A trial to reduce the need to stack arriving aircraft at London's busy Heathrow Airport will have international implications.

The trial, the second phase of a series called Topflight, aims to extend the horizon of arrival-planning for the airport's air traffic controllers beyond U.K. airspace out 350 nm into Belgium, France, the Netherlands and Ireland.

The project is led by U.K. National Air Traffic Services (NATS) and aims to reduce the aircraft stack-holding times to around 6 min. or less from the current average of 7.6 min., reducing delays for airlines, frustration for passengers and noise for city dwellers.

NATS uses a computer-based arrivals management system (AMAN) to sort and coordinate arrivals into Heathrow, ensuring that heavy-rated aircraft are not mixed into standard traffic. The flow helps controllers best utilize Heathrow's two runways, which operate at 98% capacity. However, flights arriving in London's airspace can find themselves in one of four stacks used at Heathrow: Bovingdon and Lambourne to the north, Biggin and Ockham to the south. Aircraft descend in the stack, then when instructed follow the approach down into Heathrow, depending on which runways are in use.

“Stacks are a necessary part of ensuring that controllers at Heathrow make best use of the runway capacity,” says Joe Baker, Topflight project leader at NATS. “We cannot eliminate them, but we want to ensure more efficient use of the airspace.”

NATS is now gathering data and preparing to share it over a secure network with key air navigation service providers in Europe. They will monitor the status of London-inbound flights using a cross-border arrivals management (XMAN) system. Details of flights that appear to be arriving early in the London area and are thus at risk of ending up in one of the stacks will be shared with control centers in Reims, France; Maastricht, Netherlands; or Shannon, Ireland, where controllers will request that those flights slow down.

“With early warning, even flights that slow down by 0.03 of Mach could save several minutes in the stack. That reduction in speed reduces fuel as well,” Baker says. The system will only be used after 7 a.m., once the early rush of long-haul flights from the Far East is over. “The airport operates in a different configuration during the early morning, with both runways used for landings,” he notes..

Operations using the XMAN system begin in February. The Topflight trial will collect data for three months, but use of the system will extend beyond that.

The project comes at a time when NATS is being driven by financial incentives from its customers to improve its environmental performance in the coming years.

NATS statistics show that in 2010, half of all flights into Heathrow were held in a stack, compared with one-sixth for operations at Gatwick Airport, London's second-busiest and the world's busiest single-runway airport. NATS estimates that reducing stack times by 2 min. per aircraft could save 23,000 tons of fuel per year, worth £14 million ($23 million).

The company is consulting with people who live below the airspace across the counties of Hampshire, East and West Sussex, Kent and Essex about planned modernization of the airspace structure around London. Part of this includes replacing Gatwick's stack system with a point merge system in which holding aircraft fly in an extended flight path around an arc rather than in circles in a traditional hold. When instructed, the aircraft is turned toward the point- merge and then sequenced to land. The aim is to increase the opportunity for controllers to use continuous ascents and descents, thereby reducing delays and the area over-flown at low altitudes.

The trials come as plans to increase capacity at London's airports are under scrutiny following publication of the Airports Commission interim report, which identified both Heathrow and Gatwick as potential locations for a new runway in the southeast of England. The report also proposes several short-term solutions for the two airports, including arrival management and time-based separation systems that could help improve airport flow rates during periods of strong headwinds by taking into account the effect of wind on aircraft speed.

The first phase of Topflight tested the principle of “perfect” transatlantic flights using British Airways services between London and two destinations in Canada in May to July 2013. Data from the trials showed fuel burn was reduced when the flights were optimized at every stage from takeoff, climb and cruise to landing.