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Pictures of the Future
The Magazine for Research and Innovation

Electric Mobility

A Tractor that Saves Tons of Aviation Fuel

TaxiBot in action at Frankfurt Airport. Driven by electric motors, the tractors are real powerhouses — the narrow-body model has around 500 kilowatts of drive output (approximately 800 hp).

Until now, airplanes have had to use their own turbines to travel from the gate to the runway. But thanks to drive technology from Siemens, an all-new diesel-electric towing tractor controlled from a plane’s cockpit can now perform this task. Known as TaxiBots, the tractors save fuel, extend maintenance intervals, and cut noise. Already certified for the Airbus 320, TaxiBots will soon be able to safely pull about 70 percent of all the passenger planes worldwide.

Airplanes currently travel from airport gates to runways under their own power. However, this is not very economical, because taxiing can consume up to one metric ton of aviation fuel, depending on an airplane’s size and the distance covered. A much more efficient solution would be to use a diesel-electric towing tractor attached to the nose wheel to pull the plane to the runway. As a result, the plane would not have to turn on its engines during the process.

With this in mind, Siemens, Israel Aerospace Industries, Lufthansa LEOS, and the French TLD Group, which is a market leader for airport handling equipment, have been working on an environmentally friendly taxiing solution since 2011 and have now developed such tractors. Siemens supplies the tractors with their powertrains, which consist of generators, electric motors, converters, electronics, and software.

Current TaxiBots tow narrow-body airliners such as the Airbus A320 and the Boeing 737.

Powerhouses with Great Savings Potential

These tractors, which are called taxiing robots or TaxiBots, are veritable powerhouses that apply about 500 kilowatts, or around 800 hp, of drive output to the tarmac. They have a torque of 45,000 Newton-meters, which corresponds to that of around 100 midsized cars. Last year the European Aviation Safety Agency (EASA) and the Federal Aviation Administration (FAA) in the U.S. certified the tractor for aircraft of the Boeing 737 family. After conducting extensive tests, Lufthansa is now one of the airlines that regularly use such tractors, for example at Frankfurt International Airport. According to the airline, the tractors enable it to save around 11,000 metric tons of fuel each year at Frankfurt Airport alone.

According to Lufthansa, the tractors enable it to save around 11,000 metric tons of fuel each year at Frankfurt Airport alone.

The tractor has now also been certified for the Airbus 320 family. What’s notable about this decision is that the TaxiBot will now be able to safely pull about 70 percent of all the passenger planes worldwide from the gate to the runway without consuming any aviation fuel. When the tractor is used with A320 planes, a small switch must be installed in the plane’s cockpit in order to activate the plane’s hydraulic system. This system is normally started by the plane’s engines, which must be off while the TaxiBot is in operation. Installing the switch is, however, a relatively simple matter that can be taken care of while a plane undergoes a routine inspection.

The TaxiBot’s four wheel pairs - for a Narrow-Bodyvehicle - or six wheel pairs - for a Wide-Bodyvehicle - is driven by electric motors.

Less Environmental Impact, Longer Maintenance Intervals

TaxiBots not only cut fuel consumption and emissions but also reduce strain on aircraft engines, thus extending the time between maintenance events. In addition, the tractors produce only one third as much noise as a taxiing airplane.

The team is currently working on a TaxiBot solution for wide-body aircraft such as the Airbus 380 and the Boeing 747.

For safety reasons, the TaxiBots are outfitted with redundant systems. “Two Scania diesel engines each drive a generator that produces electricity for eight electric drive motors,” explains Ulrich Sammet from Siemens Large Drives who, along with a team of experts, developed the tractor’s powerful diesel-electric drive train. “The Narrow-Body TaxiBot model for single-aisle aircraft has a total of four wheel modules, each of which contains two motors. If a fault occurs, a DC link box enables us to cut the intermediate circuit and shut off the defective side of the system. This redundancy ensures that the TaxiBot can continue to move and won’t block a taxiway even if there’s a fault.”

Every individual wheel of the Wide-Body model has its own drive motor.

Next: A Tractor for Wide-Body Aircraft

The team is currently working on a TaxiBot solution for wide-body aircraft such as the Airbus 380 and the Boeing 747. This model has six instead of four pairs of wheels and an output of 1 megawatt. It has more than 1,350 hp and a higher torque than over 300 midsized cars. One of these TaxiBots can save up to one metric ton of fuel when it tows a wide-body jet, which can weigh up to 600 metric tons. Unlike the Narrow-Body model, which is smaller, the Wide-Body TaxiBot has a drive system that can be separated into three parts in the event of a fault. “This ensures that two thirds of the drive system will work. In case a fault occurs, such as a short circuit, electronic systems developed by Siemens automatically switch off one third of the drive system,” says Sammet. The system is based on the use of permanent-magnet electric motors, which operate even more efficiently than conventional asynchronous machines. What’s more, the motors are completely integrated into the wheel module housings. Another new feature is that the TaxiBot can control each wheel’s electric motor separately. This is an advantage when the vehicle turns in place or travels slowly, for example, because a great deal of force is required to turn the wheels under an airliner’s massive load. To achieve this, the Wide-Body TaxiBot can apply different amounts of force to a pair of wheels or drive them in opposite directions.

An important consideration during the development of the Narrow Body and Wide Body TaxiBots was that the pilot must have sole control over the airplane – and thus over the TaxiBot – as required by law. This is necessary, because any careless movement by the tractor could generate considerable stress on the nose wheel, for example. Switching to the new taxiing method does not seem to be difficult. The pilots who took part in the test confirmed that the TaxiBot is as good as the airplane’s own engines at controlling taxiing maneuvers. But that’s not all. Thanks to the TaxiBot, pilots have a much better grip and more control over the plane when the ground is wet or covered with snow.

Christine Rüth / Sebastian Webel
Picture credits: Lufthansa LEOS