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.
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.
- 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.
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.