FORT MYERS, FL, USA
N853M
Bell UH-1H
According to the pilot, he was about 30 minutes into the post inspection maintenance flight at about 350 to 400 feet agl, when he heard a loud 'bang' and engine rpm decayed to zero. His choice of autorotation site required use of main rotor rpm to clear trees and he landed hard and rolled over. The T-53-L-13B engine underwent factory disassembly examination, with FAA oversight. Material analysis determined that portions of the 2nd stage GP turbine disc had fractured and separated due to peak strain low cycle fatigue, resulting in structural and imbalance damage and engine failure. No material, lubrication, or fuel control defects were identified that would have contributed to the disc separation.
On January 20, 2000, about 1330 eastern standard time, a Bell UH-1H, N853M, registered to the Lee County Mosquito Control District, operating as a Title 14 CFR Part 91 public-use flight, crashed while maneuvering in the vicinity of Fort Myers, Florida. Visual meteorological conditions prevailed and no flight plan was filed. The rotorcraft received substantial damage and the commercially-rated pilot, the sole occupant, sustained minor injuries. The flight originated from Lehigh Acres, Florida, about 30 minutes before the accident. According to the pilot, the rotorcraft was being routinely flown for off season maintenance prevention when, between 350 and 400 feet agl, he heard a loud "bang", the engine fail light illuminated, engine rpm decayed to zero, and rotor rpm indicated between 298 and 304. The pilot performed an autorotation, and the rotorcraft sustained a hard touchdown and rollover about 6 miles southwest of Southwest Florida International Airport. According to an FAA inspector, eyewitnesses reported hearing a loud "popping sound" shortly before observing the rotorcraft perform an autorotation to an area southwest of Fort Myers known as Mulloch Creek. The touchdown was hard enough to cause the left landing skid to separate and the main rotor blades to collide with the tail boom and terrain. The tail boom was severed and separated. The airframe came to rest on its side, and debris in the tailpipe was determined to be remnants of turbine disc and blades. According to engine maintenance records, the Lycoming T53-L-13B turbine engine, serial no. LE-24073, was installed in N853M on March 17, 1998, at an engine time of 774 hours since overhaul. On January 19, 2000, the engine underwent a 50-hour inspection at an engine time of 924 hours since overhaul. The engine was removed and sent to the Honeywell Product Safety and Integrity facility for material failure analysis, under FAA oversight. Adjacent to the engine data plate was affixed a Corpus Christi Army Depot, (CCAD) overhaul plate; however, the spaces for "overhaul date" and "time since new" had been marked with an "N/A". Records at CCAD revealed the engine was test cell run on April 8, 1993, but never repaired or overhauled. Disassembly inspection of the core engine by factory inspectors revealed that a section of the 2nd stage GP turbine disc, P/N 1-100-063-05, had fractured and separated due to sustained peak strain low cycle fatigue. The separation of the section of turbine disc resulted in liberation of disc and blade fragments that caused secondary damage to the downstream 1st stage PT nozzle, 1st stage PT disc and 2nd stage PT nozzle and disc, as well as lesser damage to the upstream 2nd stage GP turbine nozzle, 1st stage GP turbine disc, and 1st stage GP turbine nozzle. The subsequent disc rotational imbalance caused GP spool interference between the axial and centrifugal blades with their respective shrouds, resulting in blade tip rubbing. Additionally, the imbalance of the GP spool caused relative movement and interference between the aft internal splines of the GP spool and the aft GP turbine cone. This condition was evidenced by impressions found on the aft GP turbine cone and the fracture of the sun gear retaining bolt washer. No material defects were revealed that would have led to the disc separation. No evidence of improper lubrication or fuel servicing that could have contributed to the disc separation was revealed. The original engine was assembled at the factory, Stratford, Connecticut, in October 1974; however, the Lycoming complete service history of the military surplus 2nd stage GP turbine disc could not be determined from records provided by the operator. Factory inspectors stated they had not seen such corrosion of the 1st stage GP turbine nozzle retaining bolts and erosion of the bearing housing heat shield in a 924 hour-since-overhaul engine before. See the Honeywell Teardown Report of Model T53-L-13B Turboshaft Engine, serial no. LE-24073, an attachment to this report.
A total loss of engine power due to turbine wheel fracture resulting from fatigue for undetermined reasons that resulted in an autorotation to unsuitable terrain and hard touchdown and rollover.
Source: NTSB Aviation Accident Database
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