New ground proximity warning.

Date:         01 Dec 96 04:08:50 
From: (C. Marin Faure)
Organization: Northwest Nexus Inc.
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I had an opportunity to see an Alaska Airlines internal communication
video yesterday, and they described a new terrain avoidance system that I
think is pretty slick.  It is being installed on their 737-400s and will
undoubtedly be a feature of their new 737s as well.

A very accurate terrain map of an area-- in the example on the tape it was
Juneau, Alaska-- is put on a CD-ROM. It is then loaded into the airplane's
flight computer data base.  As the plane nears Juneau, a city with a very
tricky approach due to extremely high and steep mountains immediately
adjacent to the airport and approach paths, the terrain map is
superimposed onto the plane's moving map display.  The system uses GPS to
accurately determine the position of the airplane.  By combining the
position of the airplane and its altitude and comparing this data to the
extremely detailed and accurate terrain map loaded into the computer it
becomes immediately obvious what terrain in the area is a potential threat
to the airplane as it's highlighted in red on the screen.  If the plane is
off course and heading toward a mountain, any terrain at or higher than
the altitude of the plane shows red on the map, and a warning is sounded
if the plane gets within a certain distance.

In the example shown on the tape, the pilot elected to climb out of
danger. As the 737 increased altitude, the red zone in front of him grew
smaller until it was all green, indicating that the plane was safely above
the mountain.  Had the pilot elected to turn away from the mountain, the
danger area would have remained red, but would have moved off to the side
and rear as the plane turned away.

This seems to draw on the technology that is in the Air Force's cruise
missiles.  The ALCM flies to its target at a very low altitude, often less
than 100 feet.  However, the missile is not actively sensing the terrain
in front of it but is instead responding to accurate terrain maps stored
in its memory and comparing the maps to its present location.  It doesn't
zoom up over a hill because it senses a hill is there, it zooms because it
knows exactly where it is and its memory map tells it there's a hill in
front of it, so pull up.  This system eliminates any active emissions from
the missile, making it harder to detect.  Obviously, Alaska Airlines isn't
concerned about flying undetected, but this new system will add greatly to
the safety of their operations.

C. Marin Faure
   author, Flying a Floatplane