What was found
(CAN) IN-FLIGHT FIRE BELL SOUNDED, BUT NO FIRE DETECT LIGHTS ILLUMINATED. CHECK OF ALL THREE ENGINE FIRE DETECTION SYSTEMS FOUND ALL SYSTEMS FAILED THE TEST. FIREWALL LIGHTS WORKED NORMALLY, APU FIRE TEST INOPERATIVE. ENGINE FIRE TEST CARRIED OUT SERVICEABLE AT LOWER ALTITUDE. FIRE BELL AURIAL WARNING BOX REMOVED AND CLEANED AND THE FIRE LEADS WERE CHECKED SERVICEABLE. FUNCTION TEST OF THE SYSTEM CARRIED OUT SERVICEABLE.