3Di, which stands for three dimension inefficiency score, is a pioneering metric developed by NATS to measure the environmental efficiency of UK airspace. The metric, which originally took two and half years to develop, came about after discussions with our customers on how they would like to see us measured.

In 2012, 3Di was officially adopted to financially incentivise NATS’ environmental performance as part of our regulatory licence to operate air traffic control – a world first.

In 2014 we implemented new enhancements to 3Di and agreed new more stringent targets with our customers and our UK regulator, the Civil Aviation Authority (CAA).

We know that our airlines’ preferred flight paths include a continuous climb from their departure airport to cruising altitude, followed by a continuous descent back to the ground, with the full profile taking the most direct route. The 3Di metric helps NATS measure the deviation from the preferred profile to the actual radar track of each flight in UK airspace.

3Di allows NATS to establish a clear indication of the environmental efficiency of the service it provides, track its performance over time and identify opportunities to reduce aviation’s environmental impact, while at the same time reducing airline fuel costs.

This webpage provides a description of the 3Di flight efficiency metric and reports on NATS progress against targets.

Multi-award winning

Since its launch 3Di has been recognised by numerous external awards:

  • Business in the Community’s 2015 Big Tick Award for Sustainable Products and Services (re-validated after demonstrating further improvement)
  • Business in the Community’s 2014 Big Tick Award for Sustainable Products and Services
  • Jane’s ATC 2012 Environment Award
  • Transport Times 2012 Award for Contribution to Sustainable Transportation
  • The Operational Research Society’s President’s Medal 2011 for the best practical application of operational research in industry

How is the score calculated?

3Di Score

3Di measures the combined inefficiencies within the horizontal and vertical dimensions of flight. In the horizontal plane it compares the actual radar ground track against the most direct great circle track within UK airspace.

The difference between these two distances, which describes the ‘additional miles flown’, defines the inefficiency in the horizontal plane. A new dimension to 3Di has now been incorporated in the horizontal plane. This looks at the excess track extension added to the whole flight – encouraging NATS to consider how its airspace boundary entry and exit points may affect airline fuel burn across multiple airspace regions. The overall aim is to provide aircraft with the most direct route across their whole flight profile.

In the vertical plane, the metric compares the actual vertical profile against the airlines’ preferred trajectory shown above. Vertical inefficiency has been simplified to the amount of level flight that occurs below the airlines’ last Filed Flight Level (FFL) and the vertical deviation from FFL. Level flight at low altitude is more fuel penalising than at higher levels so the more time spent in level flight below the FFL the more penalising for 3Di. If we exceed FFL then 3Di is zero (good) for the cruise element. As aircraft performance and in particular fuel flow rates vary across the different phases of flight, the metric applies different weightings for level flight occurring in climb, cruise, and descent.

View our quick guide to improving 3Di

View our quick guide to improving 3Di

All of these factors are then combined to give a single 3Di score for each flight in UK airspace.

  • Every commercial flight, every day of the year will have a specific 3Di Score calculated. At the end of each year all the scores are combined to give a single annual average score for NATS and compared to targets set by our UK regulator in consultation with our airline customers.
  • Scores run from 0, which represents zero inefficiency to over 100.
  • As a result of enhancements to the metric in 2014 both 3Di scores and targets have been re-calibrated to reflect improved accuracy. Updated average 3Di score averages have changed and are now around 30 points, circa 7 points higher than previously (see ‘New targets and enhancements’). In 2014 the average score with the updated 3Di metric was 29.7.
  • The UK CAA has set a performance scheme that targets improvements to the 3Di score over the period to 2019.
  • NATS’ target is to reduce 3Di from 29.7 to 27.7 by calendar year end 2019.

How can NATS influence the 3Di score?

The 3Di score is mostly influenced by the underlying structure of UK airspace. NATS operates some of the busiest airspace in the world and our primary consideration is to ensure the safe and expeditious flow of air traffic under our control.

The congested nature of UK airspace means that sometimes aircraft are not always able to obtain their most direct routes or most optimal climb and descent profiles.

The 3Di metric has been designed so that it drives behaviours in NATS to deliver long term improvements in flight profiles and related fuel burn and CO2.

The biggest improvements will be delivered by changes to the design and operation of airspace and by improving access to shared airspace.

Day to day changes to the way our air traffic controllers direct aircraft can also have a positive impact on the 3Di score, and fuel burn and emissions performance.

Our challenge will be to do this in the face of factors that affect the score negatively, such as the volume of flights within our network, bad weather and limited runway capacity which leads to aircraft holding. Adapting our operation to become more resilient to these external factors will help drive the score down.

Ultimately, NATS will influence the 3Di metric by delivering more aircraft closer to the airlines’ preferred flight trajectories, including:

  • More continuous climb and descent operations to/from higher levels
  • More direct routes across UK airspace
  • Reduced airborne holding at destination airports
  • Working with neighbouring Air Traffic Control providers and military airspace users to deliver more direct routes across the whole flight profile
  • Achieving or exceeding customers preferred cruise levels (FFL)

Working to improve the score from the previous year by looking at all aspects of inefficiency, whether caused by NATS or other actors, and working collaboratively to reduce it.

New targets and enhancements

We’ve built on the success of the 3Di model by implementing:

  • Improved data and input processes to more accurately identify inefficiency
  • New tools to provide controllers with access to near real time 3Di data – helping to drive understanding, monitoring and action
  • A new dimension to 3Di which encourages NATS to improve direct routeing across multiple airspace regions (see ‘How the score is calculated’)

As a result of these changes both 3Di scores and targets have been re-calibrated to reflect the improved accuracy. Updated average 3Di score averages have changed and are now around 30 points, circa 7 points higher than previously.

The new performance regime set by the CAA features harder targets. NATS’ target is to reduce 3Di from 29.7 to 27.7 by 2019.

The new performance regime set by the CAA features harder targets. NATS’ target is to reduce 3Di from 29.7 to 27.7 by 2019.

3Di 2015 Target and Historical Performance

The 3Di metric has entered its fourth year of operation.

Year Target Performance Target Status
2012 24.0 23.9 Achieved
2013 24.0 23.7 Achieved
2014 23.0 23.2 Not achieved by narrow margin

During the first year (2012) a 3Di score of 23.9 was achieved against a target of 24, set by the CAA. This represented an improvement of 0.3 points compared with a score of 24.2 in January 2012. In 2013, NATS again delivered ahead of the CAA’s 3Di target with a score of 23.7 points, a 0.3 point improvement against the target value set. For 2014, the CAA tightened the 3Di target which was set at a score of 23, and we marginally missed this goal, scoring 23.2.

Chart 1 below shows that, while the target for 2014 has been missed by 0.2 3Di points (or within 1%), due to the efforts of NATS air traffic controllers and change teams the 3Di score has reduced significantly against historical levels. The shortfall between NATS’ performance in 2014 and the target has been carried over as part of our target for 2015.

Chart 1: UK 3Di Scores 2006-2014 (click to enlarge)

For 2015 to 2019 a new performance regime has been agreed with CAA in consultation with airline customers. As noted above, 3Di has also changed in the way it measures efficiency with updated 3Di score averages now typically being around 30 points.

2018 3Di Commentary, Quarter 4


This briefing provides information about our 3Di performance for the fourth quarter of 2018.

The chart below shows the individual monthly average 3Di score (green) and the year to date score (blue), that takes account of previous monthly performance, against the CAA’s target (dotted line). Average daily movements are also shown (red).

NATS 3Di Performance Q4 2018

3Di Performance for Quarter 4, 2018

Our regulator, the Civil Aviation Authority, has set a challenging target for our airspace efficiency in 2018, with a target 3Di value of 28.1. The NATS 3Di performance at the end of the fourth quarter of 2018 score was 29.2, resulting in an overall score of 29.2, making our performance 1.1 points behind our target and below the penalty threshold set by our Regulator. Our year end score was still an improvement on our 2017 score, which was 29.6. This is due to steady improvement in score from May through to November driven by factors such as a reduction in holding, working with airlines to improve required flight level planning and airspace changes. This improvement has been seen despite a number of days where disruption at airports adversely affected airspace efficiency. 2018 was also a year where we experienced a disproportionate number of easterly operation days; UK airspace is optimised for the predominant westerly wind conditions and operations in easterly conditions have lower airspace efficiency.

We continue to identify and target improvement in 3Di hotspots across the network. Also, we are improving the accessibility of 3Di performance reporting data for our air traffic control employees to enable them to manage airspace more efficiently, day-to-day. In addition to this we are regularly briefing our operational staff on the best ways to safely improve the flight profiles of the aircraft they control and in doing so, realise emissions reductions. These initiatives are on top of our ongoing focus on airspace and procedural changes to reduce CO2 emissions. Further information is provided under “Our Environmental Strategy” write-up below.

Our next performance update will be in April 2019.

Our Environmental Strategy

Our environmental strategy is based on delivering long-term structural improvements to airspace efficiency, as well as focusing on a targeted programme of shorter-term enhancements.

Last year NATS made significant steps towards reducing arrival holding through the implementation of a descent speed trial. Reducing holding by slowing aircraft down en route reduces the need for aircraft to ‘hold’ close to an airport, circling, burning extra fuel and generating more emissions. During 2014 NATS focused its efforts on delivering improved airspace efficiency. We delivered:

  • A number of small scale airspace changes
  • Extending the flexible use of airspace with military airspace users
  • Further improvements in continuous descent approaches and improved departure profiles
  • Additional procedures and tactical changes to deliver optimised trajectories
  • Environmentally focused network management improvements

By the end of 2014 NATS had delivered in excess of 300 changes to drive airspace efficiency since 2006. The focus on this mixture of short term tactical and procedural measures to improve airspace whilst continuing with a programme of larger scale structural changes continues into this regulatory period 2015 – 2019.

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