On September 26, 1999, about 1100 Eastern Daylight Time, an Israel Aircraft Industries IA-1124, N666K, experienced an uncontained failure of the left engine while climbing from the Nantucket Memorial Airport (ACK), Nantucket, Massachusetts. The certificated airline transport pilot, co-pilot, and two passengers were not injured. Visual meteorological conditions prevailed, and an instrument rules flight plan was filed for the personal flight conducted under 14 CFR Part 91. According to the pilot, approximately 7 minutes after departure from ACK, the left engine failed. All procedures were accomplished to secure the engine and an emergency was declared. The airplane was then landed uneventfully at the Barnstable Municipal-Boardman/Polando Airport, Hyannis, Massachusetts. Examination of the airplane by a Federal Aviation Administration inspector revealed extensive damage to the left engine nacelle and turbine section. The first stage low pressure turbine rotor disk was found separated from the low-pressure turbine shaft, and had penetrated the inner stage transition liner. The engine was removed from the airplane, shipped to AlliedSignal, Phoenix, Arizona, and disassembled in the presence of a Safety Board investigator on October 26, 1999. The disassembly revealed the first stage low-pressure turbine (LPT) had a piece of the web and rim, missing outboard of the curvic coupling. The pieces were not recovered after the accident. Metallurgical examination of the ruptured first stage LPT disk revealed a high cycle fatigue failure that originated from the subsurface location. The disk's dimensions adjacent to the fracture and the disk's material properties conformed to engineering specifications. The engine was an AlliedSignal TFE731-3-1G, serial number P77382. The engine's maintenance records revealed that the engine had a total time of 9,157.6 hours and 6,575 cycles, and time and cycles since a major periodic inspection of 452.4 hours and 348 cycles, respectively. Additionally, the records revealed that new 1st and 2nd stage low-pressure turbine disks were installed in the engine by a maintenance facility on January 16, 1998, during the major periodic inspection.