What are aeroplanes made from?
Aeroplanes are made up from aluminum because it is light weight and cheap.
A percentage is a comparison of two numbers: only one is specified here. You could have CO2 released by aeroplanes as a percentage of: all gases released by aeroplanes, all greenhouse gases released by aeroplanes, all CO2 released - whatever the source, CO2 released from man-made sources. There are other comparators.