NTSB Factual Report

HISTORY OF FLIGHT

On October 9, 2005, about 1630 central daylight time, a Cessna Citation 650 airplane, N175DP, operated by Marshall & Ilsley Corporation, experienced a loss of power and uncontained release of turbine material from the left engine, a Honeywell TFE731-4R-2S, while the airplane was in cruise flight at FL 380. The pilots heard a loud bang followed by a loss of power from the left engine. The airplane diverted to Madison, Wisconsin and landed without further incident. After landing at Madison, a hole was noted in the bottom of the left engine's cowling and there were numerous nicks and dents to the baggage door and the left side of the rudder. Daytime visual meteorological conditions prevailed. The airplane was operating on an instrument flight rules flight plan under the provisions of 14 CFR Part 91 from Milwaukee, Wisconsin to Van Nuys, California. The two pilots and four passengers were not injured.

The disassembly of the engine revealed that most of the low pressure turbine (LPT) 3rd stage blades were broken about 2 ¾-inches above the base with a few broken about 2 ½-inches above the base. The aft portion of the interstage transition duct, which surrounds the LPT 3rd stage blades, was bulged out 360 degrees around and the bottom half of the duct had a 20-inch long circumferential split in the bulged out area.

TEST AND RESEARCH

The metallurgical examination of the LPT 3rd stage blades, part number (PN) 3060690, revealed one blade that had faint fatigue progressions from the leading edge. On August 12, 2004, Honeywell had issued Service Bulletin (SB) TFE731-72-3691 regarding PN 3060690 LPT 3rd stage blade separations. The SB stated that there had been separations of the PN 3060690 blades due to high stress levels. The SB further stated that the blade separations had led to several in-flight shutdowns, and that the PN 306090 blades should be replaced with PN 3060788-1 LPT 3rd stage blades at the next airframe Phase 8 inspection, or the next major periodic inspection or compressor zone inspection, or the next time the LPT tierod was unstretched, whichever came first. At the time the blade separation occurred, the incident engine had not yet reached any of the milestones defined in the SB.

The metallurgical examination of the interstage transition duct revealed much of the fracture surface had been smeared, but the undamaged portions of the fracture surface were indicative of shear overload. The metallurgical examination also revealed the thickness of the duct conformed to the engineering drawing requirements and that the material composition, hardness, and grain size conformed to the specification.

NTSB Probable Cause

The inadequate design of the engine's interstage transition duct that yielded after the LPT stage 3 blades had separated and allowed the uncontained release of turbine debris. A contributing factor was the separation of a turbine blade.