AN MD80 APU AUTO SHUT DOWN DURING PREFLT. NO FIRE WARNING SOUNDED HOWEVER INDICATIONS OF HEAT DAMAGE WERE FOUND AROUND THE APU.

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WE HAD ALMOST COMPLETED BOARDING AND WERE READY TO PERFORM THE BEFORE TKOF CHKLIST WHEN THE APU SHUT DOWN; WITH AN APU FAULT LIGHT AND OIL PRESSURE LIGHT. WE ALSO NOTICED SOME VIBRATION RESONATING FROM THE REAR OF THE ACFT PLUS A SMELL FROM THE COCKPIT AIR CONDITIONING VENTS. FOLLOWING THIS WE ARE CHIMED FROM THE REAR OF THE ACFT; THE FLT ATTENDANT TOLD US WE HAD SMOKE IN THE CABIN. THE CAPT IMMEDIATELY ASKED THE PAX TO LEAVE ALL THEIR BELONGINGS AND DEPLANE THROUGH THE FORWARD EXIT INTO THE TERMINAL. I SUGGESTED CALLING GND AND HAVE THE CFR TRUCKS ROLL. THE CAPT AGREED. FOLLOWING THIS; I SUGGESTED THAT I GO OUTSIDE SINCE IT WAS OBVIOUSLY AN APU PROB AND SEE IF I COULD ASSIST WITH INFO FOR THE CFR AND EXPLAIN THE SITUATION. I RAN INTO 2 'ACO' MECHS -- AS I LEFT THE ACFT THEY WERE ALREADY INSPECTING THE APU AREA. THEY INFORMED ME THEY HEARD A LOUD POP AND RAN OUT TO INVESTIGATE. THEY FOUND SIGNS THAT THE APU EXHAUST WAS BLACKENED AROUND THAT AREA BUT THEY COULD SEE NO SIGN OF FIRE. THEY HAD OPENED THE APU ACCESS COWLING AND STILL NO FIRE ALTHOUGH SOME SIGNS OF HEAT DAMAGE. I QUESTIONED A RAMPER WHO INFORMED ME THAT HE SAW ABOUT A 5 FT FLAME SHOOT OUT OF THE APU EXHAUST AND HE HEARD LOUD POP. THE FIRE FIGHTERS STOOD BY WHILST THE CAPT WAS INFORMED OF THE SITUATION. WHEN I RETURNED TO THE FLT DECK; ALL THE PAX WERE IN THE TERMINAL SAFELY. IT WAS DETERMINED BY THE MECHS THAT WE HAD NO FURTHER THREAT FROM A FIRE IN THE APU AREA. I BELIEVE MAINT DETERMINED THE ACFT WAS OTS AT THIS TIME. WE LEFT THE ACFT AND WAITED FOR ANOTHER ACFT TO BECOME AVAILABLE FOR OUR RETURN TO ZZZ. JUST A COUPLE OF POINTS: THE APU WAS ONLY SUPPLYING AIR AT THE TIME IT QUIT. ELECTRIC WAS BEING SUPPLIED BY EXTERNAL PWR. THIS MADE COMS EASIER SINCE WE WERE NOT RELYING ON THE APU GENERATOR DURING THE APU SHUTDOWN. ALL CREW MEMBERS DILIGENTLY PERFORMED THEIR DUTIES. I THINK IT IS IMPORTANT TO HAVE MAINT AVAILABLE AT THE OUTLYING STATIONS. THEY PERFORMED EXCELLENTLY IN A POTENTIALLY DANGEROUS SITUATION. IT WAS ALSO CLR THAT THE STATION PERSONNEL PERFORMED TO THE BEST OF THEIR ABILITY EVEN THOUGH MORE PERSONNEL WOULD HAVE PROVIDED A BETTER SVC TO OUR PAX.