Aviation has become an important instrument for economic growth which resulted in a global rise in demand for air transport services. Based on the current annual projection of about 5%, the annual passenger total is expected to increase from 3.1 billion in 2013 to 6.4 billion by 2030 [33]. Aircraft emit gases and particles directly into the atmosphere where they impact negatively on the environment. These gases and particles alter the concentration of atmospheric greenhouse gases, including carbon dioxide (CO2), carbon monoxide (CO), ozone (O3), water vapour (H2O) and methane (CH); trigger formation of condensation trails (contrails); and may increase cirrus cloudiness - all of which con- tribute to climate change [2].
The CO2 is a product of complete combustion and a key component of the greenhouse gases. It is estimated that more than 300,000 tonnes of CO2 are generated per day from aircraft operations in Europe [34]. The CO2 depletes the ozone layer thereby by causing
1Greener Aircraft Trajectories under ATM Constraints- a tool which has been developed by Cran-
global warming. Although emissions due to air transport account for only 2% of CO2 emissions through the burning of fossils fuels, it is expected to increase to 3% by 2050 with the continuous and steady growth of air traffic [35]. Consequently, the Intergov- ernmental Panel on Climate Change (IPCC) developed a range of scenarios, IS92a-f, [2], for future greenhouse gases and aerosol precursor emissions based on assumptions concerning population and economic growth, land use, technological changes, energy availability, and fuel mix during the period 1990 to 2100. The study [2] shows that global emissions of CO2 by aircraft were 0.14 Gt C/year in 1992 which is about 2% of total anthropogenic CO2 emissions in 1992. It is projected that aircraft emissions of CO2 will continue to grow and by 2050 will be 0.23 to 1.45 Gt C/year [2] as shown in Fig. 1.2. The right-hand side scale of the figure represents a projected percentage growth of global aviation CO2 given in Tables1.2 and 1.1.
1.2.
Fig. 1.2: Total Aviation CO2 Emissions from Six Different Scenarios for Aircraft Fuel Use [2]
The Nitrogen-Oxide (NOx) is produced by the engine from the reaction of Nitrogen and air during high temperature combustions. The global NOx emissions from sub-
Table 1.1: Definitions of the IPCC Environmental Reference Scenarios (reproduced from [2])
Scenario Note
Fa1 Reference scenario developed by ICAO Forecasting and Economic Support Group (FESG); midrange economic growth from IPCC (1992); technology for both improved fuel efficiency and NOx reduction
Fa1H Fa1 traffic and technology scenario with a fleet of supersonic aircraft replacing some of the subsonic
fleet
Fa2 Fa1 traffic scenario; technology with greater emphasis on NOx reduction, but slightly smaller fuel efficiency improvement
Fc1 FESG low-growth scenario; technology as for Fa1 scenario Fe1 FESG high-growth scenario; technology as for Fa1 scenario
Eab Traffic-growth scenario based on IS92a developed by Environmental Defense Fund (EDF); technology for very low NOx
assumed
Edh High traffic-growth EDF scenario; technology for very low NOx assumed
sonic aircraft in 1992 were estimated to have increased ozone concentrations at cruise altitudes in northern mid-latitudes by up to 6%, compared to an atmosphere without aircraft emissions. This is projected to rise to about 13% by 2050 in the reference sce- nario. NOx reacts with moisture and ammonium to form smog (smoke fog) and rain acid. Acid rains are those that are unusually acidic (elevated hydrogen ions) caused by emissions of compounds such as SO2 and NOx into the atmosphere. The NOx from air- craft exhaust reacts with water molecules in the atmosphere to form nitric acid (HNO3) which has harmful effects on aquatic life, plants and infrastructures.[36]. In 1992, air- craft engine condensation trails (contrails) covered about 0.1% of the Earth’s surface, and is projected to grow to 0.5% by 2050 in the reference scenario (Fa1), see Table1.4. Contrails are formed when water vapour condenses and freezes around small particles contained in aircraft exhaust gases. The water vapour is partly due to high humidity and partly from the aircraft exhaust gases. Contrails affect the environment by block- ing heat generated by the Earth from escaping into space, thus contributing to global warming. Whilst trapping radiated heat from escaping to space, contrails do also block sunlight from reaching the Earth (serving as reflective clouds) thereby aiding the global cooling effect. Therefore, contrails can be said to be good or bad for the environment depending on which aspect is considered or rather, which aspect dominates which.
The above discussed issues sparked a series of public outcries on the negative impacts of air transport with respect to the environment and air quality. The desire to combat this trend led to the setting up of major global initiatives, research and development (R&D) projects such as NextGen of the USA, Single European Sky ATM Research (SESAR) of Europe and Collaborative Action for Renovation of Air Transport Systems (CARATS) of Japan, etc. These initiatives were introduced to modernise the present Air Trans- portation System (ATS) and cater for air transport demands in the foreseeable future
Table 1.2: Summary of Future Global Aircraft Scenarios (reproduced from [2]) Scena- rio - - Traffic growth per year 1990-2050 Annual growth rate of fuel burn 1990-2050 Annual economic growth rate Annual population growth rate Ratio of traffic 2050/ 1990 Ratio of fuel burn 2050/ 1990 Fa1 3.1% 1.7% 2.9% 1990-2025 2.3% 1990-2100 1.4% 1990-2025 0.7% 1990-2100 6.4 2.7 Fa1H 3.1% 12.0% 2.9% 1990-2025 2.3% 1990-2100 1.4% 1990-2025 0.7% 1990-2100 6.4 3.3 Fa2 3.1% 1.7% 2.9% 1990-2025 2.3% 1990-2100 1.4% 1990-2025 0.7% 1990-2100 6.4 2.7 Fc1 2.2% 0.8% 2.0% 1990-2025 1.2% 1990-2100 1.1% 1990-2025 0.2% 1990-2100 6.4 2. Fe1 3.9% 12.5% 3.5% 1990-2025 3.0% 1990-2100 1.4% 1990-2025 0.7% 1990-2100 10.1 4.4 Eab 4.0% 3.2% - - 1 10.7 6.6 Fa1 4.7% 3.8% - - 15.5 9.4
[37]. The European Union (EU) initiated three stream comprehensive projects to miti- gate the impacts of aviation on the environment and fuel resources. These are R&D for greener technology, modernisation of air traffic management systems and market based measures. This led to set-up of the Clean Sky Joint Technology Initiative (JTI) was in- troduced as the flagship of the R&D projects for the greening of air transport in Europe.
The Clean Sky JTI involves six technology evaluators. These are: Green Regional Air- craft (GRA), Smart Fixed-Wing Aircraft (SFWA), Green Rotorcraft (GR), Systems for Green Operations (SGO), Sustainable and Green Engines (SGE), and Eco Design (ED) as described in Fig. 1.3. These evaluators are set to be achieved through identification, development and validation of key technologies necessary for the realisation of ACARE2 Flightpath 2050 environmental goals. The key objectives as defined by ACARE were the reduction of CO2 by 75%, NOx by 90% and perceived noise by 65% by 2050 referenced to 2000 standard [38].
The aim of the GRA is to deliver low-weight aircraft using smart structures with low ex- ternal noise configurations and integration of other technological components delivered by the other divisions within the technology evaluator such as engines, energy man- agement and new system architectures. The SFWA is aimed at delivering light wing technology and new aircraft configuration whereas the GR aims to deliver innovative rotor blades and engine installation for noise reduction, lower airframe drag, integration of diesel engine technology and advanced electrical systems for elimination of noxious hydraulic fluids and fuel consumption reduction. The SGE is aimed to produce five engine demonstrators which would integrate technologies for low noise, lightweight, low NOx and low weight novel configurations such as open rotors and inter-coolers. The EC
focuses on the whole product life cycle based on optimal and efficient use of raw ma- terials and energies. While, the SGO is directed towards all electric aircraft equipment and system architectures and, thermal management for greener trajectories and mission which is of particular interest to this work. In this work both the flight trajectory and the anti-icing system are considered part of the global aircraft icing protection system that require effective management as illustrated in Fig. 1.3. The benefit of this is to assist commercial aircraft operators manage excessive fuel burn and emissions imposed by power off-takes while operating in real weather conditions.
Fig. 1.3: Clean Sky Setup (Modified from [3])
In view of the Clean Sky JTI discussed above, the European Council in partnership with the aeronautics industry granted funding under the 7th Framework Program for research in Europe (including the one undertaken in this work) to mitigate the impacts
of aviation on the environment. Consequently, the environmental concerns, the project funding and the level of collaboration with the industrial partners have served as a great source of motivation for this work.
An overview of aircraft icing risk and its impacts on flight safety, economy and the environment further buttresses the desire for this work. This discussion includes the factors influencing in-flight icing and the effects of ice formation on aircraft performance.