China's preliminary report into Flight MU5735 has been formed based on the present facts in line with international practice, an official from the Civil Aviation Administration of China told Xinhua News Agency.

"It is almost impossible to complete the analysis of the causes of a civil aviation accident within 30 days. Thus, the preliminary report does not involve the causes of the accident," said Li Yong, deputy director of the administration's safety office.

According to the Chicago Convention on International Civil Aviation, the country organizing the investigation is required to complete an initial report of a crash within 30 days and send it to the International Civil Aviation Organization (ICAO) and countries participating in the investigation.

According to a guideline on the investigation of air crashes issued by the ICAO, the initial report can include information on flight history, casualties, crew, the flight recorder, maintenance and wreckage distribution.

The investigation of the cause of the crash of Flight MU5735 will continue, including in-depth wreckage identification, classification, flight data analysis and experimental verification, he added.

"From past experiences, the investigation of an aircraft accident usually takes a long time. Internationally, it usually takes more than two years to investigate the cause of an accident. It takes longer in regard to a more complicated accident investigation," he said.

For example, the cause of the crash of Air France Flight 447 — a passenger jet that crashed in the Atlantic Ocean after leaving Rio de Janeiro for Paris in 2009 claiming 228 lives — was released 3 years after the accident.

It also took nearly 2 years to release the cause for Chinese mainland's last major air crash which happened in Yichun, Heilongjiang province in 2010.

The administration released a preliminary investigation report on the crash of Flight MU5735 on Wednesday, 30 days after the tragedy.

The report includes information on flight history, crew, maintenance and wreckage distribution.

Flight MU5735 departed from Kunming, Yunnan province and was scheduled to arrive in Guangzhou, Guangdong province. It crashed on a mountain in Wuzhou, Guangxi Zhuang autonomous region, on March 21, claiming the lives of all 132 people on board.

Shu Ping, director of the China Academy of Civil Aviation Science and Technology's aviation safety institute, said the investigation for an air crash involves not only black boxes but also information collected at the crash site and experimental verification.

For example, he said, if at the crash site a tree branch was seen to be pulled inside the engine, investigators can determine that the engine was running before hitting the ground. But the flight data recorder would be needed to establish the speed of the engine.

"Many objects at the crash site could be used to analyze the situation, such as the condition of the engine system," he told CAAC News.