PERTH, Australia — The search for Malaysia Airlines’ missing Flight 370 on the floor of the southern Indian Ocean is nearing an end with no sign of the plane in the area that investigators had concluded it most likely went down, prompting a last-ditch reassessment of assumptions used to calculate its final descent and draw the search zone.

At issue are estimates of how far the plane may have traveled after it ran out of fuel, notably whether it followed a tight or broad spiral down as it fell or glided toward the ocean, officials said.

“We’re really doing further work to test our assumption about the end of flight, which defines our search area,’’ said Martin Dolan, chief commissioner of the Australian Transport Safety Bureau. “It’s really testing to make sure we haven’t missed anything, and that our assumptions remain valid.’’

The failure to find any wreckage in the area also raises the possibility that the plane began descending earlier, or perhaps changed course in an attempt at an emergency landing at sea, though investigators have discounted these outcomes as inconsistent with other evidence.

There is still hope that the plane will be found in the search zone, an expanse of 46,000 square miles, about the size of England. But ocean survey vessels have scoured about 90 percent of the area and are expected to finish the rest in August.

Unless new information emerges, that is when the governments of Australia, Malaysia, and China plan to abandon the search, leaving one of the greatest mysteries in the history of modern aviation unsolved.

Flight 370 disappeared on March 8, 2014, while flying to Beijing from Kuala Lumpur, the Malaysian capital, carrying 239 passengers and crew members from 15 nations.

An analysis of radar and satellite communications data determined that the Boeing 777-200 made several turns and then flew south for five hours with little deviation. But investigators never pinpointed where the plane ran out of fuel.

Instead, they identified a 400-mile arc from which the plane most likely sent its last satellite signal. Survey vessels have been going back and forth at walking speed across that swath of the southern Indian Ocean for two years, using sonar devices towed over the seafloor to scan depths more than 2 miles below the waves.

Investigators are now reconsidering an assumption that when the plane’s engines ran dry, the aircraft spiraled into the sea without traveling a horizontal distance of more than 10 nautical miles — a relatively tight spiral.

Analysts at Boeing and elsewhere have been reexamining their models of how the aircraft operating under autopilot might have responded to an initial loss of power on one side of the aircraft, and, up to 15 minutes later, on both sides.

The simulations assume the right engine ran out of fuel first, because over its years of service that engine on the aircraft had tended to burn slightly more fuel than the left engine, according to Rolls-Royce, the engine manufacturer.

The three countries bankrolling the search for the missing Boeing 777-200 agreed in April last year not to expand the search area unless new information provided clear clues that the plane was somewhere else.

So far, no evidence has emerged that would justify an expanded search, Dolan said.

While the search for Flight 370 is the largest and most costly in aviation history, relatives of passengers on the plane have called for it to be extended, as have many scientists, pilots, and aviation enthusiasts.