# NTSB MIA95IA214: Boeing 737-3B7, N373US, Fort Lauderdale, FL, August 25, 1995 - Type: NTSB incident investigation - Classification: Incident · No injuries · Incident investigation · Final report - Date: August 25, 1995, 12:30 local (16:30 UTC) - Place: Fort Lauderdale, FL - Coordinates: 26.090032, -80.150047 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=26.0900&lon=-80.1500&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 50 uninjured - Status: Completed, report January 16, 1996 - Report: Approved by Office Director - NTSB case key (mKey): 37679 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=MIA95IA214 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/37679/pdf - Page: https://openairdocs.org/ntsb/MIA95IA214 ## Probable cause LOW ELECTRICAL RESISTANCE OF THE AUTOPILOT AILERON ACTUATOR FOR UNSPECIFIED REASONS WHICH RESULTED IN AN UNCOMMANDED ROLL TO THE RIGHT. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Airframe/component/system failure/malfunction, descent - autopilot/flight director,servo - other (cause) - autopilot/flight director,servo - undetermined - autopilot/flight director,servo - uncommanded ## Analysis 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. ## Factual narrative 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. ## Aircraft - Registration: [N373US](https://openairdocs.org/deregistered/N373US-2006-07-03), registration cancelled 2006-07-03, S/N 22952, built 1985 (OpenAirDocs) - Reported make and model: BOEING 737-3B7 - Registry model: [Boeing 737-3B7](https://openairdocs.org/model/boeing-737-3b7) (registry code 138447F) (OpenAirDocs) - Registry engine: [CFM International CFM56 series](https://openairdocs.org/engine/cfm-international-cfm56-series) (registry code 13802, turbo-fan) (OpenAirDocs) - Category: Airplane - Serial number: 22952 - Airframe hours: 31239 - Engines: CFM CFM 56-3 22000 lbs thrust turbo fan - Damage: No damage - Flight: Part 121 - Operator: Usair, Inc. - Registered owner: Society National Bank - Airworthiness certificate: Standard, Transport - Last inspection: AAIP, August 25, 1995, 4 hours before - Departed: PHL, PHILADELPHIA, PA - Destination: FLL - Flight plan: IFR ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 45; flight instructor, airline transport; airplane ratings: multi-engine land, single-engine land; instrument ratings: airplane; medical: class 1, valid medical--w/ waivers/lim; seat: left; flight hours: 10,820 total, 4 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 160° at 6 knots - Visibility: 10 statute miles - Lowest ceiling: 10,000 ft broken - Temperature: 82 °F, dew point 79 °F - Altimeter: 29.00 inHg - Observed at: FLL, 25 nm away ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Incident 19950825033929I](https://openairdocs.org/incident/19950825033929I), Aug 25, 1995: Boeing 7373B7, Ft Lauderdale, FL. Descending to land. Uncommanded roll to right. Disconnected auto pilot. Landed safely. ## Other investigations on N373US (1) From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. - [NTSB NYC96IA022](https://openairdocs.org/ntsb/NYC96IA022), Nov 2, 1995: Boeing 737-3B7 (N373US), Portland, ME, incident, minor damage. Probable cause: a bird strike during takeoff, which resulted in failure of several fan blades in the right engine and a partial loss of engine power. From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.