Investigation of weight-on-wheel switch failure in F-16 aircraft

A F-16 weight-on-wheel (WOW) switch failure investigation was conducted. It was found that the root causes were broken wires that lead to the failure of the switching mechanism of the two micro-switches in the WOW switch. Detailed investigation showed that the defective WOW switch does not have a stainless steel casing for the wires, and the routing of the harness of the WOW switch was too loose, causing frequent and severe flexing. This rendered the wires close to the bottom of the switch housing to rub against it, resulting in a cut in the wire. Furthermore, scanning electron microscope revealed that the cross-section of the cut-opened wire has a smooth flat surface, probably indicating that the copper wire has failed by fatigue. For the other failure, a bending mark was observed in the wire. Scanning electron microscope showed that the insulation had an obvious twisted crack, showing that the wire has been subjected to torsion. The cause of the torsion was found to be due to the improper combing of the wires before an outer rubber tubing was encased on it as evidence from a separate experiment.