A study of pressure patterns in the West Virginia area, at the time of the accident, was conducted for the Safety Board by the National Weather Service following the initial public hearing. The study showed that the dominant low-pressure area was elongated toward the northeast with surface pressures dropping at an average rate of 0.013 inch of mercury/hour. This would correspond to an indicated altitude increase of 13 feet/hour. The low-pressure area moved steady northeastward with little change in intensity. Although there was an extensive area of light rain, no showery precipitation (possibly indicative of more rapid fire variation) was reported within 250 miles of Huntington.
1.8 Aids to Navigation
The Tri-State Airport was equipped with an ILS localizer, but no glide slope. The localizer provided a nonprecision approach to Runway 11. The crew's Jeppesen Approach Chart depicting this procedure was dated December 27, 1968; however, the current approach chart at the time of the accident was dated November 6, 1970. (See Attachment 2.) The revised approach chart was incorporated in Southern's charter kits on November 13, 1970, by the chief pilot. Two kits were not available on that date because they were in use at that time, including the charter kit on N97S which had departed at approximately 0830 on this day the revisions were inserted. The basic differences in the two approach charts were: (1) an increase in the Minimum Sector Altitude[1] from 2,500 feet to 2,600 feet m.s.l., for the sector west of the airport (180° clockwise through 360° inclusive); and (2) the addition of holding instructions to the missed-approach procedure text.
The Localizer-Runway 11 approach required a procedure turn southof the 114° localizer course within 10 miles of the outer marker, at 2,600 feet m.s.l. The outer marker minimum crossing altitude was 2,200 feet m.s.l., and further descent was then authorized to the Minimum Descent Altitude (MDA)[2] of 1,240 feet m.s.l. The outer
- ↑ Minimum Sector Altitude—provides 1,000-foot obstacle clearance within a 25-mile radius of a navigation facility (except localizers without a nondirectional beacon). A sector may not be less than 90° in spread, and the obstacle clearance must also apply in adjacent areas within 4 miles of the sector boundary. All altitudes are mean sea level (m.s.l.) unless otherwise indicated.
- ↑ Minimum Descent Altitude is the lowest altitude to which descent shall be authorized in procedures not using a glide slope. Aircraft are not authorized to descend below the MDA until the runway environment (runway threshold, or approved lighting aids or other markings identifiable with the runway) is in sight and the aircraft is in a position to descend for a normal landing
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