Time Based Separation

[K.Haroon]

Time Based Separation

Time Based Separation (TBS) will come into operation at Heathrow Airport in the near future. The system being installed for Heathrow has been jointly developed by NATS and Lockheed-Martin and will be a world first implementation of Time Based Separation. TBS is expected to become the norm for capacity constrained airfields in the coming years.

The Time Based Separation system and procedures are expected to significantly reduce inbound Air Traffic Flow Management and airborne holding delays that result from the impact of headwinds on final approach.

With fixed distance separation between aircraft on final approach, when the headwind component increases, the groundspeed drops resulting in a reduced landing rate. This leads to increased airborne holding and delays.

Time Based Separation dynamically adjusts the separation between arrivals to maintain time separation equivalent to the distance separation with a headwind of 5-7 knots. By doing so, the reduced approach separation recovers most of the lost capacity from headwinds.

In order to prove that it is safe to apply time based separation, we measured the wake vortex using LIDAR equipment for over 150,000 flights at Heathrow including all aircraft types over a period of nearly 5 years. This data clearly shows that as the headwind component increases, the wake vortex decays faster and has allowed us to develop a new set of rules for Time Based separation, which take into account the aircraft wake vortex category and the effect of headwind on vortex decay.

Time Based Separation operates in all wind conditions, reducing the separation between arrivals as the headwind component increases and increasing the separation in still winds or tailwind conditions.

These Time based separation rules have full regulatory approval and replace the existing ICAO distance based wake separation standards for Heathrow.

Crew Procedures & Briefing

With Time Based Separation there is little or no change to the flight deck procedures as the aircraft will still be radar vectored on to final approach in the same way that they are today and with the same speeds. The main difference is that final approach separation used by ATC is time based. In practice this means that aircraft may be seen to be closer on TCAS in headwind conditions but the time separation will be constant. It is important to maintain ATC assigned speeds as accurately as possible to deliver the maximum benefit from TBS.

This page provides background to Time Based Separation for flight crew including a short video which describes the context, a briefing sheet and the associated Aeronautical Publications. In addition NATS has produced other media which help describe the concept and background and these can be accessed via the links on this page.