The History of Time-Based Separation (TBS): Celebrating 10 Years Since TBS Landed at Heathrow

NATS, Leidos and Heathrow Airport are celebrating the 10th anniversary of Time-Based Separation (TBS) – using time intervals to manage aircraft separation on approach instead of fixed distances – delivered as a world first when their groundbreaking Intelligent Approach arrival spacing tool went live in 2015. Take a look back at the early years of TBS and its development and implementation at the world’s busiest two-runway airport and the huge benefits this tool has brought, including saving almost 3 years of airborne holding time throughout the 10 years.

Having been involved in getting TBS into Heathrow, I was lucky enough to experience the benefits first-hand just three days after it went live. I was flying back to Heathrow, arriving at peak time in the early evening. While you could see on the flight status boards at our departure airport that the rest of Europe was struggling with delays due to strong winds, we arrived at Heathrow five minutes early on stand – at a peak hour during strong winds! Proof that TBS did what we had hoped it would.

When flight delays occur, particularly in the morning, it creates a domino effect throughout the day resulting in rotational delay and cancellations. In 2003, Heathrow, British Airways and NATS looked in detail at the causal factors behind the airborne holding and delays. We identified several causes and areas which we could potentially improve: This included headwind related delays, capacity constraints, airborne holding, and more.

These factors were crucial in leading to the development of Time-Based Separation (TBS). No one had done this before – the safety case needed to be proven, and the tools and procedures needed to be developed from scratch.

Members of the TBS core team celebrating 10 years since it went live.

Improving Safety Performance

NATS started actively preparing the ground for Time-Based Separation in 2007 when LIDAR data collection began at Heathrow, assisted by an on-loan Eurocontrol WindTracer LIDAR – highly accurate remote sensing technology. Five years and 150,000+ flights later, in every wind condition imaginable close to the ground and further up the approach, analysis proved that it would be safe to reduce the arrival distance gaps proportionally as the headwind component increases, significantly recovering delay time.

Whilst TBS was being developed, NATS worked closely with the UK CAA as the first regulator in the world to enable time-based separations. This safety case has now been adopted by ICAO, who published TBS separations in Procedures for Air Navigation Services Air Traffic Management (PANS-ATM) November 2024.

Collaboration & Development

Developing and implementing TBS required a huge collaborative effort; new tools and procedures were needed to enable TBS to work, and endorsement and expertise from across the industry was crucial to success. With funding and support from SESAR, the airlines, and Heathrow Airport, NATS and Leidos started the build of the final product in 2014 and it went live in just over a year later.

Support and preparation from airlines and Heathrow Airport saw a smooth introduction of TBS. We worked with the airlines to improve their speed compliance on final approach, giving ATC the confidence to vector aircraft closer together. We also worked closely with Heathrow and some of the main airlines to ensure that crews were ready for the introduction of TBS having previously been accustomed to standard fixed distance-based separation. Airlines even helped by appearing in explanatory videos to help brief everyone before the system went live. Unusually, the first two days of TBS operation saw light winds leading to a disappointing start for TBS. Then, with strong winds on day 3, the benefits were clear for all to see.

On 24 March 2015, Heathrow became the first airport in the world to use TBS when it went live in the London Terminal Control Ops Room, where it has been running ever since.

Controller Response and the Go-Live Experience

So, how did ATC respond to TBS? TBS was developed by controllers, for controllers; the development was completed in collaboration with this core team. Thanks to their involvement, TBS’ landing at Heathrow was smooth and controlled. The core team helped to simplify the tools and procedures to make it easy to learn and use.

It took only a few days to see the effects of TBS; after abnormally light winds in the first couple of days following deployment, the headwinds increased, reaching over 30 knots at 3,000ft. The system came into its own, immediately recovering the capacity that would have been lost.

Importantly, TBS was very well accepted by Air Traffic Controllers. A few days into live operation, we asked if we could turn the system off to make a small change to the software adaptation, but the controllers asked us to leave it running and do the adaptation overnight – a testament to the excellent work by the TBS Core Team that designed the tool and procedures.

The airlines and the airport could instantly see a benefit; within the first year of TBS operation, Heathrow saw a 62% reduction in headwind delays.

Results & the Future

In year one, TBS allowed Heathrow to land an average of 2.9 additional aircraft per hour on strong wind days greatly reducing delays and airborne holding. Crucially, this was all achieved without any increase in reported wake vortex encounters or go-arounds.

This was just the beginning of the story for TBS. We realised that we had a very effective new product with a strong use case and ongoing developmental potential. The TBS tool (now known as Intelligent Approach) was then used to implement RECAT EU separations and add Optimised Runway Delivery and Runway Occupancy Time separations in 2018. This enhanced the benefits of the system further. It was this standard that went live in Toronto Pearson in 2022 and Amsterdam Schiphol in 2023.

In March 2024 an agreement was signed allowing incorporation of Intelligent Approach into Indra’s ATM systems, some of the most widely used systems in control centres around the world. And, in December 2024, Heathrow became the first airport in the world to utilise Pairwise final approach separation, using wake vortex separation individually tailored for each pair of aircraft, offering even more benefits to Heathrow’s airlines, passengers and to the environmental savings.

Since its implementation, the benefits of TBS have been outstanding. Over the 10 years since TBS went live, it has cut airborne holding by a total of 1,500,000 minutes, equating to almost 3 years of holding time saved. This has saved over 100,000 tonnes of fuel – worth approximately USD 70 million based on March 2025 jet fuel prices – and helped contribute to more sustainable aviation by saving over 300,000 tonnes of CO₂.

TBS is a key feature of arrival spacing tool Intelligent Approach, which continues to add new features allowing busy airports to get the most out of their runways and offer significant environmental savings and operational resilience to any airport that adopts it.

Thank you to everyone at Heathrow Airport, NATS, Leidos, Think, Eurocontrol, SESAR, all involved airlines, our customers and the many other organisations who made TBS happen initially and who continue to support the product.