What was found
KXAS/KAUS - FLT 311 - DURING TAKEOFF AND JUST AFTER ROTATION, NR 1 ENGINE FIRE WARNING CAME ON. NR 1 ENGINE THRUST LEVER WAS RETARDED TO FLIGHT IDLE. FIRE WARNING LIGHT WENT OUT. RETURNED TO KLAX WITHOUT FURTHER INCIDENT. FOUND 13TH STAGE DUCT WITH CRACK ALONG WELD AROUND FLANGE OF DUCT. REMOVED AND REPLACED 13TH STAGE DUCT IAW B727 MM 75-31-00. LEAK CHECKED ENGINE TO PART POWER, SYSTEM OPS CHECKED NORMAL. RETURNED TO SERVICE. (X)