What was found
FLT NR 1603 - DCA - THE APU FIRE WARNING LIGHT AND BELL ACTIVATED IN FLIGHT. THE FIRE BOTTLE WAS DISCHARGED, BUT THE FIRE WARNING LIGHT REMAINED ILLUMINATED. THE AIRCRAFT WAS RETURNED TO DCA WHERE IT LANDED WITHOUT INCIDENT. MAINTENANCE FOUND NO EVIDENCE OF FIRE. MAINTENANCE FOUND THE M717 APU TAILPIPE OVERHEAT DETECTION ELEMENT AT FAULT. THE FIRE BOTTLE WAS REMOVED AND REPLACED. THE APU FIRE DETECTION SYSTEM WAS TEMPORARILY PLACARDED INOPERATIVE, PENDING REPLACEMENT OF THE FAULTY OVERHEAT DETECTION ELEMENT. (M)