NTSB Identification: ENG12IA013
Scheduled 14 CFR Part 121: Air Carrier operation of GREAT LAKES AVIATION LTD
Incident occurred Saturday, December 03, 2011 in Denver International Airport, CO
Probable Cause Approval Date: 08/08/2013
Aircraft: BEECH 1900D, registration: N247GL
NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft incident report.
During a post flight inspection the pilot discovered a hydraulic leak on the nose landing gear that was emanating from a crack in the nose landing gear end cap. The end cap was examined and found to have failed in fatigue that initiated from multiple origins at an internal diameter. The fatigue had propagated significantly into the wall thickness of the end cap and through the thickness at one location. The striation count revealed that the fatigue crack had initiated and been growing in the end cap for at least 14,898 cycles. The end cap had been inspected using an ultrasonic method recommended by the manufacturer 485 cycles prior to the incident. The crack was present at the time of the inspection which indicated that the inspection method was ineffective in detecting the crack. The timing of the end cap inspection was in compliance with the manufacturer’s recommendation. Examination of the microstructure of the material revealed that the longitudinal grain direction was oriented parallel to the shuttle valve bore in the end cap. Resistance to fatigue can be improved by aligning the principle tension stresses in the end cap with the longitudinal grain direction. It is probable that the manufacturer had determined that the grain direction was a contributing factor to fatigue failure in the end cap and revised the end cap drawing to require a grain direction along the longitudinal axis of the end cap in February 2010. No engineering data to support the drawing revision was made available.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: A failure of the nose landing gear end cap due to fatigue. Contributing to the failure were the ineffective inspection and the unknown effect of grain direction on fatigue life. Full narrative available
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