E-Taxi Systems – The Hidden Technical Challenges

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EGTS is the acronym for the e-taxi solution from Honeywell and SAFRAN, which was demonstrated this week at the Paris Air Show. A description of their product can be found here.  The photo below shows how this solution looks on the main wheels of an A320.

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This system is in competition with another e-taxi solution, WheelTug which relies on power to the nose wheel. (Full disclosure, WheelTug is a former AirInsight client)

The difference in architectures between the two systems resulted from different engineering solutions to the complex tradeoffs involved in developing an e-taxi system.  The difference is pulling using the nose wheel versus pushing using the main gear, and results in differences of speed, efficiency, turning radius, and other factors.

EGTS team are industry Goliaths compared to WheelTug – however WheelTug, which demonstrated its competing nose-wheel system more than a year earlier, already has commitments from 11 airlines for nearly 600 aircraft for its nose-gear system, and several additional customers pending announcements.  EGTS now has an agreement with Air France for tests.

Two of the major trade-offs in powering the main wheel of an airplane haven’t been publicly discussed by the EGTS team – additional heat generation, and the time required for heat dissipation.

Brakes generate massive amounts of heat. (On the A320 brake temperatures greater than 300 degrees Celsius trigger a hot brake ECAM [Electronic Centralized Aircraft Monitor] warning). Brakes must cool below a certain temperature before the airplane can begin a takeoff roll, because the aircraft would not be able to stop should a takeoff be aborted for a safely issues.  Today, carbon brakes generate significant heat that must be dissipated to ensure that in a Rejected Takeoff (RTO) that the plane can be stopped, and that hydraulic fluid fires won’t occur.

  • Airbus offers as a standard part of the package hub fans to cool the brakes. Without the fans, airlines might delay a takeoff when the brakes are still too hot. The existence of fans proves that these wheels get very hot.
  • Cool air gets sucked in through the gaps between the brake disks and blown out through the protective screen on the hub. There are holes in the flange of the wheel to permit the air pass through. (A good discussion on brake fans can be found here)
  • Now what happens if you put hardware, sealed with its own oil, which blocks the path of the cooling air? Look at the EGTS hardware; it covers the axle-side of the wheel.
  • In-flight heat is another concern.  Some  aircraft brakes do not cool down all the way in flight.6-24-2013 9-03-16 AM
  • The EGTS will is a heat source in its own right.  EGTS uses 50kw electric motors which will generate heat as the airplane taxis. All that heat is located inside the wheel. Brake hydraulic fluid has to stay below its flash point or there is a fire risk. Awareness of this issue is demonstrated on the prototype EGTS in which the hydraulic lines run on the outside.
  • The question is will this solution provide adequate brake cooling, or exacerbate the problem by blocking airflow and introducing the heat from electric motors adjacent to the brakes?


  • For airlines, time is money, and shorter turn times generate improved profitability.  If EGTS adds cooling time to a turn, it may give back the savings it achieves in lower fuel burn through slower turns.
  • The EGTS may not, itself, generate that much heat.  But any additional heat introduced to the wheel and brake area, without additional cooling, could be problematic.
  • Airlines want to get rid of wheel heat as fast as they can.  Why? Because they want to save time.
  • Airlines sell schedule if nothing else. That has resulted in Airbus making its aircraft more efficient by adding hub fans for faster cooling of brakes.  EGTS located in the airflow must lower the effectiveness of these fans.
  • What is the cost for an airline?  It is a rule of thumb that an “airline minute” is worth between $100-150.
  • Brake cooling could be a limiting factor for EGTS-enabled aircraft, as they could potentially spend more time waiting for brakes to cool than those without the EGTS system. Therefore aircraft with this system cannot be turned faster than an aircraft without the system.
  • The question is whether the e-taxi system will result in a rise again in temperatures as the aircraft taxis out for its next flight.  As we haven’t seen the final details, it is difficult to tell.  But it is certain that adding electric motors adjacent to brakes both generates more heat in a confined space, and disrupts the path for cooling airflow.

The Configuration Advantage
Clearly putting e-taxi on the main wheels impacts flight operations in a way that using the nose wheel does not because nose wheels have no brakes.  Unless the EGTS’ heat is dissipated very quickly an airline could see its flight turn times impacted.  How much does EGTS claim to save an airline per turn?  The chart says about ninety seconds.  In airline minutes that means about $300.

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Given that airlines are risk averse and the time saved is rather low compared to the possibility of waiting for brakes to cool (especially in hot weather) it would appear the EGTS solution savings are not that compelling. The savings return and the flight operations delay risk, in our view, do not have enough delta to assure a highly profitable installation. Imagine selling this solution to an airline and advising the airline to slow its operations for minimum cooling?  Particularly an LCC with twenty minute turn times.

Conversely, the nose-wheel solution from WheelTug adds no heat to braking systems, and enables the current cooling airflow to operate normally.  As a result, there will be no adverse impact on brake cooling times.

While many of the trade-offs between the differing e-taxi configurations are quite clear, such as push versus pull, total weight added and implications for weight and balance, others, such as brake cooling, are not quite as obvious, but could have significant ramifications on operational effectiveness.  We’re not certain that EGTS, while acceptable for a ground demonstration at the airshow, is yet ready for prime time in a fast-turn LCC environment.

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15 comments on “E-Taxi Systems – The Hidden Technical Challenges
  1. On the EGTS side of the “Configuration Advantage” chart, it appears they forgot to start engines…

  2. Lurker says:

    I wonder if EGTS could use electrical motor braking instead friction braking? If electrical braking could reduce the # of times friction brakes are applied, then that would seem to reduce the issues w/impeding airflow to dissipate heat from friction braking. I don’t know enough about the physics of motor size needed to brake an A320….but its something to consider.

    • Good thought, but there is no free lunch. You would be dissipating the same kinetic energy via regeneration or friction braking, which would require rejection (or storage) of similar amounts of heat (or power, with transmission losses).

      • mhalblaub says:

        There is already a smart system to store high energy inputs: KERS.
        It depends on how heavy the KER system will be an how much energy is said to be stored.
        There is a lot of energy: E = 0.5 * m * v²

        To store just some energy from braking that could be used to taxi afterwards could be smart.

        Superfluous energy could be “burned” via resistors located somewhere on the landing gear.

        The remaining question is, will the additional weight of a KERS consume more fuel than it will save during landing? Just burning the energy via resistors could be a solution to keep the brakes cooler.

  3. iPuck says:

    E-Taxi by its very nature means less brake use. Today the pilots ride the brakes to overcome the engine idle thrust. E-Taxi means braking only when needed – which includes landing, of course.

    I am pretty sure EGTS would allow today’s turn times. But reduced turn times that take advantage of not having jet blast (two door, sideways jet bridge ops)…. maybe not so much.

  4. VV says:

    This whole article just doesn’t seem to have much substance.
    How much (if any) is the airflow impeded by this system? We don’t know.
    Does it include a different fan/cooling solution? We don’t know.
    It’s fine to ask questions about that subject, but there is nothing substantial to discuss.
    More dubiously, the article seems to treat the entire benefit of the system in terms of time.
    EGTS have been crystal clear that a much broader scope of cost benefits is affected.

    • iPuck says:

      The EGTS claims are for 200k a flight. The time savings, if fully utilized, could be 20 minutes – or 15-20x that in terms of value.

  5. RSimon says:

    The weight penalty of a system capable of storing such a large amount of energy generated in a short time would be counter productive.

    WheelTug is the market leader. Their more elegant system is years ahead and weighs very much less. They have already signed up 11 airlines with 573 delivery slots.

  6. Jeroen Meijer says:

    The kinetic energy of a 60.000 kg airframe landing at 140 knots is somewhere in the ballpark of 100 MJ ( 1/2 x m x v-squared). The article states the motors are ballpark 50 kW, so let’s suppose they can be stretched to generate 100kW on landing, so say 2MW on a 20 second rollout (all very very ballpark). That is 2% of the total energy to shed. Not worth it. And it also give am idea about the massive energies involved.

    • mhalblaub says:

      3 MW or 3,000 kJ is about the energy used to accelerate a 60,000 kg airframe from stop to 10 knots. About 400 kJ could be stored in one system with a mass of 24 kg (not optimized for aircraft usage). So an additional mass of around 150 kg is needed to store such an amount of energy.

      A 787 Lithium-Cobalt batteries have a mass of 29 kg which can provide 75 Ah at 32 V equivalent to 8,500 kJ but can be charged just at ~2 kW ( 75 minutes to reach 90 % capacity).

      How many electric energy can a APU made out of 1 kg of kerosene? 10,000 kJ?

      Therefore I conclude an aircraft brakes and accelerates far to little to use KERS efficiently.

      • Jeroen Meijer says:

        @mhalblaub You correctly point out my error: I should have said 2MJ (not MW), but a mass of 60.000 kilo’s accelerated to 10 knots (18 km/h or 5 m/s most certainly does not require 2MJ. more like 0.5 x 60.000 x 5 x 5 = 0.75 MJ.

        The roughly 100MJ on landing cannot be converted (other then to heat = brakes), let alone stored anywhere, let alone within 20 seconds with the current state of technology, as you correctly point out. For comparison, 100MJ is what my household uses per day on average through the year for all heating (premises and hot water).

        [kidding on]Maybe we can heat the terminal with the excess heat of the brakes[kidding off].

        Per wikipedia, a kg of kerosine holds roughly 42MJ, but that assumes 100 conversion efficiency, which a APU most certainly does not even come close to.

  7. Uwe says:

    cue 787. Too good to be true. So many customers hooked by PR.

    Electric braking with energy dissipation in electric antiice.
    That would work quite well. E-machines in that size can
    have pretty good efficiency ( as motor _or_ genererator 95..98%)
    Most of the neccessary power dissipation could be moved onto the wing surfaces.

    So the 787 could be a perfekt fit for this 😉

  8. Pierrebus says:

    I was always wondering why parking spots are not made with a negative slope like my driveway. I never use reverse in my driveway, I simply remove the parking brake and my car backs up in the street. It would cost zillions to update current airports but for new airports, why not? Hmm, actually, how muxh would it cost to add some concrete to create a little slope at one gate just to try it out. You game, southwest? allegiant? Same for taxiways along the runways, why not put in a gentle slope in the prevailing wind direction. That one for sure would be an idea for new airports only 😉 Let the planes waiting in line roll down the gentle hill. No ailgaiting of course otherwise you ll need the engines.

    • mhalblaub says:

      I guess the slope is there to prevent heavy rain entering your garage.

      Train stations even in mountains are tried to build without any slope. That is just more secure than using a break to halt on a slope. How do you get the wheel chocks away then an aircraft rolls onto them due to the slope?

      What’s next after E-Taxi? E-assistet take-off?

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