Larger airliners, like the Airbus's, and 757s, 767s and 747s, have not been brought down because these missiles were not designed to take on aircraft with such large and powerful engines. While these missiles were originally intended for use against jet fighters operating over the battlefield, the reality turned out to be different. The most likely targets encountered were helicopters, or propeller driven transports. These aircraft proved to be just the sort of thing twenty pound missiles with 2-3 pound warheads could handle. Against jet fighters with powerful engines, the missiles caused some damage to the tailpipe, but usually failed to bring down the jet. This was first noted during the 1973 Arab-Israeli war, where the Egyptians fired hundreds of these missiles at Israeli A-4 light bombers. Most of the A-4s, with their 11,187 pounds of thrust engines, survived the encounter. Larger jets, like the F-4 and it's 17,000 pound thrust engines, were even more difficult to bring down. Smaller commercial jets, like the 737 or DC-9 (each using two 14,000 pounds of thrust engines) have proved vulnerable. But a 757 has much larger engines with 43,000 pounds of thrust, and the 747 is 63,000. Moreover, the rear end of jet engines are built to take a lot of punishment from all that hot exhaust spewing out. Put a bird into the front of the engine and you can do some real damage. But these missiles home in on heat, and all of that is at the rear end of the engine.
If terrorists target helicopters and smaller turboprop commuter airliners, or business jets, they are likely to take down aircraft better than half the time a missile is used. This takes into account poorly trained missile operators and defective missiles. And a lot of the missile operators will be poorly trained, and, like the recent incident in Mombassa, using missiles built over two decades ago. They won't be using any of the Stingers the U.S. gave out in Afghanistan during the 1980s. The custom battery packs in those missiles gave out in the 1990s. It's a lot easier to get Russian missiles, and fresh batteries for them.
Another option for terrorists is to use anti-tank guided missiles (ATGM) against larger airliners. An ATGM is more accurate and hitting a 747, taking off, in an engine or the main fuel tank between the wings, there is a good chance that the aircraft would crash. But how do you get ATGM? Simple, from one of the Chechen criminal gangs that get all manner of weapons and equipment from light fingered Russian soldiers.
Since the 1980s, Russia's problem with soldiers stealing military equipment, and selling it on the black market, has kept getting worse. Low pay, shabby housing and poor (or sometimes nonexistent) food often left the troops with no other choice. Officers either looked the other way, or realized that trying to stop this bit of entrepreneurial activity might be fatal for themselves. The problem became worse after the Soviet Union collapsed in 1991 and the size of the armed forces shrank by over 60 percent in a few years. Suddenly there were tens of thousands of armored vehicles being put into storage or junked, and thousands of poorly guarded warehouses full of weapons and equipment. This included lots of portable anti-aircraft missiles, and even more ATGMs.
Normally, there's not much of a market for anti-aircraft missiles. In fact, it's pretty obvious that anyone trying to get them is either a terrorist, or a Chechen rebel looking to shoot down Russian helicopters. And that's what the Chechen rebels have been doing for the last few months. The Chechens got the missiles because many of the major criminal gangs in Russia are run by Chechens. A favorite business for these gangs is dealing in stolen goods and weapons. A soldier looking to sell some pilfered military gear would likely get the best price from a Chechen specializing in such trade. All the Chechen mobsters had to do is let a greedy, and amoral, Russian soldier know that there would be a large price paid for portable anti-aircraft missiles. This apparently worked, for the Chechens not only have at least a few dozen of the missiles, but the missiles have been modified to disable the safety device that prevents the warhead from detonating when hitting a Russian aircraft (that is using it's IFF beacon.)
Since hundreds of Chechens were found fighting for al Qaeda in Afghanistan last year, there's an obvious connection between a major source of anti-aircraft missiles and al Qaeda. The missile fired in Kenya was a 1970s vintage SA-7 that, not surprisingly, didn't work very well. In Chechnya, the rebels have been using more up to date, and effective (at least against helicopters) models. The ATGM's are shorter bulkier than the anti-aircraft missiles, and weigh 50-60 pounds (with the launcher). The Chechen's have not been seen using ATGMs, as they find it easier to use home made mines against Russian vehicles. But the ATGMs are as easy to steal as the anti-aircraft missiles.
There are many other sources of anti-aircraft missiles. Older Russian designs are manufactured by many countries, like China and Pakistan. But these nations guard their missiles more diligently than the Russians. Non-Russian missiles in the hands of terrorists would cause a diplomatic embarrassment the manufacturing nation would want to avoid. It's one thing for terrorists to be found using Chinese or Pakistani assault rifles, but the world would be a lot more upset if the weapons were anti-aircraft missiles. Russia's leaky armories are another matter. This is tolerated because of the need to get the Russians to at least guard their thousands of nuclear warheads. This they have done. But the portable missiles are getting out, and some may be coming to an airport near you.
Terrorists trying to take down airliners with portable missiles has been a threat for a long time. Actually, over the last thirty years, it's been a reality. Some 29 commercial aircraft have been shot down by such missiles. However, the downed aircraft have been small, and most of these tragedies have taken place in Africa. The wars in Africa are the worst on the planet, so violent that most journalists avoid them. For three decades, this has kept the use of portable missiles against civilian aircraft off the front page.