FAA may be closer to weeding out a thorny safety issue persistent enough to have made the National Transportation Safety Board's "Most Wanted" list for 15 straight years: Coming up with a real-time cue to warn pilots of an impending on-airport collision.

The agency in June completed an operational evaluation of a Runway Status Lights system that uses a stoplight analogy to warn pilots or vehicles approaching a runway whether it is safe to cross. The effort is part of FAA's push to reduce the most serious runway incursionssituations where two aircraft or an aircraft and a vehicle get too close together on the airportand possibly change the status of the action item in NTSB's database from "unacceptable" to "acceptable."

The runway incursion problem is thorny for several reasons: It involves a wide range of airport workers, from air traffic controllers to pilots to anyone driving on the airside surface; solutions can't unduly restrain airport throughput and capacity, and implementations must be customized to fit the unique layout of each facility. According to FAA, there were 325 runway incursions last year, 324 in 2003 and 339 in 2002. The agency divides the close calls into four categories, with Category A being the most serious, B being somewhat less serious and C and D progressively less risky. From 2000 to 2005, Category A and B incidents accounted for about 12% of the total incursions and resulted in six collisions but no fatalities.

In parallel with launching education efforts and airport layout and taxiway paint scheme modifications to address the problem, FAA in the early 1990s also opted for a technological tool to alert controllers of an impending situation. The Airport Movement Area Safety System uses output from an ASDE-3 surface radar on the airport and custom-designed software to alert controllers when an incursion might occur based on the position and velocity of vehicles and aircraft near the runways.

AMASS tracks as many as 256 surface and airborne targets on a 1-sec. update cycle, displaying the information on a dedicated screen in the tower cab. It will warn controllers, but not pilots or other operators, with audio and visual alerts if a predefined separation criterion is breeched, allowing controllers to alert pilots or others if necessary. The $139 million system has been operating at 34 of the busiest US airports since 2003 and has been credited with at least four "saves." FAA points out that AMASS is considered a backup in that controllers retain the primary responsibility for separating aircraft.

In one case in November 2004, AMASS alerted controllers in Philadelphia that a tug was crossing the active runway as an aircraft began its takeoff roll. Once alerted, the pilots took evasive action. Though no one was hurt, the aircraft was damaged when its wing struck the tug. Last June, an aircraft arriving at New York JFK had to perform a go-around after AMASS alerted that an aircraft was holding for departure on the same runway. FAA says that on average there are one or two AMASS alerts every week.

Critics cite two persistent problems with AMASS as a backup: It doesn't work in all weather conditions, and it may not provide enough time for those in harm's way to take action.

National Air Traffic Controllers Assn. officials complain that the system's sensitivity must be downgraded during times of moderate or greater precipitation due to the physics of the ASDE-3 radar, otherwise it generates too many false alarms to be useful (it doesn't help that NATCA and FAA are currently in the midst of contentious contract negotiations). In one incident at JFK, NATCA says an Israeli pilot accidentally crossed a runway after an ABX cargo jet was given clearance to take off. The incident occurred late at night in foggy conditions with heavy rain. The ABX pilot estimated that his aircraft's nose came within 75 ft. of the El Al aircraft and the tail came within 45 ft.

FAA says AMASS limitations are widely known, which is one of the reasons the agency developed an improved version called ASDE-X. The issue with AMASS is that ASDE-3, which relies on short-range, narrow-beam radar technology, "sees" moderate to heavy rain as multiple targets and isn't capable of distinguishing "raindrop targets" from aircraft, other vehicles or people. If left to operate in its normal mode, AMASS becomes "very active" during rainstorms, FAA says, generating numerous false alarms and creating a nuisance and safety hazard for controllers. As a result, controllers downgrade the system to "limited" mode, degrading much of its conflict detection and warning capability, according to NTSB.

ASDE-X solves the problem by adding in more sources of informationsurface radar, GPS-based position reporting and multilateration, a technology that uses special ground-based antennas around the airport to receive aircraft and ground vehicles' 1090 MHz transponder signals. By having multiple independent inputs, ASDE-X works in any weather. The system was declared ready for use in October 2003 and is installed at four airports, the largest of which are Orlando and Houston Hobby. FAA plans to deploy it to 31 additional airports over the next six years, including 22 that currently have AMASS.

A second complaint comes from NTSB officials, who say it takes controllers too much time to relay the alert information to pilots or others in harm's way with either AMASS or ASDE-X. According to NTSB, simulations of AMASS performance using data from actual incursions show that accidents can occur in as little as 8 sec. after an alert. Once alerted, a controller must determine the nature of the problem, determine the location of the aircraft, identify the aircraft involved, determine what action to take, and only then issue a command or warning to the pilot.

NTSB says that in an incident at Los Angeles International Airport in August 2004, "there are strong indications" that AMASS alerted the controller beyond "the point of no return." In that incident, an Asiana 747-400 was on short final when controllers mistakenly ordered a Southwest Airlines 737 onto the same runway for departure. The Asiana cockpit crew saw the 737 and made the decision to abort the landing. AMASS had detected the threat but sounded only 10 sec. before the collision would have occurred. "Until there is a system in place to positively control ground movements of all aircraft, with direct warning to pilots, the potential for this type of disaster will continue to be high," the Board declared in its most recent annual "Most Wanted" report.

FAA's new Runway Status Lights system could address at least one of NTSB's concernsgiving a direct warning of an impending collision to pilots and vehicle operators who are about to cross an active runway. The agency developed a prototype RWSL in 2001 and began operational testing in March 2005 at Dallas/Ft. Worth International and San Diego International airports. The system uses aircraft and vehicle information from ASDE-X and five red "stop" lights embedded in taxiway centerlines from the hold line to the runway edge at runway/taxiway intersections. The stoplights are oriented in such a way that they are visible only to pilots and vehicle operators entering or crossing the runway from that location. When the combined RWSL/ASDE-X system determines a conflict, the lights turn red, alerting the pilot to stop immediately. FAA plans to test RWSL with AMASS next year.

Based on FAA's assessment that RWSL is performing well and is "highly applauded by both air traffic controllers and the pilot communities," the agency says it now is formulating a plan to expand the program to more airports. It is too soon to tell, however, if RWSL could end NTSB's 15-year rift with FAA.