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On June 26, 1996, at 1430 central daylight time, a Fairchild SA227AC, N2692P, operated by Mesaba Airlines as Northwest Airlink flight 3253, sustained substantial damage following an uncontained failure of the number two engine during climb. The flight returned to Grand Forks, North Dakota and landed without further incident. The flight crew and 17 passengers reported no injuries. The 14 CFR Part 135 flight departed Grand Forks, North Dakota, about 1430 with a planned destination of Minneapolis, Minnesota. Visual meteorological conditions prevailed and an IFR flight plan was filed.
The captain reported that airplane was at an altitude of 5,000 feet mean sea level. The propeller rpm was 100 percent and other engine indications were "normal." The loss of power on the number two engine was "sudden and immediate." Air traffic control transcripts indicate the airplane was ten miles south of Grand Forks. The flight crew declared an emergency and recovered visually to Grand Forks without further incident.
DAMAGE TO AIRCRAFT
Examination of the airplane by Federal Aviation Administration Inspectors revealed eleven impact marks to the right side of the fuselage. Two of the impacts penetrated the fuselage side. The propeller exhibited minor damage. The number two engine was removed and transported to Minneapolis, Minnesota for subsequent examination.
TESTS AND RESEARCH
The number two engine was examined on July 23, 1996, at Mesaba Airlines maintenance facility, Minneapolis, Minnesota. The direct drive fuel control (DDFC) idler gear assembly was dislodged and was protruding through a hole in the gearbox inlet section. Several fragments of the gearbox were recovered. The engine mounts were intact. The compressor bearing retainer was fractured. The tie-bolt was fractured forward of the center curvic splines. The high speed pinion was fractured at the shear section. The compressor bearing outer race was fragmented and the balls were missing. The turbine bearing was fractured in several places. The second stage compressor impeller rim was fractured. Several fragments of the impeller were imbedded in the transition liner and the diffuser. The impeller shroud exhibited a circumferential rub. Corresponding rubbing was evident on the "shroud line edge" of the impeller and impeller fragments. "Thick layers of aluminum rich deposits" were fused to the impeller.
Examination of the impeller and impeller fragments revealed multiple fatigue fractures emanating from the rubbed shroud line edge. Scanning electron microscopic examination of the fatigue areas revealed evidence of "high cycle fatigue." A section of one of the impeller fragments revealed "highly heat affected (transformed) structure emanating from the heavily rubbed shroud line edge." No material defects were noted.
Examination of the gear box fragments revealed fracture surfaces "indicative of an overload fracture mode... no material defects or cyclic fatigue propagation were observed."
An Allied Signal product integrity engineer reported that the tolerance between the impeller and shroud is accomplished via shims during engine assembly. He reported that the shims in the number two engine were all within acceptable limits. Additionally he reported that the dynamic clearance can be affected by operational conditions such as extended use of engine anti-ice at excessive temperatures. Examination of Mesaba Airlines SA-227 Pilot Operating Handbook revealed "the use of engine inlet anti-ice is restricted to ten seconds of use on the ground when the OAT is above +5 degrees Celsius regardless of engine RPM."The TPE331 installation manual specifies "inlet anti-icing should not be used above 50 F ambient conditions for more than ten seconds."
Parties to the investigation were the Federal Aviation Administration, Mesaba Airlines, the Airline Pilots Association, and Allied Signal Aerospace.