MH370 search area widened 40,000sq km

A fast response craft from Australian Defence Vessel Ocean Shield tows Able Seaman Clearance Diver Michael Arnold as he searches the ocean for debris of the missing Malaysia Airlines Flight.

Investigators seeking the wreckage of Malaysia Airlines Flight 370 in the southern Indian Ocean are widening their search area to cover the chance the aircraft fell from the sky at a shallower angle than expected.

"It's possible that the descent wasn't in quite such a tight circle as we are assessing," said Martin Dolan, chief commissioner of the Australian Transportation Safety Bureau.

That could put the Boeing 777-200 on the sea floor as far as 50 nautical miles (93km) from the seventh arc, a line drawn over the ocean where satellite communications suggest its fuel ran out.

An extra 40,000sq km has been scanned by ship-based sonars over the past three weeks, adding about 25 per cent to a high-priority search zone previously declared complete on October 26. That indicates the level of uncertainty still remaining in the hunt for the Malaysia Airlines aircraft, which disappeared on March 8 with 239 people on board.

There's a possibility that it could have gone a bit further in the few minutes between the engines stalling and the aircraft hitting the sea, Dolan said. These aircraft travel at hundreds of kilometres an hour, so a few minutes can make quite a difference.

The nine-month search for the plane, which vanished en route from Kuala Lumpur to Beijing, remains the longest such hunt in modern civil aviation history.

The best evidence of the plane's location comes from eight failed connections with an Inmarsat orbiter over the Indian Ocean, showing the plane probably travelled south from the Bay of Bengal before ditching somewhere along an arc to the west of the Australian city of Perth.

The aircraft probably spiralled anti-clockwise into the sea after its right and then its left engines ran out of fuel, the bureau wrote in an update on the search process.

Going into an increasingly tight spiral is the most likely behaviour, Dolan said. But even that isn't guaranteed.

The latest bathymetric scans, using sonars mounted on the search ship Fugro Equator, have widened the bureau's search area to stretch 50 nautical miles rather than 30 nautical miles from the seventh arc. They've also extended the zone to the very furthest south the aircraft could have come to rest, Dolan said.

That doesn't mean investigators expect it to have drifted so far. All the evidence says the most likely behaviour of the aircraft will mean it will be found within 10 nautical miles of the arc, he said.

The ship-based scan, which has now covered 200,000sq km, is the first part of a two-stage search process. It's designed to provide an accurate picture of the ocean bottom for side-scan sonar submersibles carrying out the second stage.