Calling for a taxi has taken on a whole new meaning at Frankfurt Airport, where Lufthansa Boeing 737s are quietly taxiing towards the runways with their engines shut-down.

According to Lufthansa LEOS, which is operating a fleet of hybrid-electric TaxiBot tractors at the airport, these “green taxis” are already saving fuel, reducing CO2 emissions, and accelerating flight operations at this busy hub.

Conventional tractors are currently used only for push-back, then they leave aircraft to taxi to the runway under their own power. TaxiBot, developed by Israel Aerospace Industries (IAI), is a towbar-less 800 hp. hybrid-electric tractor, controlled by the pilot and intended for towing aircraft between departure gate and runway. It is fitted with a patented nosewheel cradle that registers all the steering movements and transfers them into electronic commands that steer its wheels. The pilot steers after pushing back from the gate, then releases it at the runway. The aircraft’s engines are not started until the TaxiBot has separated from the aircraft.

At Frankfurt TaxiBots are moving the aircraft on the taxiway at a speed of 24 kt. “(They) reduce fuel consumption by 85%,” said Peter Unger, managing director of Lufthansa LEOS, who estimates that the airline’s Frankfurt hub can save up to 2,700 tons of fuel on long-haul flights per year. With engines shut down, the risk of foreign object damage is also reduced, as half of such incidents occur while taxying.

The three TaxiBots participating in the evaluations are towing Boeing 737s on regular flight operations at Frankfurt. LEOS expects that about a third of its 35-strong tractor fleet at the airport will convert to TaxiBots, and handle 70% of the Lufthansa’s narrowbody flights there. “We are not only helping to conserve fuel, but are also making an important contribution towards reducing noise and exhaust emissions,” said Kay Kratky, member of the Lufthansa German Airlines Board, Operations & Hub, at Frankfurt.

Recent operations have validated that each taxi-out reduces CO2 emissions by up to 158-315 kg., depending on operational profile. With engines shut down during taxi, noise levels are reduced by 50%, LEOS says.

The current eight-wheel vehicle is designed to support narrowbody aircraft, and is currently certified to tow the Boeing 737, with 757 and Airbus 319/320/321 approvals to follow. The company is also developing a larger 12-wheel tractor that will support Boeing 747s and Airbus A380s. Testing on a Lufthansa 747 should be completed by year’s end, to be followed by certification in early 2016.