Pilots made confused radio calls after seeing a Lufthansa Airbus A320 taxiing around Frankfurt Airport last December. Covers over the engine intakes showed that the powerplants were not running. The obvious question was: "Where is the tug?"
After years of studying how aircraft could move on the ground without using the constant thrust from idling engines, which forces pilots to tap regularly on the brakes to avoid gathering too much speed, several aerospace companies are working to bring electric taxiing systems to the market over the next four years.
Safran and Honeywell have been testing a taxiing system
Between 2% and 4% of fuel could typically be saved on short-haul flights, depending on the airline's specific operations such as sector length, number of daily flight cycles and taxi times, says Olivier Savin, programme manager at Safran and set to be executive vice-president for the future green taxi joint venture with Honeywell.
If the electricity were generated through fuel cells instead of conventional auxiliary power units (APU), kerosene consumption at Frankfurt Airport alone could be slashed by around 44t a day through the employment of electric taxi systems on narrowbodies such as the A320 or Boeing 737, according to Thorsten Mulhouse from the Institute of Flight Guidance at German research centre DLR.
Short-haul aircraft would generate the greatest savings, given their high number of flight cycles and consequently large proportion of taxi movement during their operating hours, especially at busy airports where aircraft queue for take-off or wait on the apron until their park position is free. But savings will not be limited to fuel consumption. Reduced engine running times will also have associated reductions in maintenance costs. There would be less engine damage from foreign object debris (FOD) ingestion on the ground and aircraft could push back on their own power without using a tug, generating time savings and efficiency gains. But perhaps most importantly, noise would be significantly reduced if only the APU were necessary to taxi to and from the runway.
Gibraltar-registered engineering company Borealis Exploration has been working on its WheelTug electric drive system for nose landing gear (NLG) since 2005. Currently under development for the 737, it is scheduled to enter service with Israeli launch customer El Al in 2013. But chief executive Isaiah Cox says that the technology could also be adapted to other types in the future.
WheelTug revolves around an induction motor generation, which Borealis re-engineered from conventional technology in the mid-1990s. These motors will be integrated into both NLG wheels with a total equipment weight of 300lb (136kg), including cockpit interface and system-control unit. The concept was initially trialled with an Air Canada Boeing 767 in Marana, Arizona, in 2005, with the nose wheels driven by externally installed motors. This allowed the widebody to reverse on its own and reach forward speeds "exceeding 10mph [8.7kt]" , according to Borealis.
The absence of brakes and the generally less complex landing-gear construction makes the installation of a nose-wheel drive system much simpler than for the main wheels. The concern is, however, that the low weight on the nose wheels will not provide sufficient traction in adverse conditions. As the main landing gear (MLG) is located close to an aircraft's centre of gravity, the nose wheels carry only a fraction of its weight.
DLR conducted taxi trials with an A320 modified with electrically driven nose wheels, powered by an onboard fuel cell, at the airframer's plant in Hamburg, Germany, last June. Both wheels were equipped with internally installed motors. While the twinjet has a maximum take-off weight of 73.5t, the nose wheels typically carry only 5-7t, according to project leader Josef Kallo of DLR's Institute of Technical Thermodynamics in Stuttgart.
Transmitting torque to start the 47t test aircraft moving without generating wheel spin was a challenge. While the wheels could spin at a torque of around 6,000 Newton metres (Nm), the A320 required a breakaway torque of 3,500Nm on a level surface in dry conditions. This could go up to 5,800Nm, however, if the nose wheel was standing in a dip, according to Kallo. He says the nose-wheel drive mechanism would not work on icy surfaces.
Prior to that, however, WheelTug conducted another test to prove its concept with a 737 belonging to Czech charter airline Travel Service in snowy conditions at Prague Airport in December 2010. Cox says the demonstrator system was able to move the aircraft on wet, snow-covered and even icy surfaces, while the twinjet's centre of gravity was within flight limits.
US technology company L-3 Communications, Lufthansa and Airbus were behind the trials in Frankfurt, for which they had installed an electric MLG wheel-drive system on one of the airline's A320s as a demonstrator to gather operational data and explore the aircraft's ground handling characteristics.
Using largely existing components for ground vehicles, the engineers replaced the brake assemblies of the inboard MLG wheels with drive units, each containing a permanent magnet motor and planetary gearbox.
The team tested the system over 14h, covering 40 test points to gather data such as breakaway momentum, acceleration, energy consumption, heat development and tyre deformation in different conditions. The engineers varied the aircraft's weight between 46t and 60t, and the tyre pressure. The twinjet operated in dry and wet surface conditions, in winds with gusts up to 70kt, and taxied up a 3% slope. The pilots even reversed the aircraft against the forward thrust from the idling engines.
All manoeuvres were possible using just one of the two wheel-drive units. But Joe Hoffman, executive programme manager at L-3, admits that the demonstration motors were over-designed for the test and that a final system would not need to be as powerful.
Perhaps most revealing was that the pilots found the aircraft much more responsive and easier to handle than under main engine operation, says Christian Mutz, project manager innovation at Lufthansa Technik (LHT). While turbofans offer more than enough power, they respond with a certain delay due to the spool-up. Mutz says the pilots adapted to the system with surprising ease and tried manoeuvres such as reversing into a park position and 360-degree reverse turns unexpectedly soon.
Measuring breakaway and acceleration values is central to defining the power requirements because these will, in turn, determine the weight and size of a future wheel-drive system. Airbus specifies that the aircraft needs to be able to accelerate to 20kt within 90s, Savin says.
Safran and Honeywell have been gathering the specification data with a conventionally-powered A320 since November. They plan to test a prototype of their electric wheel-drive system next year, with entry into service in 2016.
Each wheel actuator, which includes the motor, gearbox and clutch to disengage the system for take-off and landing, should weigh no more than 100kg, according to Savin. After considering whether to drive one or both MLG wheels and to employ one or two motors, Safran/Honeywell decided on one motor, to drive one wheel per MLG. However, the manufacturers will have the final say, adds Savin.
L-3 and potential system partner LHT, meanwhile, are conducting economic benefits analyses to decide whether to go ahead with the development of an electric wheel-drive system - to be determined by the third quarter, says Mutz.
What is clear for all electric-taxi developers, however, is that their systems can be installed as both retrofit and new-production equipment with only minor changes to the existing aircraft architecture. The added technology must not compromise the aircraft's dispatch reliability and maintenance intervals, but has to create a net cost saving for the operator.