During the investigation, considerable attention was focused on the height of the trees on the hill where initial impact occurred. It was determined by an FAA Runway Obstruction Survey, dated December 1, 1970, that several trees on the hill penetrated the ILS approach surface[1] and therefore constituted obstructions to air navigation as defined in Part 77, Subpart C, of the Federal Aviation Regulations. However, these standards are used in (1) administering the Federal-Aid Airport Program, (2) transferring property under Section 16 of the Federal Airport Act, (3) providing technical advice in the airport design and development, and (4) imposing requirements for public notice of construction or alteration of structures where notice will promote air safety. The criteria used in the establishment of flight procedures and aircraft operational limitations are contained in Part 97 and the U. S. Standard for Terminal Instrument Approach Procedure (TERPS). Paragraph 954 of TERPS requires that the minimum obstacle clearance in the final approach area[2] shall be 250 feet for a localizer approach. The trees did not violate this requirement.
A pen recording was made of the outer marker identifier signals as they were recorded in the CVR tape, to assist in locating the flightpath of SOU 932 through the radiation pattern. For the purpose of this evaluation, it was assumed that the receiver sensitivity of the DC-9 was the same as that of the FAA flight-check aircraft. It was also assumed that the identification tone had reached its maximum signal strength when the recorded signal stopped. Based on the calculations, it was determined that the aircraft was approximately 1,850 feet south of the outer marker transmitter when the signal stopped. Any variation in these assumptions would, of necessity, place the aircraft closer to the transmitting antenna than depicted on Attachment 1.
- ↑ Part 77.27(b) defines ILS approach surface as a surface longitudinally centered on the extended centerline of an ILS runway beginning at each end of the primary surface and extending outward and upward at a slope of 50 to 1 for a horizontal distance of 10,000 feet and at a slope of 40 to 1 for an additional 40,000 feet. This surface is the width of the the primary surface at the beginning and expends uniformly to a width of 16,000 feet at a distance of 50,000 feet from the end of the primary surface. The primary surface of Runway 11 was 1,000 feet wide and extended 200 feet beyond the threshold at each end of the runway.
- ↑ Paragraph 930(1) gives the dimensions for the final approach area as 50,000 feet long measured outward along the final approach course from a point 200 feet outward from the runway threshold, and 1,000 feet wide at the point expanding uniformly along the localizer course to a width of 16,000 feet at a point 50,000 feet from the beginning point.
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