Page:CAB Accident Report, Pan Am Flight 115.pdf/4

- 4 - Another B-707 was ferried to Gander to transport the passengers to New York. All passengers and crew continued the flight to its planned destination.

The damage consisted mainly of buckles in the lower surface skin of the right and left horizontal stabilizers and buckles in the center section web and upper surface doubler, and both wing panels were damaged including shear wrinkles in the rear spar webs and damage to the outboard aile1'ons and aileron control rods. The wing-to-fuselage fairings were damaged and a three-foot section of the right fairing separated in flight. Both wing panels suffered a small amount of permanent set. All four wing-to-strut fairing sections of the engine nacelle struts were buckled. Nos.2 and 3 nacelle shear bolts partially failed in shear and the fitting holes of all front spar-to-wing bushings were elongated.

During the early part of the descent the stabilizer pitch trim which varies the angle of incidence of the adjustable horizontal stabilizer was seen by the flight engineer to move toward a nosedown position. When Captain Lynch returned to the cockpit the stabilizer trim was in the full nosedown position. The horizontal stabilizer may be varied electrically by means of an electric trim button on the upper outboard side of the pilot's control wheels. The autopilot disengage switch is located on both pilot's control wheels immediately below the trim control switch. Copilot Peters testified that during the descent he did not touch the electric stabilizer trim switch but that he remembered pushing the auto-pilot release button. He also said that the speed brake was not used at any time.

Because of the nature of this accident, certain system and components of the aircraft were definitely suspect; these were the autopilot, Mach trim systems and their warning systems. As a part of the Board's investigation comprehensive tests of these systems were made at the Boeing Airplane Company plant near Seattle, Washington, under the direction of a CAB investigator.

The PB-2OD autopilot provides sensitive automatic, coordinated control of the aircraft. It incorporates all the necessary switches, including mode selector switches, for normal autopilot operation; in addition the system includes a comparator unit. Basically, the comparator is a second autopilot computer that monitors response of the autopilot. By keeping autopilot Signals under constant observance it monitors against step command (hardover) signals or gradual error condition buildup (slowovers) and automatically disengages the autopilot in response to such signals. The system is designed to be sensitive with respect to autopilot disengagements in the interest of safety and passenger comfort.

On several previous flights of B-707 aircraft there have been disengagements of the autopilot without the customs of a mechanical failure and after which the system functioned normally when the autopilot was reengaged. One such disengagement occurred on the immediately preceding flight of the subject aircraft. On this occasion the warning light failed to come on and the crew became aware of the disengagement by observing a 20-degree right-wing-low attitude on the horizon indicator. Before recovery was effected approximately 600 feet in altitude was lost. The warning light was Checked by the crew immediately after the occurrence and it did not light.