Environment performance 2014

Find out about our innovative way for measuring the environmental performance of the UK airspace network.

On 1 January 1 2012 NATS delivered another world first by developing an entirely new way of measuring the environmental performance of the airspace network above the UK.

The flight efficiency metric, known as 3Di, forms the cornerstone of a new incentive regime which is designed to deliver 600,000 tonnes of CO2 savings over the next three years, worth up to £120m in today’s fuel prices.

The introduction of the metric follows several years of developmental work by NATS in consultation with airline customers and the UK’s specialist aviation regulator, the CAA. The 3Di metric will help air traffic control route flight paths as close to the environmental optimum as possible by accurately measuring the efficiency of each flight in UK airspace.

By extending existing horizontal flight efficiency tracking to include vertical elements, 3Di quantifies the benefits delivered by air traffic controllers through continuous climb departures, cruise levels as requested by airspace users and continuous descents, as well as most direct point-to-point routings.

During its first three months in operation, 3Di has derived an indexed score of 24, which is the target set to deliver the expected fuel and emissions savings to airlines. This score is on track for the 2012 incentivised performance target set by the CAA. Under this scheme, NATS stands to be financially rewarded for exceeding the target or penalised for failing to deliver the expected efficiency gains.

This page provides a description of the 3Di flight efficiency metric and reports on NATS progress under the performance regime.

How is the score calculated?

The 3Di score combines together the inefficiencies calculated within the horizontal and vertical dimensions of flight.

In the horizontal plane it compares the actual radar groundtrack against the most direct great circle track – within the airspace network above the UK. The difference between these two distances, which describes the ‘additional miles flown’, defines the inefficiency in the horizontal plane.

In the vertical plane the metric compares the actual vertical profile against the airlines’ preferred trajectory shown in the figure on the left. Vertical inefficiency that results from Air Traffic Control interactions has been simplified to periods of level flight that occur below the airlines’ requested cruise level.

The vertical inefficiency is defined by the amount of flight time spent in level flight and the deviation from its requested cruise level. Level portions of flight at low altitude are more fuel penalising than at higher levels. In addition, the more time spent in level flight below the requested cruise level the more penalising for 3Di.

Lastly, because 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 phases of flight.

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

  • Every flight, every day of the year from 1 January 2012 will have a specific 3Di Score calculated. At the end of the year all the scores are combined to give a single annual average score for NATS
  • Scores run from 0, which represents zero inefficiency to over 100, with most flights typically having a score in the range between 15 and 35. Currently (Jan 2012)
  • Our regulator, the UK CAA has set a performance scheme that targets improvements to the UK 3Di score over the period to 2015.
  • To achieve the expected fuel and emissions savings to airlines NATS are working towards achieving an indexed score of 24 for 2012 and 2013, reducing to 23 in 2014.

How can NATS influence the 3Di score?

The 3Di score is mostly influenced by the underlying structure of NATS 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, limited runway capacity which leads to aircraft holding and bad weather. Adapting our operation to become more resilient to these external factors will help drive the score down.

Ultimately, the way NATS will influence the 3Di metric is by delivering more aircraft closer to the airlines’ preferred flight trajectories; more continuous climb and descent operations, reduced holding, fewer track miles and cruise levels at customers’ chosen flight levels.

Opportunities to improve 3Di

Airspace design
Controller tools
Flow management techniques
Changes to procedures
Awareness training
Flexible use of airspace
Optimised Coordination across sectors

Factors influencing the score

Number of flights
Traffic demand on sectors
Unusual events e.g. runway closure
Runway capacity

Bonus and penalty

The regulatory performance scheme that NATS is working to includes bonuses and penalties.

  • If NATS is able to reduce the annual 3Di score for the network below 21 points we enter bonus territory (max bonus £2.4m for a 3Di of 9).
  • If the 3Di score goes above 27, we risk penalty (max penalty £4.8m for a 3Di of 51). The performance scheme tightens by one point in the last year.

Head of Environment, Ian Jopson, discusses 3Di


3Di 2014 Target and Historical Performance

Our innovative airspace three dimensional efficiency metric (3Di) has now completed its third year of operation. 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.

Chart 1 below shows NATS airspace 3Di performance for the last two years. The blue line shows the rolling average 3Di score; the green line shows the monthly 3Di scores; and the red line shows the average daily traffic movements for each month. Overall, 3Di is showing a downward trend against historic levels, indicating improving airspace efficiency over time.

Chart 1: UK 3Di Scores 2013-2014

Chart 1: UK 3Di Scores 2013-2014 (click to enlarge)

The 2014 full-year 3Di score was 23.2. This means that despite delivering significant improvements across 2014 NATS has narrowly missed the CAA’s tightened target of 23.0 3Di points. Chart 2 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. This shortfall between NATS performance in 2014 and the target will be carried over as part of our targets for 2015.

Chart 2: UK 3Di Scores 2006-2014 (click to enlarge)

Chart 2: UK 3Di Scores 2006-2014 (click to enlarge)

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 in order to drive towards its tougher 3Di targets for the end of 2014. 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.


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