Back on June 30, 2011, DLR of Germany demonstrated electric taxiing using a novel fuel cell-powered electric landing gear (press release here and video link here). The premise of electric taxiing is to postpone engine start and using it as the source for propulsion and electricity while the airplane is still on the ground, turning the engine on once the aircraft is ready to take off. By doing so, this would help reduce fuel consumption and wear on the engine while the aircraft is sitting in queue waiting to takeoff, and now days the queue seems to get longer and longer.
This concept sounds great at first glance, but let’s dig a bit deeper. Fuel burn savings during taxi are really dependent on the proportion of time spent on ground taxi relative to the entire mission. For example, a 737 flight involves around 20% of its mission time on ground taxiing, while a 777 flight involves only 6%. Therefore, electric taxiing creates more value for a 737 versus 777 because the fuel consumption reduction due to electric taxi is greater on a 737 (~16%) than a 777 (~5%). The savings is not 20% for 737 because you still have to burn some fuel to power the electric motors, either through an APU or in the future, a fuel cell.
While ~16% savings is pretty significant, there are tradeoffs to consider. To start, one will have to offset the gain with the added complexity and weight of adding motors and related controls robust enough to handle the rigorous landing environment. Also, aircraft engines need to warm up prior to takeoff, depending on ambient temperature and whether it is the first flight of the day, so the real savings might be less than the 16%.
Are there other ways to achieve the same results? What about using ground tugs to tow aircraft to the runway? What do you think?