PILOTS have a tough job keeping helicopters hovering over one spot during
rescue missions at sea. Now a system being developed in Germany could make
maintaining a hovering position a matter of simply pushing a button, no matter
how bad the weather.
Even when helicopters are moving forward, they are more difficult to control
than fixed-wing aircraft. Keeping them in a steady hover while trying to pluck a
survivor from the sea is an even bigger challenge. Pilots need a fixed reference
point to lock onto, says Henrik Oertel of the Institute of Flight Mechanics at
the German Aerospace Center in Braunschweig. This is a problem, because the
surface of the water can look very uniform, and there may be squally winds to
contend with.
Oertel believes that his system could make a big difference to rescue work.
鈥淲hen the ferry Estonia sank in the Baltic in 1994, one of the main
problems was that the pilots couldn鈥檛 fly for very long,鈥� he explains. The work
was so fatiguing that pilots frequently had to cut rescue missions short.
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The new system uses a video camera trained straight down that records 25
images each second. The system scans each image for five significant features
from the scene below, and compares them with the corresponding features in the
next image.
Any change in the position of these features indicates that the helicopter is
drifting, says Oertel. The system will automatically adjust the helicopter鈥檚
controls to compensate. The features chosen are renewed frequently to account
for the changing appearance of waves. 鈥淚t can follow anything it finds,鈥� he
says. Some 鈥渃ommon sense鈥� built into the system in the form of inertial sensors
allows it to tell the difference between a wave moving up and down and a change
in altitude.
Some helicopters are already equipped with radar-based systems that help
maintain a steady altitude. But in many situations, these still allow the
helicopter to drift, Oertel says. One reason for this is because auto-hovering
works by monitoring radar reflections from below. Wash patterns created in the
water by the helicopter鈥檚 downdraft can make this difficult because it causes a
similar pattern to appear even when it moves. Similarly this can happen above
ground, says Oertel, with the wind blowing a pattern on the grass.
A former search and rescue helicopter pilot, Nick McDonald-Gibson, says: 鈥淎
system that will allow you to lock on to the survivor, rather than the water,
certainly merits trials.鈥� McDonald-Gibson is now project development manager for
Bristow Helicopters, which operates coastguard helicopters in Britain.
When small craft drift in the water, their movement is determined by the
wind, not currents in the water, says McDonald-Gibson. This presents problems
for radar-based auto-hovering systems, because they lock onto radar reflections
from the water, rather than the target itself. Another advantage of a visual
system is that it will work in weather that will defeat radar-based systems, he
says.
The project, funded by the European Commission, has been tested in two
stages. An earlier version was used to follow a black square mounted on a jeep.
The more recent version has so far been tested in a simulator. Oertel is
confident that it will work in difficult weather conditions, such as crosswinds
and turbulence.