* Drone mission cut short after depth limit exceeded
* Six hours of underwater data being analysed
* Robot may take months to scan and map search zone
By Lincoln Feast and Byron Kaye
SYDNEY/PERTH, Australia, April 15 (Reuters) - A U.S. Navy underwater drone sent to search for a missing Malaysian jetliner on the floor of the Indian Ocean had its first mission cut short after exceeding its 4.5 km (2.8 mile) depth limit, Australian search authorities said on Tuesday.
The launch of the Blue-fin 21 autonomous underwater vehicle on Monday marked a new phase in the six week search for Malaysia Airlines MH370 which disappeared on March 8 and is presumed to have crashed thousands of kilometres (miles) off course with the loss of all 239 people on board.
Searchers are confident they know the approximate position of wreckage of the Boeing 777, some 1,550 km (963 miles) northwest of Perth, and are moving ahead on the basis of four acoustic signals they believe are from its black box recorders.
But having not heard a "ping" for almost a week and with the batteries on the locator beacons two weeks past their 30-day expected life, a decision was made on Monday to launch the slow-moving undersea drone to try and locate wreckage.
"After completing around six hours of its mission, Bluefin-21 exceeded its operating depth limit of 4,500 metres and its built in safety feature returned it to the surface," the Australian agency coordinating the search and recovery operation said.
"The six hours of data gathered by the Autonomous Underwater Vehicle is currently being extracted and analysed."
Malaysian authorities have still not ruled out mechanical problems as causing the plane's disappearance, but say evidence suggests it was deliberately diverted from its scheduled route from Kuala Lumpur to Beijing.
An aircraft's black box records data from the cockpit and conversations among flight crew and may provide answers about what happened to the missing plane.
The Blue-fin robot will build up a detailed acoustic image of the area using sophisticated 'sidescan' sonar, hoping to repeat its success in finding a F-15 fighter jet which crashed off Japan last year.
It had been expected to spend up to 16 hours scouring the silty sea floor, after a two hour descent.
If it detects possible wreckage, it will be sent back to photograph it in underwater conditions with extremely low light.
Officials are currently focusing their acoustic search on an area about the size of a medium city - 600 sq km (230 sq miles) - and say it could take the underwater robot months to scan and map the whole search zone.
The search for the missing plane is on track to be the most difficult and expensive search and recovery operation in aviation history. (Additional reporting by Matt Siegel in SYDNEY; Editing by Michael Perry)