|Aircraft and wind vortex. NATS
[Avionics Today 04-02-2015] The introduction of Time-Based Separation (TBS) at Heathrow Airport is paving the way for decreased delays. Traditionally, Air Traffic Controllers (ATCs) separate flights on final approach according to distances set by the International Civil Aviation Organization’s (ICAO) wake vortex minimums or radar standards that are determined by aircraft type, size and the wake vortices they create. NATS and Eurocontrol, just completed a five-year data collection process set to examine and dynamically calculate optimal safe spacing between each aircraft in line with Single European Sky ATM Research initiatives, known as TBS, which has just gone live at the London airport.
The TBS tool analyzes information on the exact wind conditions pulled from the aircraft flying into the London terminal, which are downlinking through their Mode S transponders the ground speed, air speed and heading data from the aircraft alongside rate parameters that work to create a picture of wind conditions. ATCs also feed information on the sequence of arrivals as well as the runway they’re landing on into the TBS tool, which ultimately generates dynamic indicators to the controller that shows how far apart the aircraft can be.
“The biggest single cause of arrival delay into Heathrow is the impact of headwinds on final approach,” said Andy Shand, general manager for customer affairs at NATS, noting that an estimated 44 percent of Heathrow’s arrival Air Traffic Flow Management (ATFM) delays are caused as a result of headwinds, with significant delays on 55 to 65 days a year. “We’re operating very close to the minimum wake vortex separation all the time at Heathrow because it’s a very busy airport. When the headwind increases, the ground speed to the aircraft drops. It’s a bit like walking the wrong way up a moving walkway at the airport, so the faster the walkway, the slower you’re making progress and therefore the landing rate drops.”
This can decrease the landing rate on windy days from the average 40 aircraft an hour, to just 32 an hour. And with an airport that has essentially been at capacity every day for the last decade, airlines, ATCs and passengers were looking for a way to reduce these delays and cancellations.
“What we needed to do was to be able to demonstrate that it’s safe to put aircraft closer together in headwind conditions,” Shand told Avionics Magazine. NATS went about doing this by collecting five years worth of data alongside Eurocontrol and with Lockheed Martin’s WindTracer Light Detection and Ranging (LIDAR) system, which measures the strength and behavior of wind vortices caused by arriving and departing aircraft. This data can then be analyzed by participating airports to increase traffic and throughput at rates of up to 30 percent, according to Michael Margulis, director of WindTracer Programs at Lockheed Martin, in an earlier interview with Avionics Magazine.
“We have been able to pull together a safety case by measuring the actual wake vortex behavior both in-ground effect at about 300 feet on final approach, and out-of-ground effect at about 1000 feet on final approach, for 150,000 flights into Heathrow. And that’s all sorts of aircraft types and all sorts of conditions,” said Shand.
According to Shand, in normal conditions, there’s a 4-mile separations between two heavy aircraft such as from a 777 to another 777 or an A340. “Takes you about 90 seconds to cover about a 4-mile gap, but in 35 knot headwinds it would take you about 107 seconds to cover that same gap. The impact in 35-knot headwinds is you’re losing about 17 seconds for each heavy-heavy pair that’s arriving. So what we do is use time-based separation to go back to a 90 second gap. So, in 35-knot headwinds, we can separate the aircraft by 3.4 nautical miles. We’ve got the safety case that says it’s safe to do that,” said Shand.
Heathrow has cleared the way for the 16 other airports that have committed to deploying the TBS technology by 2024, according to Eurocontrol. As it spreads to other airports, it will likely take on other uses as well.
“Once you’ve put a dynamic, system-driven indicator in front of a controller, we’re really on the start of a journey because we can now use that indicator to be driven by anything that’s affecting the separations on final approach,” said Shand, referencing the possible ability to automatically dynamically space aircraft according to size in low visibility conditions as well as on a mixed-mode runway.
As Heathrow begins to iron out the final details of rolling out TBS and starts in on seeing benefits in reduced delays from the technology, NATS feels confident that they’ve done the work to put the safety case for the technology in place.
“We validated [the system at Heathrow] using over a year’s worth of live data and it’s really incredibly accurate. It’s way more accurate than any forecast could hope to be,” said Shand.