Clearing the Skies: The Future of Sustainable Aviation

Introduction

Imagine boarding a Boeing 777 for a long-haul flight, such as from Singapore to Los Angeles—a journey spanning approximately 8,300 nautical miles. During this 16 hours flight, the aircraft consumes around 110 tons of jet fuel, equal to tens of thousands of kilograms of carbon dioxide being released into the atmosphere. Given that aviation accounts for about 2–3% of global CO₂ emissions, the environmental impact of such flights is substantial (Flight Training School FAQ, n.d.).

As the climate crisis intensifies, the aviation industry faces increasing pressure from governments, consumers, and environmental organizations to reduce its carbon footprint. While air travel is essential for global connectivity and commerce, the challenge lies in transforming an industry traditionally reliant on fossil fuels into one that can operate sustainably.

In this article, we will explore the pathways toward emission-free aviation, focusing on solutions such as sustainable aviation fuels (SAFs), battery-electric propulsion, and hydrogen propulsion systems. We will also examine the challenges airlines and manufacturers encounter in adopting these solutions, considering economic, technical, and policy-wise dimensions that shape the future of flight.

Physics of Flying

To achieve and maintain flight, an aircraft must balance four primary forces:

• Lift: The upward force that counteracts gravity, enabling the aircraft to rise off the ground.

• Weight: The downward force due to gravity, acting on the aircraft's mass, including its structure, passengers, cargo, and fuel.

• Thrust: The forward force produced by the engines, propelling the aircraft through the air.

Drag: The resistance or friction that opposes the aircraft's forward motion through the air.

Source: Nasa

For an aircraft to take off, thrust must overcome drag, and lift must overcome weight. During level flight, lift equals weight, and thrust equals drag. Generating lift is essential for flight, but it inherently produces drag. As the aircraft's wings generate lift by redirecting airflow, they also create vortices at the wingtips, leading to induced drag. This drag increases with the square of the lift coefficient, meaning that to generate more lift, an aircraft must increase its angle of attack or the amount of air being redirected, which in turn raises induced drag.

The Role of Weight in Lift and Drag

An aircraft's weight comprises its empty weight, payload, and fuel. This total weight directly influences the amount of lift required to maintain level flight. From the previous explanation, we know that increasing the weight of the aircraft will increase the lift required and thus the amount of lift-induced drag. Therefore the heavier the plane, the more fuel is consumed to generate a greater  engine thrust to maintain flight. Something to keep in mind when examining alternative energy sourcesis that the weight of the fuel itself plays a significant role in determining the overall fuel consumption of a plane, especially in long-haul flights. 

Sustainable Aviation Fuels (SAFs)

Sustainable Aviation Fuels (SAFs) are alternative jet fuels produced from renewable resources such as used cooking oil, agricultural residues, and municipal waste. These fuels are chemically similar to conventional jet fuel, allowing them to be used in existing aircraft engines and fueling infrastructure without modifications. SAFs can be produced through several technologies including:Hydroprocessed Esters and Fatty Acids (HEFA) which converts oils and fats into jet fuel; Alcohol-to-Jet (AtJ) which transforms alcohols like ethanol into jet fuel; and Fischer-Tropsch (FT) synthesis which converts carbon monoxide and hydrogen into liquid hydrocarbons. These processes enable significant reductions in lifecycle greenhouse gas emissions by up to 80% compared to traditional jet fuel (Scheelhaase, Maertens, & Grimme, 2019; U.S. Department of Energy, n.d.). Nevertheless, SAFs are by far the easiest to implement among sustainable aviation solutions, with some long-haul flights already being partially powered by these fuels, marking an important step toward decarbonizing commercial aviation.

One of the key advantages of SAFs is their potential to reduce carbon emissions substantially. As drop-in fuels, SAFs can be blended with conventional jet fuel, facilitating integration into current aviation operations without costly modifications to aircraft or fueling infrastructure. Moreover, SAF use can improve local air quality by reducing particulate matter and sulfur emissions near airports, contributing to healthier environments (Scheelhaase et al., 2019; Chooose, 2023).

However, SAFs currently face significant challenges. Production costs are considerably higher than conventional jet fuel, often two to five times more expensive, making widespread adoption economically difficult without regulatory incentives or subsidies. Additionally, the supply of sustainable feedstocks is limited, constraining production capacity and the ability to meet the growing demand of the aviation sector. There are also sustainability concerns related to feedstock sourcing, including land-use changes and competition with food production, which could offset some environmental benefits if not managed properly (Scheelhaase et al., 2019; Chooose, 2023).

Electric Propulsion

Electric aviation offers a promising path to reduce carbon emissions and lower operational costs by replacing fossil fuel combustion with clean electric motors. Electric planes produce zero emissions during flight, generate less noise, and require lessmaintenance for their simpler propulsion systems. While small electric aircraft are already flying short routes, scaling this technology to larger commercial planes remains challenging due to current battery limitations. 

Pipistrel Alpha Electro with a seating capacity of two

Batteries still fall far short of jet fuel in energy density to sustain long-haul commercial flights. Jet fuel corresponds to about 43 MJ/kg, while even the best lithium-ion batteries provide roughly 1 MJ/kg, making batteries over 40 times heavier for the same energy. This means an Airbus A320, a relatively small short-haul jet, would need batteries weighing more than four times its empty weight to match current fuel energy, creating a cycle where heavier batteries require more power and lift. Now we have anaircraft too heavy to fly (Yıldız, 2021; Wired, 2020). Unlike liquid fuel, however, the mass of the battery does not decrease as it is used. A long-haul jet like the Boeing 777, which may start with 100 tons of fuel, will burn off most of that weight during a long flight, improving fuel efficiency as the aircraft becomes lighter. In contrast, the battery in an electric plane is dead weight when it is empty, further decreasing the range of the plane. Alongside these weight challenges, the lack of high-power charging infrastructure and the need for advanced thermal management to prevent battery fires are major hurdles to widespread adoption (Yıldız, 2021; NREL, 2022).

Thus while small electric planes with fewer passengers and shorter ranges, such as the Pipistrel Velis Electro, achieve flight times around 45–60 minutes, scaling this technology up to large, long-range commercial jets is beyond current capabilities (Yıldız, 2021; The Times, 2023). Although emerging technologies like sodium-air fuel cells show promise for much higher energy densities, they remain in early research stages and are not yet viable for commercial aviation.

Hydrogen Propulsion: A Promising Alternative

Hydrogen has long been recognized for its high energy density. In fact, it was chosen as the fuel for the Space Shuttle's main engines due to its superior energy content per unit mass. Hydrogen's specific energy is approximately 120 MJ/kg, roughly 28 times more energy dense than jet fuel  which contains about 43.5 MJ/kg. Unlike Sustainable Aviation Fuels (SAFs), hydrogen can be produced using electricity, particularly through the process of electrolysis, where water is split into hydrogen and oxygen. This method allows for the generation of hydrogen without relying on organic feedstocks, offering a potentially more sustainable and scalable solution for aviation.

However, hydrogen's low volumetric energy density presents challenges. At ambient temperature and pressure, hydrogen gas occupies a large volume for a given amount of energy. To make it practical for aviation, hydrogen must be stored either as a compressed gas at high pressures (up to 700 bar) or as liquid hydrogen at cryogenic temperatures (around -253°C). These storage methods require robust, insulated tanks and specialized infrastructure, adding complexity and cost to hydrogen-powered aviation. Additionally, the mass of the hydrogen fuel does not decrease during flight, unlike jet fuel, which burns off and reduces the aircraft's weight, thereby improving fuel efficiency. This characteristic necessitates the design of aircraft and fuel systems that can accommodate the constant weight of hydrogen throughout the flight.

Despite these challenges, hydrogen propulsion systems are advancing. Companies like ZeroAvia and H2FLY are developing hydrogen fuel cell technologies for aircraft. For instance, ZeroAvia's hydrogen-powered Dornier 228 successfully completed a test flight in the UK, demonstrating the viability of hydrogen fuel cells in aviation. These developments indicate that hydrogen propulsion could play a significant role in decarbonizing aviation, especially for medium to long-haul flights where its energy density advantages are most beneficial.

In conclusion, while battery-electric propulsion offers a zero-emission solution ideal for short-range, low-capacity flights, its limitations in energy density and weight make it unfeasible for long-haul aviation in the foreseeable future (Yıldız, 2021; NREL, 2022). For long-distance commercial flights, sustainable aviation fuels (SAFs) and hydrogen propulsion emerge as the most viable pathways. SAFs offer an immediate solution by integrating seamlessly with existing infrastructure, although they face scalability and cost challenges (Scheelhaase et al., 2019; Chooose, 2023). Hydrogen, with its superior energy-to-weight ratio, holds significant long-term potential—particularly for medium and long-haul flights—despite the technical hurdles in storage, infrastructure, and aircraft design (ZeroAvia, 2023; The Times, 2023). Thus, the future of sustainable aviation likely rests on a hybrid strategy, where SAFs bridge the transition and hydrogen leads the next leap forward, while electric propulsion continues its development for regional applications.

References 

Chooose. (2023). What is Sustainable Aviation Fuel (SAF) Retrieved from https://www.chooose.today/content/blog/what-is-sustainable-aviation-fuel-saf

National Renewable Energy Laboratory (NREL). (2022). Battery Electric Aviation: Current State and Future Outlook. Retrieved from https://www.nrel.gov/transportation/battery-electric-aviation.html

NASA. (n.d.). Four forces on an airplane. NASA Glenn Research Center. Retrieved from https://www.grc.nasa.gov/www/k-12/airplane/forces.html

Scheelhaase, J., Maertens, S., & Grimme, W. (2019). CO₂ mitigation measures for international aviation and their impact on the industry. Journal of Air Transport Management, 77, 1–10. https://doi.org/10.1016/j.jairtraman.2019.101819

The Times. (2023). Electric planes face weight dilemma. Retrieved from https://www.thetimes.co.uk/article/electric-planes-face-weight-dilemma

U.S. Department of Energy. (n.d.). Sustainable Aviation Fuel. Retrieved from https://www.energy.gov/eere/bioenergy/sustainable-aviation-fuel

Wired. (2020). The Problem With Electric Planes Is Batteries. Retrieved from https://www.wired.com/story/the-problem-with-electric-planes-is-batteries/

Yıldız, B. (2021). Why electric airplanes face a battery barrier. Nature Energy, 6, 917–918. https://doi.org/10.1038/s41560-021-00901-6

ZeroAvia. (2023). ZeroAvia makes first flight of hydrogen-electric Dornier 228. Retrieved from https://www.zeroavia.com/press-release-hydrogen-electric-dornier-flight