The Civil Aviation Administration of China (CAAC) has asked its East China branch and the Shanghai airport to investigate why a dead body was found in the landing gear of a United Airlines passenger jet from Shanghai to San Francisco on July 19.

The Shanghai airport has contacted the US side to ask for information, including fingerprints, in an attempt to identify the person's identity as soon as possible, the CAAC said on its website on July 20.

The body, suspected to be a stowaway, was discovered at about 7:30am local time in the nose gear of United Airlines flight 858 from Shanghai.

The remains appeared to be that of an Asian man in his 50s, who was wearing several layers of clothing and had few obvious injuries, San Mateo County Coroner Robert Foucrault said.

The man could have died from a lack of oxygen, from hypothermia, or from being crushed by the plane's landing gear, Foucrault said.

An autopsy was scheduled for July 20.

US Federal Aviation Administration (FAA) spokesman Ian Gregor said the death shows the perils of attempting to be a stowaway in the undercarriage of a passenger jet.

The FAA said it knows of 74 instances since 1947 of people who tried to hide in wheel wells of passenger planes. Only 15 survived, Gregor said.

"The mortality rate for this is around 79.7 percent," Gregor said, citing prolonged exposure to sub-zero temperatures.

"It's extremely dangerous and invariably leads to death. Most people die because they are crushed by the landing gear, or they freeze, or they fall out of the plane," he said.

The most recent survivor was a stowaway who hid on a flight from the Dominican Republic to Miami in 1974, Gregor added.