Sydney - The underwater drone hunting for clues to the fate of flight MH370 returned six hours into a planned 16-hour mission because it was out of its depth, officials said Tuesday.
The submersible is programmed to resurface rather than go beyond its safe maximum operating limit of 4,500 metres.
"The six hours of data gathered by the Autonomous Underwater Vehicles currently being extracted and analysed," searchers said.
The Bluefin-21 vehicle was sent down for the first time Monday to scour a 40-square-kilometre patch of the Indian Ocean about 2,170kilometres north-west of Perth, the search team headquartered in the west-coast Australian city said.
A week has gone by without any signals being picked up that could be from the flight recorders on the missing Malaysia Airlines Boeing 777. An underwater search was deemed the only option left.
Search leader Angus Houston said "aircraft wreckage needs to be visually identified before we can say with certainty that this is the final resting place of MH370."
The plane vanished an hour into a night flight March 8 from Kuala Lumpur to Beijing with 239 people on board, mostly Chinese nationals,when it made a drastic change in the flight path.
The last acoustic signals thought to emanate from MH370 plane were detected April 8. The batteries on the recorders of flight data and cockpit chatter are guaranteed reliable for just 30 days. The search is now into its 39th day and the batteries are likely to have fully discharged.
In a downbeat assessment of search results, Houston said it would "be appropriate to consult with Australia's partners to decide the way ahead later this week."The search for aeroplane wreckage has been assessed as the most costly ever mounted.
Houston said "it's very expensive and all the countries contributing to this are running up big costs."No debris has been recovered and the chances of wreckage still remaining on the surface are slim. It is also likely that any wreckage could be submerged in the silt on the ocean floor.
The search area has been plotted using analysis of where the plane would have gone down had it run out of fuel, triangulation from plane's last electronic "handshake" signals to satellites and from the acoustic signals picked up last week by a device towed by the Australian vessel Ocean Shield.
"Analysis of the four signals has allowed the provisional definition of a reduced and manageable search area of the ocean floor," Houston said.
The former Australian armed forces chief did not specify the scale of the search area for the 5-metre Bluefin-21. The drone is equipped with either sonar or a camera, takes 4 hours to journey to and from the ocean floor and can search for 16 hours at astretch.
Downloading the data aboard the Ocean Shield takes 4 hours before the mission cycle restarts. The area defined for Monday's search was 40 square kilometres. Houston warned against over-optimism that the drone will locate a debris field where the last acoustic signals were monitored. He also said that a 2-litre sample of oil had been taken from an oilslick spotted about 5 kilometres "downwind and down-sea" from where the Bluefin-21 was operating.
"It's another lead to pursue ... we either confirm or discount it inexactly the same way we've handled the vast amount of material that's been gathered during the visual search," the former military pilot said. "We look at it, we've discounted all of it so far."