Page:NTSB Southern Airways Flight 932 report.pdf/31

 Other damage to the internal mechanism of each barometric altimeter precluded positive determination of their operating capability prior to impact. Nevertheless, if the mark at the 1,250-foot point on the captain's altimeter drum was made at initial impact, the altimeter was reading 300 feet high. Similarly, the marking on the first officer's altimeter could be construed to indicate an error of approximately 300 feet. To place the significance of these markings in proper perspective, however, it should be noted that tests conducted subsequent to the accident demonstrated that the 300-foot difference could have been caused by impact forces.

Finally, evidence supportive of an altimeter error can be derived from the cockpit voice recorder. During the final stages of the descent, the first officer made four altitude callouts. All of these callouts except the first, which was made by reference to the ground, were approximately 200 feet higher than the actual altitude of the aircraft as reflected by the flight data recorder. Since the barometric altimeter is the primary source of altitude information, it would be reasonable to assume that these callouts were made by reference to that instrument.

The foregoing discussion constitutes one possible explanation for the unrecognized descent through MDA by demonstrating how an error in the static system could mislead the pilots by causing erroneous indications on the barometric altimeters and the vertical speed indicators. If such an error did, in fact, occur, then the altimeter would have read 200-300 feet high, which in turn would account for the fact that the crew did not arrest the descent until the aircraft reached an altitude of approximately 916 m.s.l. or over 300 feet below MDA.

There is one remaining factor which must be considered in evaluating the likelihood of an error in the static system instruments. Since an error in the static system would also affect the indicated airspeed, the Board calculated the effect of a static system error sufficient to cause an indicated altitude error of approximately 300 feet. The calculation assumed that a static pressure difference existed between ambient and that sensed by the altimeter so that when the altimeter indicated 1,240 feet, the actual altitude was 916 feet. By use of the calibrated airspeed of 130 knots and the