Electric Taxiing May Provide Signifficant Aircraft Fuel Savings

March 27th, 2014
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While looking at some aircraft videos on Youtube I came across a new technology which will soon be available in both new aircraft and for retrofit applications on existing airliners. The technology in question is electric wheel motors for taxiing.

While it might not seem like a big fuel user, aircraft burn significant fuel while taxing around airports after landing and before takeoff. This is because jet engines happen to be especially inefficient when idled and operating at low power settings. Therefore, once the aircraft starts up its engines and uses them to slowly move down the taxiway, it is burning significant quantities of fuel. For short-run flights, with frequent landings and takeoffs, up to 5-10% of fuel can be burned on the ground.

Airbus, Honeywell and others have been developing a system which replaces this with an electric-driven system. It uses light weight electric motors connected directly to the wheels of the aircraft. Power is provided by the aircraft’s APU. The APU uses significantly less fuel than the main engines of the aircraft.


The system has some other major benefits. Control is far more nimble and precise than is achieved using jet thrust, and less time running the jet engines on the ground means less chance for FOD to be sucked into the intakes and damage the engines. The systems also allow the aircraft to reverse on their own. Normally this would require the assistance of a pushback tractor. As a result, the aircraft can leave the gate without needing to wait for a tractor could simplify and expedite the procedures for entering and departing gates.

This technology has not been deployed earlier for a number of reasons. First, aviation tends to be very conservative about adopting new and unproven systems, especially when existing ones get the job done. Secondly, weight is always an issue, so it has required manufacturers to develop a full system of electric taxiing that is light enough that its additional weight is more than offset by the savings provided.

There have also been efforts to save additional fuel by keeping the APU off for longer periods of time or using a smaller APU through the use of more on board battery capacity. Unfortunately this has been problematic both because of the weight of batteries and because of the problems experienced with high capacity and lightweight lithium ion batteries on aircraft such as the 787.


This entry was posted on Thursday, March 27th, 2014 at 1:26 pm and is filed under Good Science, Misc. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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16 Responses to “Electric Taxiing May Provide Signifficant Aircraft Fuel Savings”

  1. 1
    DV82XL Says:

    This idea has come up on several occasions but didn’t generate much interest back then, I hope they have better luck this time. Another powered wheel idea that has been batted around is a system for spinning up the tires prior to landing in an effort to reduce wear. That puff of smoke that one often sees from the tires at touchdown is the major source of wear on these units, which need retreading surprisingly often.


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  2. 2
    Anon Says:

            DV82XL said:

    This idea has come up on several occasions but didn’t generate much interest back then, I hope they have better luck this time. Another powered wheel idea that has been batted around is a system for spinning up the tires prior to landing in an effort to reduce wear. That puff of smoke that one often sees from the tires at touchdown is the major source of wear on these units, which need retreading surprisingly often.

    I heard that a variant of an old Soviet bomber used to do that and that apparently it also made the landings a lot smoother.


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  3. 3
    DV82XL Says:

            Anon said:

    I heard that a variant of an old Soviet bomber used to do that and that apparently it also made the landings a lot smoother.

    There have been a few attempts. The model 500 Cessna Citation has a nose wheel spin-up scheme as part of a gravel runway package that helps reduce FOD, and that I believe is the reason the Soviet bomber used this system, but in general the extra weight/complexity/cost isn’t worth it.

    I brought it up only because it is a powered wheel idea that has repeatedly come up for decades, as has the idea of using them for taxi, and frankly I’m a bit surprised that the numbers are actually working out. It’s kind of funny because these were among the classic newbie ideas that just about every maintenance apprentice used to come up with within the first three months of starting in the industry to the amusement of the grizzled old hands, who would then either gently correct the kid or humiliate him depending on how much arrogance needed to be rubbed off. There is going to be a few ‘I told you so’ handed out over this, I would guess.


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  4. 4
    Anon Says:

            DV82XL said:

    There have been a few attempts. The model 500 Cessna Citation has a nose wheel spin-up scheme as part of a gravel runway package that helps reduce FOD, and that I believe is the reason the Soviet bomber used this system, but in general the extra weight/complexity/cost isn’t worth it.

    IIRC the Soviet bomber did it with the main wheels to reduce wear without anything to do with FOD.

            DV82XL said:

    I brought it up only because it is a powered wheel idea that has repeatedly come up for decades, as has the idea of using them for taxi, and frankly I’m a bit surprised that the numbers are actually working out. It’s kind of funny because these were among the classic newbie ideas that just about every maintenance apprentice used to come up with within the first three months of starting in the industry to the amusement of the grizzled old hands, who would then either gently correct the kid or humiliate him depending on how much arrogance needed to be rubbed off. There is going to be a few ‘I told you so’ handed out over this, I would guess.

    It also occurs to me that you could run the motor as a generator on landing though I doubt the aircraft batteries could absorb very much from it, maybe a resistor bank would be better but that’d be extra hardware so wouldn’t be free like pre-landing spin-up if you get electric main taxiing.


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  5. 5
    DV82XL Says:

            Anon said:

    IIRC the Soviet bomber did it with the main wheels to reduce wear without anything to do with FOD.

    Perhaps, but the FOD kicked up by spinup on gravel runways is a known issue and the Soviets were big on designing in rough-field capabilities on their warbirds, so I suspect that was in the mix of reasons.

            Anon said:

    It also occurs to me that you could run the motor as a generator on landing though I doubt the aircraft batteries could absorb very much from it, maybe a resistor bank would be better but that’d be extra hardware so wouldn’t be free like pre-landing spin-up if you get electric main taxiing.

    Most of these past schemes envisioned the wheels spun up by some apparatus driven by the slipstream, either directly or via hydraulic transmission. Its only been recently that we have had electric motors that have power-to-weight ratios that would make them candidates for this sort of service. Therein lies the major rub with this idea: for this to work well you really need to have it happen as close to touchdown as possible because there are real issues with control from having these large spinning masses coupled to the airframe and the distortion of airflow especially at landing speeds. The only practical solution is electric motors under computer control for most aircraft. Big military craft that are meant for rough field use likely would have been designed with strong enough landing gear to deal with these issues, but it wouldn’t be something that would be worth it on commercial aircraft, but maybe with computer controlled electric systems it could be practical there too.


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  6. 6
    Anon Says:

            DV82XL said:

    Perhaps, but the FOD kicked up by spinup on gravel runways is a known issue and the Soviets were big on designing in rough-field capabilities on their warbirds, so I suspect that was in the mix of reasons.

    Maybe it was, if only I knew which aircraft it was.

            DV82XL said:

    Most of these past schemes envisioned the wheels spun up by some apparatus driven by the slipstream, either directly or via hydraulic transmission. Its only been recently that we have had electric motors that have power-to-weight ratios that would make them candidates for this sort of service. Therein lies the major rub with this idea: for this to work well you really need to have it happen as close to touchdown as possible because there are real issues with control from having these large spinning masses coupled to the airframe and the distortion of airflow especially at landing speeds.

    Aren’t the wheel still spinning after take off until just before the landing gear gets retracted? That doesn’t appear to cause any problems.

            DV82XL said:

    The only practical solution is electric motors under computer control for most aircraft.

    The kinds of aircraft this would be used on would have radar altimeters and we could expect them to know their true ground speed.


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  7. 7
    DV82XL Says:

            Anon said:

    Aren’t the wheel still spinning after take off until just before the landing gear gets retracted? That doesn’t appear to cause any problems.

    No. On most airliners the main wheels with brakes are activated as soon as the gear starts to retract by the autoretract braking system.Those that have nose wheels with no brakes use snubber pads that rub on to the tire to slow it down as its retracted. There are several reasons for this. The big one is to eliminate gyroscopic precession as it is much easier to raise stopped gear than to fight a bunch of gyroscopes sitting at the end of sticks. The other is the potential for damage from spinning wheels inside the bay.

            Anon said:

    The kinds of aircraft this would be used on would have radar altimeters and we could expect them to know their true ground speed.

    Ya now it could be done relatively easily and with little equipment overhead, but like I said this idea have been around forever and this wasn’t always the case.


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  8. 8
    Robert Sneddon Says:

    It’s a neat idea but I noticed the Youtube footage from Lufthansa dates back to 2011 which indicates the concept hasn’t taken the airline industry by storm. I can see several reasons for this.

    It adds complexity to a safety-critical component of the plane, the landing gear. There has to be some sort of coupling to drive the wheels, a gearbox, a motor, shafts and bearings and if they go wrong, jam, bend or break then they can adversely affect the landing capabilities of the plane. A lot of development time and effort has gone into making the landing gear on modern jets as safe as possible in part by making it simple so less things can go wrong.

    It adds weight which must be transported from takeoff to landing. The major plane makers have been making extreme efforts to reduce in-flight weight of the aircraft they build as lower weights mean a lower fuel burn hence lower operating costs. Carbon fibre hull and wing components, lighter seats, anything that shaves weight and doesn’t compromise safety is fair game. This motor drive system is only a few hundred kilos per plane, perhaps but over a modern jet’s lifespan covering tens of millions of kilometres of travel that few hundred kilos really adds up.

    It adds cost to the plane’s build cost and is yet another safety-critical part of the plane that has to be maintained, repaired, inspected, parts replaced etc. which is an ongoing cost too.

    There’s another safety factor — I was on a flight a few months back where the plane was pushed back from the pier by the tractor and we set off under engine power as usual, only for the pilot to stop the plane after we’d gotten out onto the taxiway. He reported to us over the PA that one of the engines was reporting a fault (unspecified) and after a tug came and got us we returned to the gate to wait for an engineer to come and fix whatever the engine problem was. Taxiing to the runway from the gate gives the engines a few minutes to reveal problems after starting from cold and I’d rather they broke during taxi than at full takeoff power and V2.


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  9. 9
    DV82XL Says:

    Most of the remarks you made have some truth to them, although one would assume that the cost-benefit issue would have been looked at by Airbus and Honeywell neither of whom are lightweights in this industry, and the numbers would have had to be at least in the credible zone for them to have push it forward at all. However I must correct you on this:

            Robert Sneddon said:

    A lot of development time and effort has gone into making the landing gear on modern jets as safe as possible in part by making it simple so less things can go wrong.

    The landing gear on big aircraft are far from simple, they have several very large components that give one the illusion that there are not many in a gear, but if you look at one up close there are plenty and a surprising number of them are inside the main cylinder where you can’t see them. As well most of the locking and swing mechanism is inside the well and not in easy view.


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  10. 10
    Robert Sneddon Says:

    I’m more concerned with the extra shafts, clutches, gearboxes etc. that would connect a drive motor to the wheels than the rest of the existing mechanical spaghetti that moves the landing gear into position. At the moment aircraft wheels are located on simple shafts coupled to disc brakes so on landing the wheels spin freely at touchdown and are only later braked (I believe using ABS these days, a well-tested addition and pretty much failsafe). If a drive gearbox or clutch failed in some way and the wheel seized on touchdown then the tyres would blow out very quickly and directional control of the plane on the runway would be severely compromised.

    What might work as a substitute, although the logistics and costs are beyond my reckoning, would be for more capable plane tugs to be used to tow the plane from the jetway along the taxiway to the runway end before disconnecting and returning to the terminal area to collect another “customer”. Shuffling more and larger tugs around a busy airport would be problematic though and add cost to operations but it could be tested without modifying the planes themselves and introduced on a sliding scale for only some planes like commuters to start with.

    There are legal aspects to consider for this system — a pilot is supposedly Master and Commander of their aircraft and relegating the control of where it goes, even on a taxiway, to a tug driver is fraught with problems. Of course the masters of naval ships use tugs for manoeuvering and take on pilots for harbour and narrows transits but the responsibility for the ship’s navigation is always theirs in the end.


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  11. 11
    DV82XL Says:

            Robert Sneddon said:

    I’m more concerned with the extra shafts, clutches, gearboxes etc. that would connect a drive motor to the wheels than the rest of the existing mechanical spaghetti that moves the landing gear into position. At the moment aircraft wheels are located on simple shafts coupled to disc brakes so on landing the wheels spin freely at touchdown and are only later braked (I believe using ABS these days, a well-tested addition and pretty much failsafe). If a drive gearbox or clutch failed in some way and the wheel seized on touchdown then the tyres would blow out very quickly and directional control of the plane on the runway would be severely compromised.

    There are rather standard freewheeling clutch designs that have been around in one form or another since the mid 1800s, one would assume that something of this nature would be included in this system. Again you have to look at the players here and know that they wouldn’t even consider such a scheme if the standard caveats were not met.

            Robert Sneddon said:

    What might work as a substitute, although the logistics and costs are beyond my reckoning, would be for more capable plane tugs to be used to tow the plane from the jetway along the taxiway to the runway end before disconnecting and returning to the terminal area to collect another “customer”.

    Some airports did use this system for many years, mostly prior to upgrading facilities to deal with larger gage aircraft at the start of the jet-age, and a few in Eastern Europe did it for quite some time. It’s not untried, mostly it’s considered inconvenient now.

            Robert Sneddon said:

    There are legal aspects to consider for this system — a pilot is supposedly Master and Commander of their aircraft and relegating the control of where it goes, even on a taxiway, to a tug driver is fraught with problems. Of course the masters of naval ships use tugs for maneuvering and take on pilots for harbour and narrows transits but the responsibility for the ship’s navigation is always theirs in the end.

    Not really. On the ground an aircraft is just another wheeled vehicle in the eyes of the law and responsibility for it under tow has been long understood to be the tug operator’s. Even if the pilots still are needed to apply the brakes, they take their orders from the guy driving on the ground.


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  12. 12
    Anon Says:

            Robert Sneddon said:

    I’m more concerned with the extra shafts, clutches, gearboxes etc. that would connect a drive motor to the wheels than the rest of the existing mechanical spaghetti that moves the landing gear into position.

    Hub motors don’t need gearboxes and if they are brushless (which they would be) shouldn’t be need a clutch either.

            Robert Sneddon said:

    What might work as a substitute, although the logistics and costs are beyond my reckoning, would be for more capable plane tugs to be used to tow the plane from the jetway along the taxiway to the runway end before disconnecting and returning to the terminal area to collect another “customer”. Shuffling more and larger tugs around a busy airport would be problematic though and add cost to operations but it could be tested without modifying the planes themselves and introduced on a sliding scale for only some planes like commuters to start with.

    Right now someone has to drive the tug so that’ll probably cost more than electric taxiing (you’ll probably need more tugs if the aircraft are sent all over the airport instead of just pushed back a bit).

            Robert Sneddon said:

    There are legal aspects to consider for this system — a pilot is supposedly Master and Commander of their aircraft and relegating the control of where it goes, even on a taxiway, to a tug driver is fraught with problems. Of course the masters of naval ships use tugs for manoeuvering and take on pilots for harbour and narrows transits but the responsibility for the ship’s navigation is always theirs in the end.

    Given that tugs are already used for pushing back aircraft and occasionally towing planes all around (if they can’t move by themselves) I’d call that a solved problem.


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  13. 13
    Robert Sneddon Says:

    It’s unlikely to impossible the motors used to taxi a plane like a 737 weighing fifty or sixty tonnes ready-to-fly will be direct-drive hub motors given the torque required to get something like that moving from a standing start, never mind push it at 20 or 30 km/h along a taxiway. I am presuming that for weight and cost reasons as well as safety and reliability only the font wheels are driven in such an installation. One further problem is that on landing the wheels connected to the drive system will be turning at a runway speed of 250km/h at touchdown so any gearbox or clutch has to be able to freewheel at those speeds with 100% or near-as-damn-it reliability. The engineering is not trivial.


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  14. 14
    DV82XL Says:

            Robert Sneddon said:

    It’s unlikely to impossible the motors used to taxi a plane like a 737 weighing fifty or sixty tonnes ready-to-fly will be direct-drive hub motors given the torque required to get something like that moving from a standing start, never mind push it at 20 or 30 km/h along a taxiway.

    By the looks of the outboard housing of the installation in the video, they are using a gear train. One of the standard ways to failsafe freewheel systems is via sheerpins – if for some reason the clutching doesn’t disengage, a connecting pin breaks. Again it’s a rather standard solution that is already used at several points in modern aircraft.


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  15. 15
    drbuzz0 Says:

            DV82XL said:

    This idea has come up on several occasions but didn’t generate much interest back then, I hope they have better luck this time. Another powered wheel idea that has been batted around is a system for spinning up the tires prior to landing in an effort to reduce wear. That puff of smoke that one often sees from the tires at touchdown is the major source of wear on these units, which need retreading surprisingly often.

    I think there is a better chance of it taking this time, because of the fact that airlines seem to be willing to do a lot to save fuel. I think that presently the thinking is that the cost of fuel is high and is likely to stay very high and anything to shave off fuel use, even by a relatively small amount is worthwhile.

    Also, it is clear that Honeywell, Airbus and Safran have invested quite a bit of serious effort and money in the R&D on this. These big well established companies are not the kind who will waste their time on something that they don’t expect to go anywhere. I’d imagine that this is the result of some amount of market research they have done.

    The fact that this is not adopted yet may be related to the fact that aviation tends to be very conservative and that there are so many safety standards to consider. However, the systems that they have been developing in the past few years do seem to meet very good standards. These are not amateurish attempts.

            Robert Sneddon said:

    I’m more concerned with the extra shafts, clutches, gearboxes etc. that would connect a drive motor to the wheels than the rest of the existing mechanical spaghetti that moves the landing gear into position. At the moment aircraft wheels are located on simple shafts coupled to disc brakes so on landing the wheels spin freely at touchdown and are only later braked (I believe using ABS these days, a well-tested addition and pretty much failsafe). If a drive gearbox or clutch failed in some way and the wheel seized on touchdown then the tyres would blow out very quickly and directional control of the plane on the runway would be severely compromised.

    It should be noted that capable landing gear is not a 100% necessary requirement to land an aircraft in a manner that will result in everyone walking away without injury. Tires do blow out from time to time. Even if the wheels are completely seized, the situation is not going to be fatal. Hell, even gear-up landings usually don’t result in hull loss.

    I suppose it’s possible the aircraft could slide off the runway, but even that is unlikely. More likely it will stop more abruptly than usual, but in a very jerking, uncomfortable way and, of course, there would be damage to the undercarriage. It would not be a pleasant landing, but it would be a completely survivable one.

            Robert Sneddon said:

    What might work as a substitute, although the logistics and costs are beyond my reckoning, would be for more capable plane tugs to be used to tow the plane from the jetway along the taxiway to the runway end before disconnecting and returning to the terminal area to collect another “customer”. Shuffling more and larger tugs around a busy airport would be problematic though and add cost to operations but it could be tested without modifying the planes themselves and introduced on a sliding scale for only some planes like commuters to start with.

    As a fuel saving measure, this has actually been put into practice. I know at least one airline has tried using more tugs to bring the aircraft out, I think it might be Virgin.

    There are a lot of issues with this, however.

    The tugs can get in the way of other aircraft as they are shuttling to and from the aircraft on the taxiways and they have to be very careful, because taxiing aircraft are not very nimble. These are dangerous areas in general, with big jet engines running etc. So it creates a lot of issues having tugs shuffling around.

    Also, it is a problem on landing to get the tug out to the aircraft, because it has to get out of the way and start rolling right away. Not to mention the fact that passengers want to get to the gate right away.


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  16. 16
    DV82XL Says:

            drbuzz0 said:

    Also, it is a problem on landing to get the tug out to the aircraft, because it has to get out of the way and start rolling right away. Not to mention the fact that passengers want to get to the gate right away.

    Rotting on the apron after arrival, waiting for the tug was a legendary feature at one US Eastern Seaboard airport in the early Seventies, but my memory has failed me as to which one it was.


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