​Divers being sent to examine wreckage from AirAsia Flight 8501

PANGKALAN BUN, Indonesia -- With the weather improving, divers prepared Sunday to examine wreckage of AirAsia Flight 8501 in hopes of retrieving the rest of the bodies and finding clues to what caused the plane to crash in stormy weather a week ago.

The breakthrough in the hunt for the Airbus A320 came after sonar equipment aboard search ships detected four massive objects on the ocean floor in the Java Sea, and Indonesian officials said they were confident they belong to the plane.

The biggest piece, measuring 59 feet long and 18 feet wide, appeared to be part of the jet's body, said Henry Bambang Soelistyo, chief of the National Search and Rescue Agency.

While the plane's black boxes -- the flight data and cockpit voice recorders -- have yet to be located, the discovery of the wreckage, especially if it is largely intact, would greatly benefit the investigation.

In shallow water like the Java Sea, detection signals emitted by the boxes can be hard to find, according to Air Safety Investigator Toos Sanitoso.

"The signal is a quite a weak signal from the pinger," Sanitoso told CBS News correspondent Allen Pizzey, adding that the sound made by nearby ships can pose a problem.

"In some cases it's even also disturbing, that's why they dispatch very carefully who is eligible to go to the site and who is not," Sanitoso said.

Two U.S. Navy ships -- the USS Sampson and the USS Fort Worth -- are helping with the search effort along with three Seahawk helicopters, according to the Navy.

Strong currents and big waves that had prevented divers from entering waters eased on Sunday. About 90 divers from Indonesia and Russia were being deployed to recover more bodies that officials believe are still strapped in their seats, said National Searh and Rescue deputy chief Tatang Zainudin.

"We hope lower waves will give us a better result today," Zainudin said. "We are racing with time and weather in running this mission."

There were 162 people aboard the plane, but after a week of searching, only 30 bodies have been found floating in the choppy waters.

The plane crashed Dec. 28, halfway into a two-hour flight from Surabaya, Indonesia's second-largest city, to Singapore. Minutes before losing contact, the pilot told air traffic control that he was approaching threatening clouds, but was denied permission to climb to a higher altitude because of heavy air traffic.

It remains unclear what caused the plane to plunge into the sea. Bad weather appears to have been a factor, according to a 14-page report released by Indonesia's Meteorology, Climatology and Geophysics Agency.

"Flight 8501 appears to have been trapped in bad weather that would have been difficult to avoid," the report said.

The objects on the seafloor were discovered Friday and Saturday, and an Indonesian Geological Survey vessel was used to assess their dimensions. In addition to what appeared to be a significant part of the plane's body, chunks of debris found in the target search area measured up to 39 feet long.

Other suspected plane parts were seen scattered on beaches during an aerial survey.

Generally, aviation experts say the more passengers, luggage and parts of the aircraft that remain intact, the more likely the plane hit the water in one piece. That would signal problems like a mechanical error or a stall instead of a midair breakup due to an explosion or sudden depressurization.

Indonesian authorities announced the grounding of AirAsia flights from Surabaya to Singapore, with the Transport Ministry saying the airline did not have a permit to fly on Sundays. However, Singapore's Civil Aviation Authority said Saturday that from its end, the airline had been approved to fly the route daily.

AirAsia, which began operations in 2001 and quickly became one of the region's most popular low-cost carriers, said it was reviewing the suspension. The crash was the airline's first.