Revolutionising Taxiing

When airline pilots were asked how long they have been ‘stuck’ on the ground while taxiing for takeoff, the responses were enough to make one ponder. An Air India Airbus A320 Captain reported 50 minutes on ground due to a thunderstorm at Singapore. A Jet Airways Boeing 737-800 First Officer related his experience of inordinate delay on ground due to visibility dropping below minima while taxiing out, requiring return to bay to refuel, costing one hour of taxi fuel. An Air India Captain had his worst 78 minutes on the ground at Delhi airport because of a rise in fuel temperature while taxiing, forcing a return to bay. Others reported delays of not less than 30 minutes at Delhi and Mumbai airports.

Because an aircraft’s main engines are optimised for flying rather than taxiing, they burn a disproportionate amount of fuel during ground operations. With both engines running, a ‘classic engine’ (IAE or CFM) powered Airbus A320 consumes about 600 kg per hour during taxi. Average delay of 30 minutes corresponds to a fuel burn of 300 kg.

An efficient carrier can realistically operate five domestic flights on 1,000 nautical miles routes per day, resulting in a total taxi fuel burn of between 1,200 kg. and 1,800 kg. That much taxi fuel costs Indian carriers at least Rs. 96,300, when fuel is uplifted at Delhi or as much as Rs. 1,64,200 when uplifted at Kolkata at current price levels. Over a year, an A320 can burn in excess of a whooping Rs. 3.5 crore. With a fleet of 70 aircraft, an airline can spend at least Rs. 246 crore annually. This is equal to 31 per cent of the profits reported by IndiGo in financial year 2012-13. Shorter the flight, greater are the cycles and longer is the cumulative taxi time per day, further escalating taxi-fuel related expenses.

Paris Air Show in June 2013 marked the first public demonstration of Honeywell’s and Safran’s Electric Green Taxiing System (EGTS), a revolutionary system promising huge savings on fuel.

Sparking Off Change

The EGTS initiative was first announced at the Paris Air Show in June 2011, where Honeywell and Safran signed a memorandum of understanding to create EGTS International, a joint venture company tasked with system development, production, marketing and support of the new system. The concept involves electric motors mounted on the main landing gear bogies drawing electrical power from the auxiliary power unit (APU), enabling the aircraft to taxi without its main engines running.

The partnership between Honeywell and Safran provides systems expertise with a combined experience of eight years of research and development. A major benefit of the EGTS technology is that it draws upon Honeywell’s extensive knowledge of avionics and auxiliary power systems with Safran’s expertise in electrical components and experience in landing gear systems.

The primary aim of EGTS is reducing fuel burn during taxi. The APU, which essentially is a small jet engine for electrical power generation and not propulsion, consumes 100 kg of ATF per hour. An aircraft equipped with EGTS, will consume one-sixth of the fuel otherwise burnt during taxi. This translates to a taxi fuel-cost saving of at least Rs. 205 crore annually for an airline. This figure is compelling enough to warrant the installation of the system on aircraft undertaking short- to medium-range flights.

System Design

The system is designed with the motors installed on main landing gears (MLG) that bears 90 per cent of its weight. Each MLG employs a specifically designed forced aircooled 50 kW permanent magnet motor driving the outer wheel through a gear box and a clutch. The clutch allows the outer wheels to be engaged to the EGTS when electric taxi is required and disengaged and free turning during take-off and landing. By powering the outer wheels only, the EGTS system ensures there is adequate drive focused on each wheel to move the aircraft without adding unnecessary weight or changing brakes temperature regimes. The system can perform tight turns by driving one of the wheels or turning the wheels on either side in the opposite direction, much the way a tank can move its tracks to turn on the spot.

Powering the motors is the Honeywell APU131-9A delivering 90 kW of electricity which is insufficient to meet the total power requirements to operate the systems onboard. A modification is, therefore, incorporated to upgrade the APU to deliver 120 kW electrical power. Between the power generation at the modified APU and the electric motors at the wheels, is the motor controller and power distributor (MCPD) controlled by the EGTS controller which sits in the electronic equipment bay of the aircraft. Interfacing the pilot to the EGTS controller is the pilot interface unit (PIU) which has a speed lever to control forward and rearward movement and the taxi speed. The lever allows the pilot to set the taxi speed up to 20 knots. When pulled back and the locks removed, the speed lever can be pulled further back, to taxi backwards as during a pushback. A guarded three-position momentary switch lever controls the tight turn system. Directional control of the aircraft is accomplished using the standard nose wheel steering tiller.

The PIU is still in the prototype stage. Once fully developed, the system is expected to be controlled using a dedicated joystick and power/direction control lever that can be easily integrated into the existing flight deck. Since the EGTS sets the speed of the aircraft, braking is unnecessary unless an emergency stop is required. To reduce speed, there is no need to apply brakes. Pulling back on the speed lever decreases motor speed which immediately reduces the taxi speed. A full stop can be achieved without brakes by pulling the joystick to the position corresponding to zero knots taxi speed. A point factored into the product development is the concern of the airplane tipping over during a self-pushback using the EGTS. Onboard actuator-controlled software obviates this possibility.

Operational Impact

Existing tyre speeds are unaffected by EGTS. The only operational impact will be training required on the usage of the system, marginally enhancing cockpit workload and EGTS related maintenance.

The EGTS is disengaged for take-off and landing. The aircraft takes off and lands normally and travels along the runway under main engine power. Once on the taxiway, the pilot would power down the engines and switch to EGTS engaging the outer wheels and taxi to the gate under electric power. Likewise, on departure, EGTS would help taxi the aircraft to the runway holding point. Pilots would then start the main engines, disengage EGTS, enter the runway and take-off in the usual manner. Operational procedures are still being developed with the various regulators as well as airports with detailed definition of the process for transition from engine-powered to EGTS-powered taxiing.

Benefits of EGTS

The EGTS prototype currently adds approximately 150 kg per wheel to the total aircraft weight. In addition to other EGTS-related system weight, the total weight is expected at 400 kg. In the coming two years, as technology matures, EGTS is expected to get lighter. However, even with the weight increase, EGTS International expects airlines to save up to four per cent of total fuel consumption.

For short-haul, high cycle fleets, the savings would be very significant outweighing the small increase in weight, which would also be partially offset by the reduction in fuel required for taxiing. Further quantified benefits are brake wear, eliminating the need for tugs for pushback, reduced foreign object damage, elimination of taxi out fuel contingency and reduced greenhouse gases emissions/carbon footprint.

Other benefits include reduced noise at airports, improved safety at apron as there are no engines running and no threat of a jet blast, taxi to hangar/gate and stand positioning, increased gate autonomy resulting in improved on-time-performance (OTP), reduced ground operations damage, engine maintenance cost savings, higher precision manoeuvring and lower pilot workload when compared to a single-engine taxi. However, EGTS will understandably lead to increased APU fuel burn, additional APU maintenance costs and additional aircraft fuel burn due to EGTS weight.

Over a 1,000-nantical mile flight for an Airbus A320 cruising between FL370 and 390, an increase in 400 kg corresponds to a estimated flight fuel burn increase of just around 30 kg, while saving 125 kg of fuel for a 15-minute taxi and more for longer taxi times or delays on ground.

On May 2, 2013, the Directorate General of Civil Aviation (DGCA) issued an Aviation Environment Circular recommending single-engine taxi-in and taxi-out procedures at airports. While this may seem to dampen the claimed savings of the EGTS system, EGTS International has stated that it has been very conservative on estimating savings that the system could bring and that it has based early figures on single-engine taxiing rather than twin-engine taxiing, as many airlines around the world use single engine taxiing today. The EGTS development has apparently been focused to ensure significant savings for airlines, regardless of whether their current operations are based on single- or twin-engine taxiing.

The EGTS Retrofit

The system has been designed to be easy to install minimising costly grounding time for airlines. During system install of the market-ready version due for service entry in 2016, a minor modification will be necessary to the APU apart from generator upgrade to increase electric power output. The APU envelope, however, will require no modification. The only cockpit change is the installation of the PIU. The system is effectively a “bolt-on” solution requiring no major modifications to the landing gear without requiring removal of gear or retraction system. Consequently, the EGTS can be installed during “C Check” that takes around four days using optimised retrofit kits, making it easy and cost-effective for airlines.

Timeline and Status

In 2010, it was announced that demonstration of the EGTS was planned for end 2012. However, the project only fructified in April 2013, when a 22-year-old Airbus A320 taxied, for the first time, without a tug or its CFM56 engines. The system architecture was frozen in Q1 2012, with manufacturing starting in Q3 2012. Q1 2013 saw the commencement of system integration tests, resulting in system validation in a controlled environment (TRL-4), in Q2 2013. A fully integrated prototype system was tested on the Airbus A320 aircraft mid-2013, with prototype systems validation in a relevant environment targeted for the Q4 of 2013 (TRL-5). Launch of full-scale development is planned early 2014, followed by a targeted service entry end 2016. Solutions for other narrow-body aircraft are under exploration.

Since its maiden trial in April 2013, the EGTS fitted has logged more than 160 kilometres of operation. The next step will be to achieve full performance speed of 20 knots at maximum take-off weight. The aircraft has already flown with the EGTS installed.

The initial development of EGTS till date has mobilised more than 200 engineers working in 13 Safran and Honeywell facilities around the world. The component system and aircraft evaluation programme has accumulated more than 3,000 hours of testing on test-bench and on-ground manoeuvres in Toulouse on a dedicated A320.

Market and Industry Response

Over 50 airlines are in talks for the EGTS system with seemingly positive response from most. EGTS International has been working with UK’s airlines—EasyJet and TUIfly for over a year, helping them better understand how the technology would benefit their current operations. In addition, during the 2013 Paris Air Show, EGTS International signed a memorandum of understanding with Air France, which announced its support for the development of EGTS. As Air France operates a large number of short-range single-aisleaircraft, it will provide valuable assistance in refining estimated savings of the system and quantifying other operational benefits. There seems to be strong interest from the airline, in collaborating on the retrofit option which will be offered to current generation aircraft, in addition to the forward-fit option scheduled for future generation aircraft.

To maximise operational efficiency benefits of the new technology, Honeywell and Safran are focused on launching the EGTS for single-aisle, narrow-body commercial aircraft that operate in high-cycle, large congested airports. Mechanically, EGTS technology can work on all types of aircraft although returns are most significant for aircraft that spend a large part of their operating day on the tarmac. But Honeywell and Safran are not the only ones with a commercially inclined electric taxiing system.

L-3 together with Lufthansa tested an electric taxi system on their Airbus A320 in December 2012 at Frankfurt International Airport. An engineering team comprising staff from Airbus, L-3 and Lufthansa Technik replaced the brake assemblies of the inboard MLG wheels with drive units, each containing a liquid-cooled electrical motor, powered by the aircraft’s APU and planetary gearbox. Although the demonstration was months ahead of the EGTS, the setup was only temporary, with power supply cables and coolant hoses installed along the rear of the main landing gear, across the wing and through opened passenger windows into the aircraft’s interior.

On June 30, 2011, German Aerospace Centre’s (DLR’s) A320 advanced technology research aircraft (ATRA) taxied at Hamburg Airport propelled by an electric nose wheel. In the taxiing tests, engineers from DLR, Airbus and Lufthansa Technik demonstrated a fuel cell-powered electric nose wheel.

But the silence surrounding the L-3’s development indicates the project is possibly shelved or abandoned. DLR’s efforts still in its infancy, the only competition today is from Gibraltar-based Wheel-Tug, which was the result of work by Boeing’s Phantom Works. WheelTug uses a motorised nose-gear drive system, running off the APU. WheelTug’s first in-wheel demonstration was in June 2012 on a Germania Boeing 737-500. The WheelTug system seems to be significantly lighter at 150 kg per aircraft, involving no APU modification. However, according to Olivier Savin, Safran’s EGTS programme Vice President, “Less than ten per cent of the aircraft weight is on the aircraft nose gear, making it difficult to achieve the taxi performance required by airlines in all operating conditions.” However, WheelTug’s model is not just technically different.

The business revolves around leasing the system to aircraft operators, with revenues for the company generated by sharing cost savings. This model eliminates the need for an airline to invest in the system. The attraction of this business model reflects on the order book of WheelTug aircraft drive systems that has grown to 731 delivery slots reserved by 13 airlines from Europe, America, the Middle East, South East Asia and the Far East including for the Boeing 737NGs fleet of Jet Airways. Service entry of the WheelTug system for Airbus A320 and Boeing 737 family of aircraft is expected in 2015.

The Future

The concept of electric taxiing is not new. A patent was filed as early as 1971, for a system that uses power from the engine or the APU to drive the wheels of the aircraft using electrical, pneumatic or hydraulic systems. Forty-two years later, the e-taxi concept is taking shape with a strong business case, thanks to technology that can today make the concept viable and the heightened need in an era when spiralling oil prices are only driving airlines and original equipment manufacturers to seriously harness technologies that will help realise fuel savings. Electric taxiing joins winglets, geared turbofan engines and composites in significantly reducing fuel costs. An EGTS may in the coming years become a standard fit on singleaisle airliners used on short-to-medium routes.