Two takeoff near-collisions in July at Chicago O’Hare and Los Angeles International Airports underscored the fact that runway incursion accidents–one of the NTSB’s top safety concerns–remain a threat. In both cases, pilots were forced to lift off prematurely when another aircraft appeared suddenly, crossing the runway ahead of them.
In theory, neither event should have happened, since both airports are equipped with the FAA’s radar-based airport movement area safety system (AMASS). But even after a long and expensive development, this system has had a troubled operational history: not alerting controllers about impending accidents or alerting them too late, leaving it to the pilots to avoid potential collisions. Now, the FAA is introducing a new technique, called Safety Logic, which should overcome these shortcomings.
The AMASS computer takes inputs from a rapidly rotating “skin paint” radar–usually located above the tower–which monitors surface movements, along with transponder returns from a nearby ATC surveillance radar, and then calculates the collision risk and provides visual and audio alerts to controllers. But the surface movement radar– called airport surface detection equipment, model 3 (ASDE-3)– can be blocked by rain, and the separate surveillance radar rotates relatively slowly, scanning each airborne aircraft at roughly five-second intervals.
Lost targets are reportedly common, very short warnings–as short as 10 seconds–have occurred, and false alarms have also been a problem. And at Boston Logan, a quirk in the AMASS software prevents it from determining collision threats between aircraft operating on the airport’s converging runways. Consequently, many controllers have lost confidence in AMASS, and some reportedly ignore it.
In response, the FAA began installing a newer-technology system called ASDE-X, built by Sensis of Syracuse, N.Y. This equipment’s surface radar operates at a higher frequency, with better weather penetration and, perhaps more important, uses a multilateration (triangulation) technique, to plot the position of every aircraft transponder–whether on the surface or airborne–once per second, and with much more accuracy than the conventional surveillance radar. As a result, ASDE-X eliminates the problems that accompany amass. Reportedly, controller acceptance has been favorable, and there appear to have been no reports of near misses at ASDE-X-equipped airports.
The FAA is adding a Safety Logic feature to the ASDE-X system. Already installed at Orlando, Atlanta, St. Louis and Seattle Tacoma, the Sensis Safety Logic software provides the controller with both aural and visual alerts. The visual alerts are unambiguous, with yellow caution or red warning circles around the aircraft in potential conflict, with their callsigns plainly visible. Sensis also builds small transponders for airport vehicles, which would then appear on controllers’ screens, although these are not yet in FAA plans.
Eventually, subject to FAA funding, ASDE-X will replace ASDE-3/AMASS installations at all major airports in the continental U.S., but the cash-strapped agency has not yet published a schedule for this work. Overall, however, technologies such as Safety Logic are encouraging steps that will reduce the risk of surface accidents.
