 This "fail-safe" principle is not necessary in airplanes and helicopters that are not certificated in the transport category.If an element in a transport category airplane or helicopter is not highly critical, or if its failure is relatively benign, it is possible for that element not to be duplicated.For example, most transport category airplanes do not have duplicated nosewheel assemblies; and some do not have duplicated wheels on each main undercarriage.