It was an overcast October night at the Louisville (Ky.) Standford Field when five Boeing 727 freighters lined up for departure on the airport’s runway 35R. Four were owned by United Parcel Service (UPS) and the fifth by Federal Express (FedEx). But these airplanes were not carrying overnight packages, ready to leave for points across the nation. That would come after midnight, when as many as 150 large jets, from 727s up to 747s, would converge on the UPS freight hub at Louisville, to land, unload, reload, and depart again within a few hours.
Under U.S. Federal Aviation Administration (FAA) assessment, the five early-departing 727s were key participants in flight tests of Automatic Dependent Surveillance-Broadcast (ADS-B)–the avionics technology that is one day expected to become the cornerstone of Free Flight. The tests, called OpEval II, were part of an ongoing cooperative program of the Cargo Airlines Association (CAA) and the FAA’s Safe Flight 21 office. The FAA officials were on hand to assess whether ADS-B can allow pilots, during arrivals and departures in high density terminal airspace, to unmistakably identify other traffic in their vicinity and to maneuver themselves into an orderly and efficient, yet safely spaced, traffic stream.
Currently, this is the sole responsibility of air traffic control (ATC), which uses radar to provide steering vectors to incoming and departing aircraft. However, ATC’s procedural radar standards often result in large gaps–sometimes as long as eight or more minutes–between aircraft. The object of OpEval II is to see whether pilots can safely close these gaps by using ADS-B for self-separation, while still remaining under the ATC radar’s "umbrella" and the FAA controllers’ watchful eyes. During the air freight industry’s very busy early morning "rush hours," minutes saved in arrivals and departures across the fleet can add up to millions of dollars over a year.
The Louisville tests followed the CAA’s OpEval initial feasibility trials of ADS-B in Wilmington, Ohio, in 1999, and were the second step in the CAA’s multistage program to demonstrate–with FAA support–that ADS-B does hold the key to enhancing airspace capacity in high-density terminal areas. In many ways, the project affords an early view of the future air traffic management (ATM) concept where, under Free Flight monitoring by air traffic managers (no longer called controllers) and their radars, aircraft will fly individual "preferred trajectories," untrammeled by fixed air corridors. Free Flight will allow pilots to factor wind, temperature, payload, fuel burn and other aspects of each individual mission to derive the unique optimum route and trajectory for each flight.
Safety and Accuracy
On the night of the test flights–which were just one part of the larger, three-day, Louisville program–the main objective was to assess how safely and accurately pilots could self-separate themselves, from takeoff to landing, without ATC directions. Of course, controllers monitored the flights on their radar screens, and were prepared to intervene if safe separation between the aircraft was compromised. Pilots were to rely on their ADS-B units to maintain two-minute in-trail spacing from brake release on takeoff through runway exit after landing.
In one sequence, the spacing between five aircraft on departure was meticulous. This reporter observed the sequential takeoffs from an ADS-B equipped Rockwell Collins North American Sabreliner, which the company uses as a flying test bed for new avionics units and for systems still under development. The right side of its instrument panel was taken over by two new Rockwell Collins 8-by-8-inch (20-by-20-cm) electronic displays. It’s a tight fit for the small jet’s snug cockpit, but nicely sized for, say, the Boeing 777.
The Sabreliner was parked on the ramp, with its position shown on the screen at the tip of a white triangle. Although it was parked, its ADS-B system was still operating. So was the system in the FAA’s ADS-B equipped van, call-sign TRIOS, parked on the other side of the runway. The TRIOS van was used to simulate runway incursions in another part of the OpEval II tests.
From the airborne B727s, both the Sabreliner and the TRIOS van would appear as "targets" on the airport surface below. But watching the 727s from the Sabreliner display highlighted another ADS-B attribute: its ability to show relative data on other aircraft in view. For example, each of the tri-jets was flying at precisely 7,100 feet above the Sabreliner, rather than above ground, or above sea level. Similarly, when additional information on one aircraft, FedEx 9060, was called up by the Sabreliner’s pilot, its symbol turned green and a data box at the lower left of the Sabreliner’s screen showed 9060’s groundspeed (255 knots), its relative range (11.5 miles), and its relative closing rate (a negative 220 knots, moving away). This is vital situational awareness information for pilots, which no other avionics system has been able to provide before.
Beside the five cargo jets, the Louisville evaluations also included two FAA Convairs and the agency’s B727 test bed, plus an Aztec from the U.S. Transpotation Department’s Volpe Center. Non-government participants included the Rockwell Sabreliner, a Honeywell King Air, an Aircraft Owners and Pilots Association (AOPA) Bonanza, and four privately owned aircraft: Citation, King Air, Cessna 210, and a Piper Lance.
Most aircraft were equipped with GPS receivers, multifunction displays (MFDs) and ADS-B transceivers built by UPS Aviation Technologies (UPS/AT–formerly II Morrow Inc.) Yet much more is promised from ADS-B. UPS/AT engineers are developing computer algorithms to calculate and then display on the pilot’s screen any potential collision or "near miss" threats posed by other aircraft in the vicinity. The threats would be shown as yellow or red arrowhead symbols to alert the pilot. However, the engineers caution that this is a fairly long-term project. In nearer term development, the arrowheads of all displayed aircraft can be enhanced to carry a fine line "wand" ahead of them. The wand’s furthest tip will show each aircraft’s predicted position in two minutes time–another valuable situational awareness cue.
The display can also be pilot-selected to reduce screen clutter by showing only those aircraft within 2,000 feet above and below. And, for the Louisville OpEval II exercises, UPS/AT provided an answer to those general aviation pilots who had expressed concern that the aircraft identification included in the ADS-B data on the controllers’ screens would allow them to be precisely tracked (hence, the fear of "big brother") whenever and wherever they flew. A modification to the AOPA Bonanza installation demonstrated the solution: it simply displayed an arrowhead, plus relative altitude, but no ident, similar to the anonymous Code 1200 transponder "squawk" used by visual flight rules (VFR) pilots today.
Reducing Runway Incursions
But UPS/AT demonstrated another development that displays startling potential. The avionics manufacturer showed how runway incursion accidents, as well as tragedies like the Oct. 31, 2000, Singapore Airlines accident at Taipei, could be reduced. Here ADS-B provides pilots with a map display of the airport surface, indicating runways, taxiways, airport buildings and other facilities. These maps were used in OpEval II to assess their value in low visibility taxiing conditions, which in the past have caused pilots to become lost.
In an experimental application, the ADS-B computer on-board an approaching aircraft used a combination of the aircraft’s position, heading, altitude and landing-gear position to determine that it was about to land. This then triggered a special ADS-B transmission, which immediately caused the runway on the airport map displays in all taxiing aircraft and airport vehicles to turn red, and remain red until the aircraft landed and exited the runway. The same technique could be used by airport personnel who could generate on every aircraft’s surface map display a warning yellow or similar color along those parts of the airport’s runways or taxiways that are under construction.
Detailed analysis of the Louisville data is not expected to be completed until early 2001. It includes the correlation of many thousands of ADS-B positions with ATC radar, plus masses of other recorded data. But preliminary, though unofficial, assessments indicate that, barring unexpected anomalies, the tests have been a resounding success. As a result, CAA personnel are already planning their OpEval III program, scheduled for Memphis in April, 2001. Visit www.ads-b.com for more on this.