BEIJING, March 12 -- Search for the missing Malaysian plane faces a number of difficulties, and the biggest is to locate the area where the plane disappeared, a former director of the French air accident investigation said on Tuesday.

"The search at sea is very difficult, especially when you do not know exactly where the plane disappeared," said Jean-Paul Troadec, former director of BEA, or French Bureau of Enquiry and Analysis for Civil Aviation Safety.

"Besides, there are many pollutants on the sea surface. We cannot make sure whether the objects come from the plane or not before we approach it," he added.

He said it took six days to find the first piece of debris of Air France Flight 447, which slammed into the Atlantic Ocean and killed all 228 people onboard on June 1, 2009.

Dozens of ships and planes from around 10 countries are scouring the waters around the missing Flight MH370's last known location, but no solid clues have been found so far.

The Boeing 777 aircraft, carrying 227 passengers and 12 crew members, suddenly vanished from radar early Saturday en route from Kuala Lumpur to Beijing.

"Search for the wreckage and the black boxes can be done at the same time," he said. "As weather conditions and currents on the sea surface change quickly, the longer the search, the more difficult to locate the place where the accident took place."

Comparing the Malaysian flight with Rio-Paris Air France Flight 447, he said the biggest difference is that in the case of the Air France crash, the search scope had been narrowed down to a circle on the sea with a radius of 75 kilometers.

"But it is difficult to locate the black box under the sea," he said, despite that the black box is able to emit acoustic signals underwater in the early days, which could last roughly between 30 to 40 days depending on the recorder's battery life.

"After the battery dies, location should be done in other ways," he added.

He said when searching the Rio-Paris flight, they did not detect any acoustic signals the recorders sent. "We found the wreck using sonar," he said. "The undersea detection lasted more than two years."

Asked if there is any hope of finding the missing plane, he said "it is not impossible, but it can be very long."

"The larger the area is extended, the more time it will take to conduct the operation, and the more it will cost," he said. "The most important way is to use all possible means we have at our disposal to determine the smallest possible area for search and rescue."

Troadec said the BEA has offered its assistance to Malaysian and Vietnamese authorities, but so far both countries have not sought help from the investigation.

Owen Geach, Commercial Director of International Bureau of Aviation and British expert on air safety, said the beacon on the flight data recorder, or black box, has a range of approximately 2 miles (about 3.21 km) and can transmit under water up to approximately 20,000 feet (about 6.09 km).

The search is already exhaustive and, as long as it is exhaustive, they will find evidence of wreckage, he told Xinhua, but declined to further comment.

"The authorities are being sensible by not speculating," he said.

Scott Hamilton, managing director of U.S.-based aviation consultancy Leeham Corporation, said satellites from many countries including China, maritime surveillance airplanes, such as P-3 Orion, and radar tracking are already being used.

"The effort so far was thorough but obviously in the wrong place," he noted.

Hamilton believed the black boxes can be retrieved from the ocean floor, saying the recorders in Air France Flight 447 weren't retrieved until two years after the crash happened and useful data was still obtained.

Bill Waldock, an expert on aviation accident investigation from U.S.-based Embry-Riddle Aeronautical University, also said no progress has been made because the aircraft obviously isn't where most of the search has been concentrated.

The revelations that the transponder shut down but radar shows the aircraft changed course and turned back southwest would suggest some sort of intentional event. But "whether it was the crew or someone else remains to be seen," he said.

"The search efforts so far have been unsuccessful because the airplane kept flying for some time after the transponder stopped," he said.

Asked about the chances of finding the data recorders, he said the detection depends on water conditions, how deep the recorders are, how close the search vessels are to them, and if they are still in the tail of the plane or not.

"It took the French two years to find them (black boxes), but only they revealed what really happened," he said.

The recorders of Air France Flight 447 were retrieved in 13,000 feet (about 3.96 km) underwater. "Obviously the data can last at least two years in 13,000 feet of water," he added.