Does anyone really think airlines will allow you to work for them for nothing??? Of course they won't!!!!!!!!
Apart from shafting your fellow pilots the airlines themselves will be rather suspicious of your motives. If you dont get paid it will be more like a hobby to you, therefore your actions and consequently flying may reflect this. Apart from this the unions would have a word to say to anyone "working" for nothing in the majors!!
You could of course get away with working for nothing down at the local flying club or a small air taxi company. But you are selling yourself short.
Take no notice of any idiot claiming to have sent 100s of CVs offering their services for nothing!
