The FAA is investigating having VASI and PAPI lights begin flashing when the landing runway is occupied. Doing so could prevent “landovers,” where an aircraft continues its approach and touches down while another aircraft or vehicle is still on the runway.
The worst U.S. accident of this type was probably the night collision at Los Angeles in 1991, when a landing USAir Boeing 737 struck an AirWest Swearingen Metro holding on the runway awaiting takeoff clearance. Effectively hidden in the glare of the runway lights, the Metro was invisible to the 737’s crew. Since then, there have been a number of close calls under similar circumstances.
In a paper presented at a NASA conference in April, Kirk Swanson of Architecture Technology described preliminary tests of the final approach runway occupancy system (FAROS) installed on Runway 30 at Long Beach, Calif. His company developed the system for the FAA’s office of surface technology assessment. FAROS causes the PAPI or VASI lights to dim for a short period every two seconds whenever an aircraft or a vehicle enters the active runway, thereby providing a clear advisory message to the approaching pilot of a potential hazard.
The technique is elegantly simple and uses a method commonly applied to the control of road traffic lights. Here, a wire coil, or loop, is buried just below the road surface near the stop line of the traffic lights; any significant metal mass passing over it will briefly induce a small electrical current in the loop. This is then sent as a signal to the light-control unit, where it is processed against a traffic flow program and, in due course, the traffic lights change.
The principle at Long Beach is the same, with rectangular loops buried across the width of the taxiways leading to the runway, and there is more than enough ferrous metal in any aluminum or fiberglass skinned aircraft, or airport vehicle, to induce a current in the loop when passing over it. But in FAROS the resulting signals go to the PAPI control box, where they initiate continuous light sequences of 66-percent bright and 33-percent dim every two seconds to alert the pilot of an approaching aircraft of a possible incursion. The PAPI ceases to flash if the intruding aircraft or vehicle is simply crossing the runway, because in leaving the runway it will cross the coil buried in the taxiway on the opposite side, which sends another signal to the PAPI control unit, causing it to cancel the alert and resume normal bright operation.
Using this system, pilots are warned almost instantaneously of potentially hazardous situations, usually with ample time to take appropriate action. What’s more, pilots are already familiar with the PAPI and VASI presentations and would be immediately alerted should they change to the flashing sequence. In fact, many pilots in an early test group believed that the flashing signals made the PAPI easier to acquire visually when nearing the airport, simply because they were flashing.
By comparison, the process to warn pilots via the current advanced-technology solution deployed at the nation’s major airports can appear laboriously slow. The FAA’s Airport Movement Area Safety System (AMASS) takes in data from the local airport surface detection radar (ASDE) and then combines it with other data to determine whether a potentially hazardous situation exists. If it does it alerts the controller, who must first verify the safety hazard, then identify the conflicting parties’ callsigns, as well as the communications channels they are both using, and then notify one or more of them to take avoidance action. In some cases reported in the past, this has left as little as 11 seconds to avoid an accident.
While AMASS is generally effective, its need to work in conjunction with ASDE and other inputs leads to a complex and expensive system, which is unlikely to be fielded widely across the NAS. FAROS is significantly cheaper, with the major cost driver being the installation of the loops in the runway access taxiways. Its electronic control system requires a fairly simple computer.
The Long Beach tests have been deemed successful, and a formal FAA operational evaluation is planned in the next fiscal year. Besides flight trials, the evaluation will assess loop-placement strategies and system vulnerabilities, such as a runway service vehicle leaving an intersection by driving over the grass, thereby preventing FAROS from logging its exit.
Indeed, if FAROS becomes a future part of the airport environment, the FAA’s guidance to pilots will emphasize that it is purely an advisory system to assist in situational awareness on the approach, meaning that the decision to continue or go around remains with the pilot.
The agency is likely also to stress that the absence of flashing PAPI or VASI lights is not a positive clearance to land. After all, that runway service vehicle could just as easily enter the runway from the grass. As always, there’ll never be a substitute for keeping a good lookout.