31/01/2018
When you're settling into your seat on an aircraft, ready for departure, there's a crucial phase that often goes unnoticed: taxiing. This seemingly simple journey across the tarmac, from gate to runway or vice versa, is far more complex and resource-intensive than many realise. Aircraft engines, optimised for flight at high altitudes, are surprisingly inefficient during these slow ground manoeuvres, consuming significant amounts of fuel and emitting pollutants. However, the world of aviation is on the cusp of a quiet revolution, with pioneering technologies and innovative concepts set to redefine how aircraft move on the ground, and even how we commute through the air. This article will delve into the current landscape of airport taxiing, explore the cutting-edge solutions making it more sustainable, introduce the exciting future of air taxis, and even uncover the fascinating etymology of the word 'taxi' itself.
The duration of an aircraft's taxiing phase can vary significantly, ranging from a brisk five to ten minutes at smaller, less congested airports, to over thirty minutes or even longer at major international hubs during peak times. Factors influencing this include airport size, runway proximity, air traffic control directives, and the sheer volume of aircraft movements. During this period, traditional aircraft rely on their main engines for propulsion, burning valuable fuel and contributing to the airport's environmental footprint. Recognising this challenge, airport operators, airlines, and aircraft manufacturers are actively seeking new, energy-saving propulsion systems for ground manoeuvres, aiming for greater sustainability and operational efficiency.
Pioneering Sustainable Taxiing Solutions
The drive towards more sustainable aviation begins on the ground. Several innovative concepts have emerged, some already tested, to drastically reduce the environmental impact of taxiing. These solutions can be broadly categorised into procedural changes, ground-based systems, and on-board technologies.
Single Engine Taxiing (SET): The Immediate Impact
The simplest and most immediate way to reduce energy consumption during taxiing requires no additional equipment or complex installations. In Single Engine Taxiing, or SET, the pilot operates only one of the aircraft's engines while navigating the tarmac. The remaining engines are typically started and brought to full operation only shortly before takeoff. This straightforward procedural change can yield substantial benefits. Estimates suggest that SET can cut fuel consumption at the airport by more than 20 percent. While seemingly minor, when applied across thousands of flights daily, this translates into significant savings in fuel costs and a considerable reduction in carbon emissions. It's a testament to how even small operational adjustments can have a large collective impact on the aviation industry's environmental footprint.
Ground-Based Systems: The TaxiBot Revolution
Taking a more advanced approach, ground-based systems introduce external vehicles to move aircraft without the need for their main engines. The flagship of these systems is the TaxiBot, an autonomous aircraft tractor with a hybrid diesel-electric powertrain. This remarkable vehicle has already undergone successful trials at major airports, including Amsterdam's Schiphol Airport. The operational concept is ingenious: a driver precisely manoeuvres the TaxiBot backwards to the nose landing gear of the aircraft and securely docks with it. Once connected, the TaxiBot transforms into an external propulsion system that the pilot can control directly from the cockpit. This allows the pilot to steer and drive the aircraft to a designated holding position just before the runway, where the main engines are then switched on for takeoff. The environmental benefits of the TaxiBot are compelling, achieving fuel, carbon dioxide, and nitrogen oxide savings of over 50 percent during the taxiing phase. This not only reduces emissions but also significantly cuts down on noise pollution around airport terminals.
On-Board Systems: The Promise of E-Taxiing (ETS)
Looking even further into the future, on-board systems aim to integrate the taxiing propulsion directly into the aircraft itself. These e-taxiing systems, or ETSs, are based on electric motors that could be installed within one or more of the landing gears. The concept offers several distinct advantages. Firstly, ETSs would significantly improve aircraft manoeuvrability on the ground, giving pilots more precise control. Secondly, they would eliminate the need for traditional pushback vehicles, streamlining ground operations and reducing congestion around gates. Thirdly, and crucially, ETSs would greatly reduce noise emissions at airports, particularly in busy gate areas and during early morning or late-night operations. The power required to operate these electric motors could be generated on board the aircraft, either by the existing Auxiliary Power Units (APUs) – small jet engines that provide power for aircraft systems when the main engines are off – or by emerging technologies like fuel cells. Developers of e-taxiing systems project the potential to reduce emissions of air pollutants by more than half, marking a monumental step towards truly green ground operations.
Comparison of Taxiing Technologies
To better understand the distinct advantages of each approach, consider the following comparison:
| Technology | Description | Key Benefits | Equipment Needed | Savings Potential (Fuel/Emissions) |
|---|---|---|---|---|
| Single Engine Taxiing (SET) | Pilot uses one engine for ground movement, others off. | Simple, immediate, reduces fuel burn. | None (procedural). | ~20% |
| TaxiBot (Ground-Based) | Autonomous hybrid tractor pulls aircraft. | High savings, reduced noise, pilot control from cockpit. | External vehicle. | >50% |
| E-Taxiing System (ETS) | Electric motors in landing gear move aircraft. | High savings, no pushback, improved manoeuvrability, very low noise. | On-board electric motors & power source. | >50% |
The Dawn of Air Taxis: A Sky-High Revolution
Beyond the tarmac, a different kind of "taxi" is emerging, one that promises to revolutionise urban transport: the air taxi. These are typically electric Vertical Take-Off and Landing (eVTOL) aircraft, designed to transport passengers or cargo over short to medium distances within urban and suburban environments. Imagine bypassing gridlock by flying directly to your destination. This concept, often referred to as Urban Air Mobility (UAM), is rapidly progressing from science fiction to a tangible reality, with numerous companies developing prototypes and planning commercial services in the near future.
The Technology Enabling Air Taxis: Beyond the Horizon
The feasibility and widespread adoption of air taxis, particularly autonomous ones, hinge on incredibly sophisticated underlying technologies. One such critical area is Integrated System Health Management (ISHM). At its core, ISHM is about continuously monitoring the 'health' of an aerospace system – be it an air taxi, a drone, or a satellite. It does this by fusing vast amounts of sensor data with historical performance information from various components and subsystems. The goal is to provide actionable insights and enable intelligent decision-making regarding the operation and maintenance of these complex systems.
ISHM fundamentally relies on accurate assessments and predictions of system health, including the early detection of potential failures and the estimation of Remaining Useful Life (RUL) for critical components. This predictive capability is achieved using advanced reasoning techniques, which can be model-based (relying on engineering models), data-driven (learning from vast datasets), or a hybrid of both. The primary benefits of ISHM for air taxis and other aerospace applications are profound: enhancing maintainability, significantly improving reliability, ensuring paramount safety, and boosting overall performance.
The next evolution of the ISHM concept is Intelligent Health and Mission Management (IHMM). IHMM takes the predictive power of ISHM a step further by actively utilising real-time system health predictions to dynamically modify mission profiles. This means that if an IHMM system detects a potential degradation in performance or an impending issue, it can automatically suggest or implement changes to the flight plan to ensure continued safety, reliability, and efficiency. For example, it might recommend a shorter route, a different altitude, or even an alternative landing site to mitigate risks before a failure event occurs. This concept is particularly vital for Trusted Autonomous System (TAS) applications, where an accurate assessment of the current and future system state-of-health is integral for making operational decisions (with or without human intervention), ensuring both flight safety and mission success. IHMM systems introduce the unique capability of predicting degradation in the functional performance of subsystems with sufficient lead time, allowing for the dynamic identification of appropriate restorative or reconfiguration actions. This proactive approach ensures that the system can maintain an acceptable level of operational capability well before the onset of a critical failure event. These advancements in autonomy and predictive analytics are key enablers for the safe and reliable operation of future air taxis within complex environments like Urban Air Mobility (UAM) and Unmanned Aircraft Systems (UAS) Traffic Management (UTM).
Why Do We Call Them 'Taxis'? Unpacking the Etymology
While the future of air and ground taxiing is undeniably exciting, have you ever paused to consider why we use the word "taxi" at all? The name "taxi" is, in fact, a shortened form of "taxicab", a term that beautifully blends two historical words: "taximeter" and "cabriolet".
The "taximeter" was a revolutionary invention in 1891. This mechanical device was designed to precisely record distances travelled and calculate the fare based on that distance and time. It brought transparency and standardisation to what was previously a often-negotiated, and sometimes contentious, transaction. The word "taximeter" itself has deeper roots, derived from the Mid-Latin "taxa", which means "tax or charge", perfectly reflecting its function as a fare calculator.
The second component, "cabriolet", refers to a specific type of horse-drawn carriage that was popular in the 19th century. These were typically light, two-wheeled, two-seater carriages, often with a folding hood (hence the 'cabriolet' part, implying convertibility). A distinguishing feature of these carriages was often the driver's position, standing at the back. When motor vehicles began to replace horses, the term "cabriolet" (or simply "cab") was naturally transferred to these new motorised carriages-for-hire.
The first documented use of the combined word "taxicab" can be traced back to March of 1907 in London, a city synonymous with its iconic black cabs. This marked the official birth of the term that would become universally recognised. Interestingly, another phrase that derived from the concept of the taximeter was a "taxi dancer", a woman who sold her services for a dance at dance halls, paid by the dance, much like a taxi charges by the mile.
Frequently Asked Questions (FAQs)
How long does an average aircraft taxiing take?
There's no single average time, as it depends heavily on the airport's size, layout, air traffic volume, and the aircraft's position relative to the runway. At smaller airports, it might be 5-10 minutes. At large, busy international hubs like Heathrow or Gatwick, it can easily extend to 20-30 minutes, especially during peak hours or adverse weather conditions.
Are electric aircraft taxis common at airports today?
While electric ground support vehicles (like baggage tugs or pushback trucks) are increasingly common, fully electric aircraft designed for taxiing using on-board electric motors (e-taxiing) are still in the testing and development phases. Systems like the TaxiBot (a hybrid external tractor) are being trialled, but widespread commercial use of on-board e-taxiing systems is not yet standard.
What are the main benefits of new taxiing technologies like TaxiBot and e-taxiing?
The primary benefits are significant reductions in fuel consumption, carbon dioxide (CO2) emissions, and nitrogen oxide (NOx) emissions, leading to a much smaller environmental footprint for airports and airlines. Additionally, these technologies drastically reduce noise pollution on the ground, improve operational efficiency by reducing reliance on external vehicles, and potentially enhance aircraft manoeuvrability.
Will air taxis replace traditional ground taxis?
It's unlikely air taxis will completely replace traditional ground taxis. Instead, they are expected to complement existing transport networks, offering a premium, faster option for specific routes, particularly in congested urban areas or for rapid airport transfers. They will serve a different market segment and address different transport challenges, focusing on speed and bypassing ground traffic rather than door-to-door convenience for every journey.
Is the TaxiBot currently in widespread use?
The TaxiBot has undergone extensive testing and is certified for use with specific aircraft types, such as the Boeing 737 and Airbus A320 families. While it has been successfully demonstrated at airports like Schiphol, its widespread commercial adoption is still in its early stages. Implementation requires significant investment in infrastructure and operational changes at individual airports, but its proven benefits suggest a growing future presence.
The journey of a taxi, whether on the ground or in the air, reflects a continuous human drive for innovation and efficiency. From the simple yet effective single-engine taxiing to the sophisticated autonomous ground tugs and the futuristic vision of air taxis, the aviation industry is embracing technology to become greener, quieter, and more efficient. As we move forward, the commitment to sustainability and the pursuit of seamless, safe travel will continue to shape the evolution of how we move, both across the tarmac and through the skies, ensuring that the next generation of transport is as responsible as it is revolutionary.
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