DFW AIRPORT, TX, USA
N335AE
Saab-Scania AB (Saab) SF340B
The airplane departed runway 13L and climbed to an altitude of approximately 400 feet AGL when the flight crew 'heard a loud bang.' The right engine upper cowling departed the airplane, the right engine shut itself down, and the right propeller automatically feathered. The flight crew declared an emergency and executed a single engine landing without further incident. Examination of the airplane's right CT7-9B turboprop engine revealed that the Stage 2 turbine disk was missing and that approximately one third of the disk was found in the right landing light compartment. One end of the disk fragment progressed through a cooling air hole and there was evidence of a fatigue fracture that had initiated from multiple origins on the inboard side of the cooling air hole. Research revealed a layer of deformed microstructure and an associated zone of increased disk hardness near the origin area of the fatigue crack. Test disks were drilled using dull and worn drill bits which resulted in plastic deformation and increased hardness of the inner diameter hole surfaces. The manufacturer reported that the deformed microstructure correlates to a zone of reduced fatigue properties.
HISTORY OF FLIGHT On July 1, 1996, at 0711 central daylight time, a Saab SF340B, N335AE, sustained an uncontained engine failure resulting in structural damage to the airframe, while on initial takeoff climb from the Dallas/Fort Worth International Airport, Texas. The two pilots, one flight attendant, and 12 passengers were not injured. The airplane was being operated by Wings West as American Eagle Flight 547, under Title 14 CFR Part 121. Visual meteorological conditions prevailed for the scheduled passenger flight which originated from DFW Airport, Texas, approximately 1 minute before the accident. An IFR flight plan had been filed with Shreveport, Louisiana, as its intended destination. The airplane departed runway 13L and climbed to an altitude of approximately 400 feet AGL when the flight crew "heard a loud bang." The right engine upper cowling departed the airplane, the right engine shut itself down, and the right propeller automatically feathered. The flight crew declared an emergency and continued their climb to 2,000 feet MSL while coordinating a flight path reversal with DFW tower. The flight crew executed a single engine landing on runway 31R without further incident 7 minutes after the engine failure. DAMAGE TO AIRCRAFT Examination of the airplane's right General Electric Aircraft Engines CT7-9B turboprop engine, serial number 785-199, revealed two holes in the turbine case in the Stage 2 turbine disk plane of rotation, and that the Stage 2 gas generating disk was missing. Approximately one third of the disk exited the engine case at the 9 o'clock position, separating the engine nacelle's left nacelle longeron, penetrating the landing light fairing, and coming to rest in the right landing light compartment. The outer passenger window for seat 6C had two puncture holes in it with one having a piece of the gas generating disk fir tree lodged in the window. The skin on the right side of the fuselage and the wing root fairing was penetrated 12 times, with 6 of the holes containing debris from the right engine. The right side of the engine case was penetrated with corresponding penetrations to the nacelle from approximately 1 to 3 o'clock position. The right nacelle longeron was deformed, but was not severed. The remaining portion of the number two gas generator disk was not recovered. AIRCRAFT INFORMATION The Stage 2 turbine disk, part number 6064T12P01, serial number GAT2M710, was approximately 6 inches in diameter and was manufactured from Rene 95 nickel alloy material. The disk had 15 holes through the web area for 5 each tiebolt holes, cooling plate bolt holes, and cooling air holes. The engine and the Stage 2 disk were mated at the time of manufacture. At the time of the accident, they had accumulated a total of 10,502.9 hours and 10,133 cycles since new. They had accumulated 4,472.6 hours and 4,549 cycles since the last shop visit when the Stage 2 disk was exposed and inspected. The engine was removed from the airplane and shipped to the manufacturer's facility for disassembly and examination. See the enclosed Powerplant Group Chairman's Report for details of the examination. TESTS AND RESEARCH The NTSB's materials laboratory examined the wedge shaped fragment of the Stage 2 turbine disk which was recovered from the right landing light cavity and observed the following: One end of the disk fragment progressed through a cooling air hole. The portion of the fracture radially inboard of the hole contained a heat tinted region that ranged in color from dark blue to gray and covered approximately 2/3 of the area between the cooling hole and the bore. The crack arrest marks and river pattern within the heat tinted region were indicative of a fatigue fracture that had initiated from multiple origins on the inboard side of the cooling air hole adjacent to the radius at the front face of the disk. Examination of the origin area of the fracture with the scanning electron microscope showed that the surface of the air hole did not have any mechanical gouges or visible defects that could have contributed to the initiation of the crack. The fracture surfaces on the outboard side of the cooling hole and on the other end of the disk fragment had ductile dimple features that are an indication of an over stressed fracture. Further analysis of the disk fragment by the metallurgists from the NTSB and the manufacturer determined that the chemical composition, grain structure, and material properties all conformed to the specifications for properly processed Rene 95. There were no indications of any inclusions or handling damage to the part. A cross section of the disk's air hole adjacent to the fracture surface showed the front face of the disk, which was the drill exit side, was slightly mushroomed at the tangent to the radius. Additionally, the metallurgists found a layer of deformed microstructure and an associated zone of increased disk hardness near the origin area of the fatigue crack. See the enclosed NTSB Metallurgist's Group Chairman Report and the manufacturer's metallurgists report for details. The manufacturer reported that they drilled a number of holes, in test disks, using dull and worn drill bits. Examination of these holes revealed displaced material similar to that noted on the fractured disk from the accident airplane's engine. Additionally, the manufacturer found that the inner diameter of these test drill holes exhibited plastic deformation of the hole surface and these deformed areas were found to exhibit increased hardness. ADDITIONAL DATA The FAA Engine Certification Office, Burlington, Massachusetts, advised the NTSB that it had worked with the manufacturer to publish three service bulletins affecting the inspection of the CT7 Stage 1 and 2 turbine disk holes (see attached service bulletins). The FAA also reported that they had published an airworthiness directive (AD), which became effective April 15, 1997, requiring the inspection of all CT7 Stage 1 and 2 turbine disk holes by the end of 1997 (see attached AD). As the result of this investigation, the manufacturer has made several changes in manufacturing procedures of these turbine disks (see attached letter) and in their Overhaul Manual for the CT7 engine. The airplane was released to the owner's representative.
The manufacturer's inadequate procedure for manufacturing the number two turbine disk.
Source: NTSB Aviation Accident Database
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