Page:CAB Aircraft Accident Report, American Airlines Flight 320.pdf/15

- 15 - this analysis, it must relate to the earlier portion of the approach. Therefore, with respect to the possibility that the captain's altimeter had failed, we are of the view that such a failure did not occur before reaching 600 feet. Although the Board has renewed all available records concerning instrument failure, none appear to be of the nature and magnitude of that suggested here. As has already been indicated, no evidence of instrument failure was discovered in the examination of the wreckage.

The sole evidence of a malfunction of the captain's altimeter is Flight Engineer Cook's testimony. His observation of the captain's altimeter, after some clarification of its facial presentation, was 500 feet at impact. This was after the first officer had called out 600 feet. He made the observation intuitively after a momentary glimpse of lights through the captain's side window which alerted him to the fact that they were extremely low.

To substantiate further a single failure on the captain's altimeter, we must assume a premature descent and discount First Officer Hlavacek's testimony that he called out 900 feet over the La Guardia Range Station. This is necessary in order to rationalize approximately a 300-foot per minute descent as testified to by him.

The captain's altimeter was set at 29.83. The actual pressure at the time of the accident was 29.75, and La Guardia tower was reporting a setting of 29.77. This error in setting of the captain's altimeter would account for 80 feet of erroneous altimeter indication. Since an additional minus 30 to 145 feet of error due to static air correction must be made to the captain's altimeter, one can readily rationalize an accumulative error in which the altimeter indicated from 110 to 125 feet higher than the actual altitude near sea level.

A premature descent is substantiated by the eyewitnesses to Flight 320 just prior to and after its passage over the La Guardia Range Station. As stated earlier, the Board believes Flight 320 approached the range station at a height greater than 300 feet and probably not much higher than 400 feet above the ground because of the prevailing ceiling at the time.

Because of the novel presentation components of the altimeter, serious consideration has been given the possibility that the pilot misread the altitude indication and thereby permitted or caused the aircraft to deviate vertically from the desired flight path. While some incidents have been reported in which a pilot had misread the 1,000-foot scale on the small drum, no such error could conceivably be involved here. The altitude presentation below 1,000 feet is accomplished by a pointer the indications and appearance of which are, for all practical purposes, identical to those used by the crew in other aircraft types and the interpretation of which calls for no new or different evaluations on the part of the pilot.

The reversed sensing of the altimeter setting numerals has already been mentioned. While an erroneous setting might result from this condition, the possible order of error would be very small indeed. Unlike the primary instrument flight reference which is frequently "generalized" by approximate positions of pointers or indicators, the altimeter setting scale must be read in order to permit any substantial correction to be made. Furthermore, the altimeter setting positions which were found in the altimeters installed in this airplane are very reliable indications of the settings existing at the time of the accident. The