Clock Watching

1BStudies on increasing the resilience of an airport’s landing rate have recognised the need to overcome restrictions imposed by traditional distance-based wake turbulence separation standards, writes Ian Thompson.

NATS’ imminent implementation of a new method of separating aircraft on final approach at London Heathrow represents a ‘world first’ development which is expected to enable one of the world’s busiest airports to recover up to five aircraft landings per hour in windy conditions.

At capacity challenged airports, optimising runway capacity requires that runway occupancy time is reduced and the minimum allowable spacing is used to separate aircraft while on approach to land. Time based separation (TBS), which reduces wake turbulence separation minima in periods of strong head winds is being introduced to improve the resilience of the airport in high winds and to increase the attainment of the minimum safe spacing between aircraft pairs on approach.

Aircraft are separated by wake turbulence rules or by runway and radar separation standards, whichever is greater. While time spacing is typically used between successive departing aircraft, distance is the separation method applied between arriving aircraft.

Wake turbulence separation standards were developed during the 1970’s. Aircraft were grouped into four categories depending on their weight. For arriving aircraft, wake turbulence separation minima range from 4NM between a pair of heavy aircraft, to 8NM when a light aircraft is following a super heavy. In contrast, the base radar separation distance at Heathrow is 3NM, which can be reduced to 2.5NM when the tower runway controller can see the aircraft. Therefore, it is the wake turbulence separation standards that most often govern the spacing between aircraft on final approach.

Since their implementation, wake turbulence separation standards have been found to be very conservative and do not change when weather conditions, particularly strong winds, cause the effects to decay. While significantly reducing the potential harm presented by wake turbulence, strong wind conditions also cause a reduction in an aircraft’s ground speed. Lower ground speed means an increased time separation between arriving aircraft pairs because they are flying more slowly across the ground, causing a significant reduction in the runway landing rate.

2BA landing rate that can fluctuate on a daily basis prevents the ATM system from being able to provide a stable, highly predictable service to airlines. Aircraft often experience in-air delay and increased fuel burn, while airlines regularly have schedules disrupted. Service disruptions at key hub airports like Heathrow permeate throughout an extensive airline route network.

Andy Shand, NATS’ general manager, customer affairs explains: “At Heathrow, in normal conditions, we have a target landing rate of 40-45 aircraft per hour. We performed analysis about the impact of wind on the arrival rate, where strong winds account for about 40 per cent of the total arrival delay.”

As an example, on 11 October, 2013, the wind strength at 3,000 feet was around 45kts. The slower ground speed of aircraft on final approach resulted in a reduction in the landing rate to 38 movements per hour. An adjusted flow rate was instituted to cater for this lower landing rate, which resulted in 13,000 minutes of flow management delay. Should conditions cause the landing rate to drop to 36 movements per hour then some of the airlines will start to cancel flights.”

“We estimate that strong wind conditions impact operations at Heathrow between 50-85 days each year,” adds Shand.

Time based separation involves new procedures and air traffic control technologies to overcome the operational limitations of the existing distance based wake turbulence separation standards. Future system ATM development programmes in Europe, North America and Japan are each addressing, in some form, the development of TBS.


The TBS development at Heathrow is based on the Single European Sky ATM Research (SESAR) concept of operations. Although the SESAR target implementation date for TBS is 2018, NATS accelerated development activities so that TBS will enter operational use in 2015.

Development of new wake turbulence separation standards to be incorporated into the TBS involved a lengthy period of data collection and analysis. Between October 2008 and December 2010, 120,000 measurements of the wake vortex generated by specific aircraft types in certain wind speeds and weather conditions were taken. It also measured the impact of aircraft altitude, where wake vortices also have greater risk of persisting above 1,500ft and below 400ft above ground level.

Data was measured using a Lockheed Martin WindTracer LIDAR system with sensors placed at two locations on the approach path. The LIDAR system measured the strength and motion of the wake vortex. Lockheed Martin has subsequently been engaged to develop software tools for the ATM system to undertake processing of the TBS, although NATS has designed the tool to work with any ATC system from any manufacturer.

3NATS also assessed the impact of the wake vortex on the airspeed/flight gradient profile of various categories of aircraft operating under TBS-termed ‘Pairwise’ analysis. The impact of wake vortices at specific distances and altitudes on final approach was calculated.

The LIDAR data analysis concluded that wake vortices did not persist when the wind speed exceeded 15kts. As a result the TBS is the same as distance based wake turbulence separation in light wind conditions. The TBS is less than distance based wake turbulence separation in strong wind conditions, but greater in light tailwind conditions.

At Heathrow generic time based separation standards have been developed based on a 5-7kt headwind and an indicated airspeed of 160kts to 4nm from the runway. TBS intervals for six categories of aircraft were developed. These time intervals range from 60sec between two aircraft in the lower weight categories up to 180sec when a light aircraft is following an A380.

The base time intervals are then adjusted to take account of wind speed and resultant aircraft ground speed to derive the appropriate distance between aircraft pairs. Time separation needs to be represented in terms of distance in order for radar controllers to monitor aircraft and provide separation. For example, a 90 second TBS in a headwind of 5kts with ground speed of 160kts equates to a distance of 4.0NM. This TBS reduces to a distance of 3.5NM in 25kts headwind with a ground speed of 140kts. On the other hand, with a light tail wind of 10kts, the 90sec TBS increases the separation distance to 4.25NM.


In practice, the wind speed is obtained from a downlink of mode S secondary radar data from all of the aircraft in the area. This real time wind data is used to dynamically adjust the aircraft ground speed and therefore the distance that reflects the time separation between arrival pairs. Processing of wind data and aircraft type to derive the appropriate separation is performed by software in the ATM system.

TBS is presented on the radar displays of the final approach controller and the tower runway controller by way of a separation indicator line. It depicts the minimum distance that must be maintained between an arriving aircraft pair. The separation indicator line is dynamically presented to take account of the aircraft wake turbulence category, wind speed and aircraft ground speed.

4The separation indicator line is first presented on the final approach controller’s radar display prior to the turn on point that sets up the spacing between aircraft pairs on final approach. The final approach controller is then required to turn the following aircraft onto final approach behind the separation indicator line and then monitor/adjust the separation to maintain the minimum safe separation between arriving pairs. For the tower runway controller the separation indicator line is first displayed when both aircraft are established on final approach.

Runway capacity remains influenced by differences in the size of aircraft in a traffic sequence, when weight differences can require additional separation distance. At Heathrow, arriving aircraft enter one of four stacks. The arrival manager (AMAN) makes some adjustment to the approach sequence so that pairs of similar aircraft size are grouped together to complete the approach. However, the optimal grouping of aircraft with similar weight is difficult to achieve in a highly dynamic traffic situation.

Aircraft are required to fly their approach, to selected distances, in line with speeds that are detailed in the AIP. Andy Shand notes: “One of the problems with capacity optimisation involves aircraft slowing down too early on approach. We are working with airlines to improve speed compliance on approach by providing them information of instances where an aircraft has not complied with published requirements. Airlines are very supportive as they can see the huge benefits that TBS will bring.”

In order to implement the system changes in a little over two years, NATS has adopted an ‘agile’ process of software development. Andy Shand explains: “To meet our ambitious implementation date we had to adopt new approaches to developing and testing software changes. The agile development method involves a number of short term ‘sprint’ development activities that are tested in the simulator on a monthly basis. In essence, we have a work-off list of software changes that need to be undertaken. We complete one then move to the next. This process has enabled us to present new functionality to the air traffic controllers at an early stage and receive feedback.”

Shand adds: “We established a core team of approach controllers who are involved in the ‘sprint’ development activities, validating the TBS tools that are created. Once the TBS tool is fully developed in 2015 it will begin operation in limited operational service (LOS) with the core team. Once other controllers are trained they will be able to use the TBS tool. This LOS has worked well in other operational development initiatives NATS has undertaken. It provides for a phased implementation and enables transition activities to take place without impacting operations.”

5NATS established a core team of users comprising a number of airlines to validate new operational concepts that have been part of TBS. Engagement is now taking place more widely so that information is provided to pilots. A workshop with airlines will be held soon to confirm the TBS phraseologies that will be incorporated into the AIP.

Shand says: “Although the fundamental concept of operation at Heathrow will not change, operations may look a bit different to pilots. Aircraft will be a little closer to each other than they are today.”

The introduction of TBS is expected to reduce wind related delays at Heathrow by up to 50 per cent, while also largely eradicating flight cancellations. NATS believes that TBS will almost certainly become the standard for capacity challenged airports throughout the world.

Acknowledgement: This article referred to the paper by: Morris.C., Peters, J., and Choroba, P., Validation of the Time Based Separation concept at London Heathrow Airport, Tenth USA/Europe Air Traffic Management Research and Development Seminar (2013).

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