A 6-by-10-foot section of the cargo door that blew off United Flight 811 last year, killing nine people, was recovered in 14,100 feet of water 100 miles south of Honolulu by the Navy's deep-sea submarine Sea Cliff, the National Transportation Safety Board announced yesterday.

The three-man submarine also detected a "strong" sonar target near the recovered section that is believed to be the other half of the door, the board said. After the submarine is recharged, an effort will be made to recover that piece.

The door blew off the Boeing 747 at 22,000 feet as it climbed away from Honolulu on a flight to New Zealand on Feb. 24, 1989, tearing away part of the fuselage and sweeping nine passengers out. Five others were seriously injured.

The board determined earlier that faulty design of the door's locking mechanism probably caused a cockpit indicator to show the door was locked. Recovery of the door was considered necessary to help redesign the latch. The recovered piece contains the latching mechanism.

The Sea Cliff lifted the piece to the surface with hydraulic jaws after videotaping it, placing it on the deck of a Navy vessel. It will be taken to Boeing in Seattle for a detailed examination.

The board said a preliminary examination revealed that the door had "fractured longitudinally near the midspan latch position." The midspan latch is one of several latches holding the closed door to the fuselage, located about halfway up the door on each side.

The board found that eight latch cams, which electrically rotate around pins to keep the door closed, were nearly open even though the master handle was locked. Eight interior bolts, which are mechanically moved across the latch cams as the last function in locking the door, were in place and deformed.

The board reached no conclusions from its preliminary examination.