PERTH, Australia -- A robotic submarine looking for the lost Malaysian jet continued its second seabed search on Wednesday as up to 14 planes were to take to the skies for some of the final sweeps of the Indian Ocean for floating debris from the ill-fated airliner.

The U.S. Navy's Bluefin 21 submarine began its second 20-hour underwater mission on Tuesday after cutting short its first because the ocean waters where it was sent were too deep, officials said.

The unmanned sub is programmed to hover 100 feet above the seabed, but it started searching atop a patch that was deeper than the sub's maximum operating depth of 15,000 feet, the search coordination center and the U.S. Navy said.

A built-in safety feature returned the Bluefin to the surface and it was not damaged, they said.

The data collected by the sub was later analyzed and no sign of the missing plane was found, the U.S. Navy said.

Crews shifted the search zone away from the deepest water before sending the Bluefin back for Tuesday's mission, the U.S. Navy said.

The search coordination center said 11 military planes and three civilian planes would search a 21,000-square-mile expanse of ocean on Wednesday centered 1,300 miles northwest of the Australian west coast city of Perth. Eleven ships would also join the search.

Isolated showers were forecast in the search area with sea swells up to6 feet, 7 inches and visibility of 3 miles, the center said.

With no wreckage from the Boeing 777 yet found, authorities said this week that the days of the surface search were numbered as the hunt for the remains of Flight 370 moved under the waves.