* Undersea drone resumes scanning seabed
* Searchers prepare to abandon hunt for floating debris
* Robot may take months to scan and map search zone
* Good conditions for sea bed search
By Byron Kaye
PERTH, Australia, April 16 (Reuters) - Officials searching for a missing Malaysia Airlines jetliner were on Wednesday waiting for the results a U.S. Navy underwater drone's first full mission, after it was forced to abort its maiden deployment because it had exceeded its depth limit.
The massive air, sea and underwater search for missing Malaysian Airlines flight MH370 was continuing almost 2,000 km (1,200 miles) off the coast of Perth, where all eyes are on the Blue-fin 21 autonomous underwater vehicle.
The Blue-fin 21 drone was forced to end its first deployment six hours into what was meant to be a 16-hour operation on Monday after it exceeded its 4.5 km (14,750 feet) depth limit and was automatically returned to the surface.
After search data from that shortened mission yielded no objects of interest, the robot submarine was sent back to the depths of the remote Indian Ocean on Tuesday night, where search authorities believe the jetliner crashed after its disappearance on March 8 with 239 people on board.
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 introduction of the Bluefin marks a methodical, slower paced new phase of the search, now in its 40th day and described by the search coordinator, retired Air Chief Marshal Angus Houston, as the most expensive in aviation history.
U.S. Naval personnel have said the drone could take up to two months to scour a 600 sq km area where the plane is believed to have sunk.
The deep sea area now being searched, the Zenith Plateau, has never been mapped in detail because it is not in any country's economic zone.
However the sea floor is likely covered in "foraminiferal ooze", a sludge formed by microscopic marine organisms, which would show up any large metallic object clearly, James Cook University marine geologist Robin Beaman told Reuters.
"A sidescan is very good at detecting the difference in the acoustic return of a hard object versus a soft, muddy sea floor," he said. "This is quite a good environment for looking for wreck debris, albeit deep."
An air and sea search for floating debris continued on Wednesday, but Houston has indicated that will soon end.
Up to 11 military aircraft, three civil aircraft and 11 ships would help in Wednesday's search, covering a total area of about 55,151 square km in rainy conditions.
Authorities have targeted the remote stretch of ocean based on four acoustic signals they believe are from the plane's black box recorders. But they have not heard a "ping" for a week and with the batteries on the locator beacons now 10 days past their 30-day expected life, authorities have decided to stop searching using a Towed Pinger Locator and to use the Bluefin instead.
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