Page:NTSB Southern Airways Flight 932 report.pdf/27

 One area which the Board carefully considered is the extent to which the final stages of the descent were influenced by visual reference to lights on the surface. Conversation between the captain and first officer during the 10-second period preceding MDA passage indicated that they were beginning to see the lights on the ground, or at least the glow of the lights. It is possible that the sighting of these lights, in combinations with the knowledge that they were approaching the bottom of the lowest cloud layer, could have induced the captain to continue the descent below MDA in order to see the runway environment at the earliest moment. The descent, in fact, did continue through this period, and ground witnesses observed the aircraft clear of clouds in this area.

It is also possible that the conduct of the approach could have been affected by a visual illusion produced by the difference in the elevation of the refinery and the airport. Approximately 2 miles from the runway, the flight was approaching the bright lights surrounding the refinery and, as noted above, the crew was discussing at least the glow, and probably fleeting glimpses, of ground lights through the broken clouds. As the descent continued, the opportunity for ground reference through scattered clouds would have increased. Below approximately 1,100 feet m.s.l., the reported cloud base, the only restriction to visibility should have been the fog, smoke, and light rain.

If the approach lights or sequence flasher lights were sighted while the refinery lights were still in the field of vision, with no appreciable lights between, the pilots would have mentally visualized both light sources at the same elevation as the nearer lights. Therefore, the height above both lights would appear to be at 700 feet, whereas the actual height above the approach lights would have been only 400 feet, due to the 300-foot difference in elevation between the refinery and the approach lights. After the aircraft passed the refinery, the preconceived image would have been retained and the visual cues would have told the crew that they were approximately 300 feet higher than desired.

The remaining evidence, however, strongly suggests that the crew never obtained visual contact with the approach lights or with any part of the runway environment. The visual illusion discussed above, for example, would have prompted an increase in the rate of descent, which significantly is not reflected on the flight data recorder. Even more important, there were no comments on the cockpit voice recorder pertaining to ground lights other than those mentioned as the aircraft passed over the refinery. If any lights associated with the runway environment had been sighted, it can be presumed that some mention of the sighting would have been made. Certainly,