Wireless sensors could reduce aircraft maintenance costs

Aircraft fuselages may one day be fitted with wireless sensors that transmit crucial information on factors such as stress endured during flight to ground crew during maintenance checks. 

The technology is being trialled by EADS in an effort to reduce maintenance costs, which account for up to 22 per cent of an aircraft’s overall expenses per flight hour.

‘If you have a sensing system on the aircraft you can go to maintenance-on-demand’

Dominik Samson, EADS Innovation Works

A team of experts from EADS Innovation Works and external co-operation partners have developed data-collecting sensors to work wirelessly and power themselves by thermo-electricity. This method involves converting heat flow into electrical power with the aid of a thermoelectric generator.

PhD student Dominik Samson from EADS Innovation Works said: ‘Now you have scheduled maintenance, which means you have no sensors or maybe few sensors and then the ground crew has to check everything in the aircraft for possible errors.’

‘If you have a sensing system on the aircraft you can go to maintenance-on-demand because the aircraft tells you where to look,’ he said, adding the information could be sent wirelessly to the PDA of a ground crew engineer. ‘You can save some maintenance because you don’t have to look at everything, just points of interest.’

Josef Schalk, head of communications technology at EADS Innovation Works, said the biggest challenge for the engineering team was developing a reliable energy source for the wireless sensors. These sensors, he said, will be placed in areas such as the wing tip where cables cannot reach and batteries offer only a limited lifetime.

The team developed an energy-harvesting solution based on thermoelectricity. This method involves converting heat flow into electrical power with the aid of a thermoelectric generator.

Samson said the semiconductor device, which is the size of a sugar cube, converts thermal gradients to electrical energy using the Seebeck effect.

He said: ‘If you heat it one side and cool it on the other you get some electricity flow.’ 

‘The biggest challenge for the engineering team was developing a reliable energy source for the wireless sensors’

Josef Schalk, head of communications technology, EADS Innovation Works

The technology takes advantage of the major temperature differences that occur in and around the aircraft. Where ambient air can have temperatures ranging from -20°C to -50°C, the passenger cabin keeps a temperature of about +20°C. There are also the strong temperature fluctuations on the outer skin after take-off or during landing.

Samson said the artificial temperature difference can be created anywhere on an aircraft’s outer skin. This is done by connecting one side of a thermoelectric generator to a heat-storage facility, while the other side is connected to the outer skin and cools down more quickly. The difference in temperature generates an electric current.

The heat-storage facility is simply water contained inside small hemispheres that are adhered to the inside of the aircraft wall.

Samson said: ‘We were looking for what kind of heat storage we could use and water offers very efficient heat storage.’

He added the special spherical containers are designed to increase the thermal conductivity of the water and keep it from turning to ice as the aircraft increases elevation.

EADS also developed a special mechanism for transforming the generated voltage into a value suitable for the sensor. This is known as ‘power management’. The system also has to buffer energy so that phases without energy production can be bridged.

The researchers have tested the system in a climate chamber. According to the team, the sensor nodes consumed only several milliwatts, compared to the 20 to 50 milliwatts a light-emitting diode could use up, and there was a sufficient amount of energy produced and stored during the flight to reliably operate the sensor node.

It was noted this was enough for long-haul flights, as the ’health monitoring’ sensors do not need to be active all the time and the sensor nodes are consumption-optimised.

The next step will be to test the technology in flight. A different team from EADS Innovation Works is currently investigating ways of using the heat of an aircraft engine’s exhaust jet. An analogous technique is also being intensively examined in the automotive industry.