From: firstname.lastname@example.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 | email@example.com | (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.