(Updates with second Bluefin mission interrupted)
* Undersea drone forced to curtail second mission
* 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) - The search for a Malaysia Airlines jetliner deep in the Indian Ocean was again cut short on Wednesday when technical problems forced a U.S. Navy underwater drone to surface without finding anything, officials said.
While a massive air and sea search for missing Malaysian Airlines flight MH370 is continuing almost 2,000 km (1,200 miles) off the coast of Perth, hopes have been pinned on the Bluefin-21 autonomous underwater vehicle finding the first concrete sign of the plane in more than six weeks of hunting.
Malaysian authorities have still not ruled out mechanical problems as causing the Boeing 777'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.
A unspecified technical problem meant the Bluefin resurfaced early on Wednesday and analysis of the sonar data downloaded showed no significant detections, the Australian agency leading the search said.
It has subsequently been relaunched to continue its search.
The drone was forced to end its first deployment early on Monday after it exceeded its 4.5 km (14,750 feet) depth limit in the remote stretch of ocean where search authorities believe the jetliner crashed after its disappearance on March 8 with 239 people on board.
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.
(Additional reporting by Lincoln Feast in SYDNEY; Editing by)