Trying to remove grey background from a grey/desaturated image is trickier. I find the more contrast in colour the easier it is and less you have to do. For tricky ones I normally run along as close to the edges as possible and then fill in the rest as much as possible, takes a little more time but still much quicker and handier than tracing all of the edge exactly and usually get very good results. Though that plane nose shape would be pretty quick to just do with the shape tool too.