Re: FMS capability (was: Re: AA965 proves Airbus bashers deadly wrong)

From:         Tom Speer <>
Organization: 412th Test Wing / TSFF
Date:         07 Mar 96 02:04:42 
References:   1 2
Followups:    1
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  or MIME structure (FMCDave) wrote:
>...  A complete safety analysis would need to be
>done on this function, but if a terrain display based on an NDB update was
>to be used as the primary means for avoidance of Controlled Flight Into
>Terrain, then the criticiality of the function may be higher than most FMC
>computers are certified to.  In this configuration, I would presume to
>leave the GPWS in as the "belt and  suspenders" function due to the
>consequences of the failure.
>... Terrain display coupled with GPWS will help
>flight crews avoid these accidents by providing them information.
>However, there are no easy solutions, implementation of this function will
>be a significant systems engineering task.  If any company tells you
>otherwise, hold on to your wallet.

I missed the beginning of this thread, but for what it's worth:

The latest generation of military airplanes are implementing digital terrain
databases as an aid to terraing following, and indeed _could_ do TF entirely
based on the digital terrain elevation data.  However, if you talk to the
Defense Mapping Agency folks that put together the data, they will tell you
that the data are NOT, repeat NOT, reliable enough to serve as the sole
means of terrain avoidance.  This is in part because of the accuracy of the
methods used to acquire the data, the size of the cell for which the height
information is taken, the lack of information on manmade obstacles, etc.

Even if your terrain data were perfect, you also have the problem of knowing
exactly where you are in the terrain map.  No nav system is perfect, either,
so you may be a lot closer to that mountain than the computer says.

That being said, terrain maps can be of significant value.  In the case of
terrain following, they allow the TF system to know what's in the radar
shadow cast by the higher terrain in front of the aircraft.  They can also
be used as a navigation aid by correlating the radar altimeter history trace
with the terrain elevation.

GPWS will still be necessary for safety.  But aiding it with terrain data
would help to make it more predictive and provide better safety buffers.
Essentially, the uncertaintly in the knowledge of what lies beyond the beam
of the radar altimeter (or other GPWS sensor) would go from 100% to a much
lower value.  It could also be used to present a picture of the terrain to
the pilots even when IFR, and this would enhance their situation awareness
significantly.  Especially if the contours were presented on a HUD, as has
been done in research studies.  I haven't seen it flight tested on HUD's yet
(but it may have been).  FLIR imagery has been successfully displayed on a
HUD superimposed on the outside scene, and there isn't any difference in
principle with displaying the synthesized terrain picture.