On June 9, 2000, about 1400 Eastern Daylight Time, a British Aerospace BAE Jetstream 3201, N914AE, sustained minor damage during a descent to Blue Grass Airport (LEX), Lexington, Kentucky. The airplane was operated by CCAir doing business as US Airways Express, as flight 5224. The 2 pilots and 11 passengers were not injured. Visual meteorological conditions prevailed for the flight that originated from Charlotte/Douglas International Airport, Charlotte, North Carolina. An instrument flight rules flight plan was filed for the air carrier flight conducted under 14 CFR part 121. According to the captain, the airplane was about 20 miles south of LEX, descending from 11,000 feet to 6,000 feet. During the descent, about 10,000 feet, the fire warning system activated for the left engine, and the flightcrew experienced a severe left yaw. The captain discharged one fire extinguisher, and secured the left engine. The flight proceeded to LEX and made an uneventful landing. Examination of the left engine by a Federal Aviation Administration (FAA) inspector revealed two "fist size" holes in the accessory case. The engine, a Honeywell TPE 331-12UHR-702H, was examined by a Powerplants Group, headed by a Safety Board Aerospace Engineer. Representatives from the airplane manufacturer, engine manufacturer, and FAA, participated in the group. According to the Powerplants Group Factual Report, there were "five exit holes" in the reduction gearbox housing. Examination of the bull gear (PN 3107161-1, SN 98P27305), also known as the splined spur gearshaft, revealed that approximately 1/3 of the outer rim had separated from the gear. The outer rim had fractured into several pieces, five of which were recovered. The pieces were examined at the Honeywell Material's Laboratory under the supervision of a FAA inspector. The examination revealed beachmark fracture surfaces near the gear teeth root, consistent with fatigue. Additionally, non-uniform wear was observed on the aft portion of the gear teeth face, and the aft portion of the high speed pinion gearshaft teeth. The non-uniform wear was consistent with a misalignment between the bull gear and the high speed pinion gearshaft. Since the most recent bull gear configuration, there have been eight reported TPE331-12 engine failures involving bull gear PN 3107161-1. After the incident, on November 6, 2000, Honeywell issued three service bulletins. The service bulletins instructed operators to replace the existing bull gears with a newer production model. Operators were also instructed to replace the high speed pinion gear shaft, and provided information to improve inspections of the gears within the reduction gearbox. Prior to this incident, Honeywell Service Bulletin SB TPE331-12UHR-702H recommended a spectrometric oil analysis program (SOAP) inspection every 450 hours since new or after an oil change, and then every subsequent 205 hours. The inspection was intended to detect small metal particles in oil, indicative of an impending failure. Although this particular engine had a SOAP inspection on June 6, 2000, Honeywell issued another service bulletin to reduce the interval between SOAP inspections from 450 hours to 50 hours. The change was made in an attempt to increase the probability of identifying impending bull gear failures.