At first, I thought this was some kind of rocky cut-out. Now I see that it's an aircraft. You may want to reconsider your color choices, and possibly add in a secondary color to compliment the primary color. (Increase visual appeal.)
Otherwise, what TRH said, the (side) of the aircraft is extremely flat. There is no depth. If I were to imagine this in 3D, I can only see the main part of the craft as being cardboard-thick. The wings have a good, solid, 3D shape, though.
Reminds me of Advanced Wars. Nice style, but the front of the plane seems extremely flat.
Actually the side is flat. It's not cardboard thick though :P