INSTALLING HEAD BOLTS OVER #9 INTERSTICE SILO. CREW MEMBER STANDING ON HOLE COVER WORKING CHAINFALL. AS HEAD BOX CLEARED TANK FLANGE LOAD SHIFTED AND SLID PLYWOOD HOLE COVER WITH EMPLOYEE ON I T PAST LIP OF FLANGE. PLYWOOD TIPPED UP CAUSING EE AND PLYWOOD HOLE COVER TO FALL INTO TANK. HIS CREW MEMBER HEARD CHAINFALL TRAVEL APPROX 20' WITH EE HOLDING ON, THEN HE FELL TOO.