Aircraft efficiency starts with having the right altitude

More accurate data on an aircraft's position can help pilots fly more efficient routes, cutting the fuel consumption and emissions of today's fleet. The SESAR JU Green-GEAR project aims to quantify the impact of this change, as Tobias Bauer, project coordinator from the German Aerospace Center (DLR), explains.

What is the main objective of the Green-GEAR project and what problem(s) is it trying to solve?

When people hear the term "energy-efficient air traffic" they usually think of sustainable aviation fuels, hydrogen- or battery-powered, electric aircraft. The development and full roll-out of these technologies is unfortunately still decades away. The good news is that we can also improve aviation's climate balance through the evolution of Air Traffic Management (ATM).
The Green-GEAR project is aiming to do exactly that. The objective of our research is to enable optimum green flight trajectories and decrease both CO2 and non-CO2 climate impact at the network level.

How do you propose to approach these problems?

Green-GEAR is working on three Solutions to jointly address the climate impact of flights. Apart from this, we are striving to generate a positive effect on airspace capacity and overall safety.

What are the three Green-GEAR Solutions?

Solution 1, "Vertical guidance using geometric altimetry", examines the feasibility of switching from barometric to geometric altimetry, namely the use of satellite navigation altitude instead of air pressure measurements. In the Terminal Manoeuvring Area (TMA), the airspace near and above airports, it is critical that flights manoeuvre in relation to ground. This is where the use of geometric altimetry is expected to increase safety due to its greater accuracy, and the fact that it would remove the need for an error-prone ground pressure reference setting input.
Geometric altimetry provides continuous altitude references, which removes the necessity for a transition layer in the TMA. This would allow more efficient climb and descent operations, reducing fuel use and noise impact on the ground, and would permit more efficient airspace use.

It remains to be seen whether the use of geometric altitude measurements also benefits operations at cruising altitude but its greater accuracy is likely to help decrease existing vertical "separation minima" (Green-GEAR's Solution 2) between aircraft.
Barometric altimetry's relatively high system error necessitates a big part of today's airspace contingencies. These measurement inaccuracies increase with altitude, making it a particularly salient question for Higher Airspace Operations (HAO) aircraft.
With current procedures and the resulting requirements, it seems that we cannot easily populate additional flight levels without developing advanced modes of separation first. "Unlocking" these additional levels would have a significant environmental benefit as more aircraft could fly close to their preferred altitude and optimised flight trajectories.

Finally, Solution 3, "Green route charging" aims to incentivise environmentally friendly flight path choices.
The provision of air navigation services is fully paid for by the airspace users, and charges need to be non-discriminatory, transparent and cost-related. The current practices can unfortunately lead to unintended consequences. In our work, we observe detours through "cheaper" airspace to avoid higher navigation charges on the more direct route, which actually saves the operator money on balance even if more fuel is burned and more CO2 produced.

Our goal is to investigate to what extent the application of "modulation of charges" can be used to encourage environmentally beneficial trajectory choices like selecting the shortest route that still avoids congestion.
Ultimately, we will also take into account non-CO2 effects, where we want to incentivise the avoidance of so-called "climate hotspots". All this needs to work at the network level - the independent optimisation of every single flight will not result in a global optimum.

It is important that airspace users retain a choice, as there are valid operational reasons for certain routings, notably to catch up or avoid delays, which have their own environmental costs. Our aim is not to impose hard restrictions on airspace use, but to achieve a high degree of coincidence between economically and environmentally optimal routings. To this end, we will be in close consultations with airlines and the air transport industry throughout the project.

What's at the top of your agenda this quarter?

Our immediate work is the initial definition of the operational concepts, services and environments, and planning of the validation activities. Working at Exploratory Research level, Green-GEAR's eventual output will be formulations of the technological concepts validated in workshops, computational studies and fast-time simulations. We still have quite a way to go until deployment, but if we are successful our concepts will bring a big change for airspace users and Air Navigation Service Providers (ANSP) alike, and result in a significant reduction in aviation's environmental impact.

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