LOS ANGELES, CA, USA
N1568W
BEECH 1900C
WHILE ON FINAL APPROACH, THE AIRPLANE EXPERIENCED A RAPID, UNCOMMANDED ROLL TO THE LEFT WHEN THE 35-DEGREE FLAP SETTING WAS SELECTED. THE PILOT WAS ABLE TO COUNTER THE ROLL BY APPLYING FULL OPPOSITE AILERON AND LANDED WITHOUT FURTHER INCIDENT. AN EXAMINATION REVEALED THAT THE WING FLAPS HAD BEEN FULLY EXTENDED IN THE AFT DIRECTION, BUT THAT THE INBOARD TRAILING EDGE OF THE LEFT OUTBOARD FLAP HAD BEEN FORCED UPWARD ABOUT 3 INCHES. THE INBOARD AFT FLAP TRACK ROLLER BEARING ASSEMBLY, INCLUDING THE ATTACHING BOLT AND WASHER, HAD PULLED THROUGH THE SKIN OF THE FLAP TRACK HINGE BRACKET. AN ANALYSIS OF THE FAILED PARTS BY BEECH REVEALED THAT THE OUTER ROLLER WAS LOOSE ON THE BEARING AND COULD BE SHIFTED AXIALLY. A CIRCULAR WEAR PATTERN MATCHING THE DIAMETER OF THE OUTER ROLLER FLANGE WAS FOUND ON THE PORTION OF FLAP TRACK HINGE BRACKET WHICH HAD PULLED OUT. BEECH CONCLUDED THAT THE BEARING OUTER ROLLER HAD SHIFTED ON THE ROLLER ELEMENT BEARING AND ALLOWED THE OUTER ROLLER FLANGE TO WEAR AGAINST THE SIDE OF THE FLAP HINGE BRACKET UNTIL THE BEARING ASSEMBLY WAS PULLED OUT. THE FAILED FLAP TRACK HINGE BRACKET AND BEARING ASSEMBLY IS CONTAINED WITHIN THE INTERIOR OF THE FLAP STRUCTURE.
On March 17, 1995, at 1050 Pacific standard time, a Beech 1900C, N1568W, encountered an asymmetric flap deployment and subsequent uncommanded roll while on final approach for Los Angeles International Airport. The aircraft was operated by Alpha Aviation, Inc., dba TW Express, as flight 7980, a regularly scheduled domestic passenger flight under the provisions of 14 CFR Part 135 of the Federal Aviation Regulations. Visual meteorological conditions prevailed at the time and an IFR flight plan was filed. The aircraft sustained minor damage to the outboard left flap and its associated attachments. The two certificated airline transport pilots and the 17 passengers were not injured. Flight 7980 originated at San Francisco, California, on the day of the incident at 0730, and made en route stops at South Lake Tahoe and Mammoth Lakes, California. The Alpha Air system chief pilot was acting as the first officer for the flight, and was also the flying pilot in the incident. In a telephone interview, both the captain and chief pilot stated that the flight was on final approach to runway 24R, descending through 300 feet agl with 20 degrees of flaps selected. Shortly after the 35-degree flap setting was selected, the aircraft entered a rapid roll to the left, to about 35 degrees of bank. The chief pilot said he was able to counter the roll with full aileron application and landed without further incident. Postflight examination of the aircraft by company maintenance personnel revealed that the left outboard flap was fully extended, but forced upward at the inboard trailing edge about 3 inches. The inboard aft roller bearing center portion, with the attach bolt and washer intact, was found pulled through the mounting brackets. The bearing assembly, with associated attach hardware and mounting bracket, was removed from the aircraft and returned to Beech Aircraft for laboratory evaluation under the supervision of the Federal Aviation Administration (FAA). According to Beech, the bearing assembly consists of a standard roller element bearing pressed into a cylindrical outer roller with a flange on one end (the laboratory report and vendor drawings are attached to this report for reference). The installation provides clearance at each end of the outer roller when the bearing assembly is positioned between the supporting structure with a mounting bolt installed through the hole in the center ring of the bearing. In pertinent part, the laboratory report noted that the outer roller was found loose on the bearing and could be shifted axially on the bearing outer ring. A circular pattern of wear, similar in dimension to the diameter of the outer roller bearing flange, was found on the mounting bracket portion which pulled out with the mounting bolt. The area inside of the worn circle pattern was of a diameter similar in dimension to the inside diameter of the bearing outer roller and exhibited no perceptible wear or abrasion. The report concluded that the bearing outer roller shifted on the roller element bearing and allowed the outer roller flange to wear against the side of the mounting bracket until the assembly pulled out of the bracket. The report further noted that due to damage that occurred to the bearing outer roller during jamming and binding of the bearing, the dimensions and degree of interference fit between the outer ring of the bearing and the bearing outer roller could not be determined. No other occurrence of a bearing shift was found during a review of both Beech Aircraft records and the FAA Malfunction Defect Reports and Service Difficulty Report data bases. Review of the aircraft maintenance records disclosed that the aircraft had accumulated a total operating time of 11,711 hours and 13,291 cycles. No damage or major repair history was noted for the wing flap areas. The FAA airworthiness inspector who initially examined the aircraft reported that the failed bracket area is hidden inside the flap structure and cannot be viewed by external visual inspection methods. He also reported that both the Beech and Alpha Air Continuous Inspection Programs do not contain specific inspection requirements to look for wear at the flap hinge bracket bearing support areas.
AN AXIAL SHIFT OF THE OUTER BEARING ROLLER FOR AN UNDETERMINED REASON, RESULTING IN EROSION AND FAILURE OF THE FLAP TRACK HINGE BRACKET/BEARING ASSEMBLY. FACTORS RELATING TO THE INCIDENT WERE: THE ROLLER BEARING AND ASSOCIATED BRACKET ASSEMBLY WITHIN THE INTERIOR OF THE FLAP STRUCTURE COULD NOT BE ADEQUATELY INSPECTED WITHOUT DISASSEMBLY; AND LACK OF INSPECTION CRITERIA IN THE MANUFACTURER'S MAINTENANCE MANUAL CONCERNING FLAP ROLLER/HINGE BRACKET ASSEMBLIES.
Source: NTSB Aviation Accident Database
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