Push-back tugs and taxiing aircraft with engines powered up may well, in a few years, be seen as remnants of the past. Two exhibitors here at the Singapore Airshow are studying electric motors that would drive the aircraft’s wheels on the ground.
Safran and Honeywell have formed a dedicated joint venture (Booth Q23), while German aerospace research center DLR (Booth H95) has completed trials and an in-depth evaluation of the benefits of electric motors. The new-generation electric taxiing systems could enter service as soon as 2016, they said.
Turbofan engines are optimized for flying, not for powering an aircraft on the ground because in that mode they burn a disproportionate amount of fuel taxiing from the gate to the runway. It is more important on short- and medium-haul aircraft, which spend a relatively long time taxiing compared to the duration of a flight.
Thanks to recent progress in electric systems, motors located in the landing gear and powered by the auxiliary power unit (APU) could make the aircraft autonomous. It would get rid of complex push-back operations at airports.
Safran and Honeywell have even predicted better punctuality from the process. They also foresee less brake wear and reduced engine maintenance because, when running on the ground, engines sometimes ingest dust, gravel and other debris.
Moreover, the aircraft would burn much less fuel on the ground because the jet-A would start flowing only during engine start and warm-up. From an environmental perspective, the reduction in carbon dioxide (CO2) emissions is obvious. Additional benefits include reductions in noise and emissions pollution around airports.
Safran and Honeywell estimate overall fuel savings for an A320 making a 500-nm flight to be around 3 to 4 percent compared of what they would be with standard taxiing procedures. About 550 pounds of fuel would be saved each time an aircraft taxied, they said.
Taxi speed with the electric motors would be close to 20 knots and that speed could be reached in 90 seconds. These are the requirements that airframers and airlines have told Safran and Honeywell would be necessary to make the technology practical for scheduled operations.
The program calls for a demonstrator to start tests in 2013 and Safran has bought an A320 as a testbed. The two partners are targeting the in-development new generation of narrowbodies–including the Airbus A320neo, Boeing 737MAX and Comac C919–as the first types to use the system, and therefore potential entry into service in 2016. Honeywell and Safran are considering retrofits as well.
The partners have ruled out a hydraulic motor because it would require extensive, bulky fluid ducting. This makes an electric, air-cooled motor far more practical, with power coming from the APU.
Unlike other organizations that have studied electric taxiing, Safran and Honeywell have chosen to fit the motors on to the main landing gear, in spite of the fact that installation would be easier on the nosewheel because it does not have a brake, has very few bits of ancillary equipment and is hollow, explained Jean-Pierre Garcia, Messier-Bugatti-Dowty’s technical director for what the partners are calling the “electric green taxiing system.”
However, he pointed out that the nosewheel bears only 5 to 8 percent of the aircraft’s weight, which is deemed insufficient to ensure traction on a wet taxiway since there could be a risk of the wheel spinning. This prompted Safran and Honeywell to locate the actuator for the system on the main landing gear, which bears 95 percent of the weight. “Our choice is more complicated for installation but much more effective,” Garcia said.
But why a joint venture? Couldn’t one of the two diversified equipment makers design its own system? “Safran and Honeywell each were working on these technologies but neither was covering the entire chain,” said Olivier Savin, Safran’s program director. Brian Wenig, Honeywell’s vice president for business development, said his company is bringing “credibility to an innovative project that could be seen with skepticism.” The 50-50 joint venture, however, has not yet been legally established, a Safran spokesperson told AIN.
The required fields of competence to develop the electric taxiing system include APUs, brakes, power electronics, landing gear and wheel steering. Honeywell will probably supply expertise in APUs and avionics. Safran will use its subsidiaries for their relevant skills, namely Hispano-Suiza for power electronics, Labinal for wiring, Sagem for control electronics, Technofan for actuator cooling and Messier-Bugatti-Dowty for the system’s design and integration.
According to Honeywell and Safran, several carriers have shown an interest in learning how feasible and beneficial the system could be for them, including Easyjet, Delta, U.S. Airways, Air France, American Airlines and Alitalia.
Separately, as part of the European Union’s Clean Sky initiative, the DLR has conducted an evaluation of the impact of electrically powered taxiing on airport traffic. It ran simulations based on Frankfurt and Berlin airports in Germany, and found that although the electric motors have slower acceleration and taxi speeds, “they hardly seem to constrain faster traffic.” Paradoxically, despite the slower speeds, average taxi times are expected to be reduced slightly for departures. This is “due to the autonomous push-back, since no time is required for disconnecting the towing vehicle and for starting up the engines.”
The DLR study predicted up to 17 to 19 percent in emissions reductions at airports. The potential saving at Frankfurt airport from the use of such equipment for all A320-class aircraft is about 44 metric tons of kerosene per day. On short-haul services, an aircraft’s engines could run two hours less per day.
In autonomous electric taxiing, the DLR has a head start. In June last year, it teamed with Lufthansa Technik and Airbus to test an electric nosewheel on an A320. The aircraft (a research testbed owned by the DLR) taxied around Hamburg Finkenwerder Airport.
A fuel cell located in the aircraft’s wheel rims powered the two electric motors. One challenge was that “a very high torque had to be transmitted without making the wheel spin,” said project leader Josef Kallo. He seemed to concur with Safran and Honeywell on the drawback of a nosewheel installation. Overall, DLR prefers the fuel cell to an APU because of its higher efficiency when it comes to producing electricity.