Reporting of environmental performance

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.

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 scores for the first quarter of 2013 are presented in the graph below.  The par (target) value set by our regulator, the Civil Aviation Authority is 24 for the year.

Chart 1: Full Year 3Di Performance – since January 2011 (CLICK TO ENLARGE)

The regulated running average score for the first quarter of 2013 is 25.1, 1.1 points above the target value of 24 points (the last year’s rolling average is 24.2).

Chart 1 shows our 3Di performance since January 2011. The green line shows the average monthly 3Di scores. The blue line shows the last year’s rolling average 3Di score (this is the figure upon which we are tested against our regulated target).  The red line shows the average daily traffic movements for each month.  As agreed in advance with the CAA, 3Di performance over the course of the Olympic Games has been excluded from the overall scoring because of the temporary airspace arrangements and security restrictions that existed around London during the Games.  This exclusion period was agreed because the temporary arrangements could have generated 3Di results that were due more to those changes than to NATS’ efforts to improve airspace efficiency.

The general trend in 3Di scores for the three months of 2013 is upward, indicating that the efficiency of the airspace has degraded marginally over that period.  However, looking at the long term trend in the monthly 3Di scores, it is normally expected that 3Di scores are less favourable in the first quarter of the year – this tends to be due to adverse weather conditions that has an effect on efficiency.  Despite this trend in the first quarter, we continue to work to deliver significant improvements in airspace efficiency.  Our environmental strategy (see below) is focused on a programme of strategic and tactical change to deliver long-term improvements that will both improve resilience to seasonal variation and reduce the underlying inefficiency.

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.

In the last financial year we delivered 31 specific near term changes to procedures to improve flight profiles enabling around 6900t of fuel savings equating to 22,000t CO₂ reductions.  We also have a programme of increased awareness across the company of the need to improve airspace efficiency has contributed to the improvements.

Since the 2012/13 financial year began there has been a huge effort across NATS re-focussing and re-doubling effort to bring a specific plan together to deliver our interim target of enabling a 4% reduction in ATM CO₂ on average per aircraft by the end of 2014.  Around 30 additional fuel and emissions saving measures are being considered, much of which is expanding the scope of what we have already done successfully, or speeding-up delivery of existing projects.

Examples of measures being incorporated into the 4% Plan include:

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

Nevertheless, meeting the 4% target will still take significant effort, some innovative thinking and collaborative working with our customers.  We will therefore be calling on the knowledge and expertise of our front-line operational staff and airline customers to ensure we deliver the further CO₂ savings.

An update to NATS 3Di performance will be published on this site in July 2013.