We all need to calm down and accept the fact that, once in a while, a plane will blow up and 300 people will die. 9/11 is a rounding error compared to deaths from heart disease or car crashes, but we don't seem to be doing much about those.
About 6 or 7 years ago, I used to take lunch break in a nice grassy, woody area where no one else really went which was right under a take off area of Heathrow airport in the UK. Apart from the fact that it was a really nice place to sit and eat lunch, I loved the place because it felt like you could almost touch the planes they were so close. And it did occur to me how easy it would have been to aim a small missile, or something, at the plane of your choice with out any interference.
Of course, I presume that it's not that simple, other wise either it would have been done, or the terror threat is hugely exaggerated...
Ding ding ding, we have a winner.
All this is to say that you have about as much of a chance of shooting down a commercial airliner with an RPG as you do by throwing rocks at it.
With that said, good luck getting something with the firepower equivalent of an RPG anywhere close enough to a commercial aircraft to even be within the weapon's maximum range.