A Russian or U.S. shoulder-launched missile has a small warhead of 2.5 to 3.5 pounds, sufficient to down a fighter, but not necessarily large enough to inflict fatal damage on a modern wide-body jet. Older heat-seeking missiles go to the hottest spot on the aircraft the jet exhaust at the rear of the engine housing. The engine cowling is designed, in case of bird strikes or mechanical failure, to be strong enough to contain engine parts from flying out like shrapnel and downing the plane,. Damaging one engine would cripple the aircraft, but nearly all medium-sized and large commercial jets use two engines and can land with only one engine operating.
Live-fire testing would be conducted against GE's CF-6, the jet engine used to power the Boeing 747, 767, MD-11, and Airbus's 300 series of aircraft. Tests would be used to analyze how much damage it takes to affect flight safety and to destroy the engine. Large aircraft may be prone to being damaged by hydraulic shockwaves within fuel tanks generated by a wing hit. Further, while larger planes are believed to be more survivable due to their two (or four) distributed engines, critical components aren't as hardened as they are on military aircraft.
It is estimated that between 29 and 40 commercial aircraft have been shot down by shoulder-launched SAMs between 1975 and 1992, with a death toll of between 550 and 700 people. Exact numbers are hard to come by since the attributed aircraft losses occurred in Africa and other out of the way places where an exact cause of an aircraft crash is difficult to pinpoint. Over the past year, approximately a dozen missiles have been launched against airplanes flying over Iraq, with two confirmed hits, one an Air Force C-17 and the other a commercial DHL Airbus A300. Both planes managed safe landings, with the C-17 being repaired and the A300 being written off as not repairable. According to one Pentagon document, over 500,000 shoulder-launched SAMs have been produced worldwide, with many under "loose" (i.e. can be stolen or diverted) controls. Doug Mohney
The U.S. military has a very good understanding on how shoulder-launched surface-to-air missiles will affect (i.e. shoot down) fighters and other low-flying attack aircraft, but there's substantially less data on what could happen when a U.S.-made Stinger or Russian-made SAM is fired at a commercial wide-body aircraft. Therefore, the Pentagon has organized a multi-year effort to explore the vulnerability of larger aircraft, such as the C-17 transport and the proposed KC-767 a derivative of the civilian Boeing 767 -- to shoulder-launched missiles.