Feeling the cold

A UK-designed system for detecting ice build-up on aircraft wings, rotor blades and tail-fins is at least 20 times more sensitive – and considerably smaller – than any existing solutions, its creator has claimed.

The fibre-optic sensor can detect the formation of ice films just 0.1mm thick, according to inventor Keith Cameron of Cambridge Optical Sciences.

When ice forms on an aerofoil the increase in drag and decrease in lift can be catastrophic. In 1985, ice was blamed for the loss of 250 lives when a chartered McDonnell Douglas DC-8 crashed shortly after take-off from Canada’s Gander airport.

While today’s big aircraft usually generate enough surplus power to heat the wings constantly, smaller planes and helicopters are frequently struck down by icing, said Cameron.

The new system uses an infrared light source that is fed to the aerofoil surface using an optical fibre. This is surrounded by an array of receiver fibres that pick up the scattered or reflected light from the ice when it is forming.

The system is capable of detecting much thinner layers of ice than traditional systems, while its small size and the fact that it is not an active device means that it can be placed at the heart of the aerofoil’s danger zone.

If ice is detected by the device all the pilot has to do is change altitude by a few hundred feet, said Cameron.

Existing ice sensors use a resonance mechanism (a piezoelectric diaphragm) in which the build-up of ice on the device changes its resonant frequency. Their size, and inability to cope with extreme forces, means they are generally situated away from the aerofoil surface, compromising their accuracy.

‘On rotorcraft where the tip velocities exceed the speed of sound, you get enormous forces on any components within the rotor itself and a passive device is much more suitable for that sort of application,’ said Cameron.

‘You really need to have the sensor where the ice is beginning to grow,’ he added.

Cambridge Optical Sciences is currently working on miniaturising the device further with Luton-based ACT, a division of Agusta-Westland, where wing sections are tested in a wind tunnel equipped with an icing facility.

Martin Lawrence, managing director of COS, said the sensor will be making its first in-flight appearance on the supersonic rotor blades of AgustaWestland’s EH101 Merlin helicopter next year. Ultimately the device will be used for search-and-rescue helicopters. Lawrence added that BAE Systems and Qinetiq have also expressed interest in the technology.