Time Based Separation Goes Live At Heathrow, Should Significantly Cut Delays

Air Traffic Control, NextGen

If you haven’t heard of time based separation, you’re going to like this. NATS, the UK’s air traffic control service, worked with Lockheed Martin to develop this program which should dramatically improve on time performance at Heathrow on a whopping 15 percent of the days each year. It took only about 2 years to develop and it’s already in place. Sounds great, right? I spoke with Andy Shand, GM of Customer Affairs for NATS, to learn more.

As you may or may not know, airplanes try to land into the wind as often as possible because it allows them to maintain airspeed while having a lower groundspeed. While a nice stiff wind right on the nose can be a good thing for the airplane, it actually creates an air traffic control problem. That’s because air traffic is traditionally separate by fixed distances. As speed wind varies, so does the amount of time it takes to fly that fixed distance. Let’s use pictures to make sense of this.

First, take a look at Heathrow works in light or no headwinds.

Heathrow Light Headwinds

Heavy aircraft must be separated from other heavy aircraft by 4 nautical miles. (If a lighter aircraft is behind a heavy aircraft, the separation is greater, but let’s not worry about that right now.) With little to no winds, it takes an airplane about 90 seconds to cover those 4 miles. That translates into 40 airplanes arriving every hour, on average.

But not all days have light winds, especially in a place like London. According to Andy, about 55 to 65 days a year are impacted by high winds. When winds of, say, 35 knots kick up, what happens?

Heathrow 35 Knot Headwinds Before TBS

As you can see, with a 35 knot headwind, it now takes aircraft 107 seconds to cover that same 4 miles on the ground. The result? Only about 33 to 34 aircraft can land per hour. And that can really snarl things at a place like Heathrow which is pretty much operating at capacity. Those wind-based delays account for about 44 percent of all air traffic control delays at Heathrow, so it’s a huge issue.

The reason for aircraft being separated by so many miles is due to wake turbulence. Every airplane generates wake vortices that whip up the air as they cut through it. That can be very dangerous for airplanes following too close behind. You may remember back in the early 1990s when executives from In ‘n Out Burger died when their jet crashed after hitting wake turbulence on final approach in Orange County.

But NATS was convinced that there was a better way to manage wake turbulence than just having fixed distances, so it started diving into this problem several years back. It actually set up lidar at Heathrow five years ago to measure wake vortices at both 300 feet and 1,000 feet above the surface. After reviewing 150,000 flights, NATS discovered that the wake vortices dissipate faster as headwinds increase. That created opportunity.

With this information, NATS and Lockheed Martin developed time based separation (TBS). Using real time data from each aircraft coming into Heathrow, NATS can see the actual headwinds and calculate the separation needed. That information is given to the air traffic controllers so they can properly space the aircraft.

The end result in a 35 knot headwind is this:

Heathrow 35 Knot Headwinds With TBS

Air traffic controllers can maintain the 90 second gap and the 40-per-hour arrival rate to keep traffic moving along without any risk to those onboard. If the headwinds get too high, they won’t be able to maintain the 40 per hour rate because they won’t go lower than 3 nautical miles separation no matter what, but they can still get many more airplanes in there than they could have before.

The system first went live on January 14 but only in shadow mode for testing purposes. On March 24, it went into use and was put to the test. Just a week later on March 31, high winds were ripping through Europe and several airports were seeing major delays. Andy seemed to take a great deal of pride in the fact that his flight from Brussels into Heathrow that day went right on time.

There could be applications beyond landings as well. You can see how this might increase the departure rate too. And of course, this isn’t limited to Heathrow. Other airports can implement this with relative ease now that the research has been done and the value has been proven.

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13 comments on “Time Based Separation Goes Live At Heathrow, Should Significantly Cut Delays

    1. Oops, pressed ‘post’ before writing everything else I wanted ;)

      … at least, here in NL. I don’t think LHR uses a different departure system than their colleagues in Amsterdam, but I’d love to hear I’m wrong :).

      Pretty awesome system they made! It’s already amazing how many movements LHR sees per day and if this could prevent delays during bad weather, that can only be positive.

      1. Jorg – Good point. Though maybe it could help alter the amount of time necessary between departures depending upon the winds.

        1. Brett, the problem is that arrivals all follow the same path through the air. Departures all follow different 3d paths based on departure route and climb performance, and providing safety assurance for the path of wake vortex is infinitely more difficult, even before you consider departures follow curving paths!

  1. In the long run this won’t change anything. Since they can now run things more consistently, I think you’ll see them start to award more slot pairs at LHR–it’s just too valuable a commodity for them not to. After a while they’ll be running into the same problem.

    1. There won’t be any more slots awarded, TBS is all to increase resilience.

      We already separate departures by time.

  2. Whatever works and meets all the required safety criteria should certainly be acceptable. I assume all the possible computer simulations have been done to prove that this plan will work well for Heathrow. Is TBS already in use at other airports? I couldn’t find a mention of this.

    1. Miss Informed – No, this isn’t used anywhere else yet. Heathrow is the first but they do hope to roll it out elsewhere. Wouldn’t it be nice if JFK had it…

  3. Excellent post, Brett. One must believe that “TBS” can help a lot at some very busy airports, I hope that the always maintain that physical measure of separation, currently 3NM at LHR to cover the “What If” factor. No matter what, the guys and gals driving still need a little elbow room in which to execute a Go Around if necessary. And, if the GA is a runway-based problem, one should assume that more than one airplane will have to do so. Tighter/closer IS reasonable, but certain absolute limits must remain. If/When there is a problem ahead, airplanes cannot simply pull onto the margin and wait; without significant forward speed, the simply do not fly. As we all recognize, airplanes In The Air, but without significant forward speed, pose a rather serious problem – to themselves and to the folks below them. The blather said, I think NATS is onto something that will help at LHR and other spots. Great post!! -C.

    1. Current radar minimum is 2.5nm on final approach, and we reduce below this to about 2nm if visibility allows. In a headwind, 2nm spacing when the lead aircraft touches down is no problem. Obviously this does not apply to vortex pairs, the TBS system will not reduce vortex separation below 3nm at the moment, regardless of headwind.

  4. A lot of other airports will look to LHR for guidance as they implement time based separation.
    It is unfortunate that it would not result in additional departure slots as we talked about.

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