Optics & Photonics News

Recently, the NASA
Langley group
developed fiber-
optic sensors that
can be used as
part of a vehicular
or structural health
monitoring system.
These sensors could
someday track the
shape of an aircraft
wing during flight.

from their instrument panel to the distant view through the windshield.

A 2009 worldwide study by the Flight Safety Foundation found that HUDs “prevented or positively influenced” 38 percent of accidents overall in modern jet aircraft. Where pilots were directly involved in incidents—takeoffs, landings or loss of control in flight—the study said that HUDs increased the likelihood of accident prevention by up to 69 percent. Other studies have shown that pilots viewing flight path data through HUDs, instead of through glances at the cockpit displays, made more efficient landings with less wear and tear on the aircraft.

Despite their capabilities, some HUDs, especially older models, have the drawbacks of large size and weight. Many airplanes can’t have head-up displays, Bailey said, because they’re just too bulky and heavy for the cockpit. Some HUD systems weigh upwards of 25 lb ( 11. 3 kg). The ideal head-worn display would weigh only 1 percent of that. One U.S. airline has estimated that, for every pound of weight removed from each aircraft, the fleet would save 12,000 gallons ( 45,400 liters) of fuel annually.

Thus, Bailey’s group is studying the feasibility of designing head-worn displays for commercial and general aviation. Since fighter pilots are accustomed to wearing helmets, HUD technology was not difficult to integrate in the military world. However, airline pilots would probably resist wearing helmets during standard operations.

“A head-worn display has to be very lightweight, very unobtrusive, and relatively rugged to withstand the day-to-day wear and tear of use inside a normal airplane cockpit,” Bailey said. He envisions a display apparatus, either worn like a pair of eyeglasses or mounted on the side of an eyeglass frame, that would perform the same functions as a HUD, only with a significantly smaller cockpit footprint. Such an apparatus could track the pilot’s head movements to provide a wider field of multi-layered view than the relatively small, fixed screen of an HUD.

According to a recent article by Bailey and his colleagues, some aerospace companies are trying to reduce the cockpit volume occupied by a HUD system. For example, BAE Systems of Kent, England, recently introduced Q-HUD, a lighter-weight system (with a mass less than 10 kg) that uses waveguide optics instead of a conventional projector.

Potential markets for the head-worn displays include planes where it would be impractical or too costly to install HUDs on an after-market basis. Some cockpits may not accommodate the volume of the system, or they may not have convenient places to attach the combiner screen and wiring harnesses.

Sensing with fiber optics and
quantum dots

Recently, the NASA Langley group developed fiber-optic sensors that can be used as part of a vehicular or structural health monitoring system. According to K. Elliott Cramer, head of Langley’s Nondestructive Evaluation Sciences Branch, these sensors could someday track the shape of an aircraft wing during flight.

NASA researchers designed a three-core
optical fiber incorporating low-resistance
fiber Bragg grating strain sensors. “It’s like
having three optical fibers in one,” Cramer
said. “It allows you to write your strain
gauges in three different axes all in one
optical fiber.”
The relative change in the positions of
the gratings in the cores reveals the three-
dimensional change in position of the object to which the fiber
is attached. The agency claims positional accuracy of better
than 1 mm over a fiber up to 10 m in length.

The special fibers could be embedded on the edge of a wing, aileron or other deformable airplane part that under-goes strain during flights, and the sensors could provide an idea of the stress load the part is enduring and the amount of deformation it is undergoing in near real time. In the future, such fiber sensors could be crucial components of active feedback systems that control the amount of deformation of a dynamically shaped wing. Such controls will become more important with lighter-weight materials that bend more easily than older aircraft materials.

NASA Langley has its own drawing tower, so researchers can write the fiber Bragg gratings directly onto the optical fibers during the manufacturing process prior to