-8-
in effect, inverts all levels‘of reflectivity above 40 dBZ and displays them as a black hole ' surrounded by luminescent green areas. The 40-dBZ reflectivity level corresponds , approximately to a National Weather Service (NWS) level 3 radar echo (see section 1.7).
The display area of the PPI is about 3 1/2 inches in diameter. With a 50-nmi range selection, a weather cell with a diameter of 10 to 15 nmi would cover a diameter of 0.6 to 0.9 inch on the PPI. If the precipitation contained in the cell exceeded a 40-dBZ reflectivity, and the pilot selected contour mode,'that part of the cellexceeding the 40-dBZ level would contour and appear as a black hole on the PPI. As the range between the airplane and the cell decreased, the dimensions of the cell portrayal would remain constant, but the portrayal would move downward toward the origin point of the antenna sweep at the bottom center of the PPI. If - ground returns were being'displayed en the-PPI as the airplane approached the cell, the pilot would have to increase the antenna tilt until the ground returns were eliminated. As the airplane closedto within 2 nmi of the cell, the cell‘s radar return would begin to disappear at the base of the PPI. '
I’ The airplane's logbook showed that flightcrews had written up the weather.
V radar system seven times between June 6 and July 25, 1985. The logbook entries also showed that corrective action had been accomplished after each flightcrew entry. After July 25, no further entries concerning the weather radar were found nor were any carry— over maintenance items On this system found. '
1.7 Meteorological Information
The 16-00 NWS surface analysis weather chart issued by the National Meteorological Center, Camp Springs, Maryland, showed a weak diffuse stationary from about 60 nmi north of the DFW Airport. The 1900 NWS surface analysis chart also showed a weak diffuse cold front about 60 nmi north of the DFW Airport.
The NWS terminal forecast for the DFW Airport pertinent to the accident indicated a slight chance of a thunderstorm witha moderate rain shower. The NWS area forecast pertinent to the accident called for isolated thunderstorms with moderate rain
showers for northern and eastern portions of Texas. The terminal forecast was issued by -
the NWS Forecast Office in Fort Worth, Texas, and the area forecast was issued'by th National Aviation Weather Advisory Unit in Kansas City, Missouri. -
There were no SIGMETS, 1_4/ convective SIGMETs 1_5/, Severe'We‘ather V’Warnings, Local Aviation Warnings, Severe Weather Watches, or Center Weather - Advisories (CWA) in effect for the time and area of the accident. ‘ ' ’ ' '
The company's dispatch and meteorology department provided the flighterew with a dispatch package which contained the following weather documents: the weather at DFW Airport and at the flight's field alternate, San Antonio; a DFW Airport terminal weather forecast indicating widely scattered moderate rain showers and thunderstorms with moderate rain showers; an en route forecast indicating isolated thunderstorms, moderate rain showers over Oklahoma and northern and northeastern TeXas with afew _ isolated tops above 45,000 feet; and Delta Metro Alerts applicable to the'route of flight. . The forecasts were prepared by Delta meteorologists. ' .
1_4/ A weather advisory concerning weather significant to the safety of all aircraft. 1_5/ Convective SIGMETs are issued by the National AviationWeather Advisory Unit, Kansas City, Missouri, for lines of thunderstorms, severe/embedded thunderstorms of any intensity level, and for areas of 3,000 square miles or larger with VIP level 4 (see section 1.7.1) or greater covering” at least 40 percent of the area.' I' - '
