Re: First 757 crash (and first AA fatality since 1979)

From: (David T. Medin)
Organization: Rockwell Avionics - Collins, Cedar Rapids, IA
Date:         08 Jan 96 01:37:28 
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In article <airliners.1995.2057@ohare.Chicago.COM>, Gunnar Aaboe <> writes:
> >Does any one have any
> >idea about what happened ?  The news here said that the pilot pulled up
> >when he saw the mountain.  How could the aircraft get so far off course?
> >I wonder if pilot fatigue was a factor... Maybe the pilots woke up when
> >the Ground Proximity Warning System warning horn sounded.  I also wonder
> >if there was a problem with the ILS system at the airport.
> >The pilots may have been attempting to intercept the localizer, but maybe
> >the localizer was not in operation. Hopefully we'll learn something
> >soon...

[stuff on radio ducting deleted]

> If a pilot
> use a ILS where the glidescope is too low he will not see any problems
> and not report any.  I don't think this has been the problem in this last
> accident, but I think the people who have constructed this ILS systems
> and the people using them should know that under spesial conditions they
> do have have problem.

Ducting is not a problem on an ILS, if the procedure is correctly
flown. What can be more serious problem is picking up a false lobe of
the glideslope (happened to me once during training as an exercise),
but again flying the procedure will prevent this. The fixes used along
with altitude vary in their type (VOR, DME, NDB, RNAV, 75MHz marker),
but are not changed appreciably by ducting over the ranges and
geometries used.

The altitudes and approach fixes on the approach plan give information
that allows positive verification of the main lobe of the glideslope,
and the procedure turn or radar vectors give positive intercept of the
localizer. In other words, there is always information regarding what
altitude and in relation to what fixes the glideslope can be expected
to be intercepted, or an aircraft already on the glideslope can be
expected to pass through.  While the intercept may occur higher, the
aircraft will always pass through known altitudes at known fixes on
the way down. That's part of the cross-check used for sanity and
safety on an approach.

It sure looks like the flight crew lost situational awareness in this
case, but I don't think it was error in the navaids themselves.

       David Medin            Phone: (319) 395-1862
   Rockwell Collins ATD	      Internet:
     Cedar Rapids, IA