Minimizing engine use on the ground promises significant fuel and emissions savings
An accelerating, reversing and pirouetting, its engines silent, may be an unusual sight, but the public demonstration of electric taxiing at Le Bourget may persuade aircraft manufacturers to acquiesce to airline demands and install the fuel-saving technology.
The concept involves fitting electrically driven motors to the aircraft's wheels, which will be used in place of the main engines to taxi. The systems, which will take power from the aircraft's auxiliary power unit (APU), are designed to reduce fuel-burn and emissions, lower direct operating costs, reduce maintenance time, extend engine life and provide environmental benefits.
Unlike the taxiing system in development by WheelTug, which uses electric motors in the nosewheels, the- system demonstrated on an A320 at last week's Paris air show has motors in the wheels of the main landing gear. In development by EGTS (Electric Green Taxiing System), a 50:50 joint venture formed by the two companies in 2012, the system could save up to $200,000 per aircraft per year in fuel costs, says Jim Fusaro, Honeywell product marketing vice president and EGTS business manager.
“We've looked at the mid- to short-haul market, and we think that is where there is the greatest value,” he says. Based on the standard taxiing procedures of a typical A320 making a 500-nm flight, the company predicts the system will save up to 4% in block fuel costs. Assumptions include an average taxi time of 20 min. with a single engine operating. “We are looking at total system savings, fuel consumption of the APU versus the main engine, the weight of the system and any ancillary benefits such as savings associated with tug operations, a 2-min. shave off the push-back time, reduced brake wear and less foreign object damage to the engine,” he adds. The system will also reduce carbon emissions by 75% and nitrogen-oxide emissions by up to 50%, reducing carbon taxes at various European airports.
The decision to power the main rather than nosewheels was “predicated on the majority of the aircraft's weight being transmitted through the main gear,” says Fusaro. Less than 10% of an aircraft's weight is on the nose gear, so EGTS believes its system is therefore better equipped to handle adverse surface conditions on the ground, slopes, snow, ice and breakaway torque considerations when aircraft “sink” into soft tarmac on hot days.
Honeywell and Safran, which first announced their intent to explore an electric taxi system in 2011, have combined their expertise in avionics, power and landing-gear equipment into an integrated system designed to accelerate an aircraft to around 20 kt. in 90 sec., or 18 kt. at maximum takeoff weight (MTOW). The system will also be capable of accelerating to 10 kt. in 20 sec. to cross active runways. It will also have the power to achieve breakaway torque on a 1.5% slope at MTOW.
A 50-kw electric motor will be built into the outer wheel on each main landing-gear unit, says Olivier Savin, Safran vice president for EGTS. The installation includes power converters to drive the motors from the APU, which will need a larger generator to power the wheels, and a reduction gearbox to drive the wheel through a clutch/reaction torque mechanism which is integrated with the brake control. The installed unit is now expected to weigh around 150 kg (330 lb.) per wheel, though “there will always be further optimization,” says Fusaro.
On the A320 test aircraft the wheel-motor installation is representative of a production system, but the power and control electronics are mounted in the cabin, so they can be easily removed when the aircraft is returned to service. “We are working with the OEMs to evaluate solutions for integrating the boxes and wiring, but the mechanical aspect is close to the final solution,” Savin says.
OEMs are also looking at how an electric taxiing system would be controlled from the cockpit. For the demo, the system is controlled via a joystick.
“This has been tested with three pilots and they have been very positively surprised at how easy it is to maneuver and make difficult movements, including reverse,” Savin says. “There is no reverse today on an aircraft, so we are creating a revolution.”
To enable an aircraft to turn around its center, the wheel on one side is driven in reverse while the other side is driven forward, allowing for a “tank-style” turn which could be useful when maneuvering out of an airport gate.
Initial tests to evaluate runway conditions and calculate the necessary loads to move an A320 took place in Montpellier, France. Trials of various system elements are underway at Honeywell and Safran sites in Canada, Europe and the U.S. Nearly 3,000 hr. of laboratory testing has been achieved so far on electrical and landing-gear subsystem integration, while 2,200 mi. of equivalent distance has been amassed on qualification tests of wheels and brakes under normal and loaded conditions.
Testing on the A320 covered 160 km and 300 hr. Many hurdles have been crossed in this first phase of testing, says Fusaro. The fully integrated system is designed for both forward fit and retrofit on the A320 and 737 from 2016 onward. Productive conversations have been held with various manufactures and operators, he says.