The aircraft, on a flight from New York to Buenos Aires left the airport of Rio de Janeiro at 11:28 following an intermediate stop. A climb was started, as was cabin pressurization. At an altitude of about 12,000 feet and with a cabin pressure differential of 4.1 pounds per square inch, corresponding to a cabin altitude of about 2.000 feet, the purser heard a loud hissing noise at the cabin door. He went to the flight deck and stated to the captain, “We should depressurize because I think the door is open.” With the door warning light still on the flight engineer accompanied the purser to inspect the door. The flight engineer did not make a visual inspection through the door windows but placed his hand along the top edge of the door, whereupon the noise decreased. He then instructed the purser to place wet towels in that area to reduce the air leak and the noise. At this time the door handle was still not in the locked position, the flight engineer estimating that it was still about 25 degree from being in the horizontal position and the purser estimating it to be only about 19 degree from the vertical, or fully unlocked position. The purser then went aft in the cabin to procure towels. The flight engineer returned to his station and reported to the captain that the door seal was leaking but everything seemed normal. The captain elected to continue. The door warning light was still on. Within a minute or two the cabin door blew open. A woman passenger in seat No. 33, nearest the door, was blown out. The depressurization, of an explosive violence, caused damage throughout ‘the cabin, blowing loose ceiling panels and many sections of soundproofing and upholstery and tearing off the door of the ladies’ lavatory. Fog, caused by condensation at the lower pressure, temporarily filled the cabin. The aircraft was immediately turned back to Rio de Janeiro where it landed uneventfully at 12:13. PROBABLE CAUSE: “(a) The flight engineer’s failure to recognize an unsafe condition of the cabin door despite three completely separate warnings of that condition; and (b) the captain’s action in continuing flight while pressurized despite the several warnings that the main cabin door was not properly locked.”