What was found
BNA FLT 1647, EN ROUTE DCA TO BNA - ABOUT 50 MILES FROM BNA, THE NR 2 HYDRAULIC SYSTEM QUANTITY DROPPED TO ZERO. THE FLIGHT CREW FOLLOWED THE QRH PROCEDURES, WHICH INCLUDED AN ALTERNATE GEAR EXTENSION. THE FLIGHT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE NR 2 ENGINE HYDRAULIC PUMP. OPERATIONALLY CHECKS OK.