What was found
CLT/TYS - FLT 589 - DURING DESCENT INTO CLT AFTER STARTING THE APU, CREW ACTIVATED THE SPEED BRAKE AND RECEIVED A MOMENTARY 'APU FIRE' WARNING. THE APU AUTO-SHUTDOWN AND FIRED THE BOTTLE. THE FLIGHT LANDED AT TYS WITHOUT FURTHER INCIDENT. MAINTENANCE INSPECTED THE APU ENCLOSURE AREA AND FOUND NO INDICATION OF FIRE. REPLACED THE APU ENCLOSURE SEALS. REF: OPERATOR CONTROL NR USAAF97061 AND 97ZZZX4221.