CORSICANA, TX, USA
N770MA
Mitsubishi MU-2B-35
The twin-engine airplane was in cruise flight at 11,000 feet msl, when the pilot heard a loud bang and felt the airplane vibrate. The pilot shut down the right engine, feathered the propeller, and landed the airplane at an alternate airport. Examination of the right propeller revealed that one of its propeller blades separated approximately 8.5 inches from the hub. This blade was manufactured from 7076 aluminum alloy, which is referred to as a 'hard alloy.' Examination of the propeller blade's fracture surface revealed that it failed as a result of fatigue cracking, which originated from an area of intergranular corrosion. The propeller was overhauled 222 hours prior to the accident. According to the overhaul facility, they used a process of Alodining the blades for corrosion control, which is an approved method. According to the propeller manufacturer, the preferred method of corrosion control is anodizing due to its corrosion detection properties; however, it is a more expensive and time consuming corrosion control process. As a result of this accident, the propeller manufacturer issued a service bulletin indicating that 'hard alloy' blades should be removed from service due to their susceptibility to intergranular corrosion.
On August 24, 2000, at 0215 central daylight time, a Mitsubishi MU-2B-35 twin-engine airplane, N770MA, was substantially damaged when one of the propeller blades from the right propeller separated while in cruise flight near Corsicana, Texas. The commercial pilot-in-command and his co-pilot were not injured. The airplane was registered to and operated by International Business Aircraft, Inc., of Tulsa, Oklahoma. Dark night visual meteorological conditions prevailed and an instrument flight rules flight plan was filed for the 14 Code of Federal Regulations Part 135 on-demand air taxi flight. The cargo flight originated from Dallas, Texas, and was destined for Houston, Texas. According to the pilot, the airplane was in cruise flight at 11,000 feet msl, when he heard a loud bang and felt the airplane vibrate. The pilot shut down the right engine, feathered the propeller, and then received vectors from air traffic control to the Corsicana Airport. Upon landing, the pilot noticed that one of the propeller blades had separated from the right engine's propeller. According to the operator's chief mechanic, the right engine firewall was buckled, and the right wing was twisted. The right engine cowling and nacelle were damaged. The airplane was transported to the operator's facility and the Hartzell HC-B3TN-5E propeller was disassembled on August 30, 2000, under the supervision of an FAA inspector. The fractured propeller blade was shipped to the NTSB Materials Laboratory in Washington, D.C., for further examination. According to the NTSB Materials Laboratory report, the propeller blade (serial number C24680) was fractured approximately 8.5 inches from the butt end of the blade, and was covered with a solid layer of gray paint and black topcoat on the flat side. In addition, part of the de-ice boot remained firmly bonded in place. The blade was manufactured with 7076 aluminum alloy. The blade's fracture surface displayed characteristics of fatigue cracking. The fracture marking indicated that the fatigue had initiated on the camber surface about 2.25 inches from the leading edge. Examination of the fatigue initiation area revealed a small darker appearing region at the fatigue origin. The darker origin area displayed a mixture of intergranular separation and fatigue features. At least two secondary fatigue origins were found emanating directly from intergranular features within the darker region. The 7076 alluminum alloy blades were referred to as "hard alloy" blades. Hartzell Propeller used 2025 aluminum alloy for most other blades, which were referred to as "standard alloy." Hartzell's overhaul and maintenance manual (133C; 05/20/96) stated that "hard alloy blades can be susceptible to intergranular corrosion." According to the propeller maintenance records, the propeller was overhauled on November 11, 1999, by an FAA approved repair station at an aircraft total time of 8,779 hours. At the time of the accident, the propeller had accumulated 222 hours since its last overhaul. According to Hartzell Propellers, their overhaul procedures call for the removal of 0.010 inches of material from the entire blade surface of hard alloy blades at each overhaul. In addition, the Hartzell Propeller overhaul procedures require that the propeller blades be re-anodized or protected with a chemical conversion coating, commonly referred to as Alodine. Alodining is a more cost effective process than anodizing. The anodizing corrosion control process; however, is known to help expose deep-rooted corrosion. According to the propeller overhaul facility, they removed the required amount of material and used the Alodine process on the propeller blade that failed. Upon reviewing the NTSB Materials Laboratory Report, the propeller manufacturer stated that that the failure of the blade resulted from intergranular corrosion and that the corrosion appears to have existed at the time of the propeller overhaul. According to the propeller manufacturer, a Service Bulletin (SB 250) was issued after the accident, indicating that the "hard alloy" blades should be removed from service due to their susceptibility to intergranular corrosion. In addition, the propeller manufacturer revised their overhaul manuals to require the anodizing corrosion control process only, until the "hard alloy" blades are removed from service.
the in-flight separation of the propeller blade, which resulted from intergranular corrosion and fatigue cracking.
Source: NTSB Aviation Accident Database
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