Airspace efficiency

Measuring environmental performance


Our pioneering metric

We have developed a pioneering metric, known as 3Di, to measure the environmental efficiency of UK airspace. The 3Di metric provides a score which helps us monitor the efficiency of UK airspace by comparing the actual flight path of an aircraft to the ‘preferred profile’ (or the most efficient possible flight path).

Every year, every commercial flight in UK airspace is given a 3Di score. At the end of each year, these scores are combined to give an annual score which can be compared to targets set by the Civil Aviation Authority, our regulator, in consultation with our customers.

How is the score calculated?

The 3Di score runs on a scale from 0 inefficiency to 100+ inefficiency and calculated based on both horizontal and vertical efficiency.

The horizontal efficiency compares the actual radar ground track against the most direct track possible over ground, essentially calculating the additional miles flown.

The vertical efficiency measures the amount of level flight that occurs below the airlines’ preferred cruising level; the more time spent at a lower cruising altitude, the more penalising for a flight’s 3Di score.

An example 3Di scale

How can NATS influence the 3Di score?

The biggest improvements to our 3Di score can be delivered by changes to the design and operation of airspace, and by improving access to shared airspace. But the way our air traffic controllers direct aircraft day-to-day also has an impact. Some of the ways our controllers can positively impact 3Di include: 

  • More continuous climb and descent operations
  • More direct routes across UK airspace  
  • Reduced airborne holding time at destination airports  
  • Working with neighbouring air traffic control providers and military airspace users to deliver more direct routes
  • Achieving or exceeding the customers preferred cruise level 

The challenge for our controllers is being able to direct air traffic in a way which has a positive impact on 3Di and simultaneously dealing with a high volume of flights in our network, limited runway capacity which leads to aircraft holding, and occasionally bad weather.

We also recognise that the 3Di score is also influenced by variables and factors outside of our control, including the actions of our airline customers, airports, neighbouring air traffic organisations, the military and other airspace users.

As an indicator of the UK’s overall airspace efficiency the score is not a pure reflection of NATS efficiency. But, the 3Di measure ensures that we not only focus on what we control, but how we can improve the resilience of our operations to influences such as the weather and to work collaboratively across the industry to deliver efficiency gains.

We are making our 3D insight metric available to all

Reducing the aviation industry’s carbon footprint is one of the biggest challenges we face. Following the publication of the European Destination 2050 Roadmap, which sets to achieve net zero carbon emissions in the aviation sector by 2050, NATS is making available free-of-charge the formula of its industry-leading environmental performance metric to the wider industry to deliver efficiency gains.

If you are interested in having a conversation about 3Di, we would really like to hear from you. Simply complete our get in touch form and one of the team will be in contact.

Our controllers strive to positively impact the 3Di score

3Di score for Q3 2020

Air traffic movements (ATMs) in the UK were on the increase through July, August and September with an average of around 3150 flights per day over the quarter. This was positive news for the industry but still very much reduced compared to 2019 levels due to the impact of Covid-19.

NATS controllers have continued to deliver more efficient flight profiles for our customers and as a result the 3Di scores remained lower than we would expect for busy traffic scenarios. In particular, the amount of airborne holding was very low and for some airports there was no holding at all.

The 3Di scores for July to September were in the range 19.2 to 21.9. Despite the growth in traffic compared to the previous quarter, the Year to Date 3Di score at the end of Q3 fell to 24.0. This score is 3.8 points better than the CAA regulatory target of 27.8.


3Di score for Q4 2020

UK Air Traffic Movements (ATMs) had shown a promising increase during Q3 but this trend was reversed in Q4.  October would normally be a month of gradual decline towards the clock change and then November and December are usually very quiet in terms of total traffic numbers.  The UK Tier system for Covid-19 and the second National lockdown that ran from 5th November for four weeks meant that ATMs on average were down to 2100 for Q3. 

In addition, the autumn weather was settled for much of the time, without conditions that would normally require traffic management measures to be put in place. 

As a result of the low traffic and benign weather, the environmental performance remained strong for Q4.  3Di scores regularly dropped down into the 15-18.0 range and the monthly scores were all below 20.0.  The effect of three consecutive months of such good performance meant that the Year to Date score continued to drop and the final end of year score was 23.15, which is 4.65 points better than the CAA regulatory target of 27.8.

Despite this very encouraging level of performance, we continue to identify and target improvement in 3Di hotspots across the UK ATM network.  We have analysed the 3Di performance during the quietest traffic scenarios to see where inefficiencies still remain and we will continue to push forward airspace and procedural changes to reduce CO2 emissions.


3Di Performance
Q1 2021

Air Traffic volumes have remained flat during Quarter 1 of 2021 as the latest UK-wide lockdown has restricted domestic and international travel.  ATMs for the UK averaged just under 1400 per day through Q1, which is only just over 20% of the equivalent period in 2019.

Weather conditions have been remarkably settled through the first quarter with very few days with any disruptive conditions, and little that would require any Traffic Management intervention.  The monthly scores for January, February and March were 17.5, 17.1 and 16.9.

The year to date score is 17.2.  This is over 10 points better than the current CAA regulatory target for 2021 which is 27.5.  During March we recorded the lowest score since May 2020 of 13.3.

Although the current performance is very positive, NATS continues to identify improvement opportunities in environmental performance.  These include airspace and procedural changes, but also continued dialogue with our Airline customers to see whether there are opportunities for us to work together to identify improvements.



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