What was found
PHL - FLT 1223 - DURING DESCENT INTO PHL, BOTH MASTER CAUTION FIRE WARNING LIGHTS ILLUMINATED FOR APPROXIMATELY FIVE SECONDS. NO OTHER LIGHTS OR BELLS WERE NOTED. FLIGHT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REMOVED AND REPLACED THE LIGHT BULBS IN THE FIRST OFFICERS SIX-PACK. OPERATIONAL CHECK OF FIRE WARNING SYSTEM GOOD. UNABLE TO DUPLICATE. (M)