48 Hours investigates the stories of people who have faced life-or-death crises and lived to tell about it.  Steven Courtney and Deborah Brosnan, a Portland, Oregon couple survived the crash of a Singapore Airlines flight in Taiwan and despite extensive injuries, guided 15 passengers to safety; Joanne Calabrese and Roger Marcaldi of Boston, Massachusetts were aboard a Greek ferry which sunk after striking a rock, resulting in the drowning of 81 passengers; and Tillie Tooter, an 84-year-old Fort Lauderdale woman whose car tumbled into a swamp as a result of a collision and who was trapped in the vehicle for four days.