What was found
(CAN)DURING FLIGHT AT 16,000 FT. A LOUD THUMP WAS FELT & HEARD. ALL ENGINE PARAMETERS WERE NORMAL BUT CABIN ALTITUDE BEGAN TO CLIMB. AIRCRAFT DESCENDED TO 10,000 FT. CREW SELECTED LANDING GEAR DOWN BUT LEF T MAIN LANDING GEAR WOULD NOT EXTEND. AFTER SEVERAL ATTEMPTS LANDING GEAR EXTENDED & INDICATED DOWN & LOCKED. AIRCRAFT LANDED SAFELY. MAINTENANCE INVESTIGATION REVEALED LEFT MAIN LANDING GEAR TORSION ARM WAS BROKEN AT THE APEX BOLT HOLE BOSS.THE WHEELS HAD ROTATED 180 DEGREES & TRACKED THIS WAY ON LANDING ROLL. THIS CONDITION PREVIOUSLY PREVENTED THE GEAR FROM EXTENDING DUE TO BEING JAMMED IN THE WHEEL WELL. THE COMPLETE GEAR WAS REPLACED.