A multidisciplinary team of ARCCA experts recently published ‘Nondestructive Forensic Investigation of a Scissor Lift Fatality’ in Vol. 40 No. 1 of the Journal of the National Academy of Forensic Engineers (NAFE). This team includes Michael Stichter, PhD, PE, DFE, (NAFE 1162M), Zachary Ball, PhD, PE, Carl Jewell, PhD, and Wade Lanning, PhD of ARCCA’s Failure Analysis and Biomechanical Engineering groups, as well as assistance from ARCCA’s test engineer Jeremy Robbins, BSME.
After a worker was found fatally pinned between the top rail of a scissor lift and an overhead beam, rescue attempts were frustrated by unresponsive lift controls. ARCCA experts were called to investigate the cause of the failure. During the investigation, it was found that certain lift controls did not function or functioned intermittently. This intermittent nature of the malfunction indicated that the evidence was sensitive, therefore nondestructive techniques were required to understand the failure. X-ray imaging, computed tomography (CT), electrical testing, and engineering analysis of the lift and control system were utilized to identify improperly seated wire connections in the control console plug. A biomechanical analysis indicated that the mechanism of the victim’s skull and facial fractures required compressive forces in excess of 400 lbs as well.
Based on the evidence and analysis, ARCCA experts determined that the accident was most likely caused by a control malfunction leading to a Sustained Involuntary Operation (SIO) event. In a SIO event, contact with an obstruction pushes the operator into the controls, which causes the machine to drive harder into the obstruction. Some scissor lifts are equipped with safety devices to guard against SIO events, but the subject scissor lift was not. The engineering team designed and built a prototype automatic safety device that connected to an exemplar scissor lift. During testing of this alternative design, the ARCCA anti-SIO device greatly reduced the force exerted when the machine contacted an obstruction.