ACFT EQUIP PROB. AFT AIRSTAIR UPLATCH FAILED. SAFETY DEVICE HELD STAIRS FOR NORMAL LNDG.

Narrative

IN CRUISE ON A HEADING TO WACO VOR; 31000 FT; THE SO NOTICED A YELLOW CAUTION LIGHT INDICATING THE AFT AIRSTAIRS DOOR WAS UNLATCHED. HE WALKED TO THE AFT OF THE ACFT TO VISUALLY CHK THE DOOR. HE RETURNED TO THE COCKPIT AND INFORMED US THAT IT WAS IN FACT OPEN AND WAS FLOATING IN THE SLIP STREAM. THE CAPT THEN WALKED TO THE AFT TO VERIFY; HE RETURNED AND CONCURRED IT WAS OPEN FROM 1-2 FT. WE THEN TALKED OVER WHAT MAY HAVE CAUSED IT AND WHAT WE SHOULD DO. SINCE IT WAS MY LEG HE ASKED ME IF THE CTLS WERE AFFECTED. (THEY WERE NOT.) WE CALLED OUR ENGINEERS AT MCI AND ASKED IF THERE WAS AN EMER PROC AS THERE WAS NONE IN THE FLT MANUAL. MCI SAID THEY KNEW OF NO EMER. WE DECIDED THERE WAS NO DANGER UNTIL LNDG; AT LOWER SPDS IT WOULD FALL AND HIT THE RWY. WE DECIDED TO PASS TULSA AND DALLAS AND CONTINUE TO SAN ANTONIO. THE SO SUGGESTED HE COULD LASH HIMSELF IN WITH A ROPE; WE COULD DSND TO LOWER ALT AND DEPRESSURIZE SO HE COULD OPEN THE INNER DOOR AND TRY TO CLOSE THE AIRSTAIR DOOR MANUALLY. I AND MCI RECOMMENDED NOT TO DO THIS; BUT THE CAPT AND SO DECIDED TO GO AHEAD AND ATTEMPT IT. WE INFORMED ATC AND REQUESTED EMER EQUIP TO MEET US AT THE ARPT THEN WENT TO 3000 FT W OF SAT; DEPRESSURIZED; MOVED THE PAX AWAY FROM THE DOOR AND TRIED TO CLOSE THE STAIRS. THEY DID CLOSE BUT WOULD NOT STAY UP. THE SO THEN TIED THE HANDLE IN THE CLOSED POS WHICH KEPT THE DOOR UP. WE THEN LANDED WITHOUT INCIDENT OR DAMAGE. WE FOUND OUT THAT THE REASON THE DOOR WOULD NOT STAY CLOSED; BECAUSE THE DB COOPER SWITCH WAS IN THE WAY. HOW THE DOOR CAME OPEN IS STILL UNDER INVESTIGATION. PERHAPS INSPECTIONS OF THE DB COOPER SWITCH IS NEEDED AND A PROC FOR THE DOOR COMING OPEN BE DEVISED.