DENVER, CO, USA
N235BA
Swearingen SA226TC
When the pilot added power for takeoff, at the origination of a cross-country cargo flight, he heard an impact sound on the right side of the airplane, and the right engine began to vibrate violently. He shut down both engines, declared an emergency, and when the engines had spooled down he exited the airplane. Inspection revealed the right hand propeller had shed a blade, which caused damage to the right engine mounts, right engine nacelle, and propeller shrapnel damage to the fuselage, right wing, and right flap. Examination provided evidence that the propeller blade fractured due to a fatigue crack that emanated from a stress corrosion crack region at the mid span portion on the flat side of the blade. The coating on the surface of the blade in the area of the stress corrosion cracking had been breached. The breach in the coating allowed corrosion media to attack the blade.
HISTORY OF LFIGHT On May 19, 2000, at 0639 mountain daylight time, a Swearingen SA226TC, N235BA, operated by Superior Aviation of Kingsford, Michigan, aborted takeoff at Denver International Airport, Denver, Colorado, when the number two engine shed a propeller blade. The aircraft sustained substantial damage and the airline transport certificated pilot and sole occupant received minor injuries. Visual meteorological conditions prevailed for this Title 14 CFR Part 135 non-scheduled, domestic, cargo flight, which was departing for Montrose, Colorado, on an instrument flight rules (IFR) flight plan. The flight was cleared for takeoff on runway 35L. The pilot said he advanced the speed levers to high and brought the power levers to 800 foot-pounds of torque. As he was about to release the brakes to begin takeoff roll he said he felt an impact and noise like something hit the aircraft on the right side. He looked over at the right engine and observed it to be in a "severe state of vibration". He secured the right engine, declared an emergency to the tower, and then secured the left engine. Following "spool down" the pilot exited the aircraft and observed that the right engine had shed a propeller blade about 10 inches outboard from the hub. INJURIES TO PERSONS The pilot received minor injuries to his right hand as a result of the power lever being twisted out of his hand during the event. DAMAGE TO AIRCRAFT Examination of the aircraft revealed damage to the right engine mounts, right engine nacelle, and propeller shrapnel damage to the fuselage, right wing, and right flap. All pieces of the failed blade were recovered in the immediate area where the event occurred. AIRCRAFT INFORMATION The aircraft was a Sweringen SA226TC, serial number TC235, certificated on June 17, 1977. The engines were Garrett model TPE331-IOUA rated at 850 shaft horsepower each. The propellers were Hartzell model HC-B3TN-5C. The serial number of the right propeller was BVA6546. The propeller blade model was T10282N. The blade serial numbers on the right propeller were H21804, H22299, and H22302 (failed blade). All three blades were manufactured on January 31, 1990. They had accumulated 5,313 hours total time and 1,326 hours since overhaul. TESTS AND RESEARCH All propeller blades from the propeller that suffered the blade failure were taken to the Hartzell Propeller facility located in Piqua, Ohio, for detailed examination. The failed blade was further transported to the facilities of Sherry Laboratories in Muncie, Indiana, for metallurgical examination. The failed blade had a large portion of the tip destroyed due to impact forces. It had fractured approximately 12.75 inches from the butt end (17.25 blade station, about 1 inch inboard from the outer end of the de-ice boot). There was no visual indication of corrosion on the blade surface, and the blade did not have any evident nicks, gouges, or corrosion pitting at the surface near the fracture line. The material properties (2025-T6 alloy) were normal. The only noted abnormal condition was the presence of four layers of reflective tape over the area of the fracture line. The original vibration approval data for this blade type and application were reviewed. While stresses were within approved levels, it was noted that relatively high stress occurs in the area of the fracture. (The mid-blade area is a common location for relatively high stress in turbine engine propellers.) As previously documented, the failed blade had four pieces of overlapping reflective tape in the center of the flat side of the blade, over the fracture surface. The other two blades each had adhesive residue in the same location that suggested tape had been installed at least once since the last overhaul. This provided and indication that the propeller had been dynamically balanced as many as six times in the past 1,326 hours of operation (since the last propeller overhaul). This issue was discussed with the maintenance representative from Superior Aviation. He said that these aircraft are routinely balanced every 200 hours of operation. According to the Superior representative, the frequent dynamic balancing is believed to improve service life of the engine/propeller. He also said that these aircraft are sometimes exposed to sanded runways in the winter, which can cause rapid deterioration of propeller blades, thus dictating frequent balancing. The examination conducted at Hartzell provided findings that all the blade leading edges were worn back at the tip to nearly the minimum allowable width and they were thin in the tip region. The trailing edge of blade serial number 21804 was thin enough that the tip area could be deformed with thumb pressure. According to Hartzell, the wear was abnormally high for a blade with the reported 5,313 hours total time and indicated that the blades had been operated in a harsh environment. Sherry Laboratories was requested to perform hardness testing, chemical analysis, and failure analysis on the failed blade. Based on their tests and analysis, they came to the conclusion that the propeller had small foreign object damage indentations in the surface that penetrated through the blade coatings and into the aluminum base material. The penetration of the propeller coating allowed a corrosive media containing chloride contamination to attack the aluminum. The fracture initiated by intergranular stress corrosion cracking. The fracture then propagated via corrosion-fatigue to the point of final tensile overload of the propeller. The National Transportation Safety Board Materials Laboratory conducted a review of the Sherry Laboratories examination and conclusions. ADDITIONAL INFORMATION The propeller was returned to Superior Aviation following the examination.
propeller blade fatigue failure due to stress corrosion.
Source: NTSB Aviation Accident Database
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