A DC9 IS EVACED BY THE ACFT'S AIR STAIRS WHEN THE APU MALFUNCTIONS AND SENDS FUMES AND SMOKE THROUGHOUT THE ACFT. THE ACFT HAD JUST LANDED WITH A L HYD SYS PROB; SO THE CFR EMER GND EQUIP WAS ON HAND.

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ON FINAL APCH OF OUR FLT FROM STL TO CVG; THE L HYD SYS FAILED. WE DECLARED AN EMER; COMPLETED OUR CHKLISTS AND LANDED UNEVENTFULLY WITH THE VERY COMPETENT HELP OF THE FO. I STOPPED THE ACFT ON RWY 18R AND THE CFR CREW CHKED FOR SMOKE OR ANY PROB AND SAW NONE. PROCEEDED TO TAXI TO GATE. DEPARTING THE RWY AND ON THE TXWY; WE STARTED THE APU TO PROVIDE AIR CONDITIONING. FLT ATTENDANT L-1 INFORMED US THAT SMOKE WAS ENTERING THE CABIN. I STOPPED THE ACFT AND TOLD HER TO PREPARE FOR EVAC. WE SHUT OFF THE AIR CONDITIONING AND APU AND I OPENED THE COCKPIT DOOR TO ASSESS THE CABIN SIT. I COULD SEE THE BACK OF THE ACFT CABIN IN VERY LIGHT SMOKE OR HAZE. IT SEEMED TO BE DIMINISHING WITH THE AIR CONDITIONING AND APU OFF. THE FO AND MYSELF PROCEEDED TO SHUT OFF ALL SYS. I TOLD THE FLT ATTENDANT TO DISARM THE SLIDE; OPEN THE L FRONT DOOR AND PUT DOWN THE LOADING STEPS. I MADE A PA ANNOUNCEMENT THAT BECAUSE OF THE SMOKE WE WOULD BE DEPLANING THE ACFT THROUGH THE FORWARD DOOR; AND TO LEAVE ALL BAGS AND LUGGAGE AND DEPLANE AT THIS TIME. THE DEPLANING WAS ORDERLY; WITH NO INJURIES; WITH THE EXCELLENT HELP OF THE FLT ATTENDANTS. THE FIRE DEPT BOARDED AND CHKED THE ACFT OVER AND THE PAX WERE BUSSED TO THE TERMINAL. THE ACFT WAS TOWED TO THE GATE. SUPPLEMENTAL INFO FROM ACN 428054: THE ACFT LNDG GEAR WAS PINNED AND THE ACFT WAS TOWED TO THE GATE.