Do I hv to bring the base of the car seat..? Serious answers only pls..?

I am trvlg on a plane with my 9 mth old daughter and I bought an extra seat for her. I will be bringing her car seat but I am wondering if I need to bring the base of the car seat with me. I lost my user manual so pls don't tell me to look it up. I need advise from mothers who hv travelled with their baby and brought their car seat. Also my car seat is 18 inches wide, will this be a problem? Thanks in advance.