This August 2010 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Aircraft: Piper Saratoga. Injuries: 1 Fatal. Location: Washington, Ga. Aircraft damage: Destroyed.
What reportedly happened: According to the pilot, the airplane was in cruise flight when the engine lost power. He reported the loss of power to air traffic control and was given radar vectors towards the closest airport. He complied with the instructions, but the airplane lacked sufficient altitude to glide to the airport. The Piper came down short in a grove of trees and caught fire.
The post-accident examination of the engine’s fuel servo revealed that the hub stud in the fuel servo diaphragm assembly was fractured. This 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 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. None 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% 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 mid-plane of this groove, at the plane of maximum stress concentration.
Probable cause: 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.
For more information: NTSB.gov. NTSB Identification: ERA10FA409
