What was found
APU FIRE WARNING LIGHT AND BELL ACTUATED DURING TAXI. CAPTAIN DID NOT USE FIRE BOTTLES, WARNING INDICATORS DEACTIVATED WITH NO SECONDARIES. AIRCRAFT RETURNED TO GATE DEPLANED PASSENGERS. *S/D* THE APU TAILPIPE WAS REMOVED DUE TO GROSS DISTORTION OF THE INNER LINER. TAILPIPE WAS INSPECTED ENGINEERING AND FOUND TO BE SEVERELY BUCKLED AND INNER LINER RIVETING HAD FAILED. THIS CAUSED THE FLOW PATH FOR EXHAUST TO BE REDUCED BY 50 PERCENT AND DISTORTED THE MATING FLANGE. HOT GAS CAUSED THE FIRE WARNING.