I may be missing somehting but the way I'd do the above is:
=if(CT_YEAR = 'C','2010',if(CT_YEAR = 'L','2009',if(CT_YEAR = 'N','2011','Outside Range'))) as ACYear,
Just like a running IF expression in Excel.
Alternatively look into using a Mapping load and 'Apply Map' in the script.
Hope that helps,
Matt - Visual Analytics Ltd