The European Commission’s PRB and EUROCONTROL’s PRC have just published a joint study on the ‘actual benefits of Free Route Airspace’ (FRA). This tallies with Airspace Unlimited’s findings published in January, evidenced by our industry consultations and advanced analytics.

The good news is that the direct-route only implementation of FRA is beneficial; the study finds that 50% of airspace users have experienced a reduction in ‘actual fuel burn’, with Italy and the SECSI FRA region most commonly cited.

While the PRB/PRC study is largely positive, the implication to airlines for direct-route only implementation remains. Our simulations show that there is an opportunity for an increase in the current benefits to airlines by 3-5 times what they are now. Our understanding of the issues across flight planning to ATM processes has enabled us to outline an ATM data service to deliver these extended benefits.

This article shows what we think the key PRB/PRC insights are, but some background first.

Background

First, a quote from @Achim Bauman of @A4E: “If airlines think about optimising airspace a simplified statement is to let us ‘fly as the wind blows’.”

Air Navigation Services (ANS) and their regulators have traditionally measured flight efficiency with reference to the ground track. But fuel burn is proportional to the time the engines are running, not the distance over the ground.

But in ANS, we assume that the direct ‘great circle’ is the most efficient, and that inefficiency is the percentage deviation from it. Another quote from Achim: “Unfortunately, the misconception of ‘direct route = most fuel-efficient route’ refuses to go away.”

We’ve been wondering about Free Route Airspace (FRA) for several months and have shared our sensemaking with around 40 experts in airlines, ANS providers (ANSPs), CFSPs, Eurocontrol etc.

What we found is that most flights within FRA are direct-route only, which limits the benefits. Free route requires the possibility for airlines to route via intermediate (published or unpublished) points. Under the Single European Sky, direct route was mandated for 2018 and free route by 2022.

Our take-aways from the PRB/PRC study

1. “Where weather is a significant factor, it should enable airspace users to optimise their route to make best use of weather conditions”.

=> In fact, weather and specifically winds are always a factor in flight planning, it’s how the algorithms work through ‘shortes path’ weighted graphs (Dijkstra etc.). In ATM we design in still air and operate in winds, future airspace designs need to consider this.

2. “Optimal routings should reduce the time an aircraft spends in the air.”

=> Yes, it’s flight time that burns fuel, not ground track distance.

3. “…the Regulation does not prescribe how FRA should be implemented or define the level of flexibility to be afforded to airspace users planning and flying through the airspace”.

=> The Common Project regulation was light on detail, but the intention must have been clear. The widely implemented ‘Borealis’ CONOPS details interim way points. National regulators and the EC may need to clarify their expectation for compliance with the regulations, but it is unlikely they will need changing.

4. “While all aircraft flying in FRA are technically flying freely … in practice the flexibility is limited by the choices of the airspace users and the support available from the CFSPs.”

=> CSFPs have not implemented user-preferred points, creating up to 400km directs. This can be addressed quickly by ANSPs in an ATM data service. But please let’s not chuck some random waypoints back in and make things even more complicated than they are.

5. “A presentation from ENAV … noted that only 8% of aircraft, equating to one airline, were flight planning using free route airspace in this area”.

=> We observed that 90% of flights within the Scottish FRA were within 1% of the great circle, i.e. they were flying directs not free routing. The industry needs to switch to better performance indicators and fast – our ‘Air Distance’ is contender and we are keen to collaborate on this.

6. “Maximising the benefits of FRA would lead to a significant increase in required computing power and time required for the route calculations. Therefore, CFSPs make a compromise between time required and the optimisation of the proposed routes, which CFSPs suggest achieves the majority of the benefits of FRA.”

=> This is a core issue. CFSPs did not implement user-preferred points because the processing time was too much. This is why we see an ATM Data Service as a quick solution.

Conclusions

Whether or not the current implementation meets the regulations, the benefits to airlines lie in implementation of user-preferred points, the actual ‘free-route’ part of Free Route Airspace.

In our work in airspace and air traffic management, we typically refer to the value of saving 1% of flight time, which also covers taxi time. Normalising to cruise or total fuel burn also helps keep the numbers comparable.

The basic calculation

In the following table we step through a calculation, assuming we have a technology that can improve flight efficiency by 1%. At the UK level this equates to saving £24M and 113,000 tonnes CO₂ per year. This calculation is minimalist, excluding a range of other costs that would be in a typical cost benefit analysis. For example, average crew and maintenance costs for a 737 could add £9.6 per minute of saved flight time and add £8.1M to the annual savings, increasing the total savings to £31.8M a year.

Calculation step	Value [Ref]
Annual flight hours	1.4M [1]
Flight minutes (x 60)	84M
Saving	1%
Flight minutes saved	0.8M
Fuel burn per minute (kg/min)	43 [2]
Fuel burn saved (kg)	36M
Cost of fuel per litre	£0.53 [3]
Litres of fuel to kg	0.8
Fuel saved in L	45M
Cost of fuel saved	£23.8M
Fuel burn saved (tonnes)	36,000
CO2 saved (tonnes)	113,000 [4]

Conclusions

Over 10 years the benefits of a 1% improvement in operational efficiency in the UK could be £318M.

What if we could reach 5% efficiency savings? That is £1.5B.

What if we applied the full economic costs of flights? This could be twice or more of these savings.

This is just the UK. Extrapolating to the European level, we could save £450M fuel and maintenance costs and 1.5Mt CO₂ per year.

In aviation carbon roadmaps, for some reason, operational efficiency benefits have been positioned as long-term actions, with small improvements every year. For example, the UK’s Net Zero carbon roadmap, assumes that improvements grow slowly from 2020 to 2050.

What if operational efficiency improvements were prioritised now? This could be a strategic play -achievable short-term gains to fuel long-term solutions with two important impacts:

• Early cost savings would help airlines’ bottom lines, making other initiatives like SAF and other fuels more affordable.
• Carbon savings are cumulative – a tonne saved in 2025 is worth 25 tonnes by the end of 2050.

References

1. Eurocontrol ACE Benchmarking Report 2019 Table 0.6: Operational data at ANSP level. 2019 chosen as flights have not recovered yet from Covid in 2020.
2. Calculated weighted average (49kg/min) from 2015 standard inputs (where source = BADA) modified by 1.5% per year efficiency improvement over 9 years.
3. Platts Jet Fuel Prices microsite. https://www.spglobal.com/commodityinsights/en/oil/refined-products/jetfuel. Bbl/L = 158.987, $:£ = 0.84
4. Multiply by 3.15 as a standard conversion between Kerosene and CO₂.

By default, operational efficiency benefits have been positioned as long-term actions, with small improvements every year. For example, the UK’s Net Zero carbon roadmap, assumes that improvements grow slowly from 2020 to 2050.

What if operational efficiency improvements were prioritised now? This could be a strategic play – achievable short-term gains to fuel long-term solutions with two important impacts:

• Early cost savings would help airlines’ bottom lines, making other initiatives like SAF and other fuels more affordable.
• Carbon savings are cumulative – a tonne saved in 2025 is worth 25 tonnes by the end of 2050.

How much can we save through operational improvements?

We can save fuel and emissions in aviation through a host of operational efficiency improvements. But the business case for these savings is not always clear, or even remotely understandable. If we see a claim of 10% savings in fuel and emissions, we need to ask ‘10% of what’? So, in this article we take a look at the benefits of saving emissions through operations with a quantitative eye.

First, let’s get a sense of proportion. From today’s operations we are unlikely to be able to save more than 10% of a flight’s total fuel burn through improved airline, airspace and air traffic management initiatives. In Figure 1 we show a range of operational efficiencies for which we have normalised the estimating savings to cruise fuel burn. This adds up to over 14%. However, it is likely that the benefits overlap and there are unaccounted for trade-offs.

For example, continuous descent operations are a method of improving vertical flight efficiency, which in turn saves fuel. Compared to cruise flight the potential benefit is about 0.7% reduction in fuel. This is unlikely to be achieved in practice, however, because of the need to separate arriving and departing flights vertically, reducing the fuel savings.

A basket of measures

What should be clear from Figure 1 is that there exists a basket of operational efficiencies. There are 16 measures in the figure that are straightforward to implement and can typically save 0.5-1%. There are for sure overlaps in the benefits, but a good way to look at this is that the overlapping efforts will increase the likelihood of achieving the benefits.

How are these measures saving fuel?

We can categorise fuel saving as either by reducing thrust or flight time, although there can be some overlap.

Thrust

Aircraft are more efficient the higher they fly, as drag is reduced in thinner air, and therefore less thrust is needed. However, it takes energy to climb so the optimum flight trajectory is a continuous climb, or more practically a step climb.

It also takes fuel to carry fuel, hence the introduction of rules on ‘tankering’ in the European Union. This requires that aircraft are fuelled only for the next flight and not any following flights. ‘Tankering’ fuel for several flights increases the aircraft weight, which then requires more fuel to carry this additional weight. A related idea is to fly long haul with fuelling stopovers. The logic for it taking fuel to carry fuel comes from the ‘Breguet Range Equation.’

Flight (and taxi) time

By flight time we do not mean flying faster, which requires higher thrust, but reducing unnecessary flight time. Our work in optimising Special Use Airspace and enroute charges modulation has the potential to reduce flight times by around 2%. This also benefits from optimising to flight time, not distance.

Performance based navigation (PBN) is included in the figure as being a causal factor in benefits, but PBN was developed for increasing air traffic controller productivity, not flight efficiency. We believe it is an open question as to whether PBN really helps flight efficiency.

Taxi time reduction is very interesting at airports. From data on European taxi-times there could be as much as 2.7% of cruise-equivalent fuel burn saved by various measures from reduced-engine taxi to autonomous or remotely controlled ‘taxi-bots.’ Note that aircraft engines need to be warmed-up before take-off, some requiring as much as 9 minutes of taxi to do so (737 Max).

In January 2025 we completed a project funded by the UK Airspace Modernisation Strategy (AMS) Fund. The fund is managed by the UK CAA to support the aviation industry in modernising UK airspace, and has the environment as its overarching principle.

Our project was based on the premise that by deeply enhancing the ability to understand how flights and airspace interact, we can identify opportunities for systemic environmental improvements supported by airlines, air navigation service providers (ANSPs) and regulators.

The project was focused on saving airline fuels and emissions through digital technology

Details on these benefits and next steps

Civil use of special use airspace

What we did

An analysis of a sample of special use airspace showed that up to 20% of civil flights went around the airspace when they could have routed through it. The analysis compared flights against the airspace use plan. We call these flights ‘missed opportunities’ and the extra cost to airlines was estimated to be nearly £2M in fuel, £0.5M in other operating costs and an additional 6,000 tonnes of CO₂ emissions per year (£20M and 60,000tCO₂ over 10 years).

Air distance wind-based analytics

What we did

While this discovery is a bit of a disappointment for fans of Free Route Airspace, it can be turned into a positive to get FRA back on track and deliver the promised benefits of millions of Euros and millions of tonnes of CO₂.

Further details on the discovery are here, but the summary diagnosis is as follows:

1. Free Route Airspace removed most internal waypoints and airway segments, but these were not replaced with user-defined waypoints from flight planning systems. Why? It has proven highly challenging to flight plan service providers (CFSPs).
2. Flight planning is now a complicated challenge with updating the Route Availability Document (RAD) restrictions. These seem to have effectively replaced the fixed route structure.
3. The ATM industry measures flight efficiency as a proxy; it compares ground track to great circle. A direct route is pretty much on the great circle so looks very efficient but only so in still winds. Our Air Distance indicator reveals this issue.

Hence, the ATM system is inadvertently constraining traffic flows to be less efficient while we have a measurement method that makes inefficient routes look efficient.

Next steps

Bold and fast action can be taken by ANSPs supported by regulators to address the current short comings. The good news is that the solution will be a fraction of the cost and effort already expended in developing direct routing. Airspace Unlimited are working on a prototype solution that can fit into current processes.

Analytics for charges modulation

What we did

Charges modulation is an air traffic services concept to address the impact of high user charges (enroute unit rates) on airline flight routing. As airlines tend to fly the lowest cost routes, high charge differences between neighbouring States’ leads to longer flights and more emissions.

The UK is surrounded by low-cost charge areas (Ireland and Norway), which distorts flight routes and adds costs and emissions to some flights. We identified two areas where a charges modulation could benefit UK flights and simulated a ‘charges zone’ in which a charges modulation (lower charge) could be applied.

An example is in Figure 3, which shows a narrow charges zone with reduced unit rate, to encourage flights from Manchester to Tenerife to route through the UK airspace rather than redirecting through Irish airspace. This simulation showed a 3% reduction in flight time, showing the potential for lower flight costs and emissions.

Next steps

We presented the results at the iCNS conference in April 2025 and are continuing to update our aviation industry colleagues. While the concept of charges modulation has been around for over 15 years, the AMS funded work is the first to show how it could be employed in detail.

AirOpt-Design, an advanced airspace design capability

What we did

A key requirement for airspace design is to be able to show how flights will route in the new design. However, most, if not all, current airspace design tools are set up to make incremental changes to the existing airspace. This means that these tools reflect either fixed or direct route airspace but not ‘free route’. This is the same with flight planning systems.

Our aim was to develop an advanced airspace design tool that is superior to existing tools and flight planning systems to support highly-modular airspace designs. This included assessing airspace designs according to other special use airspace (current and future), user charges and winds so that we could future-proof designs according to different scenarios. AirOpt-Design is also fast to execute in spite of the high number of variables this entails.

The AirOpt simulation is based on finding the lowest cost path through a ‘graph’, which is used to simulate airline routing in the same way that wind-optimal flight planning is performed. Graphs comprise sets of points or ‘nodes’, which are connected by edges. In our AirOpt simulations we apply a high-density graph, meaning that the nodes are 0.125° in latitude and longitude apart or about 7.5NM. Nodes are connected by edges separated at up to the fourth nearest neighbour, which gives a variety of routing angle options.

We refer to our routing capability as ‘freely routing’ and it represents how flights would ideally route if not subject to route or capacity management constraints (Figure 4). The idea is to support designers in finding the best routes and waypoints to improve the underlying airspace – beyond incremental design.

Next steps

We have been road-testing AirOpt-Design in a variety of projects, including for EUROCONTROL and a European Air Force. We are developing further functions of AirOpt-Design as a ‘pre-design’ function, supporting rapid development of future airspace designs with multiple scenarios. Once an optimum design concept is reached simulations can focus on waypoint positioning before applying other tools, such as NEST, to optimise flow management.

Conclusions

The AMS-funded project for a digital transformation of airspace management has been ground breaking:

• new civil-military analytics have shown the potential for combined stakeholder action to save millions of tonnes of fuel and emissions;
• an extraordinary finding that Free Route Airspace is only Direct Route, identifying millions of tonnes of fuel and emissions savings to begained;
• the first project to validate designs for enroute charges modulation, which has the potential to save up to 3% of flight time on certain key routes; and
• advanced airspace design tools that pave the way for highly-modular airspace designs, with the potential for significant military exercise productivity, and airline emissions reductions.

Next steps

• We need to scale the outputs of this project to deliver the identified benefits.
• States need to be able to independently monitor the performance of their airspace with wind based analytics that better reflect how airlines actually fly.
• We are developing an approach to turn direct route airspace back into free route airspace.
• In the medium term, the static approaches in this project can be made dynamic, reflecting the fact that aviation is a wind-driven activity, albeit currently measured and managed as if in still air.

There’s an old joke about the cockpit of the future, which is so highly-automated as to comprise of a pilot and a dog; the dog was there to bit e the pilot if they tried to touch the controls!

Re-framing this joke, might the air traffic controller position of the future be a staffed by controller and a dog? This time the dog is trained to bite if the controller tries to offer a ‘direct’.

Why?

‘Directs’ look like short cuts to controllers and pilots (who may request them), but they may be directs into a headwind, burning more fuel. Asking for and giving a direct assumes that the controller or pilot know better than the airline’s flight plan, which is based on predicted winds and minimum costs (fuel etc.)

As airlines increasingly invest in advanced flight planning systems, we should consider that the flight planned route is likely the best and there is no advantage in a direct.

There are some caveats to this:

(a) The airline may not have submitted an optimum flight plan, either due to their system or how they resource it.

(b) An airspace reservation (special use airspace) may have been deactivated (you would have to imagine some pretty strong winds to think that a direct would not be beneficial in this case). A caveat to this caveat is that where the destination is likely to have airborne holding for the arrivals stream.

(c) The pilots have updated forecast winds sent to their Electronic Flight Bag (EFB).

Image ChatGPT, not a real ACC.

This is an update of our January sense-making article on FRA.

What is Free Route Airspace?

Free Route Airspace (FRA) is an airspace within which airlines may freely plan their routes between an entry point and an exit point. It was introduced in Europe to give airlines better flight routing opportunities than could be achieved with the fixed route structure. A fundamental component of Free Route Airspace is the use of ‘user-preferred points’, with connecting segments, which enable airlines to actually ‘free’ route.

The potential benefits of FRA were believed to be considerable, with various benefits estimated at 1.3% ¹ up to ~2% ² of flight distance at the network level. Implementation did not require new technology, just changes to airspace and operations, creating a strong business case for it.

What’s the problem?

The user-preferred points don’t exist. So, we have to ask if the benefits exist? Let’s unpack this. We first need to consider that there are two versions of FRA:

• Direct, which comprises direct (great circle) routes between FRA entry and exit points.
• Free Route, which does not have any published routes and enables flights to be planned along published or user defined waypoints within the FRA.

Figure 1: Example direct routing through Free Route Airspace (Scottish FIR)

Over the last ~10 years, the fixed route structure has been steadily dismantled, waypoints and route segments have been deleted. This means that the only way to get ‘free’ route is for the flight planning service providers to supply user-preferred waypoints.

How do we know this?

Airspace Unlimited analysed routes through Free Route Airspace. We found that up to 90% of the routes were within 1% of the great circle distance. This is impressive track keeping born of the age of PBN. But to quote Airlines for Europe (A4E): “Unfortunately, the misconception of ‘direct route = most fuel-efficient route’ refuses to go away”.

Maybe there was no wind when we analysed the data? We have a metric for this called ‘air-distance’. It is the equivalent distance travelled by a flight between two points with a non-zero wind component. If that does not make sense, think about walking along a travelator. If you walk in the direction of the travelator, you get there quicker, as if you had gone a shorter distance, and vice versa. Air distance looks very different to ground distance. In the following figure, 90% of flights are within 1% of the great circle by distance, but in terms of air distance, 40% are actually shorter than the great circle (i.e. tail wind).

What ‘free’ route is supposed to do is to help the flight planners improve the flight trajectory by taking account for winds (avoid head wind, take a tail wind). What we know from our air distance analysis is that this isn’t being done.

What does the industry say?

To explain our findings, we have also been consulting with airlines, ANSPs and flight planning service providers. This led us to identify three main factors:

• The difficulty for computer flight planning systems to model and determine user-defined waypoints and segments for a given day of wind conditions, alongside the myriad other roles of these advanced automation systems.
• The ever growing complexity from constraining traffic flows through the RAD.
• The measurement of flight efficiency according to the ground track, not the flight time.

In summary, the current situation is that the ATM system is inadvertently constraining traffic flows to be less efficient while we have a measurement method that makes inefficient routes look efficient.

What can we do to bring free route airspace back on track?

Our conclusion is that we need to revisit how we design FRA and consider how we help the flight planning process produce better outcomes for airlines. We are currently working on a solution to do this, which will both simplify the problem for flight planning companies and lead to fewer RAD restrictions. This will be a win for airlines, ANSPs and the environment.

For further information contact Airspace Unlimited (Martin Hawley or Doug Meyerhoff).

1. Gaxiola C. FRA CBA study. PhD Thesis. 2019 ↩︎
2. https://www.eurocontrol.int/concept/free-route-airspace ↩︎

The need for live flying training for air forces

For western militaries who have reduced their combat air fleets since the mid-1990s, there is an imperative to maximise training for their 4th, 5th and, in future, 6th Generation fighters.  Sensor and weapons ranges have increased and, with the reduction in live flying in favour of synthetics, every flight hour is precious.

A war in Europe, increasing instability in the Middle East and a growing threat from China has focussed the need for realistic and collaborative training, and yet, the same basic airspace structures and Special Use Airspace (Box 1) has been in existence for decades. Air force thought leaders see that this construct is no longer fit for current, nor next generation air forces that require coordinated, collaborative multi-domain training; utilising Red Air, EW, tankers, crewed & uncrewed vehicles in a Live Virtual Constructive environment that could (and should) extend across FIR boundaries.  This is increasingly important as the F-35 user community grows across the European NATO countries and with the USAFE.

Box 1: Special use airspace

Why the scarcity of airspace is an issue

NATO States have now committed to deploying up to 750 F35s in Europe by 2033, including those of USAFE, mostly based at RAF Lakenheath. Deploying these aircraft to full capability requires extensive Force Generation exercises, which States are struggling to accommodate given the current limits of airspace reservations (RSAs / SUAs) set against the demands of F35 sensor and weapons ranges. Force Generation is the purpose of air forces, and visibility of effectively coordinated exercises acts as a strong deterrent to NATO’s adversaries.

Alongside the demands on airspace from the F35, and other 5^th generation fighters, there are growing demands on airspace through civil traffic growth and new aircraft types, as society embraces Uncrewed Air Systems (UAS), Advanced Air Mobility (AAM), High Altitude Long Endurance (HALE), and Space Launch and Recovery operations.

This growth in both civil and military demands for airspace must also be set against the commitment of European governments to Net Zero by 2050. The challenge is to grow traffic, maximise Force Generation and minimise emissions. Airspace has a key role to play in meeting this challenge as: (a) there is currently a lack of appropriate special use airspace to support live flying; and (b) geographically fixed volumes of special use airspace are not conducive to optimum civil flight routing.

The future deterrence of Europe depends on air forces increasing the amount of live flying

The concept of the ‘flexible use of airspace’ (FUA) has been implemented in Europe for 25 years. Flexibility means that military users book the airspace that they plan to use, use it, and if they no longer need it for the day of operations, they return it to civil use. In doing this the military have no sight of the routes that are most useful to airlines. They book the fixed geographical areas they want, and the airlines submit flight plans that fly around those areas.

Because civil flight trajectories are influenced by winds, each day has a new optimum trajectory, but there is no change in the routing because the special use airspace is fixed. So, on some days the special use airspace booked by the military is blocking significant traffic flows, on others it blocks minor flows.

Can we re-engineer the system to be more flexible?

To reduce the impact on civil flights and deliver more daily training we see the need to leapfrog the next incremental changes in special use airspace with a bold conceptual and technological step.

This means a new paradigm about how we design and manage airspace.

Counterintuitively, we believe we should make special use airspace larger, but more finely structured. With more space to plan exercises in, we can shape exercises to fit the mission, the traffic flows and therefore the climate; avoiding heavy civil traffic flows allows for larger airspace volumes at the same time as giving civil flights shorter paths, saving fuel costs and emissions (Box 2).

This is beyond the current flexible use of airspace, giving fluidity to the whole airspace, shaped by winds on a daily basis. This is ‘smarter airspace management’ and the implications for special use airspace are profound, so that it is:

1. enlarged, potentially by 50-100% – imagine joining areas across the North Sea;
2. activated according to impact on civil traffic flows, not fixed volumes – imagine minimising the impact on civil flights before they flight plan;
3. finely segmented, comprising elemental volumes that are 5-10NM in area by 5000ft in altitude – imaging shifting an exercise by 10NM to finesse a carbon reduction;
4. divided into training volumes based on daily mission needs not predefined segments – imagine planning the mission being allocated the airspace volume that is just right;
5. as tightly packed as mission volumes and safety buffers require – imagine accelerating training through more exercises in parallel.

In an industry that changes glacially, this may seem a big leap, but it is not rational to meet the challenges of this century with incremental solutions. And the solutions are not so much bold as living up to the expectations of what the civil-military concept of the ‘Flexible Use of Airspace’, introduced just twenty-five years ago, should mean.

These ideas align with other concepts, particularly the SESAR Dynamic Management of Airspace and Military Mission Trajectory concepts. This has even been referred to in SESAR as ‘floating’ military airspace¹.

Box 2: Because airspace is fixed, it is rarely efficient

Smarter airspace will accelerate sustainable aviation

Around the world, governments are placing increased pressure on all sectors of the economy to decrease carbon emissions in line with the IPPC’s 1.5 degree global warming target, also referred to as Net Zero 2050. These demands also include Defence. In the UK, the MoD accounts for approximately half of government carbon emissions and 40% of this is the responsibility of the Royal Air Force; most caused by burning aviation fuel.

The RAF is working on new fuels to meet its Net Zero obligations, but there are other levers that it can pull. Through the smarter airspace concept, we can use airspace in a way that minimises interactions with civil traffic flows, reducing civil aviation emissions at the same time as increasing military mission effectiveness.

What tools are needed?

The tools for smarter airspace management are being developed by Airspace Unlimited, and their development has been supported by the UK’s Airspace Modernisation Strategy Fund. From our interactions with the RAF over the last two years we have been encouraged around these bold concepts.

During 2024 we developed airspace design tools to rapidly assess multiple hypotheses for design and management of the airspace, with realistic traffic flows with winds and route charges factored-in. Being able to design airspace changes and concurrently measure the impacts on traffic means we can drastically speed up airspace change and re-envisage how multiple users can use the skies above us in the most effective and efficient ways.

We are delivering these tools as a tiered capability to transform airspace, to support States in: analysing the airspace to understand how it may be improved; designing the airspace to be more flexible, using ‘Variable Profile Area’ 5-10Nm design rules; and ultimately managing the airspace on a rolling 24-7 basis.

Our medium-term goal is to reduce the impact of special use airspace on civil flights and save 1% of civil flight time, which could amount to 2Mt CO2 saved a year across European NATO States. In the shorter term we are focusing on highly-modular ‘Variable Profile Areas’. By disaggregating airspace into elemental volumes, say 10Nm x 10Nm x 5000ft, we can pack more exercises into existing airspace design. This means we can achieve more exercises on a daily basis and accelerate training and force generation programmes, all while reducing the interaction with civil traffic flows.

The future is in sight

Air Forces, including the Royal Air Force, are seeking to optimise, to get the best out of what they have; however, they are constrained in their training while attempting to increase combat effectiveness. It simply doesn’t make sense to conduct live training in airspace that is not fit for purpose – we should strive for the win-win-win scenario that airspace optimisation can bring.

This work is also addressed at the growing demands on airspace driven by multiple new users types: Uncrewed Air Systems (UAS), Advanced Air Mobility (AAM), High Altitude Long Endurance (HALE), and Space Launch and Recovery operations.

1. SESAR Solution 04.07.07 Operational Services and Environment Description. ↩︎

What is Free Route Airspace?

Free Route Airspace (FRA) is an airspace within which airspace users may freely plan their routes between an entry point and an exit point. It was introduced in Europe to give airlines better flight routing opportunities than could be achieved with the fixed route structure. Free Route Airspace was also mandated under the EU ‘pilot common projects’ regulation: EU 716 (2014).

SESAR Solution #33 further describes ‘free routing’ as “the ability of an Airspace User to plan/re-plan route according to the User defined segments (i.e. segments of great circle connecting any combination of two user defined or published waypoints)”.

Implementing FRA has not been a simple matter, requiring traffic simulations to ensure that the airspace did not introduce multiple conflict points from unpredictable entry and exit points into the free route airspace.

The potential benefits of FRA were believed to be considerable, with various benefits estimated at 1.3%¹  of up to ~2%²  of flight distance at the network level. Implementation did not require new technology, just changes to airspace and operations, creating a strong business case for it.

So, what’s wrong?

We were intrigued that most flights through FRA seemed to be direct from entry to exit point, with little other variation. In comparison, our AirOpt simulation tool in ‘freely routing’ mode shows a variety of routes, based on a high density of nodes (waypoints). For example, when optimising flights around highly modular special use airspace (SUA) designs or simulating the effect of enroute charge modulations (reference SES 2+). We can also clearly see the effect of charges on airline routing, so why not winds? (See the NE corner of the Scottish FIR in the figure below for flights that take a 30° turn as they skirt the higher FIR charges.)

By analysing ADS-B flight tracks we confirmed them to be mostly direct through FRA, meaning that they follow the shortest distance between two points on the earth, known as a great circle.

An issue for most simulations and cost benefit analyses over the last ~25 years is that it is assumed that the wind has no effect on routing, or that winds average out. It is also difficult to include the winds without sophisticated computer models and high power processing. In other words, the ATM system appears to be designed and measured in still winds but operated in a wind-variable world. This is why we developed the ‘Air Distance’ metric, in various forms, presented later in this article.

Our hypothesis was that free route airspace has become direct route and that, if so, there is an opportunity for increasing flight efficiency by getting free routing ‘back on course’. To explore the issue we have studied ADS-B tracks from Plane Finder through the Scottish FIR. The figure below shows tracks from 25 January 2025. It is striking that the majority of the tracks closely follow the great circle from entry to exit point of the FRA. A track is highlighted (coloured dots) that diverges from the great circle slightly, shown by the blue line. This may show some wind based routing but it is hard to tell from this data.

Figure 1: Example routing through Free Route Airspace (Scottish FIR)

We concluded that we had some evidence of direct routing being the norm and decided to consult experts within the industry, including with flight planning experts. We also compared horizontal flight efficiency (HFE) measurements using ‘ground distance’ and ‘air distance’. This led us to see that there are three parts to this story: computerised flight planning systems, route ‘availability’, and flight efficiency measurement.

Computerised flight planning systems

Computerised flight planning systems came into being in the 1990s. Computers made wind-based routing a reality, advised pilots on how much fuel to carry, navigate ATC and regulatory rules and generally save substantial amounts of fuel per flight. Free route airspace was to play to the strengths of these systems. However, the thing that makes flight routing algorithms work was also being taken away – the waypoints and the routes between them. We try to explain how this works in Box 1, but note that the flight routing algorithms only work when they have nodes (waypoints) and edges (airways) to route along. Taking away the nodes (waypoints) is not like going off road in a 4×4, it’s like seeing a hole in the road ahead and having to go back to the start to take a different route.

Box 1: How routing works for flight planning

Route availability

Route availability refers to routes that are published as being subject to traffic flow rules defined within the ‘Route Availability Document’ (RAD). Originally updated every 28 days, RAD updates are now almost daily. The RAD was introduced in 2006 and has played an important role in standardising and protecting routes from overload. It has grown to control free route airspace and fine tune flow management. Indeed, early experience with FRA led to the realisation that RAD restrictions could be used to manage the flow of traffic into and out of FRA at specific points.

Flight planning companies have two concerns though. The first is that the effect of RAD restrictions in FRA is to imitate the previous fixed route structure. This gives rise to the second concern, that growing complexity and almost daily changes to the RAD make it difficult to integrate changes into their databases. This can lead to errors when their customers’ flight plans are submitted through the IFPS, which cause flight plans to be rejected. Airlines will then fall back on ‘company preferred routes’, which are static and suboptimal for the day of operation.

Flight efficiency measurement

By measuring flight efficiency, ANSPs will know that they are delivering the optimal conditions for airline flight planning. This is currently measured with distance-based horizontal flight efficiency (HFE).

Air distance metric

While the great circle is the shortest distance, it is not always the shortest time through the airspace, i.e. when there is a head wind. What is confusing about directs is that they look the most efficient on the map but often they aren’t. If they were, then there would be no need for wind considerations in flight planning. To understand wind-based routing we created the ‘air distance’ metric, which accounts for the wind vectors along the flight track.

To better scope the issue, we have calculated the actual horizontal flight efficiency (HFE) known as ‘KEA’. This indicator compares the ground tracks (GD) with the great circle distance (GC) as flight inefficiency = GD/GC -1. From a sample of 526 flights from the 25 Jan data, the flight inefficiency varies as shown in the histogram below, where 89% of the flights were within 1% of the great circle distance. This is impressive track keeping born of the age of PBN.

We can use air distance (AD) in the same way, calculating flight inefficiency as AD/GC-1. The following histogram results, showing a wide distribution where ~40% of the flights had an air distance less than the great circle due to tail winds and the remainder ~60% had air distances greater than the great circle due to head winds.

Knowing the air distance would normally give us insight into the flight plan, e.g. why was one route taken over another? But when most flights are taking a direct, does this mean a direct was always the best route? We think this unlikely, and more likely that the multiple complicated factors involved in flight planning now constrain free routing to direct only.

It is likely that distance-based KEA is not an accurate measure of flight efficiency for calculating the benefits of free routing. Aircraft use fuel in accordance with how long the engines run for, not the distance over the ground. When HFE/KFE was introduced, it was a good high level indicator, but it was never intended for operational problem solving.

Conclusion

Free Route Airspace hasn’t been implemented as we expected, but rather as ‘direct’ route. This has arisen from three main factors:

• The difficulty for computer flight planning systems to model and determine user-defined waypoints and segments for a given day of wind conditions, alongside the myriad other roles of these advanced automation systems.
  
• The ever growing complexity from constraining traffic flows through the RAD.
  
• The measurement of flight efficiency according to the ground track, not the flight time.

Measuring the route system with distance no longer makes sense while striving to increase real flight efficiency for net zero targets. Worse than this, it makes direct routing look better than wind-based routing.

In summary, the current situation is that the ATM system is inadvertently constraining traffic flows to be less efficient while we have a measurement method that makes inefficient routes look efficient.

Our conclusion is that we need to revisit how we design FRA and consider how we help the flight planning process produce better outcomes for airlines. This could even mean adding waypoints and segments back in, but does not mean we have to go back to the fixed route system.

1. Gaxiola C. FRA CBA study. PhD Thesis. 2019 ↩︎
2. https://www.eurocontrol.int/concept/free-route-airspace ↩︎

Who has heard the old saying ‘Do more with less’? While this strategy is not sustainable, increasingly reality is rooted in the idea of doing more with what you have.

Optimisation, efficiency gains and operational enhancements are words that mean do more without additional resources. This should be the aim of every individual or organisation.

Waste should neither be encouraged or accepted. Unfortunately, there is waste in any system with multiple stakeholders and external influences. The air traffic management (ATM) system is no exception to this reality, our mission should be to identify where there are gaps in the overall performance and move to close these holes.

Many of the efficiency gaps within the ATM system are rooted in legacy airspace structures and operating procedures. Aircraft capability has far outrun the suitability of airspace structures particularly in the area of military training and airspace requirements.

Airspace Unlimited acknowledges that there will never be 100% efficiency, but there are real gains to be had by managing the assets we have more effectively. These structural gains come at a hugely discounted rate compared to the cost of developing new engines and fuels to fire them, and most importantly, are available now.

See what our Chair, Justin Reuter has to say in the latest Air and Space Power Association Bulletin:

https://airspacepower.com/wp-content/uploads/2024/02/Air-Space-Power-Association-Bulletin-Spring-2024-26-Feb-2024.pdf

We would like to thank the Air and Space Power Association for publishing this article. To read more visit www.airspacepower.com

Airspace Unlimited has been nominated for the Innovation of the Year Award through the #ScottishKnowledgeExchangeAwards.
Working alongside Dr. Sandy Brownlee of #StirlingUniversity, we have improved the overall performance of our system.
Focused on improving the #environmental footprint of the #aviationindustry, this video helps describe our collaboration with the University.