Reducing runway emissions

Innovative automotive engineering could help aviation reduce its carbon footprint.

The challenge to drive down CO2 emissions in the aerospace industry has seen billions of pounds spent on smart materials, fuel cells and improvements in engine design. However, up until now, little work has been carried out on one of the most promising areas of carbon reduction in aircraft flight – the taxiing phase.

According to Airbus, taxiing its aircraft between the main gate and the runway produces around 18m tonnes of CO2 per year and is estimated to cost airlines in the region of $7bn-8bn (£4.2bn-4.8bn). It is also a source of foreign-object debris, resulting in around $350m worth of aircraft maintenance and flight delays each year.

These figures are rising alongside increasing airport traffic. Currently, an aircraft can take anything between 45 minutes to two hours to leave the ground, during which time it is running its main propulsion jet engines and burning one tonne of fuel for every 17 minutes that it remains idle.
Attempts to tackle the problem have come in the form of powerful tug vehicles. In 2006, Virgin Atlantic began an initiative to create airport ‘starting grids’ that would allow aircraft to be towed closer to a runway before take-off. However, like early tug designs, Virgin’s attempts were abandoned due to maintenance problems with the aircraft landing gear as a result of continuous jerks inflicted by the towing vehicles.

Three years on from these trials, a new solution called the ‘Taxibot’ has been designed to overcome these problems. Developed by Israel Aerospace Industries (IAI) in collaboration with Airbus and Ricardo, the system is claimed to be able to tug a 400-tonne Boeing 747 without applying any stress to the aircraft landing gear. This is done using IAI’s patented turret and energy-absorption system, which the group claims eliminates the stop-start action inflicted by conventional tugs. Ricardo is contributing its engineering expertise to the project and has manufactured the first full-scale demonstrator vehicle of the design.

“An aircraft is very bouncy. It has lots of tyres, struts and springs and a lot of oscillation” RICHARD GORDON, RICARDO

Richard Gordon, project manager at Ricardo, said that successfully eliminating wear on the landing gear could save the industry billions of pounds a year in wasted fuel. ‘This is a significant change to current systems’, he said. ‘Instead of connecting the tug directly to the nose wheel, it’s attached to a platform that can move axially. On this platform we’ve fixed the vehicle turret to which the aircraft nose wheel is clamped and can rotate as the pilot steers the nose wheel. This prevents a continuous stop-start action that would otherwise lead to component damage.’

The demonstrator vehicle is based on a Krauss Maffei PTS-1 aircraft tractor, donated to the project by Lufthansa LEOS. Over the past 15 months, the team at Ricardo has been working to redesign and modify the tractor to meet the brief provided by IAI and Airbus.

Gordon added: ‘With this system, the pilot can apply the brakes to the aircraft to stop the Taxibot. These brakes are incredibly powerful and underbraking has a load transfer onto the nose wheel that effectively pushes the tug down into the ground. So we’ve had to calculate all those forces and then design all of the structures that can handle those forces safely. Before we started, the demonstrator vehicle was about 32 tonnes and now that we’ve finished it, it’s closer to 52 tonnes, so a lot of metal has been added.’

In addition to installing the turret on the base vehicle, Ricardo has added chassis extensions to include an additional axle set. The 750hp vehicle has been designed to tow a Boeing 747, however the team hopes to increase the vehicle’s power enough to tow larger wide-body aircraft such as the Airbus A380. Doing this would mean modifications to the control systems to manage the additional forces on the landing gear. Ricardo has developed a simulation tool that it claims will enable it to accurately model these forces with any size of aircraft.

‘This simulation tool is important for designing the control systems that allow us to determine how the aircraft behaves in relation to the tug,’ said Gordon. ‘Modelling these forces is difficult as an aircraft is actually very bouncy. It has lots of tyres, struts and springs and a lot of oscillation that we have to model dynamically in order to analyse its movement.’

The dynamic modelling has led to the development of a hydrostatic drive system to ensure the Taxibot can maintain a high level of speed and force. The system uses a hydraulic transmission to control the ratio of the flow of oil between the pump and the motor and lets the operator change gears by changing the rate of oil being fed in. Gordon said: ‘The benefit of this is you can go forwards and backwards just as easily and there are no steps or jerks when you change gear, which prevents fatigue on the aircraft.’

“With financial incentives we will soon see these vehicles used in airports globally”

RICHARD GORDON, RICARDO

In parallel with the demonstrator vehicle programme, Ricardo has built a 100-tonne test trailer capable of simulating large passenger-aircraft tyre drag. The trailer has been built with a Boeing 747 cockpit and nose landing gear to reproduce the processes of towing, braking and flight-deck control. ‘The only area where it’s definitely not accurate is the total system weight’, explained Gordon. ‘A 747 at take-off is around 400 tonnes and the trailer can go up to 100 tonnes. But in the trailer we’ve got a hydrostatic drive system so we can absorb energy and make the tug think its pulling a 747 from a friction point of view, but not from the momentum because we haven’t got the real weights.’

The team is currently testing the prototype vehicle using the test trailer at the Dunsfold aerodrome in Surrey. If these trials are successful, it is hoped that the demonstrator vehicle will be shipped to Toulouse airport in February next year, where the Taxibot will be used in further tests with an Airbus A340-600.

Gordon said: ‘The crucial differentiator with the Taxibot is that it is pilot controlled so feedback forces and braking are better managed and separate tug driving is not needed. I’ve personally steered the Taxibot around simulated taxiways and it is very easy to control. It has been designed so that a pilot doesn’t notice any change to the steering or feedback.’

The current prototype assumes that a driver is present in the vehicle, however the system controls are also able to support autonomous tug operation. While Gordon believes that an autonomous Taxibot is still a long way off, he added that the current incentives to provide an effective tug device to airlines could see the widespread use of the Taxibot in the near future.

‘I believe with increasing financial incentives and pressure to cut emissions we will soon see these vehicles used in airports globally,’ Gordon concluded.

An alternative take

Taxibot is not alone. Gibraltar-based technology firm Wheeltug (a subsidiary of Chorus motors) has developed a fully integrated system based on hightorque electric motors built into the hub of the aircraft’s nose wheels. According to the company’s chief executive, Isaiah Cox, one of the attractions of the system, which is powered by an aircraft’s auxiliary power unit (APU), is that it doesn’t require extra traffic around the airport. He said the system avoids potentially damaging tugs and jolts that could result from external systems. Cox also claimed that because it reduces the amount of fuel a plane needs to carry, the system is weight neutral. Following proof-of-concept trials in 2005, the firm plans to retrofit the system to a Boeing 737