altimeters in instrument approaches over level and irregular terrain. However, the Southern Airways DC-9 Operating Manual did not make such a distinction, but rather accentuated its use for all instrument approaches by stating that, "Two separate radio altimeter systems... are provided to obtain precise altitude information above the ground at the minimum decision [sic] altitude (MDA). This information is essential to the pilot in his decision to land or initiate a go-around maneuver." Notwithstanding the fact that the crew may have been formally trained to use the radio altimeter as a secondary reference, the tabulation comparing the available altitude references indicated that the first officer may have relied on the written material and was using the radio altimeter for altitude information.
If the first officer was making altitude callouts by reference to the radio altimeter, as hypothesized above, the remaining question concerns the extent to which the captain relied upon, and was misled by, such callouts. Sound operating procedures dictate that the captain should have been using his barometric altimeter during the approach, and therefore should have been aware of the disparities between altitudes reflected by that instrument and the first officer's callouts. Why these disparities were apparently not detected by the captain is difficult to explain. It is possible that he, like the first officer, was relying on his radio altimeter. A second possibility is that he was not using his barometric or radio altimeter, but rather was relying solely on the first officer for altitude information. Finally, he may have been including his barometric altimeter in his instrument scan, but was concerned with other items during the final stages of the approach to such an extent that he did not notice any variations.
On the other hand, there are several weaknesses to the theory that the radio altimeter was being used for altitude information. First, and perhaps most important, the radio altimeter is not intended for use during an approach over unknown or uneven terrain, and it is therefore difficult to accept that qualified, experienced pilots would resort to that instrument in conducting the approach at Huntington. The theory also assumes an unlikely dual human failure in that the captain was either also using his radio altimeter or did not recognize the differences between the barometric altimeter and the altitude information called by the first officer and was relying on the latter. Finally, the rates of descent between the calls of "Seven hundred feet," "Two hundred above," "Four hundred," and rotation, if made from reference to the radio altimeter, do not correspond to the rates of descent recorded by the flight data recorder for the same periods.
This variation is demonstrated in the following calculations. By using the terrain elevation established by the flightpath analysis for the position
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