The way forward: ADS-B

UPS Airlines is playing a crucial role in pioneering the application of ADS-B, the enabling technology for the USA's next-generation air transport system

In the skies over Kentucky a team from express package carrier UPS Airlines and safety avionics specialist ACSS are getting to grips with the reality of implementing automatic dependent surveillance - broadcast (ADS-B) in a crucial initial step on the long road towards the future US air traffic management system.

The two companies, together with the US Federal Aviation Administration, are in the midst of flight trials of SafeRoute, an ADS-B-based system developed by L-3 Communications/Thales joint venture ACSS, which is due to be certificated for use by mid-year. SafeRoute uses ADS-B and satellite navigation combined with Mode S transponder datalink capabilities to give pilots surface area movement management (SAMM) displays at airports and merging and spacing (MAS) traffic monitoring capability in the air.

Fuel savings

For UPS, a long-time advocate of ADS-B and the initial user of SafeRoute, the promised benefits include improved safety on the ground and in the air, as well as significant fuel savings from a more efficient arrival sequencing at its Louisville, Kentucky hub through the merging and spacing function. UPS estimates this could save them up to 4 million litres (880,000USgal) of fuel a year if used 80% of the time, while ACSS estimates continuous descent approaches (CDA) using the system can cut emissions by 3% and noise by 30%.

Flight tests with a SafeRoute-equipped Boeing 757 began in November 2006 and, after a short break during UPS's peak activity period around the year-end holiday, resumed in early January. The integrated SafeRoute cockpit display of traffic information (CDTI) is presented on a Boeing-developed electronic flight bag (EFB). "This is currently mounted aft of the pedestal, so it is visible from the jump seat and therefore not yet optimised for pilot viewing," says ACSS SafeRoute test pilot Rick Ridenour. The EFB is to be repositioned aft of the nosegear control tiller on the flightdeck sidewall, but for the moment "we just rigged up something we can go out and test", he says.

Early tests in November looked at aspects of surface movement management, plus some core software routines "to track ADS-B targets and correlate those with active TCAS [traffic alert and collision avoidance system] targets," says Ridenour. "We did a few runs where we'd fly out towards St Louis and south of there and come back in with the 757 and a lead ship - an ADS-B 'out' [transmitting]-equipped Airbus A300. We'd co-ordinate with Indianapolis and Kansas air traffic control centres and Louisville approach, and fly CDAs from cruise flight levels to 3,000ft [900m] and capture the ILS [instrument landing system]."

To validate the ability of the system to help crews cope with a variety of scenarios, the tests have included flights with the 757 and A300 normally spaced at 2-2.5nm (3.7-4.6km) in trail the two aircraft flying too closely together so the trailing aircraft was forced to drop back and the two flying too far apart so the trailing aircraft was forced to accelerate. "On one night we were right in the jetstream with a 130kt [240km/h] tailwind, and we came out of that and were trying to match the speed across the ground. It was pretty tough, but that's what you want to test the system," says Ridenour.

To accomplish the correct continuous-descent profiles the team has had to create sets of waypoints for the flight management system (FMS) to enable the crew to follow special initial approach procedures. These involve descending at a constant Mach 0.78 from 34,000ft to 29,000ft during which, depending on atmospheric temperature, the monitored speed transitions to 300kt indicated. The crew then flies indicated airspeed to reach 11,000ft at 240kt at which point the FMS constructs a descent path.

But for the aircraft to follow a precise CDA trajectory to fit into the arrival sequence "like beads on a string" - a phrase coined by UPS's SafeRoute project manager Capt Karen Lee - "you need to be on the same string", Ridenour says, To achieve this UPS has designed a series of descent profiles to enable aircraft with different deceleration characteristics to converge at the same rate. Ideally the "slightly draggier aircraft come in at slightly higher flightpath angles to maintain the same constant speed," he says. However, some will have to come in at 1/10th of a degree shallower angle than the optimum. "But the little bit of inefficiency it costs you is far outweighed by the overall improvements of the CDA to the other fleets," he says.

Reduced workload

Flight tests have also tested new software to reduce the pilot workload created by the need to make frequent speed changes. "In November we identified this as one of the potential issues, and we want to make this as workload-free as we can. So we've gone back and found a couple of things to better help manage the speed changes," says Ridenour, who adds: "That's the trade-off when you don't have a totally coupled system like an autothrottle."

February sees tests with the FAA of another features known as CAVS (CDTI-assisted visual separation) in which, hope ACSS and UPS, the crew will be allowed to use the cockpit display to help follow designated traffic on the approach. The flight number for the leading aircraft is entered into the FMS and a unique icon appears on the display showing the specific spacing. Compared with trying to follow the aircraft visually against a background of city lights or in hazy conditions, "it is far better than the Mk1 eyeball in terms of working out distance and closure rate", says Ridenour, who adds "there is still a fair amount of scepticism within the FAA on this".

Following a series of additional development tests, the team plans to move into certification flights on the 757 in the second quarter once Boeing has installed the finalised EFB display unit. The system is due to be progressively rolled into UPS's fleet from this year, starting with the 757, 767 and new 747-400F. This will be a key milestone in the FAA's move to an ADS-B-based next-generation system.

Source: Flight International