visible detection by controllers and/or vehicle or aircraft crew members to enhance safety during night operations or periods of restricted visibility.
  • Does not depend on the availability of clear audio channels.

At the request of the FAA, Lincoln Laboratory reviewed runway incursions in the United States between 1997 and 2000 at 100 of the busiest airports, concentrating on those incursions that involved at least one large passenger jet and were classified as "high hazard" or had a miss distance less than 100 feet. The study determined that RWSL might have prevented or mitigated 75% of the 167 identified incursions. The study suggested that the efficacy of RWSL stems from their ability to directly alert pilots of the runway status with minimal latency. Furthermore, RWSL helps prevent the occurrence of incursions—the predecessors of accidents—by increasing the situational awareness of pilots on runways and taxiways.

According to the FAA and the NTSB, RWSL is a viable and important technology for reducing runway incursions. In addition, RWSL has gained widespread support among user groups. It requires no human processing or warning, does not increase the ATC procedural workload, and does not interfere with other pilot procedures and tasks. Pilots, pilot union officials, air traffic management, and the airport operator at DFW all agreed that RWSL works as intended and has no known negative impact on capacity, communication, or safety. NTSB officials stated that RWSL is a promising technology for addressing its long-standing recommendation to provide pilots with direct warnings of potential runway conflicts.

As a result of successful operational evaluations of prototype RWSL systems at DFW and San Diego International airports, the FAA announced in 2007 its decision to install RWSL at 23 major airports in the U.S. National Airspace System. The FAA contracted with industry to produce the Lincoln Laboratory–certified system for delivery in 2009 to Los Angeles International Airport. The same system has also been deployed at Boston's Logan International Airport. Lincoln Laboratory is currently working with industry and the FAA to complete the technology transfer of RWSL.

"It has been a great project to work on," Eggert concludes. "It's easily understood. Red means stop. People understand that."

"We still have to be very efficient in getting planes into the air," Shank says. In case an error is made, all the pilots have to do when they see a stop red light is to contact the tower for further information or instructions. "RWSL makes our safe aviation system even safer," Eggert concludes.


top of page

Newer and more sensitive satellite-tracking telescopes are helping to solve the first problem of locating objects down to less than 10 cm in size. Once each object is located, it needs to be continuously tracked to define its orbit. Now, Lue's analysis comes into play. The six-hour frequency conjunction mentioned above is for a simple sphere. Lue proposes that if the orbits can be defined more accurately, elongated ellipsoids of potential future locations of objects will not overlap as often and the "ellipoidal time between conjunctions" can be extended to 15 days—a significant improvement over six hours. With these tools—more accurate measurements of orbiting objects and improved algorithms for defining future locations of the objects—only those very close conjunctions will require a notification to satellite owners to suggest that they move the satellite to avoid the collision. As an added bonus to the collision avoidance, "there shouldn’t be as many additional influxes of debris into the Space Catalog," Lue says.