subscribe to Jimmy Yeh's feedBack 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?
Tugs would be a no brainier, especially compared to the added complexity and cost of another aircraft system. The spectator group (aka the FAA) would need to create regulations instead of continuing to under serve the industry. Also some of he potential fuel savings would be lost in flying the system around with the plane.
Hey Steve, I find your statement a little interesting above “……fuel burn savings during taxi are really dependent on the proportion of time spent on ground taxi relative to the entire mission” – I am not sure that I agree with how you have isolated this potential efficiency gain.
The FAA and EUROCONTROL (and indeed CANSO) have an agreed method that looks at all inefficiency from a phase of flight approach and doesn’t look at it relevant to the overall trajectory. If this was the case, you could very well argue that there is very little need to finish an approach with an RNP from A030 after a 10 hour long-haul flight, as it would not even save 1% of the total fuel burn – but you and I both know we don’t focus on the savings as a proportion of the flight. Otherwise the Cathy Pacific A330 that fly’s the visual approach into Brisbane every day would have very little incentive to save 1/4NM and fly the RNP transition!!!
In fact the analysis on the combined taxi pool for US/Europe (in excess of what is required to taxi unimpeded) is 10.2 min extra per flight. This actionable pool is so significant it should keep us up at night looking at ways to reduce the excess (A/CDM, traffic synchronisation etc) before we ever get concerned about saving a standard taxi with an electric motor!!!
Tim
Tim, I agree that there is a fundamental issue with the surface movement.I didn’t mention that another way to solve the fuel consumption issue on the ground is by optimizing aircraft movement on the ground, thereby reducing taxi time.
Alos, there is a recent article about SAFRAN and Honeywell’s effort on Electric Green Taxiing System. They are advertising 4% fuel burn benefit.(http://www.aero-news.net/subscribe.cfm?do=main.textpost&id=d13f5465-4b46-4d1a-9cc0-bf3248a87a0b)