Aircraft Taxiing: Cutting Fuel & Emissions

The sky's the limit for air travel, but the journey often begins and ends on the ground. While the roar of jet engines at cruising altitude captures our imagination, the often-overlooked phase of aircraft taxiing—the movement of planes on the ground before take-off and after landing—plays a surprisingly significant role in the aviation industry's environmental footprint. As global demand for air travel continues to soar, so too does the imperative to address its impact on our planet. This article delves into the critical issue of aircraft ground emissions and explores the ingenious methods being developed and implemented to make this vital phase of flight cleaner and more sustainable, ensuring a greener future for air transport.

The Environmental Footprint of Air Travel

Air transport is an undeniable engine of economic development, fostering trade, communication, and tourism across the globe. However, this growth comes with a substantial environmental cost. Since 2013, the global airline industry's daily jet fuel consumption has exceeded 5 million barrels, contributing significantly to carbon dioxide (CO2) emissions. The International Energy Agency (IEA) highlighted that in 2021, aviation transportation was responsible for over 2% of global energy-related CO2 emissions, a rate of growth faster than that of road, rail, or shipping sectors. This escalating impact necessitates urgent and innovative solutions.

Recognising this challenge, the International Civil Aviation Organization (ICAO) has, since the late 1970s, been at the forefront of developing measures to mitigate environmental problems caused by aircraft engine emissions, particularly around airports. Their primary environmental goal is clear: to limit or reduce the impact of aviation emissions on local air quality (LAQ).

The LTO Cycle: A Closer Look at Ground Emissions

Aircraft emissions occur in two primary phases: the cruising phase and the ground phase. While cruising emissions contribute to global climate change, it's the ground phase that directly degrades airport local air quality during what is known as the Landing and Take-Off (LTO) cycle. This cycle encompasses four distinct stages: landing, taxi-in, taxi-out, and take-off. Although the LTO cycle accounts for a smaller percentage—typically 5% to 10%—of an aircraft's total emissions, its impact on the immediate airport environment is profound.

During the LTO cycle, aircraft engines operate at lower, less efficient power settings for extended periods, leading to incomplete fuel combustion. This process generates a considerable amount of exhaust emissions, including carbon monoxide (CO), hydrocarbons (HC), and, crucially, carbon dioxide (CO2). Despite these emissions being quantitatively less than those produced during flight, their concentration within airport boundaries and surrounding communities makes the study and reduction of LTO cycle emissions essential for improving local air quality and public health.

Global Efforts for Cleaner Aviation

The urgency to address climate change has led to significant international agreements and national policies. In 2016, 195 countries signed the landmark Paris Agreement, setting ambitious targets for global action to abate climate change beyond 2020. The aviation sector, acknowledged as one of the most challenging industries to decarbonise, plays a vital role in achieving the Paris Agreement's 1.5 °C and 2 °C temperature targets. Consequently, countless studies have been conducted at global and national levels to assess and predict CO2 emissions from the aviation sector.

Researchers have employed various methodologies to quantify airport and civil aviation industry carbon emissions. For instance, studies have assessed air pollutants at specific airports (e.g., Tbilisi International Airport), estimated CO2 emissions using tiered approaches (e.g., Isparta Suleyman Demirel Airport), and even established long-term aviation CO2 emissions inventories for entire nations (e.g., China). Beyond historical data, scholars have explored future emissions reductions through scenario analysis, proposing policy frameworks that include carbon taxes, R&D support for fuel efficiency, and carbon offset schemes. The Logarithmic Mean Divisia Index (LMDI) has also been used to quantitatively identify factors driving CO2 emission changes.

Pioneering New Taxiing Methods

While existing research has focused on calculating and predicting carbon emissions, a crucial area of investigation involves the actual effects of emission reduction technologies. The taxiing phase, being a significant contributor to CO2 emissions within the LTO cycle, has become a focal point for innovation. Several alternative taxiing methods have been proposed and tested, generally categorised into two main types: operational methods and technological methods.

Operational methods aim to optimise airport-level procedures and include strategies such as Single-Engine Taxiing (SET) and ground traffic optimisation. These methods seek to improve efficiency without requiring significant changes to aircraft hardware.

Technological methods, on the other hand, focus on employing more environmentally friendly technologies that often bypass the need for aircraft engines during ground movements. Examples include Dispatch Towing (DT) and Onboard Systems (OS).

Airports worldwide have begun experimenting with these new taxiing methods. For example, Delhi International Airport in India has implemented the use of TaxiBots to tow aircraft to the Tug Disconnection Point (TDP). This initiative has demonstrated substantial savings, approximately 532 kg of CO2 emissions per aircraft over an average 14-minute TaxiBoting time, significantly reducing overall emissions.

Operational Innovations: Smarter Ground Movements

Single-Engine Taxiing (SET) involves operating only one engine during taxiing instead of both, thereby reducing fuel consumption and emissions. This method requires careful operational planning to ensure safety and maintain appropriate thrust for movement. Studies, such as one using London Heathrow Airport as an example, have modelled SET operations and predicted significant impacts on fuel consumption and pollutant emissions. Ground traffic optimisation involves improving the flow of aircraft on taxiways, reducing idle times and unnecessary movements, which in turn leads to less fuel burn and fewer emissions.

Technological Advancements: Engine-Less Taxiing

Technological solutions offer a more radical departure from conventional taxiing. Dispatch Towing (DT) involves using specialised tow tractors to move aircraft to or from the runway, eliminating the need for the aircraft's own engines during these phases. The aforementioned TaxiBots are a prime example of this, using a diesel-electric hybrid system to efficiently tow aircraft. Research has shown that such systems can lead to considerable reductions in fuel consumption and CO2 emissions.

Onboard Systems (OS) represent an even more integrated approach, where an electric motor is placed directly at the wheels of the aircraft, allowing it to move independently without engaging its main engines. This system offers maximum flexibility and potentially the greatest emission reductions during ground operations. Studies comparing SET, DT, and OS have consistently concluded that all three methods can significantly reduce both operational costs and environmental emissions.

Comparing the Impact: Fuel & Emission Reductions

Academic research, government bodies, and industry players have shown increasing interest in these new taxiing methods. Comprehensive studies have calculated the pollutant emissions of various new taxiing methods in detail, comparing their characteristics and effectiveness. Beyond the three core methods, research has also investigated the effect of optimising surface traffic management (OSTM) on aircraft pollutant emissions. Some studies have even compared four taxiing methods: conventional, single engine-on, external (like towing), and onboard systems, noting that onboard systems performed best in emissions reduction.

The collective evidence suggests that by changing taxi-out procedures and remodelling ground aircraft times and tug times, significant decreases in airport carbon emissions can be achieved. As countries like China draft comprehensive carbon reduction plans for civil aviation (e.g., the China Civil Aviation Green Development Policy and Action), the adoption of these innovative taxiing methods becomes increasingly vital.

Comparative Analysis of Taxiing Methods

While specific reduction percentages can vary based on aircraft type, airport layout, and operational efficiency, the general benefits are clear:

The Path to Carbon Peaking: Future Prospects

While existing studies have provided valuable insights into pollutant emissions from various new taxiing methods, a gap remains in comparing the carbon reduction effects across different aircraft types. This specific analysis is crucial for policymakers and airport operators to select the optimal taxiing method for particular aircraft models and operational contexts. Furthermore, unlike studies relying purely on scenario analysis, the exploration of carbon peaking prospects for practical operational methods offers a more tangible and actionable path towards sustainability.

Future research aims to quantify the precise working conditions of primary emission sources, such as engines and Auxiliary Power Units (APU), during the taxiing phase across various methods. This in-depth understanding will provide a robust basis for further academic and industrial studies. Moreover, exploring the emission reduction effects of each taxiing method on several popular aircraft types will offer more specific guidance for implementation. By predicting the carbon peaking potential for these practical operational methods, researchers can provide concrete suggestions for their future application and inform the formulation of rational carbon emission reduction policies, helping airports achieve their ambitious climate goals.

Frequently Asked Questions (FAQs)

Why are aircraft ground emissions important?

Aircraft ground emissions, particularly during the Landing and Take-Off (LTO) cycle, are crucial because they directly impact local air quality around airports. While they represent a smaller percentage of total flight emissions, their concentration in populated areas contributes to air pollution, affecting human health and the immediate environment.

What is the LTO cycle?

The LTO cycle stands for Landing and Take-Off cycle. It encompasses all aircraft operations below 3,000 feet (about 915 metres) altitude. This includes the phases of landing, taxi-in (after landing), taxi-out (before take-off), and take-off. It's a key focus for local air quality studies due to the emissions produced during these ground-intensive activities.

What is Single-Engine Taxiing (SET)?

Single-Engine Taxiing is an operational method where an aircraft taxis using only one of its main engines, rather than both. This practice significantly reduces fuel consumption and associated emissions during ground movements, contributing to both cost savings and environmental benefits.

How do TaxiBots work?

TaxiBots are semi-robotic, hybrid-electric towing vehicles designed to push back and tow aircraft from the terminal gate to the runway, or vice-versa, without the aircraft needing to use its own engines. They connect to the aircraft's nose gear, allowing the pilot to control the speed and steering from the cockpit. This eliminates engine use during taxiing, leading to substantial fuel and emission savings.

Are these new taxiing methods widely adopted?

While promising, the widespread adoption of these methods is still in progress. Operational methods like Single-Engine Taxiing are becoming more common at various airports. Technological solutions like Dispatch Towing (e.g., TaxiBots) and Onboard Systems are being trialled and implemented at a growing number of airports globally, but broader adoption depends on infrastructure, investment, and regulatory frameworks.

What are the main pollutants during taxiing?

During taxiing, aircraft engines produce a range of exhaust emissions due to incomplete fuel combustion at lower power settings. The main pollutants include Carbon Monoxide (CO), Hydrocarbons (HC), and Carbon Dioxide (CO2). While CO2 is a greenhouse gas contributing to climate change, CO and HC directly impact local air quality.

In conclusion, the journey towards a more sustainable aviation sector is multifaceted, and addressing emissions during aircraft taxiing is a critical component. From smart operational adjustments like Single-Engine Taxiing to groundbreaking technological innovations such as Dispatch Towing and Onboard Systems, the industry is actively pursuing solutions to mitigate its environmental impact. These advancements not only promise significant reductions in fuel consumption and harmful emissions but also contribute to healthier local air quality around airports. As research continues to refine our understanding of these methods and their application across diverse aircraft types, the collective effort to embrace greener taxiing practices will undoubtedly pave the way for a cleaner, more efficient, and sustainable future for air travel.

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