The 26th session of the Conference of the Parties (COP 26) to the United Nations Framework Convention on Climate (UNFCCC) was finally held from 1-12 November 2021, in Glasgow, UK. With climate change intensifying, scientists are warning that humanity is running out of time to limit global warming to 1.5°C over pre-industrial levels. The Emissions Gap Report 2021 shows that new national climate pledges combined with other mitigation measures put the world on track for a global temperature rise of 2.7°C by the end of the century.[i] That is well above the goals of the Paris climate agreement and would lead to catastrophic changes in the earth’s climate. To keep global warming below 1.5°C this century, the aspirational goal of the Paris Agreement, the world needs to halve annual greenhouse gas emissions in the next eight years.
Civil air flights continue to see very high growth especially in major developing nations and emerging economies. This includes both passenger and cargo movement. New airports are being built and old modernised to cater to the increasing demand. Aviation affects the environment in many ways: people living near airports are exposed to noise from aircraft; streams, rivers, and wetlands may be exposed to pollutants discharged in storm water runoff from airports; and aircraft engines emit pollutants to the atmosphere. India is amongst the top five fastest growing markets. Besides flight and ground safety, environmental protection is the most important issue for all aircraft operations.
Global aviation contributes about two percent of global greenhouse gas emissions and are growing with growth in aviation. But aviation supports eight percent of the world’s economic activity in terms of GDP. As a result of massive increase in air travel by 2025, it is estimated that the total CO2 emission due to commercial aviation may reach around 1.5 billion tons. The amount of nitrogen oxide (NO) around airports, may rise from 2.5 million tons in 2000 to 6.1 million tons by 2025. The number of people who may be seriously affected by aircraft noise may rise from 24 million in 2000 to 30.5 million by 2025. However, analysts believe that the aviation related greenhouse gas emissions figure should peak at around 3 percent due to sustained actions being evolved by the governments and industry.
Many actions need to be taken. The aircraft engines have to be made more efficient with lesser emissions. Managing the airport construction related pollution, operating waste, e-waste, noise and chemical emissions are many of the concerns requiring technological solutions. Ecological airport redesign, changes in air and ground operating procedures, and eco-friendly initiatives can alleviate environmental pressures without causing passenger and operational stress. The terms ‘Sustainable Aviation’ or ‘Green Aviation’ are increasingly being used to address the technological and socio-economic issues facing the aviation industry to meet the environmental challenges of twenty-first century. The environmental programs have to be scientifically evolved specific to each airport. Balance has to be maintained between social, economic and environmental imperatives. The ultimate goal is to produce the greatest improvement in the quality of life of the citizens.
Greenfield Airports and Biodiversity
Airports have considerable effect on city’s urban development and have negative impacts on the environment. At a local level, even though noise seems to be the main concern, air emissions, resource (energy and water) availability, waste and water management, and ecosystems and land use planning constitute issues that are directly linked to local communities’ tolerance. Environmental impact and sustainability require life cycle sustenance. Selecting a site for airport or its expansion, must look at ecological balance, bird and animal habitats, compatible land use, landscape deterioration and biodiversity damage. We need to avoid building on green spaces and work with local communities and organisations to conserve biodiversity on sites near airports.
Climate Change
Internationally, aviation is considered one of the fastest growing sources of greenhouse gas emissions. Even though aircraft emissions are not included in Kyoto protocol, emissions that are directly controlled by airport operators are ground-based, and therefore are subject to national targets. Air pollution caused due to various reasons including the burning of aviation fuel greatly contributes to climate change. Disruptive weather affects aviation the most. The most important measures require improvements in energy efficiency and conservation, ground fleet conversions, low emission power generation plants on site or renewable energy supplies. Geothermal, hydropower, solar or wind power is used to cover a significant proportion of energy needs. Many airports focus on achieving carbon neutral operations by offsetting carbon emissions that they cannot eliminate.
Air Pollution
Degradation of local air quality is another issue. The most significant sources of air pollution (lead emissions) are aircraft, airside and landside vehicles, ground support equipment, fuel storage, engine testing, fire training and road traffic. Burning of aircraft wheel tyre rubber during landing and take-off contributes to particle matter in the air, and fuel transfer and storage facilities contribute to increased volatile organic compound (VOC) concentration. Key pollutants of concern include oxides of nitrogen, carbon monoxide, hydrocarbons, particulate matter, sulphur oxide and carbon dioxide. The most common applied measures to control air pollution include air quality monitoring systems, air traffic management, promotion of green transport, reduction in commercial vehicle trips to-and-from airports by providing efficient public transport like airport metro etc.
Noise Pollution
Noise disturbance is a difficult issue to evaluate as it is open to subjective reactions. There are significant consequences on the surrounding areas as take-off and landings are a major source of noise. Large airports normally install noise monitoring systems, put operating restrictions and limits, manage air traffic, create anti-noise barriers, and support home insulation etc. Adverse effects on people living close to an airport, could include interference with communication, sleep disturbance, annoyance responses, performance effects and cardiovascular and psycho-physiological effects. Aircraft flying at a height of 10,000 ft above ground do not usually produce ‘significant’ noise impact. Noise monitoring computer software models produce aircraft-wise noise footprints to help calculate noise levels around the airport. These noise ‘contours’ can then be placed on a map to see which communities are subjected to different degrees of noise levels.
All commercial aircraft are supposed to meet the International Civil Aviation Organisation’s (ICAO’s) noise certification standards. The ‘balanced approach’ is reduction of aircraft noise at source; land-use planning and management measures; and noise abatement operational procedures and restrictions. Avoiding overflying residential areas hospitals and schools as far as possible; using least affected runway(s) and routes; using continuous descent approaches and departure noise abatement techniques; avoiding unnecessary use of auxiliary power units by aircraft on-stand; building barriers and engine test-pens to contain and deflect noise; towing aircraft instead of using jet engines to taxi; limiting night operations; applying different operational charges based on the noisiness of the aircraft, are some of the measures.
Supersonic/Hypersonic Flights
Concorde was the only supersonic airliner in commercial use. Many countries did not permit its operations or even overflights in view of sonic booms and resultant high sound and vibrations. Even military aircraft are allowed supersonic training flights in restricted areas away from population centres. Sonic booms over hospitals have resulted in premature deliveries of babies. However, the human beings want to travel faster. Hypersonic flight is already a reality. Hypersonic airliner could do Mumbai to New York in just two hours. The saving grace is that hypersonic flight would normally be at very high altitudes closer to space.
Waste Management
Airports generate large amounts of waste, including a considerable proportion by companies involved in cargo handling, retail, flight catering, and aircraft maintenance. As most of the waste produced at airports is generated by customers and contractors, it is important to encourage good waste management practices. A holistic waste management approach would include efficient disposal and recycling of engineering material and human waste.
Water Management
As airports cover large areas of land, it creates large amounts of runoff water which has to be effectively managed to comply with environmental standards before being discharged. Water is a valuable resource, one that needs to be used sparingly. Airport water run-offs are known to contain high levels of chemicals and toxic substances coming from aircraft and airfield de-icing, fuel spillage, fire-fighting foam, chemicals and oils from aircraft and vehicle maintenance, detergents used for aircraft and vehicle cleaning etc. Waste water and effluents need proper management to avoid polluting the environment. Most common measures applied against these are waste-water and sewage plants, drainage systems, surface and ground water quality monitoring, oil/hydrocarbons and grease separators, use of biologically degraded de-icing and anti-icing agents etc. As infrastructure providers, airports use significant volumes of water in operations. Regular water usage monitoring, leak detection and targeting, and introducing water conservation practices are important. Airports may install various leak detection systems, install water reduction devices and implement water recycling operations to reduce the demand of potable water. Drainage and rain water harvesting have to be inbuilt.
Need for Green Aero-engines
Among the many factors requiring attention, the aircraft engine requires special addressing. Most airliners nowadays fly at above 30,000 feet (9 km) altitude. Therefore, the majority of aircraft emissions are injected into the upper troposphere and lower stratosphere (typically 9 – 13 km in altitude). The resulting impacts are unique. The impact of burning fossil fuels at altitude is approximately double that due to burning the same fuels at ground level. This requires technological innovations and intervention. New aircraft and engine designs/technologies, and alternative materials need to be evolved. Interestingly, the most important role in an airplane’s fuel efficiency is also of the engines. Any solution must thus look at both. The two most-widely used aircraft today—the Boeing 737 and the Airbus A320 have shown that newer models of the same aircraft, with better engines, can not only carry more passengers and payload, but do so while burning nearly 25 percent lesser fuel.
Sustainable and Green Aero-Engines
Sustainable and Green Aero-Engines (SAGE) initiatives are being taken both in the European Union and in the USA, to develop aero-engine technologies, with new engine architectures that offer opportunities for reduction in CO2 emissions relative to current turbofans. Emissions of CO2, H2O, O2 and N2 which are products of hydrocarbon fuel combustion are all function of engine fuel burn efficiency. Areas being addressed include lightweight low pressure systems for turbofans; composite fan blades and high efficiency low pressure turbine; advanced engine externals and installations including novel noise attenuation; high efficiency Low Pressure (LP) spool technology while further advancing high speed turbine design; option of an aggressive mid turbine inter-duct; high efficiency and lightweight compressor and turbine; and low emission combustion chamber for next generation rotary-craft engine. Developments in controls and electronics, lightweight metallic and composite materials, hydraulic and pneumatic systems, and novel manufacturing methods, specific aero-engine parts, like casing, tanks, pipes, high temperature materials such turbine blades, and sensors would require attention.
SAGE 2 Project
European Union’s SAGE 2 project headed by Rolls-Royce and Safran focuses on demonstrating the technologies such as composite propeller blades with aero-acoustic optimisation, electric de-icing system and equipment. The gas generator used in the SAGE 2 open rotor demonstrator is derived from a Snecma M88 engine. The Airbus A340-300 MSN001 aircraft is being used as a flight test vehicle, with one full size Contra Rotating Open Rotor (CROR) pusher engine attached to a representative pylon and engine mount. Open rotor technologies offer the potential for significant reductions in fuel burn and CO2 emissions relative to turbofan engines of equivalent thrust. Open rotor engines remove the limitation by operating the propeller blades without a surrounding nacelle, thus enabling ultrahigh bypass ratios to be achieved. Installation of the open rotor engine on the airframe has its complexities, as the airflow through the propellers interacts with the supporting airframe structure in a different manner. The trend for Very High Bypass Ratio (VHBR) engines requires technology developments across a broad range of complex gas turbine systems, from fan inlet through the complete compression, combustion and turbine to exhaust.
CAEP Targets
The aircraft engines account for most of the noise and fuel consumption characteristics of airplanes. The International Civil Aviation Organisation (ICAO) has a Committee on Aviation Environmental Protection (CAEP) since 1983. Aircraft are required to meet the engine certification standards adopted by ICAO. Of particular relevance is the Standard for NOx, a precursor for ozone, which at altitude is a greenhouse gas. Standard for NOx was first adopted in 1981. It was made more stringent in 1993, 1999, 2005 and 2011. CAEP/8 standard was set in 2010. The CAEP medium and long-term NOx technology goals was to target reduction by 45% of CAEP/6 standard by 2016; and 60% by 2026. GE clean-sheet engine GE9X class engines employ modern technologies give better specific fuel consumption (SFC). It means 10 percent lower fuel costs even when compared to the 300ER. The engine has 15db noise levels well within stage 4 margin, and 29 percent emissions within CAEP/8 margin. Novel cycles that increase bypass ratios, incorporation of lean burn technology is evolving. ICAO is developing the first non-volatile PM (nvPM) standards (covering soot or black carbon particles) for turbofan/turbojet engines. Similarly, standards are being set for turboprops, helicopter turbo-shaft, and APU engines. The nvPM standard will help better assess impact.
Design Considerations
Changes in engine design or operation might include ultra-high bypass turbofans; open rotor engines; use of alternative fuels; relocating engines on the body of the aircraft such that engine noise gets deflected upwards. An example of a ‘green’ design change can be seen in the blended wing and body of the subscale, flying X-48B aircraft prototype. Other concepts may include capitalising on the potential of advanced electrical power technologies such as batteries or fuel cells to reduce the amount of fuel needed. Using High-tech engines, propeller efficiency, advanced aerodynamics, low-drag airframe etc. can result in higher fuel saving and less gaseous emissions. Improvement in performance can be achieved by moving from a component-based design to a fully integrated design by including wing, tail, belly fairing, pylon, engine, high lift devices etc. into the solution. At the April 2018 ILA Berlin Air Show, a high-efficiency composite cycle piston-turbofan hybrid engine for 2050, combining a geared turbofan with a piston engine core was presented. The 2.87 m diameter, 16-blade fan gives a 33.7 ultra-high bypass ratio. The 11,200 lb. (49.7 kN) engine could power a 50-seat regional jet. Although the engine weight increases by 30 percent, the overall aircraft fuel consumption is reduced by 15 percent.
New Engine Concepts
Two new engine concepts currently under investigation include the ‘Combined Brayton Cycle Aero Engine’ and ‘Multi-Fuel Hybrid Engine’. Even though modern engines are supposedly very efficient, a large part of the energy input is ejected as waste heat (over 50%). Improving performance by heat recovery is the requirement. A heat exchanger integrated in a turbofan core can convert recovered heat into useful power which can be used for onboard systems or to power an electrically driven fan to produce auxiliary thrust. A dual combustion chamber, with first stage between HP Compressor and HP Turbine burning cryogenic fuel like Hydrogen/Methane or liquid natural gas, and the second combustor at an inter-stage uses kerosene/bio-fuel in the flameless combustion mode is being considered. High temperature generated in the first stage, allows flameless combustion in the inter-stage, thus reducing CO, NOx etc. Cryogenic bleed air cooling can enhance the engine thermodynamic efficiency by cooling the bleed air thus allowing increase in temperature of the fuel. contra-rotating fans (CRF) can use boundary layer ingestion to reduce both noise emission and improve propulsive efficiency.
Next Generation Innovations
Developed under the US Department of Defense’s Adaptive Versatile Engine Technology (ADVENT) and adaptive Engine Technology Development (AETD) programs, is the GE Adaptive Cycle Engine (ACE). Unlike traditional engines with fixed airflow, the GE ACE is a variable cycle engine that will automatically alternate between a high-thrust mode for maximum power and a high-efficiency mode for optimum fuel savings. ACE is designed to increase combat aircraft thrust by up to 20 percent, improve fuel consumption by 25 percent to extend range by more than 30 percent, and provide significantly more aircraft heat dissipation capacity. These adaptive features are coupled with an additional stream of cooling air to improve fuel efficiency and dissipate aircraft heat load. The joint GE/U.S. Government investment of more than US$ 1Billion, the ACE engine will incorporate both heat-resistant materials and additive manufactured components. In the ADVENT program, GE reached the highest combined compressor and turbine temperatures ever. The Adaptive Engine Transition Program (AETP) is underway. The challenge remains in going to higher overall pressure ratio engines due to increasing cooling air temperatures, constraints imposed by developing new material technologies and detrimental weight and drag impact on ultra-high bypass ratio engines. GE Aviation’s Passport engines feature a higher-pressure ratio and a compressor made of new—and unnamed—advanced materials. GE predicts that the engines will achieve 8 percent lower fuel consumption and considerably lower NOx emissions. The pulse detonation engine (PDE), which has the potential to radically increase thermal efficiency, is one of the more exciting propulsion technologies being researched. PDE uses detonation waves to combust the fuel and oxidiser mixture. Instead of burning it, it explodes it. In theory it can be used up to Mach 5.0.
Some of the statistics around aero engines can explain the challenges of engine technologies, and why very few manufacture modern engines. Each wide-chord fan blade exerts a centrifugal force of around 70 tons, equivalent to the weight of a modern locomotive; each high-pressure turbine blade generates the same amount of power as a Formula 1 car; and the turbine discs will now have a “dual microstructure” to give different mechanical properties at the centre and at the edge of the disc.
Electric and Solar Engines
A number of electrically powered aircraft, such as the QinetiQ Zephyr have been designed since the 1960s. Some are used as military drones. In 2007, France flew a conventional light aircraft powered by an 18 kW electric motor using lithium polymer batteries, and became the first electric aircraft to receive an airworthiness certificate. Solar-powered manned aircraft designed to fly both day and night without the need for fuel are already under development. Solar electric propulsion have been performed through the manned ‘Solar Impulse’ and the unmanned NASA ‘Pathfinder’ aircraft. Many big companies, such as Siemens, are developing high performance electric engines for aircraft use. Small multi-copter UAVs are almost always powered by electric motors.
Hydrogen Fuel Cells
Hydrogen fuel cell technology is fast evolving. A hydrogen fuel cell is an electrochemical device that uses a chemical process to convert hydrogen to electrical power, which in turn can drive one or more electric propulsion motors on the unmanned aerial vehicle. Electricity, water and heat are the only products of this chemical process, which makes hydrogen an extremely clean fuel. Hydrogen fuel cells are smaller, lighter, more versatile and more resilient than alternatives like batteries or small gasoline and diesel engines. Unlike batteries, hydrogen fuel cells do not need to be recharged. Simply connect a carbon fibre hydrogen storage tank to the fuel cell, and fly! Drones powered by a hydrogen fuel cell have much longer range and flight duration than a comparably sized battery-powered aircraft. Typical rotary and fixed wing platforms can fly up to three times longer with hydrogen fuel cell onboard. UAVs are already flying far beyond the capabilities of drones powered by batteries or gasoline engines. Operators of fixed-wing or multi-rotor platforms can fly up to three times longer with proven hydrogen fuel cell onboard.
The 600-watt and 1200-watt liquid-cooled hydrogen fuel cells and compressed hydrogen fuel source are ideal for military and commercial missions of all kinds, including intelligence, surveillance and reconnaissance (ISR), search and rescue, law enforcement, infrastructure and agriculture inspections, and other missions where silent operation and long duration flights are critical. The hydrogen fuel cell advantages can be summed as nearly three times the range or flight time of batteries, no need for recharging, all-temperature performance, faster turnaround times between missions, no environmental footprint, virtually noise-free, logistic simplicity, liquid-cooled technology operates more efficiently at high altitudes than air-cooled fuel cells, and longer service life. Hydrogen fuel cell technology will be increasingly used on larger aircraft.
Flight Planning Tools
The flight efficiency approach requires choosing optimum flight routes. All aircraft operators and computerised flight plan service providers exchange and compare their flight plans with the best filed flight plan accepted by the integrated initial flight plan processing system. Special software tools show shortest route plans. Dynamism through the application of the Flexible Use of Airspace (FUA) concept, under which the military release airspace to civil aviation helps. The flight planning from aircraft start-up to switch-off can be a great tool to reduce engine use and fuel consumption. This allows substantial savings in distance flown, time, fuel and environment. The air and ground crew, the airline operator, air and radar controllers, among many others can play a significant role.
India’s Aviation Environmental Regulations
India’s Directorate General of Civil Aviation (DGCA) which is responsible for all aspects of enforcement and regulation has an Aviation Environmental Unit. All operators such as the airlines, navigation service providers and airport authorities too, have environmental cells. It is mandatory to submit to DGCA monthly fuel consumption data to set up a carbon dioxide emission inventory. The DGCA sponsored noise study for Indira Gandhi International Airport (IGIA) Delhi has now become the standard for all major airports in India. DGCA has laid down guidelines for noise measurement and monitoring at airports, including noise mapping, validation, action plan, noise reporting and proposed aviation noise limits. The Indian Ministry of Civil Aviation’s Green Aviation Policy, 2019 aims to achieve the sustainable and inclusive growth of the aviation industry in the country and remedy the ecological concerns posed by the industry. The policy creates a regulatory framework to remedy the environmental issues created by the civil aviation industry by identifying key areas that require guiding principles and regulations.
Environmental Initiatives – Indian Airports
Indira Gandhi International Airport (IGIA), New Delhi, was the first Indian green-field airport build with international best practices keeping environmental excellence and sustainable work practices in mind. The focus was on natural resource conservation, pollution preventions and environmental skill development. All the aspects and associated impacts due to services and operations is based on ISO 14001:2004 Environment Management Systems (EMS). IGIA ensured building green infrastructure, renewable energy initiatives, climate change & greenhouse gas management, followed international environmental standards and controls, and resource conservations (water, energy, & materials). Noise abatement is one of the key areas. Automatic aircraft noise monitoring System is installed in approach’s of all runways and identify noisy aircraft. Distribution of aircraft movement across the three runways is based on time of the day and individual aircraft noise levels. Inputs from noise complaint system are also factored in. Continuous decent approach is followed to reduce noise. IGIA has target of net zero carbon emissions by 2030. Other major Indian airports have introduced many energy efficient technologies such as energy efficient air-conditioning and water chillers, solar water heating, solar boundary lighting, Compressed Natural Gas (CNG) or electric ground vehicles, LED lighting, waste water treatment plants, and high efficiency pumps. Cochin, Delhi, Mumbai and Chandigarh airports have already installed solar power plants. Bangalore and Hyderabad airport solar projects are under implementation. Ultimate aim is to make the airports carbon neutral. Bangalore has 273 hectare of green belt and 971 hectare of natural greenery. Chandigarh International airport uses only natural light during day and mostly LED lighting thereafter. It also has a transparent glass roof with low heat gain that cuts down air conditioning requirement.
Conclusion
Advances in engine architecture, aerodynamics, and materials have resulted in today’s aircraft engines consuming 40 percent less fuel — and emitting 40 percent less CO2 — than engines manufactured in the 1970s and 1980s. Each kilogram of fuel saved reduces carbon dioxide (CO2) emissions by 3.16 kg. Modern aircrafts are 30-40% more efficient than those of 15 years ago. Fixed electrical ground power can reduce the amount of fuel burn used on ground power by up to 85%.
However, we cannot be satisfied with the pace of progress from the past. The next set of engine technologies, including open fan architectures, hybrid-electric and electric propulsion concepts, and advanced thermal management concepts, offer the potential to achieve at least a 20 percent additional improvement in fuel efficiency compared to today’s state of the art single-aisle aircraft engines. Industry initiatives to approve and adopt 100% Sustainable Aviation Fuel (SAF) and investigate hydrogen as the zero-carbon fuel of the future should deliver. Aero-engines of the future will be more and more fuel efficient and environment friendly. The future of flight will be defined by how the aviation industry innovates to lower emissions and improves fuel efficiency. Global warming is causing global mean sea level to rise in two ways. First, glaciers and ice sheets worldwide are melting and adding water to the ocean. Second, the volume of the ocean is expanding as the water warms. On future pathways with the highest greenhouse gas emissions, sea level rise could be as high as 8.2 feet (2.5 meters) above 2000 levels by 2100. More than 260 airports are at risk of getting submerged due to such a sea level rise or coastal flooding. Up to 20% of flight routes could be disrupted. Therefore, time to act is now, lest it becomes too late.
Author Brief Bio: Air Marshal Anil Chopra, PVSM, AVSM, VM, VSM is a QFI, test pilot, and a pioneer of Mirage-2000 fleet. He was AOC J&K, ACAS (Inspections) and retired as Air Officer-in-charge Personnel (AOP). Post retirement, he served as a member of the Armed Forces Tribunal. Presently, he is the Director General, Centre for Air Power Studies (CAPS), New Delhi.
[i] https://www.unep.org/resources/emissions-gap-report-2021
