At capacity-constrained airports around the world, high winds are a major cause of flight delays and cancellations, often as a result of the knock-on effects of reduced airport flow rates.

Landing aircraft currently have to maintain set distances apart due to the wake vortices occurring from the wingtip, but research has shown that in strong headwinds, ground speeds on approach are reduced and the effect of the vortices is quickly dissipated, potentially allowing the distances separating the aircraft to be reduced.

Now engineers at U.K. air navigation service provider National Air Traffic Services (NATS) are putting the principle into action. After more than two years of collecting wake-vortex data at London's Heathrow Airport, NATS is preparing to introduce a new time-based method to separate landing traffic there.

Starting next February, an initial cadre of trained air traffic controllers are due begin using the time-based separation (TBS) tool, developed in the conjunction with Lockheed Martin at Heathrow. All controllers are expected to complete the transition in 1-2 months, allowing the system to be used on a full-time basis and making the airport the first in the world to use such a system.

Heathrow will perhaps be the ultimate test of the principle. It is the busiest dual-runway airport in the world, with flow rates of up to 42-44 aircraft per hour on the landing runway. But strong winds that are encountered typically 60-80 days a year—likely to increase to 90-100 days this year, after an exceptionally blustery winter—cause up to 40% of the airport's delays, equivalent to 160,000-200,000 min. of delay each year.

According to NATS officials, winds of just 15 kt. at 3,000 ft. can have a small impact on the flow rate, reducing it by 3-4 aircraft depending on the wind direction. Stronger headwinds, of 40 kt. and above, can lower flow rates to 34-35 aircraft per hour on the landing runway.

Through the use of TBS, however, NATS believes it can eliminate 50,000-80,000 min. of delay and improve the airport's resilience against flight delay and cancellation. In an interim report, the U.K. Airports Commission recommends a move toward a TBS system at Heathrow in December, as it looks into improving capacity in U.K. airports.

“We had initially planned the introduction of a time-based separation capability as part of Single European Skies [Sesar], in 2018,” says Andy Shand, general manager for customer affairs at NATS. “The airlines and the airports said we need to get this into use, so we took an agile approach to our R&D effort.”

As part of the work, NATS used a Lockheed Martin laser imaging detection and ranging (Lidar) device to map the vortices produced by aircraft arriving at Heathrow. The system was initially placed on a British Airways maintenance building to measure the vortices close to the ground and then further out on the approach.

Between October 2008 and December 2010, NATS collected 120,000 aircraft tracks that it then analyzed to begin preparing a tool that integrates data from the Mode-S information provided by the approaching aircraft with the weather state to give the air traffic controller information on how to dynamically separate aircraft flying the approach. Once implemented, the TBS tool will display a series of magenta-colored separation indicators along the runway track onto which controllers will vector the aircraft, giving them the optimum separation for the aircraft's wake category and the weather conditions.

On a benign day, an aircraft travelling at 150 KTAS will be doing 150 kt. ground speed, covering 4 nm in 90 sec. But an increased headwind means ground speed could be reduced by 20 kt. and the aircraft would be just 3.5 nm in 90 sec. With that principle in mind, the distance between aircraft could be reduced to 4.2 nm from 5 nm between two narrowbody aircraft in the Airbus A320/Boeing 737 size.

In one simulated trial, a group of air traffic controllers was given a 15-min. briefing on the system and tool and then tried them out. “They found it very easy to adjust to, and feedback was excellent,” Shand says. “This can be applied to any system in the world. It's already in the road map for Sesar and the FAA's NextGen implementation.”

NATS is working with Heathrow's customer airlines to introduce the system. A key area will be to train pilots to ensure they maintain speeds requested by air traffic control and an even speed balance during TBS operations. “They see the benefits of the system and we have great cooperation from them, and the regulator is extremely supportive of our efforts,” Shand says.

At Heathrow, the TBS system has also been optimized to provide data on the diagonal spacing of aircraft when the airport is using its Tactically Enhanced Arrival Measures (TEAM). Because Heathrow's two runways are not wide enough apart to allow simultaneous parallel operation, landings on both runways have to be staggered, particularly during busy periods of the day.

TBS will only apply to arriving aircraft, but studies are underway to see how the principle could also apply to departing aircraft.