WILLISTON, ND, USA
N118UX
BEECH 1900D
DURING AN INSTRUMENT DESCENT FROM CRUISE, THE ELEVATOR TRIM CABLE FAILED. THE CREW EXPERIENCED INCREASING NOSE DOWN PITCH REGARDLESS OF INPUT. A SAFE LANDING WAS ACCOMPLISHED BY BOTH PILOTS PULLING AFT ON THE YOKE TO OVERCOME PRESSURE, AND LOWERING FLAPS AND LANDING GEAR 50 AND 20 KNOTS ABOVE THEIR RESPECTIVE OPERATING SPEEDS TO REDUCE THE TRIM FORCES AND HELP SLOW THE AIRPLANE. INSPECTION REVEALED THAT THE TRIM CABLE HAD BEEN INCORRECTLY ROUTED AROUND A CABLE GUIDE DURING INITIAL PRODUCTION, AND HAD NOT REACHED THE FIRST REQUIRED INSPECTION WHICH WOULD HAVE ALLOWED THE DISCREPANCY TO BE DISCOVERED.
On January 12, 1995, at 1220 central standard time (cst), a Beech 1900D turboprop, N118UX, operated by Great Lakes Aviation, Limited, of Spencer, Iowa, as United Express flight number 5841, lost elevator trim control during a descent from cruise to Sloulin Field International Airport, Williston, North Dakota (ISN). The crew was able to override the trim forces, and the airplane was landed with no other damage or injuries. The flight was conducted under 14 CFR Part 135, as a scheduled domestic passenger flight. Visual meteorological conditions prevailed at ISN, and the airplane was in instrument meteorological conditions when the incident occurred. Flight 5841 had departed from Bismarck, North Dakota, at 1208 hours central standard time. The crew stated that operation of pitch trim in either direction resulted in increasing nose down trim. Pitch control to override this nose down condition required both pilots pulling aft on the control column, with the copilot placing both feet on the instrument panel. In order to maintain control of pitch and airspeed the crew decided to lower the trailing edge flaps at an airspeed of 240 knots, and the landing gear at 197 knots. Maximum flap operating speed (Vfe) is 188 knots at 17 degrees of flap, and the maximum landing gear speed (Vle) is 180 knots. The airplane had accumulated 410 hours of total time since its initial delivery after production. Post incident inspection revealed that the elevator trim cable running over the upper right crossover pulley at the top of the vertical tail was misrouted over a cable pulley guide pin and failed while the airplane was trimmed for a descent. Safety Board metallurgical examination of the failed cable confirmed fatigue failure due to wear across the guide pin. Inspection also revealed that the misrouted cable rubbed a groove in the uppermost empennage structural rib. The unrestrained cable was discovered unwound from the forward trim cable drum located in the pedestal between the pilots. The right side cable turnbuckle located at the bottom of the empennage was bent. N118UX was maintained under a continuing airworthiness inspection program, and the first inspection after delivery which would have accessed the control cable inspection area in the upper empennage was at 750 hours. No access had been made to the area of the incorrectly routed cable since initial aircraft delivery. Inspections of all BE-1900D airplanes still in production discovered no similar misrouting. Immediate inspections prior to further flight of all 131 BE-1900D airplanes in registered United States service revealed no similar discrepancies. A review of the tape recording from the Cockpit Voice Recorder (CVR), Fairchild A100A, serial number 61866, was performed at the Safety Board CVR lab, and no transcript was made. Review of the last 30 minutes of the flight confirmed that the crew became aware of a trim difficulty during descent from the 10,000 foot above sea level (MSL) cruise, and jointly discussed options to maintain control of the airplane prior to altering the airplanes configuration. Both the captain and the first officer agreed that they were above Vfe, but that it was necessary to extend the trailing edge flaps to reduce the nose down control pressure. Both crewman discussed and agreed to extend the landing gear above Vle in order to reduce airspeed prior to attempting to land. Throughout the last 15 minutes of the flight, the crew discussed each maneuver prior to taking the action, and stated that they had to land the airplane while they had the strength remaining to do so. A review of the data from the Fairchild Solid State Flight Data Recorder (FDR), F1000, serial number 01063, at the Safety Board FDR lab revealed that the airspeed during the descent remained between 240 and 220 knots down to 2,600 feet MSL, approximately 600 feet above touchdown. At this point the crew executed a missed approach, declared an emergency, and began their efforts to gain control of airspeed and pitch. The airplane then climbed back up to 4,000 feet MSL, and remained approximately 240 knots until approach flaps were extended, and engine torque was reduced. The airspeed decreased to approximately 200 knots until the landing gear and full flaps were extended. At this point engine torque increased while airspeed steadily decreased as the airplane descended from 3,000 feet MSL to touchdown at 1,980 feet MSL.
the improper installation and inadequate quality assurance inspection of the elevator trim cable during original manufacture, which resulted in fraying and failure of the cable prior to the first routine scheduled inspection of the system.
Source: NTSB Aviation Accident Database
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