Page:NTSB Southern Airways Flight 932 report.pdf/33

 to the static system. But until this or other efforts produce positive evidence of a phenomenon which could cause offsetting errors of the type discussed above, the Board cannot conclude that a static system error is supported by sufficient evidence to be termed a casual factor in this accident.

The second reasonably possible explanation for the unrecognized descent below MDA is that the first officer was using the radio altimeter as the primary source of altitude reference, and the crew was thereby misled into believing the aircraft was higher than it actually was because the ground surface in the approach area is at some points substantially lower than the field elevation. Support for this theory can be derived from an analysis of the altitude callouts on the cockpit voice recorder. There were at least four references to altitude after the flight passed the outer marker inbound. Since the crew had no way of determining the elevation of the terrain below them, the values could have been either read directly from the radio altimeter or calculated mentally by subtracting the field elevation from the barometric altimeter reading. The following tabulation shows (1) the first officer's callout, (2) the flight recorder indication, (3) the terrain elevation at that point, (4) the calculated radio altimeter reading, based on the flight recorder altitude minus the terrain elevation, and (5) the flight recorder altitude reading minus the field elevation (828 feet).

Any reliance on these figures must include recognition of their limitations. The computed flightpath of the aircraft may be affected by such