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Beyond Expectations

Intelligent Approach to delivering airport resilience

For years, airport managers, airline operations staff and controllers have been looking out of their windows and at screens during peak times at busy airports, thinking to themselves "How can we get more landing capacity?" Until now, controllers have set the separation distances between aircraft on approach through applying a fixed set of distance based separation criteria – with at least 5 nautical miles between a large wide-body aircraft followed by a medium sized airliner – based on a series of general aircraft characteristics. In this Distance Based Separation procedure, separation distances have been calculated to ensure that the turbulent airflow which follows a large aircraft – also known as the wake vortex – will have time to dissipate to avoid turbulence affecting the next aircraft in the landing queue.

Building resilience

The stronger the headwind the longer it takes an aircraft to fly a fixed distance. However, it is also true that the stronger the headwind the more quickly the vortices dissipate. If the exact speed and direction of the wind and the exact wake turbulence characteristics of a particular type of aircraft were known, separation distances could be applied based on actual, rather than theoretical, events. This would make airports more resilient, able to handle more aircraft in times of strong wind.

Headwinds at airports are a major cause of delays. At Heathrow, for example, strong winds are responsible for 44% of all air traffic flow management (ATFM) delays. NATS and industry partner Leidos (formerly Lockheed Martin) introduced time-based separation (TBS) into operation at Heathrow in March 2015 and from the start it became clear the benefits were far greater than the original models suggested.

"November 2015 was a windy month with 15 days when there were strong headwinds; the worst was November 15th when we had headwinds of 60 knots straight down the runway," says Andy Shand, General Manager Customer Affairs at NATS. "We estimated that we saved over 25,000 minutes of delay in November alone." TBS has on average enabled 20 extra landing aircraft per day over distance based separation at Heathrow. When the headwinds have been more than 20 knots this goes up to 49 extra landing aircraft per day.

Detailed measurements of wake turbulence at Heathrow between 2008 and 2013 using Light Detection and Ranging (LIDAR) laser-ranging equipment loaned by EUROCONTROL to measure 150,000 aircraft tracks from two different glideslope locations as part of the Single European Sky ATM Research (SESAR) programme, showed that as the wind speed increases the wake decays more rapidly.

TBS went from initial trials in January 2015 to operational mode in March 2015 after which it has been in almost continuous use at Heathrow. "TBS has brought real benefit to our customers through reducing delays in high winds," said Andy Lord, British Airways Director of Operations. Such is the potential for TBS to improve resilience at busy airports it has been adopted as a Pilot Common Project, a SESAR project which has been mandated by the European Commission for deployment at the continent's busiest hubs.

"Time based separation addresses the biggest single cause of arrival delay at the airport. Heathrow has been operating at 98% capacity for over a decade so technology like this is essential providing the best service possible to our passengers," says Derek Provan, Operational Strategy Director at Heathrow Airport Limited.

Accurate and real-time information on wind is obtained from aircraft Mode-S transmissions – but could also be obtained from alternative sources such as automatic dependent surveillance – broadcast (ADS-B) or LIDAR – and, as the tool is modular, new data sources can be added at any stage. The safety regulator was incorporated into the research programme at an early stage so the safety case could be developed and approved. The Intelligent Approach suite of tools is modular and portable enabling it to work with existing systems to provide controllers with clear separation parameters which they can apply to aircraft on approach.


minutes of airborne holding saved per annum


of all ATFM delays were caused by headwinds



minutes of wind related ATFM delays per annum


days per annum wind would have caused significant delays


minutes saved by TBS in November 2015


reduction in ATFM delay

Headwinds no longer feature as a significant cause of flight cancellation


extra movements
per hour


extra movements per hour with a headwind
of ›20 kts


fewer spacing related go-arounds

"SESAR members, such as NATS, have thoroughly tested the Time Based Separation for Arrivals (TBS), successfully demonstrating its capacity to improve efficiency at the world's busiest dual-runway airport. The solution fits into SESAR's wider ambition to deliver innovative ways of maximising existing resources and capacity at busy airports."

Florian GuillermetExecutive Director of the SESAR Joint Undertaking

reduction in reported Wake Encounters <6000ft


Watch the Intelligent Approach video

Strong Headwinds


139 secs
113 secs


107 secs
90 secs


160 secs
135 secs

Super Heavy

Wake vortices dissipate quicker so separation distance can be reduced safely.

TBS – the benefits so far

Drag the slider to see the savings


Arrival-Arrival Distance Separations Pre and Post TBS

In moderate wind conditions (0-20kts) Arrival distance separations have reduced as follows:

Arrival-Arrival Distance Separations Pre and Post TBS

In strong wind conditions (›20kts) Arrival distance separations have reduced as follows:

  • Heavy-Heavy
  • 4.50nm
  • Heavy-Medium
  • 5.53nm

Across all wind conditions:


total cumulative distance saving
per day


extra landings
per day


additional movements per hour


reduction in
ATFM delay

Reduced cancellations

Improved consistency of final approach spacing

No Increase in wake turbulence encounters reporting or Go Around Rate

In strong wind conditions:


extra landings
per day


total cumulative distance saving per day


additional movements per hour

* Based on implementation data gathered by NATS between 01/05/15 and 31/07/15 at Heathrow Airport.

...but TBS is just the start

The success of TBS spurred NATS and Leidos to look at other ways airports could boost resilience and capacity using similar advanced software tools, not just at times of high winds but all year round. The Intelligent Approach suite of tools based on a common platform is designed to help all airports increase capacity and resilience at a fraction of the cost of adding taxiways and runways.

Pairwise Separation, which takes the revised separations from RECAT (the recategorisation of existing wake vortex standards), can help to further reduce the safe separation between aircraft types, taking a more granular approach than today's wake vortex categories.

We have just completed promising SESAR simulations and are now developing tools that will allow controllers to see on their radar screen the exact separation distances that need to be applied for each specific aircraft type for all wind conditions – rather than have to rely on a mental calculation of generic types such as "heavy or medium", which can encompass a range of different aircraft types and hence speeds, and then apply the required separation distances.

"The Intelligent Approach suite allows us to deliver concepts such as pairwise separation into the operation, delivering even more efficient spacing and increased benefits. This additional capability could allow a further two landing movements per hour at congested airports."

Sarah ForsterAirspace Solutions Expert at NATS

Single runway capacity boosts

The Mixed Mode capability helps boost capacity at airports with single runways.

"On a single runway operating in mixed mode – a mix of arrivals and departures – we would normally apply a six mile gap between arrivals to allow time to get a departure away," says Janet Singhal, Queue Management and TBS Programme Manager at NATS. "If the departure is a heavy aircraft like a 747 then six miles is usually required. However, if the departure requires less runway length to take-off, as is the case with light twins like Boeing 737 or Airbus 320 then it could be that only a five mile gap between arrivals is needed." Trials and simulations as part of SESAR have shown that feeding departure sequence information into the arrival management tool – thereby tailoring arrival separations to match the departure runway occupancy requirements – could deliver up to two added movements an hour. It may not sound much but for a capacity constrained airport it is an important gain.


extra movements per hour

"The SESAR simulations we have recently carried out are really promising and show that a Time-Based separation tool that dynamically calculates pairwise separations based on real-time wind conditions could deliver significant benefits. Instead of having a 4 nautical mile separation distance between a Boeing 777 following a Boeing 747, for example, that could potentially be reduced to 3.3nm. Over a day, that's a big gain."

Claire Pugh NATS TBS R&D lead

"For me, as a Heathrow Approach controller, introduction of TBS has made a significant difference to the landing rate on strong wind days. Before TBS it was very frustrating (for us and the airlines) to see much reduced throughput, more holding and longer delays. TBS was developed with the controllers for the controllers and this helped to make implementation in to live operations relatively seamless. I am proud to be part of this 'world first'."

Shona Chalmers Operational Controller

"We believe that with partners like NATS, London Heathrow and British Airways, we have what it takes to push the boundaries of innovation by focusing on delivering solutions that are cutting edge and increasing the performance of Europe's ATM system. All of which is critically important for the sustainability of aviation and air transport and forms part of the EU's aviation strategy."

Florian Guillermet Executive Director of the SESAR Joint Undertaking

increase of airport capacity when using the Low Visibility Procedure in fog

Enhancing low visibility resilience

According to NATS researchers, the Low Visibility Procedures tool can increase airport capacity by up to 30% when the fog descends. By carefully modelling the impact of individual aircraft on interference patterns to the instrument landing system (ILS) localiser beam, controllers can tailor approach procedures to individual aircraft even in dense fog. When visibility is low, controllers normally have to decrease the landing rate at the airport by up to 50%; aircraft flying an ILS approach have to land and vacate the runway before the next aircraft in the queue can make an ILS approach, as the landing aircraft interferes with localizer beam signal which the following aircraft needs to access. But different aircraft have different interference patterns and the larger the aircraft the wider the area which is sensitive to interference. The Low Visibility Procedures tool gives controllers an exact measurement of the interference area for each aircraft, allowing them to understand more precisely the separation distances that will need to be maintained.

When runways intersect

And another capability, Dependent Runway Operations, has been developed to tell controllers when it is safe to clear an aircraft for departure at airports with intersecting or converging runways. This capability highlights how the departure will interact with aircraft operating on the conflicting runway, ensuring these two operations work in concert to maintain safety while optimising the use of runway capacity.

"All these tools have been developed to provide more capacity and resilience without increasing controller workload," says Andy Shand. "They can be adapted to whatever operating system is in place. And if they can work at Heathrow, one of the world's most capacity-constrained major hubs, they can work anywhere."

From theory to operations – the NATS fast track approach

NATS has been working with a number of research and operational partners to accelerate the process of moving theoretical benefits into the real world. The TBS concept was originally developed through SESAR research and development programme but, once the benefits of the procedure to customers became clear, was fast tracked by NATS and its technology partner Leidos for early implementation. This meant that a programme which was originally planned for deployment in Winter 2017/2018 became operational two years before then. The regulator, airlines and airports were directly involved from the start of the process, with the use of video materials and crew briefings to engage with customers and airspace users.

"We had great support from the airlines and Heathrow Airport which has really helped to make it a successful programme," says Andy Shand. "And we have seen no increase in wake turbulence encounters or go-arounds. The controllers like it and the pilots have not really noticed any difference to day-to-day operations, except for the fact they get less delay than they would have seen before TBS."

Watch our webinar

Recorded live in July 2016, watch this recording on demand to find out more about how airports can maximise revenue by increasing runway throughput and by improving resilience. This webcast was presented by Kevin Hightower from Leidos and Andy Shand from NATS. Fill in the form below for access.

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