I recently sent 24 speculative CVs to airlines and received 16 replies. 66% - not bad. I send mine to the recognised contact person in HR and this seems to be the best way to elicit a response. In other words, the CV is processed straight through the administrative system they have set up, rather than languishing on the desk of the Chief Pilot or whoever. On the other hand, I might be getting lots of nice letters from HR while those of you who approach the Chief Pilot directly are getting jobs!!
It's luck of the draw and what works in one airline won't work in another!