AFTER TAKEOFF FLT ATTENDANT REPORTED AFT AIRSTAIR PARTIALLY OPEN. FLT ENGINEER CHECKED & CONFIRMED AIRSTAIR OPEN APRX 18 INCHES. REMEDIAL ACTION PERFORMED TO NO AVAIL. UNSCHEDULED LANDING PERFORMED WITHOUT INCIDENT. AS ACFT TURNED ONTO TAXIWAY AIRSTAIR CONTACTED GROUND. INVESTIGATION DISCLOSED, PRIOR TO DEPT MAINTENANCE SUPERVISOR PLACARDED AFT AIRSTAIR WARNING LIGHTS IN THE COCKPIT 'INOPERATIVE.' SUPERVISOR THEN MADE LOGBOOK ENTRY 'AIRSTAIRS CHECKED UP AND LOCKED.' INSPECTION REVEALED PLUNGER ON AFT AIRSTAIR BUNGEE ASS'Y WAS BOUND INSIDE ITS HOUSING DUE TO CORROSION & DIRT INSIDE THE BUSHING THROUGH WHICH PLUNGER SLIDES. MAINTENANCE MIN EQUIP & DISPATCH PROC WHICH WAS PERFORMED WAS TO VERIFY UPLATCH MECHANISM ENGAGED WITH UPLOCK ROLLERS ON AIRSTAIR. A NON-VALID TWA ACFT MAINTENANCE BULLETIN STATED THE ABOVE PROCESS DOES NOT ENSURE THE UPLATCH MECHANISM IS LOCKED AND THAT THE SOFT SURFACES MUST BE IN CONTACT TO VERIFY THE CRANK ARMS ARE LOCKED OVER-CENTER.