Inside the US spy plane

The US spy plane that made an emergency landing in China on Sunday after colliding with a Chinese fighter jet belongs to an elite, highly classified reconnaissance unit.

The EP-3E Aries II is the US Navy's principal long-range electronic surveillance aircraft, described by one expert as "a really big flying tape recorder".

It's a really major intelligence disaster

US aviation expert Jim Eckes

It is packed with sensitive receivers and antennae capable of intercepting and analysing military and civilian radio, and other electronic communications including e-mails, faxes, and telephone conversations.

All the information is fed for analysis into a huge on-board computer which sends information back to defence officials at the Pentagon in Washington.

The crew on this particular flight included Chinese linguists to monitor voice data.

Top secret

Admiral Dennis Blair, commander-in-chief of the US Pacific Command, refused to go into details about the capability of the aircraft involved.

The aircraft is very cramped inside

But it was certainly carrying top-secret information that experts say China would be very interested to discover.

"It's a disaster if that equipment is analysed by the Chinese Government," said US aviation expert Jim Eckes. "It's a really major intelligence disaster.

"It's one of the most sensitive aircraft in the US fleet.

"It's totally designed to intercept communications anywhere in the world."

Damage limitation

The US crew, knowing they were going to land in China, would have gone through routine procedures to destroy codes and wipe computer disks. They may even have smashed key parts of the aircraft's intelligence-gathering machinery.

Admiral Blair would not give details about the plane

But experts agree the Chinese could still glean much valuable information, especially if they dismantled some of the on-board equipment.

One theory is that the US aircraft was on a mission to track China's most advanced destroyer, which carries a Russian-made supersonic missile. If that is the case, experts say the EP-3 would have been trying to track its electronic signatures, so they could pick up the ship's signals even when the ship was not visible.

Robert Karniol, Asia-Pacific editor of Jane's Defence Weekly, said it was routine for US planes to patrol China, and for them to be challenged - in this case, by two F-8 fighter planes.

"The only thing unusual about this particular incident is that it led to a collision," he said. "And certainly the collision was not planned by either side."

Specifications

The EP-3 came into operation in 1969, and is based on the P-3 Orion anti-submarine patrol aircraft. The US Navy has about 11 EP-3s, although there are other aircraft that carry out similar tasks.

It is powered by four Allison T-56-A-14 turboprop engines, and can fly for more than 12 hours at a time, with a range of more than 3,000 nautical miles (5,555km).

Each plane carries a crew of 24, including seven officers.

The plane is 32.28m long (about 106 feet), with a wing span of 30.36m.

The US Navy operates two squadrons of the aircraft, according to the Federation of American Scientists. One is based on the US west coast at Whidbey Island Naval Air Station in Washington state, and also operates out of bases in Guam and Japan, while the other is based in Spain.