What was found
MTC DIVERSION DUE TO FLAP ISSUE ON TAKE OFF WITH FLAPS 20, ONE FLAP DID NOT MOVE, OTHER FLAP STOPPED AT 18 DEGREES DECLARED EMERGENCY, BURNED OFF FUEL, LANDED AT IAD TRUCKS ROLLED AT TOWER'S DIRECTION (FLIGHT DID NOT REQUEST) PAX DID *NOT* BRACE. FLAPS FAIL CAUTION MESSAGE ON TAKEOFF. FLAPS 18 DEGREES ON LH SIDE. FLAPS 20 DEGREES ON RH SIDE. MANUALLY EXTENDED FLAPS TO 20 DEGREES. FLAPS REQUIRE RIGGING. C/W THE RIGGING CHECK OF FLAP IAW RIL 8847 & AMM 27-52-29-820-802