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 Later in 1996, the FAA commissioned the DOT's Volpe National Transportation Systems Center for a second study that examined the effectiveness of GPWS and enhanced GPWS in preventing CFIT accidents involving airplanes operating under 14 CFR Part 121 and 135 or their foreign equivalents. The center studied 47 domestic and 104 foreign CFIT accidents that occurred between 1985 and 1995; 38 of the domestic accidents and 96 of the foreign accidents involved fatalities. The center developed a methodology and scheme for selecting a representative sample, and nine accidents were selected for detailed study and analysis.

The Volpe center found that four of the nine accidents (44 percent) should have been prevented by the basic GPWS equipment that had been installed. In two of these four accidents, the GPWS equipment either was disconnected or malfunctioned; in the other two accidents, poor flight crew coordination after the GPWS warning led to inaction rather than decisive recovery maneuvers.

The Volpe center further found that, for all nine accidents, enhanced GPWS would have provided more warning time than GPWS (which was assumed to be 12 to 15 seconds). For seven of the accidents, warning times with enhanced GPWS would have exceeded those of GPWS by more than 20 seconds; two of the accidents would have involved differences of more than 1 minute. The center concluded that "in general, [enhanced GPWS] should have provided an additional margin in which flight crews could assess their situation, discover errors, regain situational awareness, and take appropriate action before impact." The center noted only one accident for which an assumed enhanced GPWS warning duration would have been only slightly above the 12- to 15-second GPWS warning. The center argued that this case, which involved a pilot's fatal wrong turn toward mountains, might have been prevented by the visual forward-looking terrain display installed in enhanced GPWS. Thus, the center believed that it was reasonable to assume that enhanced GPWS could have prevented all nine (100 percent) of these accidents.

The Part 121 and 135 study credited GPWS as a significant factor in reducing the frequency of CFIT accidents since 1975. However, the center concluded that "there is compelling evidence of the potential effectiveness of [enhanced GPWS] in preventing CFIT accidents." The study emphasized that CFIT accident prevention would result not only from the increased warning time after the enhanced GPWS detected terrain threats but also from the system's continuous terrain display, which would enable flight crews to perceive terrain threats and respond to them well before enhanced GPWS would generate its warnings.

1.18.2.4 Previous Safety Board Recommendations on Traditional and Enhanced Ground Proximity Warning Systems

In 1971, the Safety Board began issuing numerous recommendations to the FAA regarding the installation and upgrade modification of GPWS. (The FAA first mandated