What was found
PHL - FLT1612 ON CLIMBOUT FROM PHL, EN ROUTE TO BOS, THE TOP OF THE CAPTAIN'S L1 WINDSHIELD STARTED TO ARC, THEN CRACK. FLIGHT RETURNED TO FIELD AT PHL AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REMOVED AND REPLACED THE CAPTAIN'S L1 WINDSHIELD PER JOB CARD J356-11-01. ALSO, REMOVED AND REPLACED THE WINDOW HEAT CONTROL UNIT PER MM 30-41-11, AS PRECAUTIONARY. OPERATIONAL CHECK GOOD.