MILWAUKEE, WI, USA
N123JN
Cessna 525
After being cleared to climb from 6,000', the pilot began advancing the throttles to climb power, when he heard a loud bang & noted indications of a #2 engine failure. He secured the #2 engine & made an uneventful landing. Examination of the failed Williams-Rolls FJ44-1A engine revealed it had sustained an uncontained high pressure turbine (HPT) disk blade retention post failure. Examination of the HTP disk (PN 48629) showed that a blade retention post had fractured adjacent to the bottom of the blade slots & that 3 adjacent HPT blades had broken loose. Subsequently, the upper portion of the fuel-oil heat exchanger mount pad was fractured, which resulted in an oil leak. Metallurgical examination revealed the primary failure was the release of a blade retaining lobe just below a fret zone. Creep fatigue was found at the failed blade retaining holes. No evidence of gross overheating was noted. The failed disk was of an 'older' design. As a result of the incident, Williams-Rolls Alert Service Bulletins No. FJ44-A72-30 and -31 were issued to describe a procedure for eddy current inspection (ECI) of the high pressure turbine (HPT) disk blade retention posts to locate possible cracks and to describe a procedure for replacing the existing HPT disks with an advanced-design HPT disk. Also, Airworthiness Directive (AD) 97-01-05 was issued to require initial and recurring ECIs of the HPT disks and for replacement of FJ44-1A HPT disks with improved disks. A more durable fuel-oil mounting pad was being developed for the FJ44-2 engine.
On August 29, 1996, at 1330 central daylight time, a Cessna 525, N123JN, piloted by an air transport rated pilot, experienced an uncontained engine failure during cruise climb from 6,000 feet above mean sea level (msl) to 10,000 feet msl. Visual meteorological conditions prevailed at the time of the incident. The 14 CFR Part 91 flight was operating on an IFR flight plan. The pilot and two passengers reported no injuries. The flight departed Timmerman Airport, Milwaukee, Wisconsin, exact time unknown. According to the pilot's written statement accompanying NTSB Form 6120.1/2, the pilot "...advanced the throttles to climb power..." after he was cleared to climb to 10,000 feet msl. He said, "Before I could advance the throttles very far, I heard a loud bang from behind me on the right side with a fast spool down sound... ." He said he secured the right engine and made an uneventful landing at General Mitchell International Airport, Milwaukee, Wisconsin. N123JN was equipped with 2 Williams-Rolls FJ44-1A turbofan engines. The engine that had the uncontained failure had a total time and cycles of 714.6 hours and 497 cycles respectively. An on-site investigation revealed the right engine's upper cowl had a 1 inch by 2-inch hole in it. The cowl's lower half was oil soaked. The engine's oil cooler upper bolt hole flange was fractured next to the hole. The engine's exhaust nozzle was oil soaked and its cone and shroud were sooted. The first stage fan blades machined about 1/16th of an inch of metal from its shroud. Globular- shaped pieces of metal were found resting on the bottom of the second and third stage disc's shroud. The high pressure turbine disc (disc) was missing one blade retaining pedestal that was fractured just above the bottom of the blade slot. A second adjacent pedestal was also missing part of the upper lobe. Three of the disc's 43 blades were fractured transversely across the lower lobe of the blade root. Parts of one blade root remained in one of the slots. The remaining blades were fractured across the airfoil section about 3/4 inch above the blade root platform. A section of the disc's blade retainer (retainer) was missing at the fractured pedestal's location. The left, or counter-clockwise, side of the retainer spilt was broken about 1/4-inch from the split. The right, or clockwise, side of the retainer was broken about 3- inches from the split. The high pressure turbine disc was examined at the disc's manufacturer with representatives from Williams International (Williams), the United Kingdom's (U.K.) Defence Research Agency, and the U.K.'s Air Accident Investigation Board (AAAIB) representative observing. The AAIB represented the NTSB during the examination. The examination revealed the primary failure was the release of a blade retaining lobe just below the fret zone. Creep fatigue was found on the failed disc's blade retaining lobe in the lower serration about 1/2 inch from the front face and about 1/32 inch outboard of the inner edge of the loading witness mark. According to the examination report, the disc had not been over-heated. An investigation into the material records of the failed disc revealed its material composition exceeded the minimum specifications for Inconnel 718 material. The minimum specification called for a hardness value of 42HRC (Hardness, Rockwell "C" scale). The failed disc's hardness value was tested as 45HRC. The examination of the failed disc revealed it met the specifications for hardness, chemistry, grain size, and microstructure. The fracture mode on the failed disc was from dwell time fatigue crack propagation. According to the manufacturer's report, crack initiation and growth "can result under stress and temperature conditions of the disc post that occur during flight." The failed disc was an "old design" type according to the manufacturer. Because of this event, and a subsequent similar event, the manufacturer has established an inspection and recall schedule for all the older style discs. According to the manufacturer, the "newer design" disc has blades with a 31 percent reduction in blade weight and are positioned on the disc at an angle 10 degrees less than the older blades. The new blades had had their chord reduced on the average of about 13 percent. The disc's blades also had their thickness reduced on the average of about 26 percent. The newer design disc has a 0.010 inch greater thickness at its outer perimeter.
an uncontained failure of a high pressure turbine disk blade retention post due to fatigue. A factor relating to the incident was: inadequate design of the high pressure turbine disk. Fretting on the leading side of the lobe near the fracture origin may have contributed to premature initiation of the fatigue.
Source: NTSB Aviation Accident Database
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