Re: AA changes rules after BDL incident

From:         ehahn@wren.mitre.org (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
Date:         21 Dec 95 03:33:09 
References:   1 2
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In article <airliners.1995.1953@ohare.Chicago.COM> jsteele@AZStarNet.com (Jerry Steele) writes:

   In article <airliners.1995.1927@ohare.Chicago.COM> John Harvie/CAM/Lotus <John_Harvie/CAM/Lotus.LOTUS@crd.lotus.com> writes:
   >American Airlines quietly implemented new safety rules in the wake of last
   >month's near-tragedy at Bradley International Airport outside Hartford,
   >instructing its pilots to fly 100 feet higher than the required minimum
   >elevation for non-precision landings at all airports.

   Question from non-pilot (frequent passenger) to all you pilots:

   What happened to the ground proximity warning system?
=====
I do not know the specifics of the incident.  I will say the following
general statements about GPWS.

During landing, by definition the aircraft will get close to the
ground.

To reduce false alerts during normal approaches, the GPWS monitors the
flap setting and gear position, and MODIFIES it's warning logic to
allow normal operations.  This is intended catch unstabilized, high
descent rate approaches.

Also, during PRECISION (ILS) approaches, the GPWS monitors the
glideslope needle deflections to alert the crew if they descend below
the ILS glideslope.  Most air carrier instrument approaches tend to be
ILS approaches.

Begin speculation:  From what I gather from the press reports, the
aircraft was on a NON-PRECISION (no ILS) approach.  It appears that the
aircraft was in landing configuration at the time, and the aircraft
appears to have been on a stabilized approach (although it may have
been lower than usual due to the alleged altimeter setting error).
Because of these factors, there may have been no way for the GPWS to
determine that the aircraft was NOT over the runway at the time.  I AM
NOT SAYING THAT THIS IS WHAT HAPPENED.

That being said, the industry has been looking at ways of improving
the GPWS, through combined use of GPS position and a high-resolution
terrain database, for several years.  The recent availability of cheap
memory and computing power now make it possible to make the GPWS more
bullet-proof.  While avionics are under development, no certified
boxes have yet been manufactured.

Disclaimer:  I know nothing about the specifics of this incident aside
from press reports and postings on this group, but am familiar with
the general principles of GPWS operation.

ed

--------   Ed Hahn | ehahn@mitre.org | (703) 883-5988   --------
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.