This is an odd one, I've not come across this before.
On an AdvancedDataGrid when I am debugging I can see the exact propety I need, it is an array called visibleColumns. However, it isn't available in the live docs documentation nor will Flex let me reference it. It is protected but I have a workaround for getting at the protected propety of an adg. It's strange as I can see other similar protected properties such as visibleData, visibleCellRenderers and visibleHeaderInfos in the debugger and these all appear in the documentation and I can reference them when I extend the AdvancedDataGrid. Why can't I see / reference visibleColumns though?