AN ACR B747 RETURNED AND LANDED WITH THE LOWER ROTATING BEACON OUT.

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DURING PUSH BACK IN CHICAGO; THE MECH SAID THE LOWER ROTATING BEACON WAS INOP. MY SO CALLED OUR CONTROLLING MAINT FACILITY THEY DEFERRED THE LIGHT FOR DAYLIGHT FLT ONLY. WE RECEIVED A DEFERRAL # AND NAME FOR THE ACFT LOG. AS WE WERE # 1 FOR TKOF IN ORD. WE DEPARTED FOR IAD. ENRTE; DISPATCH CALLED ME AND TOLD ME TO RETURN TO ORD AS I WOULD LAND AFTER OFFICIAL SUNSET IN DULLES. I RETURNED TO ORD AND LANDED. IT WAS QUITE DARK AFTER FLYING BACK INTO OVER 100 KT WINDS. SUPPLEMENTAL INFO FROM ACN 262155: WE WOULD BE LANDING AT IAD JUST AT SUNSET; WHILE RETURNING TO ORD WELL AFTER DUSK. IF WE WERE ILLEGAL AT IAD WE WERE DEFINITELY ILLEGAL FOR ORD. THE DISPATCHER WAS NEW; HAD NOT COORDINATED WITH SUPVR NOR MAINT. IT APPEARS OUR TURNBACK WAS UNNECESSARY. SUPPLEMENTAL INFO FROM ACN 262074: ACFT HAD BEEN ON AN EXTENDED WX RELATED GND HOLD. I ADVISED THE CREW OF MEL LIMITATIONS WHICH ALLOW DEFERRAL FOR DAYLIGHT OPS ONLY. IT IS THE CAPT'S DECISION WHETHER TO RETURN TO BLOCKS FOR MAINT OR TO CONTINUE THE FLT. DISPATCH SHOULD HAVE ADVISED THE CREW AND CONTINUED THE FLT TO IAD.