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All flight control systems were carefully studied for any indications of possible control malfunctions. Absolute positions of control linkages and cables could not be established, because of the extensive breakup. However, there was no evidence of any control system failure or malfunction except those associated with in-flight breakup or ground impact. The stabilizer trim jackscrew was found positioned to within 3⁄32 inch of the aircraft nosedown mechanical stop. This is the stopping point of the jackscrew when it is operated electronically.
There was no evidence of arcing, burning, or electrical overload on any of the generators. All available wiring bundles were examined for evidence of electrical arcing or bending but none was found. There was no evidence of a lightning strike on any of the wreckage. A portion of the fuel vent system in the No. 1 reserve tank was never recovered, however, the remainder of the venting in both wings was unobstructed and showed no fire damage. There was no evidence of internal wing tank fires prior to initial breakup. In addition, no evidence of hail damage was found on the nose section, or the leading edges of the wing, tail, or engine coverings.
Examination of the aircraft instruments revealed that the nosedown rotational pitch stops of both vertical gyros, which furnish pitch and roll displacement intelligence for the HZ-4[1] and other devices, received severe impact damage as a result of a rapid rotation of the aircraft about its pitch axis. The compass instruments were indicating northeasterly headings at the time power was interrupted.
Selected samples of the aircraft wreckage were sent to the Federal Bureau of Investigation (FBI) laboratory for examination. However, no explosive residues were found.
- ↑ Northwest's 720B aircraft are equipped with an HZ-4, combined flight director and attitude indicator, for each pilot. The captain's is powered by the essential bus and the first officer's by the No. 2 bus. The instrument face is four inches in diameter and displays actual aircraft pitch and roll, as well as glide slope, and localizer or VOR computed information. The visual display of attitude is accomplished by movement of a servo-driven ball. As the ball rotates, a white centerline representing the horizon is displaced in relationship to a fixed "miniature airplane." The line moves 0.037 inch for each degree of pitch change, up to 85 degrees, at which time control precession occurs. A vertical scale of short bar-like marks is placed on the all black face of the ball to indicate face displacements of 10, 20, and 40 degrees. A marking of "2" and "4" is found at the 20 and 40-degree marks, and the oars denoting nosedown pitch are slightly longer than those on the noseup scale. In this type presentation the horizon line tends to disappear from the face of the instrument in extreme pitch attitudes because of the curvature of the ball. A roll attitude of up to 60 degrees in either direction is displayed on a scale at the top of the instrument. Since the roll pointer is attached to the ball, which remains stabilized with the actual horizon, and the aircraft "moves around the ball," the pointer is displaced the correct number of degrees on the scale, but indicates this displacement on that portion of the scale opposite to the direction of the turn.
