N373US

On August 25, 1995, about 1230 eastern daylight time, a Boeing 737-3B7, N373US, registered to Society National Bank and operated by USAir, Inc., as flight No. 93, a 14 CFR Part 121 scheduled, domestic, passenger service from Philadelphia, Pennsylvania, to Fort Lauderdale, Florida, experienced an uncommanded roll to the right during descent for landing. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The aircraft was not damaged and the airline transport-rated pilot, first officer, 4 flight attendants, and 44 passengers were not injured. The flight originated from Philadelphia on August 25, 1995, about 1027.

The captain stated to USAir personnel that about 25 nautical miles northeast of Fort Lauderdale, the flight was descending to 6,000 feet, on a 222-degree heading, at 250 knots. The "B" autopilot system was on longitudinal and vertical navigation. The first officer was flying the aircraft. They felt a small "bump" and the aircraft suddenly rolled uncommanded to the right to about 30 degrees of bank angle. The captain disconnected the autopilot system and returned the aircraft to wings level with aileron control. There was no yaw associated with the roll event. The flight continued to Fort Lauderdale without further incident.

Postincident readout of the digital flight data recorder was performed by USAir personnel. The data shows the aircraft was descending through 6,250 feet on a 225-degree heading when the aircraft rolled to the left about 4 degrees and then rolled to the right to about 20 degrees of bank. It took about 4 seconds for the aircraft roll from 4 degrees left bank to 20 degrees right bank. See the Aircraft 373 Flight Recorder Data.

Postincident examination of the aircraft showed the "B" system aileron actuator had low electrical resistance when tested on the aircraft. Additionally a shear rivet was found failed in the aileron autopilot lever arm which connects the aileron autopilot actuators to the aileron bellcrank and aileron position sensor. Examination of the failed rivet showed it had been failed for a long period of time and the second shear rivet had moved in its hole causing elongation of the hole.

Testing of the "B" system autopliot actuator was performed by the manufacturer. The actuator manifold and electrical hydraulic valve failed the insulation resistance test and the de-energize friction test. See E Systems Failure Analysis Report.

Testing performed by Boeing Commercial Airplane Company after previous uncommanded roll incidents showed that low electrical resistance within the aileron autopilot actuator would cause uncommanded rolls. See Boeing 737 Autopilot Review.