Inflation of side curtain air bag with protection for out-of-position head

A side curtain air bag system deploys downward during a crash event into a protection zone overlapping an out-of-position head region. An inflator discharges an inflation gas upon the occurrence of the crash event. The system employs an inflatable bag including first, second, and third chambers. The second chamber has a respective deployed position coinciding with the out-of-position head region. The first chamber has a respective deployed position between the B-pillar and the deployed position of the second chamber. The third chamber has a respective deployed position along a lateral edge of the inflatable bag so that the second chamber is disposed between the first and third chambers. The chambers are configured so that the third chamber substantially fills prior to expansion of the second chamber, whereby the inflatable bag substantially completes its downward movement prior to full expansion of the second chamber.