Missing Malaysia Airlines Flight 370: Nothing spotted in air search Friday, Australia reports

(CNN) -- Despite better weather, the first of five search planes dispatched to look for floating debris that could be related to missing Malaysia Airlines Flight 370 returned to base Friday without spotting anything of note.

The surveillance planes are looking for two objects photographed by a commercial satellite on Sunday bobbing in the remote and treacherous waters of the southern Indian Ocean off the west coast of Australia.

Aircraft and a merchant ship scoured the area Thursday, but found nothing in a search hindered by poor weather.

Flight 370 vanished 14 days ago with 239 people aboard, and the announcement Thursday by Australian officials that they had spotted something raised hopes of a breakthrough in the frustrating search.

On Friday, Australian Prime Minister Tony Abbott defended the decision to announce the find, saying that Australia owes it to families f those missing "to give them information as soon as it's to hand, and I think I was doing that yesterday in the Parliament."

But he reiterated a warning that two objects spotted by satellite in a remote area of the southern Indian Ocean, which are now being sought by aircraft and ships, may not be related to the search for the plane.

"It could just be a container that has fallen off a ship," he said during a visit to Papua New Guinea. "We just don't know."

His words have focused worldwide attention on Australia's part in the massive international hunt for the jetliner, which disappeared March 8 over Southeast Asia with 239 people on board.

Almost two weeks after the Boeing 777-200 dropped off radar screens, authorities still don't know why it veered dramatically off course or where it ended up.

Because of the "anxiety and apprehension" experienced by relatives of the people aboard the plane, Abbott said

Search teams that flew over the area where the two objects are thought to be located drew a blank Thursday, with poor visibility reported. Flights to the zone by long-range reconnaissance planes resumed Friday, Australian authorities said.

The search area, thousands of kilometers southwest of Perth, the main city on Australia's west coast, is "about the most inaccessible spot you could imagine on the face of the earth," Abbott said.

Visual search

Aircraft from Australia and the United States have staggered their departures to the area. The first plane returned without finding anything, and two other aircraft are now inside the search area, Australian officials said. A fourth plane was due to arrive in the zone soon.

The weather conditions Friday are better, said John Young, emergency response manager for the Australian Maritime Safety Authority said. Flight crews are searching for the objects visually rather than using radar, he said.

"That's encouraging," he said. "But we have no sightings yet."

Given the distance from Australia to where the objects were spotted by a commercial satellite, the aircraft will only have about two hours in the search area before having to start the return journey.

Young cautioned that the search zone is "a big area when you're looking out the window trying to see something by eye."

The flight crews may have to repeat sequences of flights like those undertaken Thursday and Friday "a few times" before they can be confident of having covered the whole area, he said.

Along with the aircraft, a motley collection of merchant ships are heading to the search area, where they will join a massive Norwegian cargo ship diverted there Thursday at the request of Australia.

The sailors aboard the Norwegian ship worked throughout the night looking for the objects, said Erik Gierchsky, a spokesman for the Norwegian Shipowners Association.

Locators needed

If the objects are determined to be debris from Flight 370, experts say that search teams may then use locators to try to find any pings in the area emanating from the plane's flight data recorder.

Malaysia's Acting Transport Minister Hishammuddin Hussein tweeted Friday that the search and rescue teams working across a wide area of the Indian Ocean are in need of the locators, also known as hydrophones. He said not many countries have them.

Hishammuddin said he would be speaking to U.S. Defense Secretary Chuck Hagel on Friday.

The batteries that power the pings from the flight recorder have a life expectancy of 30 days, meaning they will probably last about another 16 days.

But one expert warns that the depths of the Indian Ocean may make it hard to put the hydrophones to use.

"At this water depth, the range is very limited on listening to those pingers," said Mike Williamson, who runs a Seattle engineering firm specializing in deep-sea searches of planes and shipwrecks.

Deleted files sought

Malaysian authorities say they believe that the missing plane was deliberately flown off course during its scheduled flight from Kuala Lumpur to Beijing. But they haven't so far found any clear evidence to indicate who might have changed the plane's path and why.

The pilot and first officer of

the plane have come under particular scrutiny, especially in light of information suggesting a sharp turn in the flight path had been programmed into the plane's flight management system before one of the pilots gave a routine sign-off to Malaysian air traffic controllers.

Question marks remain over data that authorities say was deleted from the hard drive of a flight simulator found at the home of the plane's pilot, Capt. Zaharie Ahmad Shah.

On Thursday, a U.S. official familiar with the investigation told CNN that an FBI team is confident that it will be able to retrieve at least some of the deleted files.

Investigators will also analyze websites that Zaharie and the first officer, Fariq Ab Hamid, may have visited recently, the official said on the condition of anonymity.

CNN's Mitra Mobasherat, Kyung Lah, Chelsea J. Carter, Mike Pearson, Brian Walker, Elizabeth Joseph, Evan Perez and Shimon Prokupecz contributed to this report.

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