What was found
IN FLIGHT , AT 90KIAS, A BANG WAS FELT IN FLOOR, STUDENT IN MARTIN BAKER SEAT DETERMINED THAT THE LEFT SLIDING DOOR UPPER BRACKET BROKE AND DOOR WAS HANGING BETWEEN FUSELAGE AND STEP (ONLY ATTACHED BY AFT SLIDER). STUDENT IN AFT SEAT HELD ON TO DOOR DURING APROACH AND LANDING. AS AIRFLOW DECREASED, DOOR FELL OUT OF AFT SLIDER TRACK (NOW NOT ATTACHED TO AIRCRAFT AT ALL), BUT STUDENT IN BACK RETAINED CONTROL OF DOOR UNTIL LANDING. MAINTENANCE REPLACED UPPER CAR ASSEMBLY AND REINSTALLED DOOR IAW AMM 52-13-00,4-2. MOC OK.