NTSB Identification: ERA10FA409
14 CFR Part 91: General Aviation
Accident occurred Tuesday, August 10, 2010 in Washington, GA
Probable Cause Approval Date: 12/19/2011
Aircraft: PIPER PA-32R-301, registration: N220ST
Injuries: 1 Fatal.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
During the cruise portion of a positioning flight, the pilot reported a loss of engine power to air traffic controllers. He was then radar vectored towards the closest airport but was unable to reach it. The airplane impacted trees, was partially consumed by a postimpact fire, and the pilot was fatally injured. Examination of the engine's fuel servo revealed that the hub stud in the fuel servo diaphragm assembly was fractured, which would have resulted in the fuel servo being unable to properly meter fuel. Review of maintenance records revealed that an engine overhaul had been completed approximately 18 hours prior to the accident and that the fuel servo was shipped to the fuel servo manufacturer, where the unit was overhauled using the manufacturer's components.
Examination of the hub stud revealed that it fractured as a result of fatigue cracking. The most likely cause of the fatigue cracking was a lack of braze material, which should have filled the gap between the hub stud and the hub and would have supported the shoulder of the hub stud. The manufacturer's brazing process documentation indicated that a visual check would have been performed to ensure that the braze did not exceed a certain measurement over the hub or hub stud, but there was no specific indication that a check would have been made to ensure that the braze was visible at the joint edges as required by the braze process specification, nor did any of the records provided by the manufacturer indicate the quantity of braze to be used for each assembly or how it was applied.
The hub stud from the airplane and a hub stud from an exemplar assembly (from the same batch) were also found to be significantly softer than specified by the manufacturer's assembly drawing. Based on the hardness measurements, the ultimate tensile strengths of these hub studs was only about 80 percent of the expected tensile strength. Fatigue resistance of the hub stud would have correlated with the tensile strength, so the reduced hardness relative to the specification likely played some role in the rapid onset and propagation of the fatigue cracking. The reduced hardness of the hub studs indicated that the thermal history for the brazing process was either incorrectly specified or that the process was not properly controlled for this lot of assemblies.
Also indicative of the manufacturer’s poor quality control was the lack of conformance to the drawings for the hub stud from the airplane and the exemplar hub studs; these hub studs had a groove perpendicular to the axis of the stud at the termination of the threads on the hub end, which was not in the drawing, and may have increased the stress concentrations. The hub stud from the accident fractured at the midplane of this groove, at the plane of maximum stress concentration.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The manufacturer's inadequate quality control and improper manufacture of the fuel servo diaphragm assembly, which resulted in fatigue cracking of the hub stud and subsequent loss of engine power due to fuel starvation. Full narrative available
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