On average, a single-aisle jet burns more than 65 gallons of fuel while taxiing, which translates to almost $130 per flight (considering the price of jet fuel at $2 per gallon) and around $650 per day (assuming 5 flights a day). The use of electric taxiing systems (ETS) could offset this cost, but the installation of motors required for ETS will increase the overall weight of the aircraft which can result in higher fuel consumption throughout the flight.
In order to address this, aircraft manufacturers are developing independent taxiing systems that work without a tug or the main engine. The systems have not seen deployment on a commercial scale and most development efforts are still in the lab but are supported by many carriers around the world. A few solutions are scheduled for commercialization in the short to mid-term term.
Cost-Intensive Solutions
High up-front cost is common in the capital-intensive aviation industry, making it challenging for bleeding full-service carriers or new market entrants following the low-cost carrier (LCC) model to test new technologies such as electric taxiing systems. In order to bring down ground handling costs, smaller carriers are more likely to opt for systems like WheelTug due to its innovative business model, leasing out systems to airlines for a price which also accommodates a percentage of realized cost savings. This provides a level of assurance to the purchasing airline. WheelTug system has shown savings of over $1 million per aircraft per year.
Short-Radius Turning Requirement
Nosewheel-controlled taxi operations have a limited turn radius because the upper limit of steering angle is typically around 60 degrees. Thus, conventional nosewheel steering is not appropriate for short-radius turning for larger aircraft, such as the A380 or the upcoming 777X, and would require main landing gear steering systems.
Even if the main landing gear steering system is used, sharp turns should be at low taxi speeds to reduce centripetal forces, which along with cross-wind forces affect aircraft taxiing stability. These two forces can overturn the aircraft. Nosewheel loading also can interfere with steering control. Taxiing backward is even riskier, especially when done under aircraft power. For this reason, reversing aircraft is often done with a tow vehicle.
To avoid overturning an aircraft, they are moved slowly over the ground, which increases both taxi time and fuel consumption.
One of the major advantages of ETS is safer taxiing at higher speeds in both forward and reverse.
The Electric Green Taxiing System project was discontinued in 2016, but it triggered the development of new sustainable taxi solutions such as Safran’s initiative to increase aircraft efficiency through aircraft landing gears. Israel Aerospace Industries collaborated with ground-support equipment manufacturer TLD to develop electric taxiing solutions appropriate for aircraft that undergo frequent pressurization cycles, like planes flying short-to-medium-haul flights.
A conventional tug can cause stress on the nosewheel during braking. TaxiBot, a semi-autonomous electric vehicle which can be controlled by the pilot, addresses this by allowing the pilot to use main landing gear to brake to avoid fatigue. This can be useful for heavier, twin aisle planes.
Effectual thermal management and power source to accentuate adoption
Because aircraft brakes generate significant heat, ETS requires proper thermal management. For single-aisles, brake temperatures may exceed 300 degrees Celsius. The brakes must cool down to a certain level for the aircraft to take off. This is accomplished by cold atmospheric air seeping through gaps between the brake disks and exiting through a protective screen in the hub. Air also can pass through holes in the wheel flange while integrated fans help dissipate the heat.
With the implementation of ETS hardware on brakes, the cooling path of air may be disrupted, but an auto transformer rectifier unit, or ATRU, can be used to avoid this. Safran and Thales are developing a lightweight ATRU.Despite the environmental and sustainability benefits of ETS, it needs a power source like an auxiliary power unit (APU). Airbus, in collaboration with EGTS International, tested an APU-driven taxiing system on an A320 aircraft, which was able to move across the tarmac without its main engines.The system was developed by a Safran-Honeywell joint venture and consists of one wheel on each main gear equipped with an electric motor, reduction gearbox, and clutch assembly to support movement during taxi operations. The power electronics and system controllers enable pilots to control the speed and direction. Finding the ideal balance between the voltage, output power and keeping it lightweight is a challenge that needs to be addressed with improving scalability.
Carriers Can Drive Adoption
Air France-KLM, EasyJet, GoAir, and Icelandair have already supported the development and evaluation of ETS. Ideally, a profitable low-cost carrier will adopt it for a test-run. For example, in Warsaw, Poland, Ryanair is the only airline that operates from Modlin Airport. Using an ETS in there would be simpler than at Chopin, Warsaw’s main airport, where multiple carriers operate. The Modlin airport does not have an aerobridge facility and is operated mostly by narrow-body jets. With limited number of carriers, ample space and unavailability of space-occupying equipment, ETS could be easily tested.
EasyJet has saved an average of 20 minutes of taxi time per flight, translating to four million miles a year and around 4 percent of its total annual fuel consumption for ground taxiing. To further reduce its fuel usage, EasyJet signed a strategic partnership agreement with Cranfield Aerospace to test the feasibility of a hydrogen fuel cell driven A-16m e-Taxi System. The sustainability factor is enhanced with fuel cells since the emitted by-product is water, which can also be used to refill the aircraft’s water system. The ETS contains a cell container for storing the energy produced by fuel cells, kinetic energy recouped from the wheels through regenerative braking, and from photovoltaic cells. The cumulative energy output, in turn, can be used to perform aircraft pushback during aircraft taxiing operations.
Analogous to regenerative braking in automobiles, the system allows energy capture as the aircraft brakes on landing, charging the system’s batteries while the aircraft is on the ground. This energy is used while taxiing. With electric motors in the main wheels along with electronics and system controllers, pilots can control speed, direction and braking during taxi operations, thus eliminating the requirement of tugs while moving in and out of gates, improving turnaround times and on-time performance.
Role of Academia
Academia can play an important role in the adoption of ETS across the global aviation industry. The regenerative braking concept conceptualized by EasyJet is similar to the work of a team of researchers from the University of Lincoln in the United Kingdom. The initiative was meant to analyze the feasibility of capturing energy from a landing aircraft. The researchers aimed at harvesting around 3 MW of peak available power from the momentum created by a landing Airbus A320. The approach aligns with the more electric configuration or the ultimate all-electric aircraft designs.
Safran Landing Systems has been working closely with academia to explore potential benefits of electric taxiing at some of the international airports in Asia and Europe. The agreement signed with France’s national civil aviation school ENAC and the Civil Aviation University of China is expected to accentuate the go-to-market strategy for the electric taxi system Safran is developing in partnership with Airbus.
Conclusion
In order to increase profit margins and reduce overall operating costs, airfield operation expenses need to be reduced. One such sustainable practice is the use of APUs supported by a single engine during aircraft taxiing, and this method can be further optimized with respect to cost and environmental viability through the use of ETS.
Aircraft manufacturers have declared their interest in ETS, which can be applied to aircraft nosewheel/main landing gear of an aircraft. Avoiding the use of tow trucks will translate to minimal personnel and fuel costs and associated vehicle maintenance. Lastly, this is also expected to create opportunities for engineering service providers that have been working closely with planemakers, MROs and carriers across several research projects from design and development to implementation.
Avimanyu Basu is the Lead Analyst at Information Services Group (ISG). With 8 years of experience in market research and consulting, Avi’ s engagements have been focused on providing strategic recommendations to both public and private sector clients across Europe, Middle East and Asia Pacific across commercial aerospace, energy and automotive industries.