There are more MANPADS per square mile in Libya than anywhere else in the World. Many of those have likely made their way into Syria. The MANPADS in question are likely the SA-18, SA-16, and SA-14. The SA-18 is the most modern but all 3 are good at killing helos. You can train a monkey to use these AA weapons. A highly skilled operator can kill fixed wing aircraft with either of these weapons.
Not to mention the russkies t we’re mentioned as being around.
Would have been a nice revenge for our stingers in Afghanistan to knock down a couple planes .