Event Details

Title:Roll to the right during descent, Boeing 737-3B7, August 25, 1995
Micro summary:This Boeing 737-3B7 veered to the right during descent.
Event Time:1995-08-25 at 1230 EDT
File Name:1995-08-25-US.pdf
Publishing Agency:National Transportation Safety Board (NTSB)
Publishing Country:USA
Report number:MIA95IA214
Site of event:Fort Lauderdale, FL
Departure:Philadelphia International Airport, Philadelphia, Pennsylvania, USA
Destination:Fort Lauderdale-Hollywood International Airport, Fort Lauderdale / Hollywood, Florida, USA
Airplane Type(s):Boeing 737-3B7
Flight Phase:Descent
Type of flight:Revenue
Serious Injuries:0
Other Injuries:0
Executive Summary:

NTSB short summary:

Low electrical resistance of the autopilot aileron actuator for unspecified reasons which resulted in an uncommanded roll to the right.

NTSB synopsis:

The flight experienced an uncommanded roll to the right during descent for landing. The 'b' autopilot system was engaged at the time. Post incident examination showed the 'b' autopilot aileron actuator had low electrical resistance. Tests by boeing commercial airplane company have shown that low electrical resistance within the autopilot aileron actuator will cause uncommanded roll occurrences.

NTSB factual narrative text:

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.
Learning Keywords:Operations - Upset - Uncommanded or excessive Roll
Systems - Autopilot/Autothrottle
Close match:ILS interference with autoland, Boeing 757-3CQ, G-JMAA
In-flight upset, China Airlines, Boeing 747 SP, N4522V, 300 Nautical Miles Northwest Of San Francisco, California, February 19, 1985
Uncommanded roll to the left, Boeing 737-3B7, July 18, 1995
Uncommanded roll and yaw, Douglas DC-9-34, April 30, 1998


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