We’ve all experienced it. Our flight is approaching its destination. In theory we should be landing soon. But there’s a delay, and so instead of starting its descent the plane is directed into what’s known as a holding stack, waiting its turn to land.
At Heathrow there are four such holding stacks, and every minute that an aircraft spends in one of them means more fuel being burned – fuel that costs money, and adds to the flight’s overall emissions.
But what if the pilot could anticipate the delay and slow down the approach so the aircraft could remain at a more fuel-efficient cruising altitude rather than waiting around in a stack?
That’s exactly what is now happening at Heathrow thanks to the Cross Border Arrival Management (XMAN) system, a collaborative air traffic management procedure implemented by NATS, the UK’s main air traffic control provider.
“Obviously waiting has inefficiency built in,” says Ian Jopson, NATS’ head of environmental and community affairs. “With this system, when an aircraft is up to 350 miles away from Heathrow, we can get a message to the crew saying, ‘can you slow down by 10 or 15 or 20 knots, because we’ve got excessive holding.’
“And what that does is that rather than spinning around at low level, you take that wait at cruise level where you’re more efficient. You’re not delayed any further, you’re just eliminating the most inefficient part of the flight.”
This linear holding system has already reduced waiting time by an average of one minute per flight, he adds. “One minute doesn’t sound all that much but when you think about the fact that 10,000 flights use the hold every month, that’s an awful lot of reduced fuel burn and reduced emissions.”
In fact, the system has seen an annual reduction of around 35,000 tonnes of fuel and 105,000 tonnes of CO2 emissions. And XMAN is just one of the ways that NATS is working with Heathrow to help the airport achieve its goal to make air travel altogether more sustainable.
“Heathrow is the busiest dual runway operation anywhere in the world,” explains Heathrow’s airside director Jon Proudlove. “It operates in excess of 98% of its available capacity, so ensuring there is resilience in the operation every day is a constant challenge.
“Managing Heathrow sustainably is part of the airport’s licence to operate. Air traffic control is intrinsically linked to many of the emissions reduction initiatives, from reduced engine taxiing to airspace modernisation, all of which help us maintain a keen focus on our environmental impact.”
In 2008, NATS voluntarily set itself a target to reduce air traffic-related CO2 emissions by 10% by 2020 compared to a baseline of 2006 levels – the first air traffic controller in the world to do so.
Continuous descents
NATS’ commitment to efficiency goes even further back. In the 1970s, for example, they pioneered the idea of continuous descent. Planes are kept as high as possible for as long as possible, and then brought smoothly down to land with their engines in idle, effectively coasting to the runway and using significantly less fuel.
“It’s a bit like living at the bottom of a hill and on approach at the top of the hill you take your foot off the accelerator and just coast,” says Jopson. “It reduces fuel burn and it also reduces noise.”
For a Boeing 747 approaching Heathrow, this can save half a tonne of fuel, which translates as 1.5 tonnes of CO2 in terms of emissions. Nine in every 10 flights into Heathrow are now flying continuous descent approaches.
Smooth descent is one of the factors tracked as part of another NATS emissions reduction solution: the Three Dimensional Inefficiency (3Di) Score. Designed to measure the overall efficiency of the organisation’s airspace, 3Di compares the 2.3m flights that operate within it annually and calculates how close each one was to what it could have been. As well as descent, other measurables include climb, cruise, holding and horizontal track.
Jopson says: “Because there are other aircraft around, you can’t always get the perfect flight, so when we’re operating the airspace, we’re always asking, ‘How can I get this aircraft into the most efficient route and position from when you take off to when you leave our airspace?’”
Since 2008, NATS has saved 7.7m tonnes of CO2 and is halfway to its 2020 goal of 10% emissions reductions. This translates to £1.4bn in fuel savings. And there’s always something new in the pipeline. The air traffic control provider and Heathrow are pioneering a time-based spacing tool that will maintain runway capacity in strong winds, in order to tackle build up within holding stacks. So far, that’s reduced headwind delays by around 50%.
“The way I see it, aviation is a big global industry and it delivers big social and economic benefits to the UK and globally,” Jopson says. “But it has its negatives – we make noise, we burn fuel, we release climate change gases. We want to keep enjoying the benefits of aviation, working together to minimise our impacts.”
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