Electric Taxi Systems Aim To Save Airlines Fuel, Money Getting To the Runway
Efforts to reduce the thousands of gallons of jet fuel now being burned each year just to move aircraft to and from runways are very much in evidence at the 2012 Farnborough International airshow. No fewer than four new products vying for the attention of airline and airport managements, including efforts by Israel Aircraft Industries (IAI), L-3, Safran/Honeywell and WheelTug, and they each have taken a different approach.
L-3, WheelTug and the joint venture between Safran and Honeywell have both taken a novel approach of installing motors directly on landing gear to allow aircraft to taxi autonomously using power from the auxiliary power unit (APU). By contrast, IAI’s TaxiBot system involves a semi-robotic tow tug that is controlled from the cockpit by pilots.
Yesterday [Monday] IAI signed a memorandum of understanding with Lufthansa LEOS that should see the ground handling company begin operational trials with TaxiBot. The vehicle recently completed some trials at Chateautroux Airport in France, following earlier evaluations last year involving Airbus A340-600 and Boeing 747-400 aircraft.
According to TaxiBot program director Ran Briar, one of the main differentiators of IAI’s system is that it can be used to tow larger aircraft. Rival systems using motors fitted to landing gear are generally only designed to handle narrow-bodied airliners. He said that widebodied airliners typically would need bout 1,500 HP for taxiing and that typically APUs can only deliver about 150 HP for this purpose.
According to Briar, another advantage is that the path to certification is far simpler for a tow tug because there is no need to install systems on the aircraft and provide power and cooling. He also argued that the weight penalty associated with landing gear-mounted systems means that additional fuel burn inflight cancels out savings while taxiing for flights lasting more than two hours.
TaxiBot has two diesel generators providing power to electric motors on its wheels. The vehicle can attain a normal aircraft taxiing speed of up to around 23 knots. Briar claimed that rival systems taxi more slowly and so hold up other aircraft at busy airports that are taxiing under the power of their main engines.
According to IAI, a 747 taxiing for 17 minutes burns about 1,250 liters of fuel, while the same movement with TaxiBot would require no more than 30 liters. TaxiBot also emits far less carbon dioxide than would be the case if the aircraft were taxiing on main engine power.
For the pilot there is no difference in terms of how he or she controls the aircraft while taxiing with TaxiBot fitted to nose gear. The system uses a special interface with a rotating turret on the tug and when the gear moves it measures the angle and ensures that the wheels are in the correct alignment.
Braking is handled safely using a special pendulum and an energy-absorbing mechanism. This senses the braking load applied by the pilot and softens the braking process so as not to transfer the load to the landing gear in a way that would result in excessive fatigue.
The unconfirmed list price for the TaxiBot system is expected to be around $1.5 million for the narrowbody version of the vehicle (with eight wheels), rising to $3 million for the larger model that can handle widebody airliners. IAI believes that each vehicle would be able to handle about 20 aircraft each day.
One drawback with TaxiBot is that it is only really suited to towing aircraft out to the runway for takeoff since it would cause delays to have to attach it after landing. After the pilot has completed the taxi movement, a member of ground staff drives the tractor back to the apron.
L-3 Teams With Crane
Also progressing towards market entry is the new GreenTaxi electric taxiing system. On Monday, L-3 Space and Propulsion Systems signed an agreement under which Crane Aerospace & Electronics will develop and market the system.
GreenTaxi integrates electric wheel drives into the main landing gear, drawing power from the APU. L-3’s engineers opted to use the main gear because they felt this would ensure better taxiing performance in poor weather conditions. It chose an electric motor over a hydraulic motor on the grounds that it would deliver higher torque more efficiently.
According to L-3, more than three percent of total airliner fuel consumption comes from taxiing. In December 2011 trials at Germany’s Frankfurt Airport for Lufthansa, the company claims to have proved that the technology can deliver worthwhile fuel savings, environmental benefits, improved airline autonomy and reduced infrastructure costs. It achieved taxi speeds of just over 16 kts.
Under a 50/50 joint venture Safran and Honeywell have used an Airbus A320 and a Boeing 737-800 for ground tests, including recent trials with prospective launch customer Easyjet at London Luton Airport. The partners have estimated that its electric motor-based system could save 3 to 4 percent of fuel burnt on medium-haul flights.
Its “Electric Green Taxiing System” uses actuators fitted to both main landing gear legs. Each unit, including the motor, gearbox and clutch weighs around 220 pounds.
Safran deputy CEO Marc Ventre told AIN that the partners have purchased an A320 and will use it for further trials in 2013, including a demonstration at next June’s Paris Air Show. Entry into airline service is planned for 2016, and so would make the system a potential feature for the new A320neo and Boeing 737 MAX narrowbodies.
However, to date, it is the WheelTug system that has attracted the highest number of conditional airline commitments. In a letter of intent signed at the Farnborough show on Monday, Turkey’s Onur Air agreed to purchase the U.S. company’s Aircraft Drive Systems for 22 of its Airbus fleet.
According to CEO Isaiah Cox, the system consumes 80 percent less fuel than conventional taxiing, burning four pounds of fuel per minute. It will offer the equipment for lease or under power-by-the-hour terms. A video showing the system being demonstrated can be viewed at http://media.wheeltug.com.