Black boxes are usually easily found after air crashes because of their colorful exterior and signal, but it generally takes months to complete a report of their content, according to a civil aviation expert.

When black boxes are recovered in fairly good condition, investigators can analyze and produce a report on their content in about three months, but it can take much longer when they are damaged, said Li Xiaojin, a professor from the Civil Aviation University of China in Tianjin.

One of the black boxes from the crashed China Eastern Airlines Flight MU5735 was found on Wednesday and sent to Beijing for analysis, said Zhu Tao, head of aviation safety for the Civil Aviation Administration of China. The other black box has not yet been recovered.

Both were made by Honeywell in the United States.

The black box that was found contained the cockpit voice recorder, and the missing one holds the flight data recorder－with information including altitude, speed and heading.

A preliminary investigation at the crash site determined the exterior of the black box with the cockpit voice recorder was seriously damaged and the inner storage units were damaged to a lesser degree. But the device seemed to be in relatively good shape, according to the CAAC.

"It takes time to decode the flight recorder. If the storage units were damaged, it may take longer," Zhu said at a news conference on Wednesday.

Mao Yanfeng, director of the CAAC's disaster investigation agency, said at a news conference on Friday that they are unable to offer a timetable at this stage on completing the black box's data download and analysis work.

Li, of the Civil Aviation University, said that based on the information on damage to the first black box, it would likely take longer to analyze and report on the cockpit voice recorder.

"Based on the current technological development, there is no technical difficulty in decoding and analyzing data from the flight recorder. In addition to the investigation team from China, Boeing－the aircraft's maker－and Honeywell may also participate in the investigation," he said.

The investigation will also involve information collected at the crash site and experimental verification, said Shu Ping, director of the China Academy of Civil Aviation Science and Technology's Aviation Safety Institute.

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 said.