Green Operations

Taxi Electric

Powered wheel-drive systems promise to save fuel and reduce emissions at airports
Graham Warwick/Washington

Taxiing to and from the runway on engine power may soon be a thing of the past as development of electric wheel-drive systems progresses, with the promise of reducing fuel consumption, emissions and noise—and potentially increasing airport capacity.

EasyJet will be the first airline to test an electric taxiing system being developed by Honeywell and Safran. Operational trials on the Airbus A320 are expected to begin in 2013. Lufthansa and L-3 Communications tested an electric-taxi technology demonstrator on an A320 at Frankfurt in December. WheelTug plans to demonstrate a system on a Boeing 737-800 in May, “almost certainly in North America,” the start-up says.

The Honeywell/Safran electric green taxiing system (EGTS) uses electric motors on the main landing-gear wheels, powered by the aircraft’s auxiliary power unit (APU). L-3’s GreenTaxi system also powers the main wheels, but WheelTug’s approach is to drive the nosewheels. German aerospace center DLR demonstrated a powered nosegear on an A320 at Hamburg in June 2011, with the electrical power coming from a fuel cell.

WheelTug is testing its motors in the laboratory and expects to begin tests of the complete wheel package in March, says CEO Isaiah Cox. Details of the May demo have yet to be announced. WheelTug is on track for certification and first deliveries for the Boeing 737NG in mid-2013, he says, and talks have begun with A320 operators. Israel airline El Al has signed a memorandum of understanding, and Cox expects to have “1,000 aircraft in backlog” by late this year.

The Honeywell/Safran partnership has acquired an A320, based at Montpelier, France, which it is using to understand loads and deflections on the landing gear. This aircraft will be used for tests of the EGTS beginning this year, says Brian Wenig, vice president of business development at Honeywell Aerospace. Certification of the system for retrofit and forward fit is targeted for 2016.

Lufthansa Technik and teammates L-3, Airbus, Lufthansa and Frankfurt Airport operator Fraport are scrutinizing data from 14 hr. of taxi trials and will complete an economic benefit analysis in March before deciding whether to proceed to a prototype system, says Christian Mutz, project manager of innovation for Lufthansa Technik.

The GreenTaxi demonstrator used off-the-shelf vehicle motors from German subsidiary L-3 Magnet-Motor. Installation required removal of the brakes from the outer wheel on each pair. A container in the cargo bay provided power conditioning and liquid cooling, which Mutz says proved not to be needed, as the motors stayed cooler than the brakes.

Weight is critical, and the Lufthansa team is looking at a lighter pushback-only system as an alternative to the full capability, as well as a hybrid of the two. A pushback-only system would propel the aircraft at 3 mph, compared with a full system that would be capable of taxiing the aircraft at 30-40 mph, Mutz says.

Able to propel the aircraft at up to 28 mph, WheelTug weighs 300 lb. but is “flight weight”-neutral, as less fuel is required for taxiing, the company says. Minimum fuel burn for a 737 taxiing on one engine averages 15 lb./min., and crews typically add 30 min. of fuel weighing 450 lb. for contingencies. Powering the wheels from the APU cuts taxi fuel burn by up to 85%. “Instead of over 200 kg [440 lb.] of taxi contingency fuel, I can load 60 kg, and have the same operational flexibility,” says Chief Pilot Joseph Goldman.

WheelTug’s business plan is to supply systems and spares free to airlines and share the demonstrated savings. “We can show savings to airlines of $600,000-800,000 a year,” says Cox. “That compares with $1.5-2 million a year for reengining.” Honeywell “conservatively estimates” that the EGTS will save an average $160 per segment, and 130 tons of fuel per year per aircraft, for an A320 or 737 operator, says Wenig.

There is still debate over the right approach. Honeywell/Safran and L-3/Lufthansa say the capability to taxi in all conditions can only be provided by powering the main wheels, as there is not enough weight on the nosewheels to provide the required traction. “We have done tests, and demonstrated operation in ice and snow,” counters Cox.

Other potential benefits of electric taxiing include shorter turnaround times and the ability to position aircraft on the runway at noise-constrained airports so takeoffs can begin as soon as the curfew is lifted, generating more slots. But engine makers are concerned about the lack of warm-up time that could result.

A picture from Chorus Motors, who owns WheelTug, the motors are in the nose wheel which has no braking system so has lots of room for the motors (don’t get me wrong these are powerful motors for their size weight.)

The issue mentioned above and main reason that this is starting on the 737 and A319 is that these are both big and small at the same time.

Compare the landing gear in the pictures below one of a 737 the other of its biiiiig brother the 777.

There is much more, % wise, weight on little bro’s front wheel. With friction etc you have a practical propulsion system even in crappy frigid/icy weather, after the planes been sitting for a long time, and the tires have flat spots where they contact the ground. With the 777 there is no way that the front wheel can haul that big boy around.

The article discusses work on main gear but here you have to integrate around the brakes. Now there are reasons this would be good beyond taxing but the brakes are hot and dirty, a much tougher environment and for a big boy you would need a lot more than two driven wheels which makes cabling and drives even more of an issue. But at the end of the day this is probably the way aircraft will move on the ground in twenty some years and those future versions of us will wonder why it took so long.

Testing out a main gear electric taxi concept, L3/MagnetMotor and Lufthansa.