Page:CAB Aircraft Accident Report, American Airlines Flight 320.pdf/18

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The Eclipse-Pioneer autopilot, model PB-20E, used by American Airlines on its Electra airplanes is designed to permit completely automatic control of the airplane from initial climb through an ILS approach. Automatic navigation and approach, and automatic altitude and pitch trim control are incorporated in the system. An automatic ILS approach utilizing the PB-20E is possible only where a glide path is suitably paired with a localizer course. Since no glide path is available, an automatic approach could not be made on the backcourse of the La Guardia ILS.

Testimony of the first officer and flight engineer indicates that Captain DeWitt was making a "heading mode" autopilot approach and that the autopilot was still engaged at impact. In this method, the pilot uses the autopilot as an intermediate system to operate the flight controls instead of operating them directly through the control wheel and rudder pedals in the conventional manner. The airplane is steered by selecting a desired heading on the course deviation indicator; the autopilot than directs the airplane to this heading. Captain DeWitt had been using this procedure during the entire flight after the takeoff climb. According to testimony, he was actuating the pitch trim wheel, which is mounted in the autopilot controller on the pedestal to his right and slightly aft, with his right hand and was leaning forward in his seat to reach around the control wheel with his left hand to rotate the CDI cursor just prior to impact. Since no glide slope is available, proper altitude over fixes must be checked solely by reference to the altimeter.

The FAA-approved portion of the Electra Airplane Operating Manual contains no limitations as to the type of approach for which the autopilot may be used; nor does it establish any minimum altitude limitations for use of the autopilot. While it is recognized that the autopilot system installed in the Electra is capable of complete automatic control of descent path and direction in an ILS approach, the Electra Airplane Operating Manual specifies that the autopilot must be off before leaving a holding point for final approach.

Although Captain DeWitt had made many actual instrument approaches to La Gaurdia Airport in other than Electra aircraft, and had made several simulated instrument approaches in the Electra, the Board could find no instance where he had made a previous backcourse IIS approach to La Guardia Airport in an Electra under actual instrument conditions. Records were not available to determine how much previous experience Captain DeWitt had in making simulated or actual instrument approaches in the Electra using the "heading mode" autopilot setting.

Most of the wreckage was found within a 200-foot radius circle, the center of which was located approximately 4,891 feet short of the threshold and 610 feet to the right of the extended centerline of runway 22. It was determined that the horizon director indicator and course deviation indicator would indicate full-scale deflections if any aircraft were approximately 300 feet to the right of the localizer centerline and approximately 5,000 feet from the localizer shack. The lateral displacement of this aircraft from the localizer course is excessive and indicates that the pilot might not have been maintaining proper alignment with the localizer course during the latter portion of the approach.

The captain's and first officer's RMDI's, when removed from the water, read 205 degrees and 219 degrees, respectively. Although the heading indications of the two RMDI's reasonably agree with the instrument approach heading to be flown, neither