A twin-engine general aviation aircraft, Ted Smith Aerostar, crashed into a wooded area and experienced an extensive post-crash fire. In a subsequent inspection of the right engine, the fire damaged remnants were examined. The fuel servo meters fuel flow in proportion to the mass airflow through the servo’s throttle body. Airflow is sensed via the venturi and impact air system within the regulator section, which establishes a differential pressure signal corresponding to airflow. This signal, together with cockpit throttle position, controls the fuel metering valve to deliver the appropriate fuel flow. The servo therefore regulates fuel delivery to maintain the proper fuel–air ratio supplied to the engine cylinders.
Our Approach
ESi was asked to investigate the fuel servo and whether its internal bellows failed in a flight causing fuel mis-metering to the engine. The fractured small metering bellows was made of electroless nickel. It is manufactured by plating onto a wax form by deposition from a chemical bath. Post-deposition the form is melted away leaving the bellows. The process allows for very uniform bellow thickness to precisely control its spring constant.
Optical microscopic and scanning electron microscopic examination of the fractured bellows pieces’ edges was performed. It was alleged that the surfaces exhibited evidence of cyclic fatigue fracture from bellows oscillatory flexing in service. However, it was also noted by ESi at this inspection that the bellows material was extremely fragile and brittle, further fracturing during exam handling. The electroless nickel deposition process results in significant quantities of the embrittling element phosphorous to be alloyed with the nickel. ESi recognized that elevated temperature exposure of the bellows, from the post-crash fire, embrittled it, causing it to readily fracture during post-accident disassembly.
ESi undertook a two-prong approach to evaluate and demonstrate this through exemplar testing. First, exemplar bellows were cyclically loaded (the “shake”) to failure purposefully creating fatigue fracture surfaces for comparison with the subject fractures. Second, another set of exemplar bellows were thermally annealed (the “bake”) at varying temperatures and then tensile tested to fracture. This testing confirmed that the thermally exposed bellows were brittle with very low ductility and strength to fracture.
Comparisons of the fracture surfaces of both sets of tested exemplar bellows to that of the subject also confirmed that instead of the subject bellows suffering fatigue failure, its fracture surfaces were simply consistent with brittle fractures. Therefore, the subject below did not fail in service, rather it fractured due to handling in its post-crash, fire-exposed brittle condition.
Services Utilized
Expertise Utilized
The Outcome
Prior to trial in this litigation matter, an undisclosed settlement was reached. This project demonstrated the seamless interactions between multiple ESi disciplines: materials, mechanical and aviation.
