Monday, September 25, 2006

Fiber Optics on a Plane

A new type of optical switch could help aerospace engineers replace heavy copper wires with fiber, making planes lighter.

By Kate Greene

Bundles of insulated wire on airplanes are heavy and costly to maintain. Sending signals via a single fiber-optic line could save hundreds of pounds and simplify maintenance. (Credit: Corbis)

There's more than a hundred miles of electrical wires in the average plane, controlling nearly everything from landing gear to flight-attendant calls. All that insulated copper wire is a big liability: it's heavy, it's susceptible to electromagnetic interference, and when not properly maintained, it can cause system failures and fires.

Now researchers at Texas A&M University in College Station have found a way to replace some of these wires with fiber-optic technology, which is lighter than copper wire, less prone to electromagnetic interference, and immune from electrical shorts. They've developed a novel optical switch that could be incorporated into cockpit controls to manage operations that need to be turned on or off--for example, landing gear, displays, and manual switching between fuel tanks.

Currently, on-off switches in a cockpit are attached to separate wires that snake through a plane, controlling various functions. The maintenance cost for such a setup can be high, says Zhaoxia Xie, a researcher on the Texas A&M project. If a switch isn't working because of wiring problems deep within the plane, hunting down the offending line can take lots of time and effort, because scores of wires are bundled together.

The switch that Xie developed with the late Henry Taylor, professor of electrical and computer engineering at Texas A&M, can sense whether a button has been pushed from "off" to "on." Information from the fiber-based device could be routed to a main fiber artery, which would carry hundreds of signals simultaneously. Xie says that would eliminate the bulk of wires, simplify maintenance, and cut costs.

At the heart of the device is a 10-millimeter-long sensing element, embedded in fiber with a diameter of 125 micrometers. This sensor is composed of two mirrors that are transparent as well as reflective, allowing light to enter, reflect back and forth between them, and then pass through. As light waves bounce between the mirrors, they form interference patterns that change according to the position of the mirrors. The researchers took advantage of this property by connecting their switch to a cantilever that slightly adjusts the position of the mirrors when a cockpit button is pressed. Changing the spacing of the mirrors adjusts the interference pattern, thus signaling whether the switch is on or off.

Though engineers have been working to replace plane wiring with fiber for years, they've had only moderate success. Boeing's 777--built in the mid-1990s--uses a fiber-optic communication network, but the design and implementation were "more or less an experiment," says Dan Martinec, technical director of industry activities at ARINC, an Annapolis-based aviation communication company. The network wasn't a critical system, he says. In addition, it was "overdesigned," with more room for error than would be cost-effective if it were widely implemented in the industry. However, he says, Boeing's 787 planes, a new fleet scheduled to fly in 2007, will have a more cost-effective optical communication network onboard.

Martinec is skeptical that the Texas A&M research will ever make it into planes. But Fazi says that replacing the electrical wires connected to switches might produce sufficient weight savings to make the technology cost effective. Even if just a couple of hundred pounds is lost from a plane, he says, "that's still significant."

Much more work needs to be done to make the optical switch ready for implementation, Xie says. The next step is to test the setup on a plane to determine the temperature extremes and vibrations it can withstand.