Everyone has heard the old joke that the most dangerous part of flying is the drive to the airport. But what fewer people realize is that once you reach the airport and board the aircraft, the next most dangerous part of the trip happens when the aircraft is moving at its slowest, while taxiing out for takeoff, or taxiing in after landing.
Since 1990, airport surface accidents have been at the top of the National Transportation Safety Board’s (NTSB) "Most Wanted" list of transportation safety improvements. But media reports of surface incidents — or more commonly, "near misses" — invariably cover aircraft-to-aircraft encounters and rarely mention aircraft-to-vehicle incidents.
Reports in Dallas/Fort Worth newspapers earlier this year were exceptions to that rule. In April, it was reported that a tug towing a Boeing 777 at DFW was instructed to hold short of the active runway before being cleared to cross. But because of the tug’s speed or the aircraft’s inertia, it was clear to the watching controller that it would fail to come to a full stop before reaching the runway. The tug entered the runway, causing a landing MD-80 to take evasive action during its post touchdown rollout.
While further details were unavailable at this writing, the overall circumstances typify a runway incursion event of the type FAA is working hard to reduce. In the DFW case, only the controller’s vigilance appears to have been the key in averting a possible disaster. But this does raise the question of how such situations would unfold in low visibility, under moderate to heavy rain conditions.
Air traffic controller reports have suggested that legacy airport surface movement primary surveillance radars (SMR), which rely on "skin paint" returns, compared to transponder-interrogating secondary radars (SSR), can have difficulty in clearly detecting surface targets in heavy rain conditions. SSRs are not normally positioned for optimum airport surface coverage and, even if they were, they could not have helped at DFW, since aircraft being towed are not required to have their transponders operating.
Nevertheless, the tug and its 777 in tow would still have presented a fairly large visual and radar target. But small, more numerous airport vehicles are much more difficult to see by tower controllers, radar and pilots, and these pose a constant threat to surface safety.
FAA’s 2007 Runway Safety Report underlined that fact, noting "as many as 1,000 vehicle operators could work at a single large airport at the same time." Added to the problem is that from their lower vantage points, vehicle drivers are usually less able than pilots to discern runways and taxiways in poor visibility, and are often less familiar with the airport’s layout.
While the number of aircraft/vehicle accidents is still relatively small, the FAA report indicated a significant upward trend in overall vehicle incursions during 2006. A recent analysis of 110 high-risk runway incursions by the International Air Transport Association (IATA) indicated that 7 percent involved surface vehicles. With airport operations forecast to steadily increase in the future, vehicle monitoring is becoming essential.
Tracking Devices
The solution lies in the adoption of individually identifiable tracking devices on all vehicles that routinely enter the airport movement area, particularly on or near runways and taxiways. Such devices exist, and are already in wide use at a number of major airports around the world, although they have not yet seen significant adoption in the United States.
Ironically, most of the world’s airport vehicle tracking units are built by one of two U.S. companies — Era Corp., of Reston, Va., or Sensis Corp., of Syracuse, N.Y. Era in April announced the delivery of its 1,500th "Squid" tracking unit, and claims dominance of the market. Squid units are being used at airports including Amsterdam Schiphol, Singapore Changi and London Gatwick, according to Era.
It is expected, however, that other manufacturers will enter the market over the next two or three years. In March, VNIIRA JSC of Saint Petersburg, Russia, exhibited its Elf-N Mobile Radio Beacon for Automatic Dependent Surveillance-Broadcast (ADS-B), offering comparable characteristics to the American units, at the ATC Global conference in Amsterdam.
The Era and Sensis units, respectively called Squid (for squitter detection) and "VeeLo" (for vehicle locators), are small, lightweight Mode S "squitter" transmitters. The transmitters, linked to an internal GPS receiver, transmit low-power, 20 watt, ADS-B signals once per second at 1090 MHz, similar to higher-end airborne Mode S units. None currently operate at the general aviation 978 MHz Universal Access Transceiver frequency.
The squitter message includes the vehicle’s assigned ICAO 24-bit address, its radio call sign and a message element indicating its characteristics e.g., tug, rescue vehicle, snow plow. On the airport controller’s display, ground vehicles are easily distinguished from aircraft by their very different icons.
In the future ADS-B Air Traffic Management environment, with a tracking unit installed on a tug or another type of vehicle, both the tower controller and any approaching ADS-B-equipped aircraft would see a potential collision developing well in advance, under all weather conditions, eliminating the risk of a runway collision or a last-minute avoidance maneuver by pilots.
Importantly, too, the introduction of vehicle tracking is expected to bring significant benefits to crash/fire/rescue (CFR) operations. With a direct link from the controller’s surface monitoring system to the airport’s CFR Command Center, each vehicle dispatched to an accident could be surgically deployed via radio link to an optimum location with respect to fire, wind, smoke and evacuation conditions, and could be quickly redeployed should those conditions change.
Some specialists envision the day when each CFR vehicle would also have an in-cab display similar to that at the command center, allowing even greater flexibility at an accident scene.
Advisory Circular
FAA planned to issue an Advisory Circular, which was in draft form at this writing, to define standards for ADS-B-based airport vehicle tracking equipment. Informal discussions had been held with industry, and a final document was expected to be published for public comment in September. Following Federal Communications Commission and other approvals, a final rule is expected in 2010, but informed sources say because of the growing need, FAA will probably permit conditional use of the equipment at selected sites prior to that date.
FAA’s approach is to link the operation of the units with its Airport Surface Detection Equipment, Model X (ASDE-X) deployment, which is underway at 35 of the nation’s major airports and due for completion in 2010.
ASDE-X, a Sensis product, is an advanced terminal area and airport surface surveillance system that fuses the inputs of three separate sensor systems — ADS-B, surface movement radar and multilateration — to obtain accurate positioning of all targets, both on the ground and in the air. Also, ASDE-X introduces a unique Safety Logic function, where the system’s data processor computes the future movements of air and surface traffic in order to anticipate and then alert on potential conflicts.
FAA’s Advisory Circular also was expected to call for WAAS augmentation of the tracking units’ GPS receivers, to greatly enhance basic GPS accuracy and to bring the capabilities of the units to the level demanded in FAA’s airborne ADS-B requirement for aircraft surface movements in low visibility. Sensis had already incorporated WAAS in its newly introduced VeeLo NextGen model, which is programmable to only transmit within pre-defined airport areas, thereby decreasing possible congestion on the ADS-B 1090 MHz frequency.
Frequency congestion at 1090 MHz was a FAA and industry concern well before the vehicle beacon program, as the number of aircraft operating within high-density terminal areas has gradually increased. As a result, FAA’s Advisory Circular is expected to initially limit the number of operating vehicle systems to 50 per airport at any specific time. But it seems likely that limit will be raised to 200 once initial operating experience is accumulated and later could be increased.
On the other hand, FAA is expected to only approve installation of squitter beacons on vehicles that are required to enter the critical runway and taxiway areas, and to disallow installation on vehicles used exclusively on terminal ramps and maneuvering areas. Similarly, it is expected that beacons will be required to be turned off, either manually or automatically, when an equipped vehicle leaves the critical runway and taxiway area.
Nevertheless, one industry observer commented, even a ceiling of 200 vehicles could seriously limit essential vehicle movements at major airports. Meanwhile, another technical specialist said 50 equipped vehicles transmitting simultaneously would have roughly the same impact on the spectrum as the ADS-B transmissions of a single airliner, due to the low power and relatively short range of the beacon units. At Amsterdam’s Schiphol airport, there are more than 350 airport vehicles equipped with ADS-B squitter beacons, with reportedly no interference problems.
While FAA’s ASDE-X program is currently limited to 35 airports, this certainly does not limit the use of vehicle beacons at other airports. At many overseas locations, aircraft and beacon-equipped vehicles are tracked by airport Advanced Surface Movement, Guidance and Control Systems (A-SMGCS) that employ combinations of ADS-B, SMR, SSR and multilateration, so the specific lack of ASDE-X is not a deterrent. Consequently, we can expect that, as the implementation of NextGen proceeds, more and more airports in the United States will enjoy the added safety benefits that these small vehicle devices bring.
Options Evaluated For The Detection Of Foreign Objects
Although less well known, airport runways have always harbored another potential safety threat, that of foreign object debris (FOD). This was tragically demonstrated near Paris in July 2000, when a tire of an Air France Concorde burst upon striking a 16-inch titanium strip that had fallen onto the runway from a previously departing aircraft. Tire fragments penetrated the fuel tank, creating an instant fireball and causing the aircraft to crash, killing all 109 people on board and four on the ground.
While such an accident has not re-occurred, annual FOD damage and repair costs to engines, airframes and tires are estimated at over $4 billion worldwide.
This year, FAA is evaluating FOD detection systems that continuously monitor the runway surface for debris and automatically report its location to controllers. These systems also employ precision cameras, allowing controllers to assess the nature of the objects, such as metal pieces, bird carcasses, small animals, and even wind-blown newspapers.
Techniques under evaluation range from high-power millimeter wave radar/camera systems from QinetiQ, of Farnborough, U.K.; stand-alone cameras from Stratech, of Singapore; runway light mounted radar/camera units from XSight Systems, of Rosh Haayin, Israel, and a runway inspection vehicle radar/camera unit from Trex Enterprises, of San Diego.
FAA plans to issue an Advisory Circular in mid-2009 specifying airport FOD detection system standards, including their funding eligibility under the Airport Improvement Program.
Different Approaches To Safer Surface Movements
A common pilot remark following landing at a large but unfamiliar airport at night is that the outside scene is a "sea of blue," a reference to the massive number of runway and taxiway edge lights stretching into the distance. Finding one’s way through these to the terminal can be a challenging task.
Overnight package carrier UPS has a "cutting edge" technology here, in that its aircraft are gradually being equipped with ADS-B units that feed Electronic Flight Bag (EFB) cockpit displays. EFBs not only show the aircraft’s position on a plan view map of the airport’s runways and taxiways, but also, via an ADS-B-In mode, show the position and movement of all other ADS-B "squittering" aircraft on the surface. UPS had not formally stated its intention to equip ground vehicles with squitter units at the airports it serves, but the company seems likely to be an early purchaser of such units at its main hubs.
Two other companies have pioneered different non ADS-B solutions to assist in safer airport surface movements at night or in low visibility.
Honeywell’s Runway Awareness and Alerting System (RAAS) compares an aircraft’s location using GPS against a database of airport runways to pinpoint its location. A software enhancement to the company’s Enhanced Ground Proximity Warning System, RAAS provides pilots with audible warnings if they are taxiing onto an active runway unintentionally. It confirms the runway identification to ensure a pilot is taking off or landing on the correct runway.
Honeywell introduced RAAS in 2004 and reports installations on 200 air-transport and 1,470 business aircraft, with another 800 airline systems on order. In July, RAAS was added to the Airbus e-catalog as optional equipment, making it available directly through Airbus.
Rockwell Collins’ Flight Dynamics division developed intuitive pilot guidance via its Head-up Guidance System (HGS). Here, a similar database/GPS combination is used to present the pilot with a correct perspective and scale view ahead of the runway, its exits and its taxiways. Earlier demonstrations of a prototype system were impressive. The HGS program used velocity inputs to indicate safe runway exit points, showed the required deceleration to reach them, displayed the correctly centered exit path and subsequent taxi route, along with all named taxiways and intersections as they were approached, plus any required stopping points.
In the event of pilot uncertainty, the HGS display could be instantly switched to an airport plan view that clearly indicates the aircraft’s current position and heading, thereby providing immediate orientation.