Graham Warwick/Atlanta

In late August, NASA's Boeing 757 testbed was to be seen taxiing around Atlanta, Georgia's, Hartsfield Airport, occasionally taking off, only to land a few minutes later. Despite the excellent weather, NASA was testing technology which comes into its own when visibility deteriorates. The Low Visibility Landing And Surface Operations (LVLASO) programme is a key element in a drive to allow full-tempo airport operations to continue as weather worsens.

The goals of NASA's Terminal Area Productivity (TAP) programme are to allow more operations per runway and more runways per airport. Specific objectives are to increase single-runway throughput in non-visual conditions by 12-15%, and reduce lateral spacing for independent operations on parallel runways to below 1,040m (3,400ft), says programme manager Rose Ashford.

Approach operations are now limited by buffers inserted into the aircraft arrival spacing by air-traffic control (ATC) to allow for imprecise flight-management and wake-vortex uncertainties. Separation requirements for parallel- runway operations are set by aircraft navigation performance, communication time delays and wave-vortex concerns. "End deliverables" of the TAP programme will include the Aircraft Vortex Spacing System (AVOSS).

"With a four-dimensional FMS[flight-management system], the aircraft knows where it is and where it is going, but ATC can't access that information," says Ashford. Providing the ATC-automation system with access to FMS information will reduce spacing and arrival uncertainty, she says. This will allow ATC to optimise approach spacing and routes dynamically as weather changes.

Increasing arrival capacity , however, will create congestion on the ground, unless steps are taken to move aircraft off the runways and along the taxiways more efficiently. This is where the LVLASO system comes into play. The goal of this element of the TAP programme is to achieve safe and efficient runway and taxiway operations in instrument conditions, says programme manager Wayne Bryant.


System elements

The LVLASO system has three elements. The Roll-Out and Turn-Off (ROTO) system is intended to help pilots locate and use high-speed exits quickly and safely on landing, to minimise the time spent on the runway. The Taxiway Navigation and Situational Awareness (T-NASA) system is designed to provide taxi guidance and traffic information in low visibility. The Dynamic Runway Occupancy Measurement System (DROMS) is intended to help ATC determine approach spacing.

The DROMS creates a database of current runway-occupancy times by aircraft type and weather conditions and is designed to be used in conjunction with the AVOSS to determine arrival spacing. While spacing between a heavy aircraft and a following small aircraft would be determined by wake-vortex considerations, the spacing between a small aircraft and a following heavy aircraft would be determined by runway occupancy, Bryant explains.

Demonstrations at Atlanta concentrated on the airborne elements of the LVLASO system, and their integration with the ground-surveillance system which provides the information needed for their operation. Atlanta's Hartsfield Airport was chosen because it already has the ground-infrastructure required.

NASA's 757, in its first use as an experimental testbed, was equipped with head-up and head-down cockpit displays linked to the ROTO and T-NASA systems, which were hosted on Silicon Graphics workstations in the main cabin. The objectives of the LVLSO prototype demonstration were to validate previous simulations and assess the system's performance, says co-principal investigator Denise Jones.

The aim of the LVLASO programme, Jones says, is to improve the safety and efficiency of roll-out, turn-off and taxi operations in conditions down to Category IIIb, defined as a runway visual-range of between 45m and 215m. The ROTO system is designed to prevent time spent on the runway increasing as visibility decreases, by providing the pilot with steering and braking cues to arrive at the selected exit with the appropriate speed. The T-NASA system provides the pilot with an airport moving-map display showing the taxi route and surface traffic and generates steering cues to follow the cleared route in low visibility.

The ROTO system uses information from the aircraft's global-positioning and inertial-reference systems to generate head-up display (HUD) guidance symbology. The NASA 757 was equipped with a Flight Dynamics HUD for the Atlanta demonstration. On approach, the HUD presents a "virtual" runway made up of edge cones and shows the exit selected on the LVLASO pilot-input device. On touchdown, the system calculates the distance to the exit and displays ground speed, the predicted exit speed for current deceleration, and the resulting ground-speed error (see diagram).

Near the exit, the HUD symbology changes to include a "TURN" command, which appears 3s before the turn and begins to flash 1.5s before. A line appears across the virtual runway indicating where the turn should be started, along with an oval "football" symbol showing the location at which the desired exit speed will be achieved. The aim is to place the football just short of the line, Jones explains.

While the ROTO system is being developed by NASA Langley, the T-NASA system is being developed by NASAAmes. "Pilots comment that surface operations are often the most difficult part of a flight," says Dr David Foyle, a research scientist with NASA Ames' human-factors division, noting that the current method of taxi navigation is "Mk1 eyeball and a chart".

The T-NASA system is designed to replace situational-awareness cues which are missing in low visibility, by providing a head-down moving map and three-dimensional audio alerts of surface traffic. The latter feature, not demonstrated at Atlanta, would provide the pilot with an aural warning appearing to come from the direction in which the traffic lies, Foyle says.

The system is designed to allow the pilot to taxi in low visibility in the same way that he or she would in clear weather. This is achieved using "scene-linked" symbology on the HUD which provides speed and guidance cues overlaid on the outside-world view.

The prototype system installed in the 757 for the Atlanta demonstration used the Flight Dynamics HUD and a large-format Rockwell-Collins liquid-crystal head-down display. The airport map was developed by Jeppesen. Information displayed on the HUD and map came from the airport's surface-traffic control system. This includes a Northrop Grumman ASDE-3 surface-movement radar with AMASS conflict-detection software, and a proof-of-concept airport traffic-identification system (ATIDS) supplied by Cardion.

The ATIDS is a key element of the TAP programme and the main reason that Atlanta was selected for the LVLASO demonstration. The system identifies aircraft from their Mode S transponder responses and can track targets using either multi-lateration or automatic dependent-surveillance - broadcast (ADS-B). The NASA 757 was equipped with an ADS-B pallet supplied by Collins.

For the T-NASA demonstration, the taxi route cleared by ATC, traffic information from the ASDE/ATIDS and hold bars generated by the AMASS conflict-detection software were datalinked to the aircraft for display. The map shows the aircraft's location on the airport, along with that of other traffic. The taxi route is displayed in magenta, and red hold bars appear when a runway the aircraft is required to cross is in use. At the same time, a virtual taxiway is displayed on the HUD, using the same edge cones as on the ROTO display. The T-NASA symbology includes runway-centreline countdown warnings to a virtual turn sign which would be familiar to road users (see diagram).


Datalink focus

Also on board the 757 were two VHF datalink radios, a Mode S datalink transceiver, and a global-positioning system, all supplied by Collins. One VHF data-link was used to uplink traffic and runway status to the T-NASA system, while the other was used to send differential-GPS corrections from a Collins ground-station for use in both the ROTO and T-NASA systems. The Mode S was used for ADS-B and for controller-pilot datalink communications.

ATIDS transceivers distributed around Hartsfield were used for the Mode S datalink, and a controller's workstation with speech recognition was installed at the airport. This enabled the controller to send instructions via datalink to the aircraft. Spoken instructions were recognised by the workstation, transmitted to the aircraft and displayed on the moving map. The T-NASA system was also able to remove hold bars automatically at a spoken command from the controller.

The Atlanta trials included 53 test runs, mainly at night, but always in clear weather. The main aim of the demonstration was to validate simulations conducted at Langley and Ames, Jones points out, and further simulator runs are planned to refine the system. The tests were conducted by two NASA pilots and four guest pilots, from American, America West, Delta and United Airlines. The majority of runs included a take-off and landing, but some involved only taxiing.

Although there was no intent to demonstrate reduced runway occupancy in this trial, at least one pilot commented that the ROTO system enabled him to make a high-speed turn-off which he would otherwise have missed. Foyle says that all pilots responded that the T-NASA system allowed them to taxi faster, more safely, with more situational awareness and less mental workload, and better communications.

NASA plans a more-comprehensive demonstration of all the TAP technologies in 2000 and is seeking operator input on what should be demonstrated. "Airlines are not quite sold on the technology," Bryant admits, although NASA has done some cost/benefit analyses which indicate that airlines will save money with the LVLASO system. "We must look at accelerating the transition into service," he says. "Otherwise we are just demonstrating a neat technology that will languish."

Source: Flight International