calls at 100 feet above minimums and at minimums, with a positive statement at the latter point as to whether or not the runway was in sight. Finally, there was no report that the rate of descent exceeded 1,000 feet/minute, although the rate of descent for the 10-second period prior to the level off was 1,350 feet/minute.[1]
Apart from the first officer's deviations with respect to callouts, the captain also deviated from prescribed procedures by failing to level off the aircraft at or above what he believed to be MDA. Thus, when the first officer called out "Two hundred above," the captain should have anticipated reaching MDA, and should have taken action to assure that the aircraft would be levelled off by the time the aircraft reached MDA. Instead, the captain did not start to rotate the aircraft until several seconds after the "Four hundred feet" callout, with the consequence that the aircraft sank an additional 90 feet before the descent was finally arrested.[2]
It is difficult to assess the impact of the above deviations on the descent of the flight below MDA. Although strict adherence to optimal approach procedures is of critical importance in executing a nonprecision approach under actual instrument conditions and might have made a difference in this instance, it nevertheless appears that the crew was aware of altitude, as reflected by the cockpit conversation, and in fact initiated a go-around when they believed they had reached MDA.
The remaining and critical question is why the descent through MDA was not recognized by the crew. After carefully studying the evidence bearing on this question, the Board is of the view that there are only two reasonably possible explanations.
The first of these possibilities is that the crew was using the barometric altimeters to determine their height above MDA and the vertical speed indicators to monitor the rate of descent during the approach, but that these instruments were providing erroneous information. It is
- ↑ The standard procedure of selecting 25° flaps until the runway was in sight was also not followed, since the flaps were apparently lowered to 50° at the outer marks. However, this decision by the captain was basically sound, as demonstrated by the subsequent change in Southern's procedures.
- ↑ The flight recorder altitude trace reflected an altitude of 1,005 feet m.s.l. when the "Four hundred" callout was made, whereas, the initial impact with the trees occurred at 915 m.s.l.
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