A MD11 ON FINAL APCH AT 1500 FT AGL SELECTED FLAPS 50 DEGS. HEARD LOUD THUMP. REQUIRED 90 DEGS R AILERON FOR LEVEL FLT. FLAPS TO 35 DEG AILERONS NEUTRAL. ON LNDG; FOUND L FOWLER FLAP MISSING.

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ON APCH TO RWY 23; PLANNED FLAPS 50 DEG LNDG FOR IOE STUDENT TRAINING. STABLE AT 1500 FT AGL; FLAPS 35 DEGS. SELECTED FLAPS 50 DEGS; FELT A SLIGHT 'THUD;' SIMILAR TO AN AFT BIRD STRIKE. THIS REQUIRED ALMOST 90 DEGS R AILERON; FLAPS WERE MOVED BACK TO 35 DEGS WHERE AILERON WAS NEAR NEUTRAL. SELECTED CONFIGN PAGE AND RESELECTED FLAPS 50 DEGS WITH THE SAME RESULTS. THERE WERE NO ABNORMAL INDICATIONS ON CONFIGN PAGE OR ANYWHERE ON ACFT. AFTER LNDG; PERFORMED AFTER LNDG CHK. RECEIVED HYD 2 FAIL LEVEL 2 ALERT. HYD 2 QUANTITY WAS ZERO. MAINT ADVISED 16 FT OF FLAP HAD DEPARTED ACFT FROM L WING. FLAP WAS FOUND 5-6 NM FROM EXTENDED RWY CTRLINE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED WHEN FLAPS 50 DEGS WAS SELECTED; A SLIGHT THUD OR THUMP WAS HEARD AND FELT. THE RPTR SAID R AILERONS UP TO 90 DEGS OF WHEEL DISPLACEMENT WAS REQUIRED TO LEVEL THE WINGS. THE RPTR STATED RETURNING THE FLAPS TO 35 DEGS ALLOWED THE AILERONS TO RETURN TO NEUTRAL. THE RPTR SAID WHEN CHKING THE AIRPLANE ON THE GND; IT WAS DISCOVERED THE L FOWLER FLAP HAD DEPARTED THE ACFT. THE RPTR STATED MAINT HAS NOT RELEASED ANY FINDINGS TO THE CREW OF THE CAUSE OF THE FLAP FAILURE.