(Portland, OR)  --  Transportation officials say a broken carabiner is to blame for an incident in which a woman was hit in the head by a metal panel that fell from the Portland Aerial Tram.  The Portland Bureau of Transportation says the Aerial Tram experienced a failure Tuesday with a backup safety device meant to prevent panels from falling off the tram.  The bureau says one of the carabiners holding the panel to the roof of the north cabin broke, though it's not yet clear how.  Police say the woman was walking along Darlene Hooley Pedestrian Bridge in southwest Portland Tuesday when the thin panel fell, striking her.  She was treated at the scene.