Failure analysis of a removable support of a cockpit seat in a STOL airplane

Abstract

During an adjustment of position before a flight, a cockpit seat in an airplane presented fracture of one of its supports. A hook bracket on the base of the seat was broken. The failed metallic support (removable hook bracket) of the crew seat, which belongs to a STOL (Short Takeoff and Landing) airplane was analyzed. Failure surface was examined via optical microscope, stereo microscope and scanning electron microscope to identify zones of ductile fracture and brittle fracture, patterns of deformation and fatigue evidences. Material properties were studied via microstructural analysis, micro hardness tests and EDS. Service conditions of the seat were also identified. They included, stress caused by contact between surfaces, landing forces, as well as several adjustments of the seat and vibrations. Paint layers and plastic deformation near the fracture zone were observed via stereomicroscope. The element was modeled in CAD software and stress calculation was performed using finite element analysis. The simultaneous, harmful service conditions caused surface stresses, plastic deformation, subsequent stress concentration, fatigue and final fracture. Recommendations were made to improve the material performance of a new hook bracket and improve security during flights.