The windshield on the co-pilot side in the cockpit shattered at a cruising altitude of 32,000 feet. Photos of the plane show that part of the windshield was blown out, a phenomenon that an unnamed maintenance personnel said was rare, as the windshield is too strong to even crack.

With the flight making an emergency landing at Chengdu Shuangliu International Airport in Chengdu, capital of Southwest China's Sichuan province, the Southwest Regional Administration of Civil Aviation Administration of China immediately launched an investigation in cooperation with Sichuan Airlines.

Chen Jianguo, president of flight expert committee affiliated to Aircraft Owners and Pilots Association of China, gave four possible reasons why the windshield broke.

Chen said any operating airplane must carry out maintenance. Different checks, including windshield dismantlement and installation, will be conducted according to different maintenance levels. If a wrong screw is used or the windshield cracks due to improper maintenance, it may cause potential risks.

However, it has not been confirmed whether Flight 3U8633 underwent windshield dismantlement and installation.

Another reason may be the quality of the windshield. A windshield normally can endure 30,000 take-offs and landings. Substandard windshield will age faster. Sometimes windshield may be replaced.

Flight 3U8633 rolled off the production line in 2011 and flew more than 10,000 times in the past seven years. Sichuan Airlines claimed that the windshield of the plane had not been replaced before.

Even the windshield is proper, it still could break. During the flight, as the outside temperature is very low, windshields usually need to be heated. But if it is heated unevenly or encounters short circuit, the windshield may experience intensity change and break due to difference of the pressure inside and outside the cabin.

Plus there's the possibility of an intruder. The windshield may be hit by something, such as a bird or hailstones. The windshield usually has three layers. Outer layer is shockproof and inner layer can bear pressure and has the highest intensity. When hit, usually the outer layer may shatter and inner layer remains fine, which will not disturb the flight.

But Chen said at an altitude of 32,000 feet, there is no hailstone and hardly any bird flies that high. The possibility of being hit is rare.

As the windshield broke and blew out, the cockpit lost pressure, with the co-pilot nearly sucked out from the plane with only his seat belt saving him. The accident made the equipment malfunction and the flight went into a nosedive that lasted five to six seconds.

Facing low temperature of minus 40 degree Celsius, tearing wind of 800-900 km/h, and deafening sound, the pilot grabbed a oxygen mask and manually controlled the plane, displaying extraordinary professionalism and will power.

The pilot heroically saved more than 100 passengers and crew members on the flight.