B737-500 ACFT WAS TAKEN OTS AFTER A DENT WAS DISCOVERED THAT EXCEEDED ALLOWABLE TOLERANCES.

Narrative

CAPT COMPLETED PREFLT WALKAROUND AT APPROX XA25. AT APPROX XB00; RAMP SVC CONTACTED CAPT BY HEADSET THAT PUSHBACK WAS BEING DELAYED DUE TO RAMP SVC HAVING DISCOVERED A DENT IN THE BOTTOM OF THE APU EXHAUST SHROUD DURING THEIR INSPECTION AND THAT A MECH WAS COMING TO INSPECT. AFTER APPROX 1 HR DELAY; PLT CREW WAS ADVISED THAT DENT EXCEEDED ALLOWABLE TOLERANCES AND THAT FLT WOULD BE CHANGED TO ANOTHER ACFT. AFTER JETWAY WAS IN PLACE AND PAX DEPLANED; BOTH CAPT AND FO EXAMINED THE DISCOVERED DENT. IT WAS APPROX 6 INCHES LONG; 6 INCHES ACROSS; AND 1 INCH DEEP (IN A CROSS SHAPE) AND HAD FLECKS OF YELLOW PAINT IMBEDDED IN THE HORIZ CROSS PORTION. VIEW OF THIS COULD NOT BE DISCERNED DURING THE COURSE OF THE NORMAL WALKAROUND (EVEN WITH FLASHLIGHT); BUT REQUIRED STANDING DIRECTLY UNDER THE APU EXHAUST TAIL TIP AND LOOKING STRAIGHT UP. SUPPLEMENTAL INFO FROM ACN 603266: THE CAPT STATED TO MYSELF AND THE PUSHBACK CREW THAT HE HAD NOT SEEN ANY DAMAGE DURING HIS WALKAROUND. MAINT WAS NOTIFIED; INSPECTED THE DENT; AND DETERMINED THAT SAID DENT EXCEEDED TOLERANCES.