Windowbags (window airbags)

Window airbags, more often called curtain airbags, are inflatable cushions stowed along the roof rail above the side windows that deploy downward to protect occupants' heads in a side impact or a rollover. They exist because side collisions and roll-overs leave very little crushable space between an occupant's head and the hard structure of the door pillars, glass and intruding objects — far less than the long crumple zone available in a frontal crash — so a dedicated cushion is needed to bridge that narrow gap in milliseconds.

The airbag itself is a long fabric tube, often more than a metre and a half in length, folded into the headlining and concealed behind the trim that runs from the A-pillar to the C-pillar. When crash sensors — typically pressure or acceleration sensors in the doors and along the sills — register a side impact of sufficient severity, the control unit fires a pyrotechnic or hybrid inflator. Gas floods the bag in roughly 20 to 30 milliseconds, and it unfurls like a curtain to span the side windows, sitting between the occupants and the glass. Unlike a frontal airbag, which vents almost immediately, a curtain airbag is designed to stay inflated for several seconds so that it can keep protecting occupants through the multiple impacts of a rollover.

The protection it offers is twofold. By cushioning the head against the window aperture and pillars it markedly reduces the risk of skull fracture and traumatic brain injury, which dominate the serious harm caused in side crashes. In a rollover it also forms a barrier across the window opening that helps prevent occupants — particularly unbelted ones — from being partially or fully ejected, a scenario with very high fatality rates.

Curtain airbags are usually one element of a layered side-restraint package. Seat-mounted or door-mounted side airbags deploy alongside them to shield the torso and pelvis, while the curtain concentrates on the head. Coverage has expanded over the years from front occupants only to full-length bags protecting both rows, and some vehicles now add far-side or centre airbags to stop front occupants colliding with one another. Deployment is coordinated with seat-belt pre-tensioners, which take up belt slack at the same instant to position the body correctly against the cushions.

There are practical points to bear in mind. Curtain airbags are single-use and must be replaced after deployment, along with the inflators and often the headlining. Aftermarket sunroof or trim modifications can interfere with their path, and roof-mounted accessories should respect the deployment zone. They are calibrated to work in concert with a properly worn seat belt; an unbelted occupant may be thrown out of the bag's protective area before it can help.

Within the wider supplemental restraint system, window airbags complement frontal and side airbags, belt pre-tensioners and the energy-absorbing crumple zones of the body, forming the part of the restraint network specifically aimed at side and rollover events.