Over and over again, experts (on something) tell us that "Anti-aircraft missiles could migrate out of Syria and threaten commercial aviation." Yes they could. How big is the threat of portable anti-aircraft systems, or MANPADS?
Sounds like it's pretty big: "Since 1975, 40 civilian aircraft have been hit by MANPADS, causing at least 28 crashes and more than 800 deaths around the world...". That comes out to about 21 deaths a year, but there's always the possibility of some catastrophic attack to make anyone regret being glib about the number. Better to look at some unexpected facts relating to the statistic.
First, there are the circumstances of the attacks. I haven't found a complete list of them. If the available summaries and examples are any indication, 'civilian aircraft' is correct but misleading.
With a lone exception to be discussed shortly, every single example in ABC's list comes from a war zone, These include Vietnam in 1975 and Iraq in 2003, Rhodesia and Somalia as well as Georgia, the Congo and Angola during their civil wars. Over a third were cargo, not passenger planes. Flying in contemporary combat zones pretty well guarantees that potential attackers will already have MANPADS, so it's a little hard to see how supplying the Syrians will greatly increase the danger in these cases.
Second, there's the time of the attacks.
The vast majority of the attacks preceded 9-11. Consider what that means. At least into the early 1970s, if you wanted to take a couple of guns and a couple of pounds of weed on a US domestic flight, the smart thing to do was to put it in your carry-on. No chance you'd be searched. Even when things got tighter, it happened quite gradually. The distance between airport security before and after 9-11 is so vast that it makes no sense to base an assessment of current dangers on pre-9-11 experiences.
Since 9-11 there has been exactly one attack on a civilian airliner not in a war zone: two MANPADS were fired at an Israeli passenger jet on takeoff from Mombasa Kenya. No one was harmed; both missiles failed to hit their target. No doubt Israeli passenger aircraft run extra danger of attacks. But they also have extra protection: Israeli carriers such as El Al equip their planes with countermeasures. (These countermeasures are theoretically available for all commercial aircraft, but retrofitting is considered too expensive.) So the Kenya example doesn't tell us much about some general danger to civilian passenger planes.
So we have one very atypical and atypically less alarming case of an attack on a passenger plane outside a war zone in the dozen years after 9-11. Not one passenger outside a war zone has been injured. Nothing to be cavalier about, but airport security today is anything but cavalier.
Third, there's the crucial question: how much would supplying the Syrian rebels with MANPADS increase the danger of attacks? The answer seems to be: minimally at most.
The reason for this verdict has to do with the futility of restricting MANPADS proliferation, discussed in more detail here. Consider the huge difference between MANPADS in a military role and in a terrorist role. A military role (such as neutralizing the Syrian air force) requires at least hundreds of weapons, and they need to be pretty advanced models. A terrorist role requires one or two.
About 750,000 MANPADS are out there. Some of the countries the West worries about most, like Iran, manufacture them. Many more such countries possess them. Thousands supposedly lie in the Libyan desert. Perhaps more important, at least several dozen are in Syria already. Dozens more are bound to appear even if the West doesn't deliver them. So it seems very unlikely that supplying still more to the Syrian rebels is going to make much difference. If terrorists want MANPADS, they can certainly get them. In fact this suggests that, given current airport security and the absence of incidents, terrorists don't think a MANPADS attack is very practical.
All things considered, the danger of attacks on civilian passenger planes outside war zones is very small. Against this small possibility there is now exceptional vigilance. Potential terrorists already have ample sources of MANPADS; the availability of a few more won't substantially increase the threat. So the civilian aircraft excuse is a terrible reason for not arming the Syrian rebels against Assad's air force.