What was found
NR 4 DU FAILED IN FLIGHT WITH SMOKE IN COCKPIT ONLY. REMOVED AND REPLACED NR 4 DU. DISPLAY TESTS PASSED. ALSO CLEANED REMAINING DU COOLING INLETS. INSPECTED INST COOLING FAN & FILTER, NO DEFECTS FOUND. OK FOR SERVICE. REF 31-43, 31-61. P/N OFF/ON 4082170-912, S/N OFF 02770892, S/N ON 31491628.