Recently, the NASA
optic sensors that
can be used as
part of a vehicular
or structural health
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
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
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
[ Sensing with fiber optics and quantum dots ]
Three separate cores
measure three different
strain fields dependent
on bend radius of fiber
gratings in each core
Illustration by Phil Saunders
October 2011 | 43