China Airlines (CAL) said on Dec. 21 that it is probing the opening of a cabin door on a Boeing 747-400 flight from Taipei to Los Angeles earlier this month.

"We are investigating this incident, and we apologize to the passengers on that flight," Chen Peng-yu, head of CAL's press office, told reporters.

"When the plane took off from Taiwan's Taoyuan International Airport, the crew made sure all the cabin doors were fastened, so we do not know why it opened in mid-air. We have asked the Boeing Company to explain it," he said.

There were no reports of injuries and the incident did not cause any damage to the Boeing jet, he said.

The Chinese-language Apple Daily reported the incident after being informed by one of the passengers on the Dec. 8 flight. CAL, Taiwan's largest airline, confirmed the report on Dec. 21.

According to the passenger, surnamed Tan, he noticed something was wrong approximately five minutes after flight CI008 had left Taipei for Los Angeles at 11:52pm.

Tan, who was sitting in the Business Class cabin, said he heard commotion and crying in the Economy Class cabin, and learned that a cabin door in the back of the jet had opened.

The door was on the left side of the Boeing jet, near rows 46 and 47. A passenger seated near the door could hear it rattling, indicating the door was not completely closed.

As wind blew into the cabin, some passengers began to cry as crew members tried to close the cabin door more tightly.

When they were unable to close the door of the jet -- which was flying at an altitude of about 300m -- the pilot decided to dump the plane's fuel into the sea and return to the Taoyuan airport.

After ground checks and a delay of four hours, the plane with 264 passengers aboard took off again for Los Angeles, the Apple Daily said.

CAL wants Boeing to probe why the door opened in mid-air, and if the cabin door was not fastened when the plane took off, why the warning light had not gone off.

Aviation expert Lee Shih-ping said that the sudden opening of a cabin door was extremely dangerous for passengers and could damage the aircraft.

"If the cabin suddenly loses pressure, passengers can suffer from hypothermia or pass out from loss of oxygen. The cabin door or even passengers could be sucked out of the plane," he said.

Taiwan's "Civil Aeronautics Administration" (CAA) said on Dec. 21 that the incident was not an aviation accident, nor will the airline be penalized because of it.

Lee Wan-lee, director of the CAA's flight standards division, said the airline already reported this incident to the administration when it occurred.

Lee said that they found the instrument panel had shown nothing out of the ordinary about the cabin doors when the airplane was preparing for take off.

It is likely that the door was not completely closed and the sensors did not detect anything abnormal, he said.