Technology and Design

The growth in the aviation sector since the 1950s has delivered major benefits. However, there have been increasing concerns over the associated environmental impacts. Development of new aircraft technology, and its incorporation within advanced designs that are cleaner and quieter, is one of the key ways to mitigate the environmental impact from aviation.

The EU and EFTA have aircraft and engine environmental certification standards [19] which refer directly to the equivalent International Civil Aviation Organization (ICAO) standards [20], [21], [22]. ICAO’s Committee on Aviation Environmental Protection (CAEP) is responsible for maintaining these standards, and an overview of the noise and emissions certification measurement procedures can be found in Appendix D.

This section of the report contains certified data for aircraft and their engines, which allows to compare the environmental performance of different products. Additional interactive graphs are available on the European Aviation Environmental Report website.

Aircraft engine NOX emissions

Turbojet and turbofan engines

Engine technology has continuously evolved over the last 70 years, and reduction in fuel burn has always been a driving force behind this progress. More fuel efficient engine cycles, often made possible through the use of new materials, has led to increasing pressures and temperature within the combustor. Since this tends to increase the emissions of nitrogen oxides (NOX), the control of these emissions through the combustor design is a significant challenge. The ICAO regulatory limits for engine NOX emissions has been gradually tightened over time, and are usually referred to by the corresponding CAEP meeting number (CAEP/2, CAEP/4, CAEP/6 and CAEP/8). The engine NOX standard, and the new aeroplane CO2 standard, contribute in defining the design space for new products so as to address both air quality and climate change issues.

Figure 2.5 illustrates certified NOX emissions data of aircraft engine models above 89 kN thrust in relation to the ICAO CAEP NOX limits [25]. The regulatory NOX limits are defined as the mass (Dp) of NOX emitted during the Landing and Take- Off (LTO) test cycle and divided by the thrust of the engine (F00). The limit also depends on the overall pressure ratio7 of the engine. The current ICAO technology goals for NOX are also shown. These goals, which were agreed in 2007, represent the expected performance of expected ‘leading edge’ technology in 2016 (mid-term) and 2026 (long term).

Each point in Figure 2.5 represents EASA certified data for an engine model, and the different colours provide insight into the trend over time. The dataset represents engine models typically fitted to single-aisle aircraft (e.g. A320, B737) and larger aircraft (e.g. A350, B777, A380). No further versions of the leading edge GEnx engines (lower green dots) have been certified since 2015. However, the most recent data (purple diamonds) illustrate that other manufacturers on different product development cycles have optimised new and existing combustor designs.

7 Ratio of total pressure at compressor exit compared to pressure at engine inlet.

New standards

The latest global environmental standards were adopted by ICAO in 2017. These cover both aeroplane CO2 emissions and aircraft engine non-volatile Particulate Matter (nvPM) mass concentration. EASA has subsequently supported the process to integrate these standards into European legislation [19], and will implement them as of the applicability date of 1 January 2020.

The CO2 standard provides an additional requirement into the design process that increases the priority of fuel efficiency in the overall aeroplane design. It is an important step forward to address the growing CO2 emissions from the aviation sector, and will contribute to the climate change mitigation objectives of the UNFCCC Paris Agreement [26].

The nvPM mass concentration standard is expected to ultimately replace the existing Smoke Number requirement. ICAO is also working on future standards for both nvPM mass and nvPM number, which are based on the emissions that occur during landing and take-off operations. These proposed standards will be discussed at the CAEP/11 meeting in 2019. If agreed, it is expected that they too will be implemented into the European legislative framework.

One of the substances considered to cause significant damage to the ozone layer is Halon. Additional measures for the protection of the ozone layer were adopted in 2008 under the Montreal Protocol. Consequently the European Commission and EASA are working with industry to conduct research that supports compliance with the on-going phase out of Halon in aircraft [27], while avoiding the risk of phasing in alternatives with high global warming potential.

Supersonic aircraft

Different types of new civil supersonic aircraft are currently under development, and may be in-service as early as the mid-2020s. The design process to develop and certify such aircraft faces various environmental challenges.

When an aircraft transitions through and flies faster than the speed of sound (Mach 1), the phenomenon of ‘sonic boom’ occurs. For this reason the Concorde was limited to subsonic speeds when flying over land and near coastlines. In recognition of this problem, the ICAO 39th Assembly adopted, in October 2016, an ICAO Resolution ‘ensuring that no unacceptable situation for the public is created by sonic boom’. A flight demonstrator is currently being built in the USA to research specifically shaped aircraft designs that may reduce the sonic boom, and to establish a noise dose-response relationship through community noise tests [28]. A European research study known as RUMBLE is also supporting the development of new regulations for low-level sonic booms [29].

Compared to subsonic aircraft, these supersonic aircraft will operate at higher cruise altitudes in the sensitive high troposphere and stratosphere (15-18 km altitude). Although future civil supersonic project aeroplanes will be more fuel-efficient than Concorde, their fuel burn is still expected to be higher in comparison with current subsonic aircraft of a similar size because drag increases with speed. Research also suggests that the climate change effects due to non-CO2 emissions from supersonic aeroplanes, operating at significantly higher altitudes, could be considerably greater than the non-CO2 effects from subsonic aeroplanes [30].

The noise and emissions produced from supersonic aircraft operations in and around airports is also a critical aspect. Engines optimised for supersonic operation typically have a trade-off between lower noise during take-off (high bypass ratio) and lower drag / higher fuel efficiency in supersonic cruise (low bypass ratio).

There are currently no noise or CO2 certification requirements for supersonic aircraft in Europe, and the existing supersonic engine emissions standards are considered to be outdated according to ICAO guidance material. Europe is therefore actively working to update these standards.

New technology

The aviation industry is evolving into new areas, with existing and new start-up companies investing heavily in novel technology. In addition to recent developments of electric and hybrid engines, ideas to enhance urban mobility have also emerged including fully autonomous aircraft that can provide rapid point-to-point connectivity. New aircraft concepts and innovative types of operations have already applied for certification by EASA. These include the redesign of conventional aircraft as well as innovative electrical vertical take-off and landing (VTOL) aircraft. While the traditional noise certification procedure may be appropriate for the first category, drones and VTOL aircraft are more of a challenge. Based on an EASA Opinion, the European Commission is currently finalising proposals for noise requirements for drones that weigh less than 25 kg.

While these novel technologies bring new challenges, they also represent new opportunities to draw on a wider pool of expertise and innovative approaches from other non-aviation sectors to address the sector’s environmental challenges. An in-depth life cycle analysis will be required to assess the environmental impacts of these new concepts in comparison to conventional aircraft. EASA is working closely with applicants to assess the environmental characteristics of these products, and put in place appropriate certification requirements. This will need to take into account new aircraft designs, required infrastructure and their operational characteristics which potentially brings aviation noise much closer to EU citizens [31].

Hybrid and electric aircraft
Various European companies, such as Pipistrel, are currently developing electric power plants for aircraft. The electricity can be generated through a variety of methods including batteries, solar cells, ultracapacitors and fuel cells. In this case, the conventional engine is replaced by a hybrid or electric engine with similar performance. An evaluation of the conventional noise requirements and limits for these types of products will need to be performed.

Urban mobility - Air taxis and vertical take-off and landing (VTOL) aircraft
The number of active projects in this area has increased significantly over the last few years, such as Volocopter and Lilium. Different concepts have emerged with nonconventional designs. Specific studies of the design technologies, and operational procedures close to large populations, will need to be performed in order to identify appropriate noise certification requirements.

An Unmanned Aerial Vehicle (UAV), also known as a drone, is an aircraft without an onboard human pilot. UAVs are a component of an Unmanned Aircraft System (UAS) that includes a UAV, a ground-based controller and a system of communications between the two. There is a wide range of UAVs ranging from light and simple to heavy and complex aircraft, which operate with various degrees of autonomy and a diverse set of missions.


Clean Sky
The Clean Sky 2 initiative (2014-2024), part of the EU Horizon 2020 programme, is a Joint Undertaking of the European Commission and the European aeronautics industry [32]. It builds on the original Clean Sky 1 programme (2008-2017), and contributes towards achieving the ‘Flightpath 2050’ environmental objectives set out by the Advisory Council for Aviation Research in Europe [33]. Bringing together the aeronautics industry, small and medium sized enterprises, research centres and academia to drive forward innovative results, Clean Sky 2 also strengthens European aero-industry collaboration, global leadership and competitiveness. Clean Sky 2 has a total budget of €4 billion, and currently contains over 600 unique entities from 27 countries.

Clean Sky 1 envisioned technologies and procedures that would reduce CO2 emissions per passenger kilometre by 75%, NOX emissions by 90%, and perceived noise by 65% relative to the capabilities of a typical new aircraft in the year 2000. The objectives of Clean Sky 2 are to reduce CO2, NOX and noise emissions by 20 to 30% compared to “state-of-the-art” aircraft entering into service as from 2014.

Clean Sky 2 expects to develop innovative, cutting-edge technologies for more aerodynamic wings, advanced and lighter structures, more efficient engines including the emerging field of hybridization and electrification, advanced control, actuation and guidance systems (including increased digitization), brand-new aircraft configurations, and a more sustainable aircraft lifecycle. The scope of the programme includes large, regional, and commuter aircraft, and rotorcraft.

The Programme aims to accelerate the introduction of new technology in the 2025-2035 timeframe. By 2050, 75% of the world’s fleet now in service (or on order) will be replaced by aircraft that can deploy Clean Sky 2 technologies. The direct economic benefits are estimated at €350-€400 billion and the associated indirect benefits of the order of €400 billion. Clean Sky 2 technologies are expected to bring a potential saving of 4 billion tonnes of CO2 between 2025 and 2050. This is in addition to approximately 3 billion tonnes of CO2 emissions savings that Clean Sky 1 should deliver [34].

Stakeholder actions

AeroSpace and Defence Industries Association of Europe (ASD)

ASD is the European Aeronautics, Space, Defense and Security Industries with 16 major European companies and 24 national associations from 18 countries. In 2016, 843,500 people were employed by more than 3,000 companies generating a turnover of €220 billion. European Member States and ASD are working together, primarily through the Clean Sky 2 programme, to address aviation environmental challenges. An overview of some of these research projects is provided below.

1. Hybrid-Electric E-Fan X

The Airbus, Rolls-Royce and Siemens ‘E-Fan X’ hybrid-electric technology demonstrator is anticipated to fly in 2020 following a comprehensive ground test campaign, provisionally on a BAe 146 flying testbed with one of the aircraft’s four gas turbine engines replaced by a two megawatt electric motor. These types of propulsion systems are among the most promising technologies for reducing aviation’s dependence on fossil fuels.

2. Sage2 Counter-Rotating Open Rotor (CROR)

In 2017, the Sage2 CROR successfully demonstrated new technologies including composite propeller blades, pitch control system, contra rotating reduction gearbox and aero acoustic optimization at the Safran test facility. This full scale demonstration confirmed the technical feasibility of a CROR, the expectation of significant fuel burn improvements (-30% vs year 2000) and the capability to satisfy the current ICAO Chapter 14 noise requirements.

3. Laminar wing demonstrator (BLADE)

The Airbus A340 laminar-flow Flight Lab test demonstrator aircraft has been engaged in successful testing to explore the wing’s characteristics in flight since 2017. The test aircraft is the first in the world to combine a transonic laminar wing profile with a true internal primary structure. BLADE has been running since 2008 with 20 key partners and 500 contributors from all over Europe. It is tasked with assessing the feasibility of introducing laminar flow wing technology that aims to reduce aircraft drag by 10% and CO2 emissions by up to 5%.

4. Ultrafan

Rolls-Royce is developing a new civil aviation propulsion architecture that allows the fan and the turbine to be independently optimized by introduction of a power gearbox capable of operating at anything up to 100,000 HP to deliver greatly improved propulsive efficiency. This architecture will be proven through a programme of engine demonstrators that will culminate in a flying test bed. It will deliver 25% improvement in fuel efficiency compared to the first Trent engines and is being designed to meet potential noise and emissions stringency levels for aircraft entering service before 2030.

5. Additive 3D manufacturing

This new technique for building aerospace parts involves adding material, layer upon layer, in precise geometric shapes. This enables complex components to be produced directly from computer-aided design information. It allows quicker and more flexible production, and reduces material waste compared to traditional approaches such as milling. It also results in much lighter parts which reduces aircraft weight and consequently fuel burn. 3D-printed parts are already flying on Airbus A320neo and A350 XWB test aircraft (e.g. cabin brackets, bleed pipes, combustor fuel nozzles on the CFM LEAP engine).

6. Electric green taxiing system

This system, jointly developed by Safran and Airbus, enables aircraft to pushback and taxi at airports without having to use their main engines or call upon airport towing services. Two of the main landing gear’s wheels are equipped with an electric motor powered by the aircraft’s auxiliary power unit. It improves both economic and operational efficiency, with up to 4% fuel savings on a short to medium range mission compared to current dual-engine taxi operation, plus reductions of other pollutants and noise. The on-going development of a hydrogen fuel cell to power the electric motor will further reduce the environmental impact of aircraft ground operations.

7. Circular economy

Advanced manufacturing capability is at the heart of the aerospace sector, which relies on essential skills to optimize resources and processes. European aviation has also been at the forefront of developing capabilities and processes for end-of-life aircraft dismantling and recycling of parts. TARMAC AEROSAVE, a jointly owned company of Airbus, Safran and Suez, has recycled over 135 aircraft since it was established in 2007. Today, 92% of the total weight of an aircraft is recycled.