Page:CAB Aircraft Accident Report, American Airlines Flight 320.pdf/17

- 17 - upon approximate pointer position rather than consciously to read the numerical indications associated with each pointer position. Such a tendency is, of course, heightened at night when precise reading of instruments is more difficult won with optimum instrument lighting. Accordingly, there is a strong suggestion that a pilot with limited exposure to this particular instrument might be led to accept an excessive rate of descent because of a general appearance of instrument indication being within a range normal for an instrument approach when using the older instrument.

A computation of the times reported over New Rochelle and the La Guardia Range Station indicates that the ground speed of Flight 320 on the approach between these fixes was approximately 129 knots. A computation of the times reported over New Rochelle and the La Guardia Range Station by the five flights preceding Flight 320, and an analysis of the winds aloft reports at Idlewild Intemational Airport and surrounding areas taken at 1900, February 3, and 0100, February 14, indicates that Flight 320 would be making its approach into a mean wind of approximately 25 knots from about 210 degrees. The indicated airspeed between New Rochelle and the La Guardia Range Station appears, therefore, to have been in the order of 150 knots. At this indicated airspeed and at the power settings which the flight crew were using, a rate of descent higher than that necessary for this portion of the approach procedure appears to be likely.

According to the testimony of the flight crew, the aircraft passed over the New Rochelle marker at 1,500 feet. The procedure called for the flight to cross the La Guardia Range Station approximately 11.8 miles southwest at 800 feet. At 110 knots indicated airspeed, this distance would, in a no-wind condition, require approximately 2:03 minutes to traverse. Since the flight had approximately 700 feet to descend between these two fixes, a rate of descent in the order of 350 feet per minute would have sufficed. If the pilot at New Rochelle were to adjust the attitude so as to obtain an apparent 350 feet per minute, based upon his experience with the older vertical speed indicators, he would have realized between 700 and 1,000 feet per minute. The result of an excessive rate of descent would be for the airspeed to increase. The evidence of record shows that the airspeed was in fact higher than the 140 knots, to which the crew testified.

We regard it significant that the ground trainer in which the captain received initial training on the Bendix Flight Director System had installed the conventional vertical speed indicator and not the instrument which was actually installed in the Electra.

The possibility of misreading either the altimeter or the vertical speed indicator could hardly of itself satisfy the Board's search for a probable cause of the accident. There exist far too many other sources of cross-check for the pilot; moreover, the copilot has full facility for monitoring and cross-cheating through a completely independent flight instrument panel. Presumably, misinterpretation of one condition of flight should not normally result in gross displacement of aircraft position. However, preoccupation with any one other cockpit problem may set the stage for a dangerous drift from a desired flight path when a certain combination of circumstances exists, particularly when that environment includes any insidious or misleading assurances of a safe flight condition. (See /Appendix C/.)