Can Single-Engine Taxiing Cut Airport Emissions?

The skies above the United Kingdom are becoming increasingly busy, and with growing air traffic comes a heightened focus on the environmental impact of aviation. While much attention rightly falls on emissions during flight, a substantial, yet often overlooked, portion of an aircraft's fuel consumption and emissions occurs right on the ground: during taxiing. As aircraft navigate the intricate network of taxiways to and from runways, their engines typically run at low power settings, but for extended periods. This process contributes significantly to air pollution and noise at airports. In response, a practice known as single-engine taxiing (SET) has emerged as a promising strategy to mitigate these effects. But can single-engine taxiing truly deliver a meaningful reduction in airport emissions, and what are the practicalities and challenges involved?

Understanding Single-Engine Taxiing (SET)

Single-engine taxiing, as the name suggests, involves an aircraft operating with only one of its engines running while moving on the ground. For multi-engine aircraft, such as the vast majority of commercial jets, this means shutting down one or more engines after landing, or delaying the start-up of one or more engines before take-off. The remaining operational engine provides the necessary thrust to propel the aircraft at taxi speeds, while the other engines remain off. This practice is primarily applicable to twin-engine aircraft, which constitute the backbone of modern commercial fleets, though some larger aircraft with three or four engines might also employ variations of this method.

The concept is deceptively simple: if fewer engines are running, less fuel is consumed, and consequently, fewer emissions are produced. However, the implementation is far more complex than merely turning off an engine. It involves careful consideration of aircraft systems, operational procedures, pilot training, and air traffic control coordination. The aim is to achieve environmental benefits without compromising safety or operational efficiency.

The Environmental Burden of Conventional Taxiing

Before delving into the benefits of SET, it’s crucial to understand why conventional taxiing is an environmental concern. During typical taxi operations, all engines on a multi-engine aircraft are running. While power settings are low, the sheer volume of aircraft movements and the often-long taxi times at busy airports mean that cumulative fuel burn and emissions can be substantial. Aircraft engines, even at idle or low thrust, release a cocktail of pollutants into the atmosphere, including:

• Carbon Dioxide (CO2): A primary greenhouse gas, contributing to climate change.
• Nitrogen Oxides (NOx): Contribute to smog formation and acid rain, and can have respiratory health impacts.
• Hydrocarbons (HC): Unburnt fuel that can contribute to ground-level ozone.
• Carbon Monoxide (CO): A toxic gas, particularly problematic in high concentrations.
• Particulate Matter (PM): Fine particles that can penetrate deep into the lungs, posing health risks.

These emissions are concentrated in the immediate vicinity of airports, affecting air quality for airport staff, local communities, and passengers. Furthermore, the noise generated by multiple engines running at the gate and during taxiing adds to the overall noise pollution experienced by those living near airports. Any strategy that can reduce these outputs during ground operations offers a tangible benefit to both local air quality and the broader climate challenge.

How Single-Engine Taxiing Reduces Emissions and Fuel Burn

The mechanism by which SET reduces emissions is straightforward: by operating with fewer engines, an aircraft consumes less fuel. Each engine, even at idle, requires a certain amount of fuel to run. By shutting down one engine, that specific fuel flow is eliminated. While the remaining engine might need to operate at a slightly higher thrust setting to compensate for the reduced power, the overall fuel consumption is still significantly lower than having all engines running.

Studies and airline trials have consistently shown that SET can lead to significant fuel savings during taxi operations. These savings typically range from 5% to 15% of the total fuel consumed during taxi, depending on the aircraft type, taxi duration, and airport layout. For an airline operating hundreds of flights daily, these seemingly small percentages translate into millions of litres of fuel saved annually, and consequently, thousands of tonnes of CO2 emissions avoided.

Beyond CO2, reductions are also seen in other pollutants. Less fuel burned means less NOx, HC, CO, and PM released into the atmosphere. This directly improves local air quality around airports, which is a critical concern for public health and environmental regulations in the UK and globally. Additionally, operating with one less engine can contribute to reduced noise levels on the apron and taxiways, albeit the difference might be less dramatic than the fuel savings.

Operational Considerations and Challenges

Despite its environmental benefits, the widespread adoption of single-engine taxiing is not without its operational complexities. Airlines and air traffic control (ATC) must navigate several factors to implement SET safely and efficiently:

1. Safety Protocols and Procedures

• Engine Start-Up Time: Before take-off, the shut-down engine must be restarted. This process takes time, typically a few minutes, and requires a clear area to ensure no foreign object debris (FOD) is ingested. ATC must account for this delay in their sequencing.
• Thrust Asymmetry: Operating with only one engine creates asymmetric thrust, requiring pilots to use rudder pedals to maintain a straight path. While pilots are trained for this, it adds to workload, especially in gusty wind conditions or on slippery surfaces.
• Emergency Readiness: In an emergency, all systems must be available quickly. Pilots need to be confident that the shut-down engine can be restarted reliably and rapidly if required.

2. Pilot Workload and Training

Pilots need specific training on SET procedures, including engine start-up/shut-down checklists, managing asymmetric thrust, and communicating intentions to ATC. The increased workload during critical phases like taxiing can be a concern, particularly in complex or unfamiliar airport environments.

3. Engine Wear and Tear

Some argue that repeatedly shutting down and restarting engines, or operating one engine at a slightly higher thrust setting for extended periods, could lead to uneven wear and tear. While modern engines are robust, airlines need to assess the long-term maintenance implications and costs associated with SET practices.

4. Airport Infrastructure and Air Traffic Control

• Pushback Requirements: For departure, aircraft often need to be pushed back from the gate before starting engines. With SET, the second engine start-up might need to occur further down the taxiway, requiring coordination.
• Taxiway Gradients: Steep inclines on taxiways can make single-engine taxiing more challenging, as the remaining engine might struggle to provide sufficient thrust, especially for heavier aircraft.
• Taxi Times and Delays: While SET can save fuel, it might lead to slightly slower taxi speeds or require specific routes to allow for engine start-up, potentially impacting overall flow at busy airports. ATC must manage these factors to prevent congestion.

5. Aircraft Type and Configuration

Not all aircraft types are equally suited for SET. Factors like engine type, aircraft weight, and specific operational procedures dictate the feasibility and safety of the practice. Airlines must consult aircraft manufacturer guidelines and conduct thorough evaluations.

The Broader Context: Airport Sustainability and Alternatives

Single-engine taxiing is one piece of a larger puzzle in the pursuit of sustainable aviation. Airports and airlines are exploring various other initiatives to reduce their environmental environmental footprint:

Electric/Hybrid Taxiing Systems

More advanced solutions are emerging, such as electric taxiing systems. Companies like WheelTug or Safran's Electric Green Taxiing System (EGTS) aim to allow aircraft to taxi under their own electric power, using motors mounted on the landing gear. This would eliminate the need to use jet engines for taxiing altogether, offering even greater reductions in fuel burn, emissions, and noise. While promising, these technologies require significant investment and integration into existing aircraft fleets and airport infrastructure.

Optimised Taxi Routes and Procedures

Air traffic control can play a crucial role by optimising taxi routes to minimise distance and time spent on the ground. Implementing advanced ground movement guidance systems and better coordination between ATC and airlines can significantly reduce unnecessary idling.

Electric Ground Support Equipment (GSE)

Replacing diesel-powered ground support vehicles (e.g., baggage tugs, pushback tractors, catering trucks) with electric alternatives drastically reduces emissions and noise on the apron, contributing to overall airport air quality.

Sustainable Aviation Fuels (SAFs)

While not directly related to taxiing, the development and adoption of SAFs are critical for reducing emissions across all flight phases, including taxiing, when jet engines are running. SAFs can significantly reduce lifecycle carbon emissions compared to traditional jet fuel.

Comparative Analysis: SET vs. Conventional Taxiing vs. Electric Taxiing

To put single-engine taxiing into perspective, let's compare its impact and characteristics with conventional taxiing and future electric taxiing systems:

Frequently Asked Questions About Single-Engine Taxiing

Is single-engine taxiing safe?

Yes, when implemented with proper procedures and pilot training, single-engine taxiing is considered safe. Airlines and aircraft manufacturers develop specific guidelines and checklists to ensure that all safety parameters are met. Pilots are trained to manage asymmetric thrust and to conduct engine restarts safely and efficiently if required. The key is adherence to established safety protocols and continuous training.

Do all aircraft types support single-engine taxiing?

Most modern twin-engine commercial aircraft are capable of single-engine taxiing. However, the specific procedures and feasibility can vary between aircraft models and manufacturers. Airlines typically consult their aircraft operating manuals and manufacturer recommendations before implementing SET for their fleet. Larger aircraft with three or four engines may also employ similar practices, but the benefits and procedures are often optimised for twin-engine operations.

What are the main benefits of single-engine taxiing?

The primary benefits are significant reductions in fuel consumption and associated emissions (CO2, NOx, PM, etc.) during ground operations. This leads to cost savings for airlines and improved local air quality around airports. There can also be a slight reduction in noise levels on the apron and taxiways.

What are the main drawbacks or challenges?

The main challenges include increased pilot workload due to asymmetric thrust and engine management, the time required for engine restarts before take-off, potential for uneven engine wear (though this is debated for modern engines), and the need for careful coordination with Air Traffic Control, especially at busy airports. Steep taxiway gradients or adverse weather conditions can also complicate operations.

Is single-engine taxiing widely adopted at UK airports?

While there isn't a universal mandate, many airlines operating at UK airports have adopted single-engine taxiing as a standard operating procedure, where feasible. Major UK airports like Heathrow, Gatwick, and Manchester are accustomed to aircraft performing SET. Its adoption is driven by both environmental sustainability goals and the economic incentive of fuel savings. However, its application is always subject to ATC clearance, taxiway conditions, and specific airline policies.

Conclusion

Single-engine taxiing presents a viable and increasingly adopted strategy for reducing airport emissions and fuel consumption. By simply shutting down one engine during ground movements, airlines can achieve tangible environmental benefits and cost savings without requiring significant infrastructure investments or aircraft modifications. While operational complexities, such as increased pilot workload and the need for careful ATC coordination, must be meticulously managed, the practice demonstrates a clear commitment to mitigating aviation's environmental footprint. As the aviation industry strives for greater sustainability, SET stands out as a practical, immediate step that contributes to cleaner air and a quieter environment around UK airports, paving the way for a more sustainable aviation future, potentially complemented by advanced electric taxiing systems and other green initiatives.

If you want to read more articles similar to Can Single-Engine Taxiing Cut Airport Emissions?, you can visit the Taxis category.