Time Based Separation (TBS) continues to improve resilience at Heathrow

Time Based Separation (TBS) has been proving its worth in a windy November with 15 days of strong headwinds and we estimate that over 25,000 minutes of delay has been saved versus traditional distance based separation in November 2015 alone. For example on 10 November with 39 knot headwinds at 3000ft, there were no arrival regulations applied and we were landing c.3-4 more aircraft per hour than we would have seen before TBS was implemented. Our analysis also shows that even on days without strong headwinds TBS is delivering benefits with an average of 1.2 additional landings per hour.

The initial phase of live operations of the Time Based Separation (TBS) tool began on 24th March 2015, with approval for full operational service being gained from 1st May 2015. The system has been jointly developed by NATS and Lockheed-Martin and is a world first implementation of Time Based Separation. TBS is expected to become the norm for capacity constrained airfields in the coming years.

The TBS 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 and initial indications are very favourable. 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.

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