Page:CAB Accident Report, Pan Am Flight 115.pdf/5

- 5 - Functional tests of the autopilot system subsequent to the accident showed it to be operable in a. normal manner with the following exceptions:

1. In several instances the autopilot disengage warning light did not function properly after disengagement of the autopilot.

2. The pitch trim potentiometer did not recenter after autopilot disengagement. The mechanical centering of this potentiometer is necessary for the autopilot upon reengagement to have available full noseup or nosedown trim.

The principal components of the autopilot were bench checked. These also functioned in a normal manner with the following exceptions:

1. When testing the amplifier-computer it was found that the operation of relay K-14 was erratic. Two of the contacts within this relay serve the autopilot disengage warning light circuit. Detailed inspection of this unit disclosed a considsrable misalignment of its internal structure, suggestive of the type of damage which might be expected if the relay had been dropped. The damage found resulted in certain of the contact pressures being below standard. In all other respects the relay appeared to be normal. Testing of the vertical gyro transmitter disclosed that the rate switch was inoperative.

2. The vertical gyro transmitter is an electrically driven gyro that provides the vertical reference for the automatic pilot. Incorporated in the assembly is a rate-of-turn control consisting of a hermetically sealed rate gyro and switch. At preset turn rates the switch turns off the vertical gyro erection system, thereby preventing erection of the vertical gyro to a dynamic vertical during turns. Examination revealed that the rate are motor windings were open. A crack was also noted in the glass dash-pot tubing. Subsequent to the examining group's inspection, Boeing engineering personnel observed that the rate switch would occasionally stick when closed manually.

3. As had been previously observed during the autopilot system checks, the pitch potentiometer failed to recenter when the autopilot was disengaged. The unit was opened and it was observed that travel of the actuating lever was excessive, causing a slight binding of the lever. Subsequent to the group's examination, this assembly was inspected by Eclipse-Pioneer Division personnel at their plant. They reported finding some of the solenoid sealing compound within the solenoid in such a way as to make it sticky.

4. Comparison unit tests indicated that a five-degree pitchup change of attitude as a step was required for disengagement as against the feta-degree step change specified. It was also observed that the alarm would occur intermittently at 10 degrees noseup without the step change of pitch attitude normally required for an alarm.

The integrity of the aircraft wirig associated with the autopilot and Mach trim system was checked by making accurate resistance measurements of all circuits. In addition, the wiring in the two control columns, which forms the major part of the autopilot manual disconnect Circuit, was removed and given a detailed Visual inspection.

These checks disclosed no irregularities in the pertinent aircraft wiring.