Lufthansa is making final preparations for a six-month trial of the pilot-controlled Taxibot tow-tractor for scheduled flights at Frankfurt airport from August.
The semi-robotic aircraft tug, developed by Israel Aerospace Industries (IAI), will be used to move the German flag carrier's departing Boeing 737s from the ramp to the runway, to avoid using the main engines for regular taxiing. The first of three beta-version Taxibots was delivered to the airline's ground handling arm, Lufthansa Engineering and Operational Services (LEOS), in early June.
The objective of the trial is to evaluate the Taxibot's suitability for daily operations at a major airport and to determine actual fuel savings versus theoretical estimates, says Franz-Josef Kirschfink, technology projects director at Lufthansa Technik (LHT).
The airline wants to assess the pilots' handling of the hybrid-powered ground vehicle, different taxiing speeds, how the towed 737s fit into the regular aircraft taxiing traffic, and any potential disruptions. Particular attention will be paid to the attachment and disconnection procedures. Finding the optimal moment to detach the tug without holding back surrounding aircraft, and starting the main engines for the minimum required warm-up time before take-off, are among the most "exciting" questions of the trial, says Kirschfink.
He adds that the airline is not planning to start up the engines while the aircraft is being towed by the tugs.
Pilots will only use the Taxibots to manoeuvre their aircraft from the stand to the runway for take-off, while arriving 737s will conventionally taxi under own power to their parking positions.
Taxibot lifts up the nose landing gear like a conventional tow-tractor. The attachment and disconnection procedure is conducted by the tug driver, who remains in the vehicle throughout the entire operations. However, the aircraft's wheels stand on a rotating platform, which allows free nose-gear steering movement and translates the pilot's control inputs - via the nose-wheel tiller in the cockpit - into directional inputs for the tug.
Once the nose landing gear is secured on the Taxibot - and the driver hands over control to the pilot - the vehicle starts moving forward. The pilot can stop and control the speed with the aircraft's wheel brakes. This handling is in principle similar to that of a car with an automatic gearbox.
The nose-wheel platform also allows some lateral movement to absorb loads and avoid nose-gear damage. After Taxibot tests with an ex-British Airways Airbus A320 at France's Châteauroux airport in November 2012, Airbus senior vice-president for business development Frédéric Pochet said that the procedure creates no excessive loads and has "absolutely no impact" on nose-gear fatigue life.
In 2006, Virgin Atlantic trialled using conventional tractors to tow 747s to the runway for departure, to avoid using the main engines on the ground. But the tests were prematurely stopped due to excessive loads on the aircraft's nose landing gear structure.
Lufthansa selected its ageing 737-300/500s for the Taxibot trial, because the approximately 35-strong short-haul fleet has an appropriate size to employ the tractors for all respective flights in Frankfurt, says Kirschfink. If the vehicle was tested on a long-haul type, this would lead to operational peaks in the morning and evening hours, and thus not provide a balanced picture throughout the day, he adds.
The airline's A320 fleet, on the other hand, is too large for test vehicles to be assigned to all flights.
Taxibot is powered by electric motors in each wheel. These in turn are supplied with energy generated by a diesel engine. IAI is developing differently sized versions of the tractor for both narrow- and widebody aircraft up to the A380. Certification is targeted for attainment by year-end.
Lufthansa has implemented a wide-ranging "Score" cost-cutting programme aimed at delivering a €1.5 billion ($2 billion) improvement in the group's operating result by 2015, but Kirschfink says the cutbacks will not affect technology initiatives: "Especially in the fuel-saving area, we will conduct even more research than in the past."