Finding the black box flight recorder of the missing Malaysia Airlines passenger jet is simply ''untenable'' as things stand at the moment, the US Navy officer who will lead the search has conceded.
The deeply pessimistic assessment from Captain Mark Matthews came as the Royal Australian Navy vessel Ocean Shield was being loaded with a ''pinger'' locator and an underwater drone critical to recovering the black box .
However, ADV Ocean Shield will not arrive in the 319,000 square kilometre search area for three to four days, while the beacon on the black box could have only four days of battery power left.
Australian defence vessel Ocean Shield.
Loaded with a ''pinger'' locator and an underwater drone: Australian defence vessel Ocean Shield. Photo: Amanda Hoh

''It all depends on how effective we are at reducing the search area,'' said Captain Matthews, a search-and-recovery expert who was involved in the two-year search to find the black box of Air France flight 447, which crashed in the Atlantic Ocean.
''Right now, the search area is basically the size of the Indian Ocean, which is an untenable amount of time to search.''
The black box - with its cockpit and flight data recorders - is the key to understanding why flight MH370 disappeared more than three weeks ago.
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Search mission: US Navy Captain Mark Matthews with an acoustic "pinger", to be used in the hunt for flight MH370's black box. Photo: Getty Images
Highlighting the enormity of the task, Captain Matthews revealed that the ''towed pinger locator'' can detect emissions from the black box only if it is within about 1.6 kilometres of its beacon. And it must be towed at a snail-like 5km/h to be effective.
Captain Matthews said the black box's beacon could operate for a maximum of 45 days.
Further complicating matters is that even if debris of the plane is found, crew on Ocean Shield will then have to wait for oceanographers to complete the complex job of estimating where the plane actually crashed before the pinger locator can be usefully deployed.
Its point of impact will be hundreds of kilometres from where any debris floating on the surface of the Indian Ocean is found.
Malaysia Airlines flight MH370 vanished 23 days ago, veering dramatically off its course to Beijing after its communications system was disabled.
Eight ships and 10 aircraft were again hunting for remnants of the aircraft on Sunday, although one of the Malaysian Air Force C-130s was unable to depart due to technical problems.
A Malaysian frigate will join the search on Monday as part of the expanding international effort, which includes five Chinese vessels, one of which is understood also to have a black box locator on board.
While many objects have been picked up by the vessels in the past two days, none of them have been identified as part of MH370.
Chief of Navy Ray Griggs said the new search area was located in a ''more conventional shipping lane'', meaning there would be more flotsam than further south.
Meanwhile, former chief of defence force Air Chief Marshal Angus Houston has been appointed by Prime Minister Tony Abbott as the overall leader of the rescue and investigation effort.
Vice Admiral Griggs said that Air Chief Marshal Houston ''will deal with high-level international co-ordination'', while Commodore Peter Leavy will command the multinational search-and-recovery operation.
Air Chief Marshal Houston's role is understood to be primarily related to a push by a number of nations for Australia to be the base of the air crash investigation, as revealed by Fairfax Media on Saturday.
While Malaysia will formally still lead the investigation as required by international convention, the analysis of wreckage and the black box would take place in Australia.
The Malaysian government has yet to comment on the proposal. However, government and aviation officials in Kuala Lumpur agree the nation did not have the capacity to deal with an investigation of this magnitude.
''Really, we are out of our depth,'' said a senior Malaysian police officer involved in the investigation.