Qantas said Tuesday it would put some of its superjumbos back into service but will keep A380s off routes from Australia to the United States while it investigates whether extra thrust it uses to power those flights puts too much stress on the engines.

CEO Alan Joyce announced that two of Qantas' six A380s would resume commercial flights starting Saturday — 23 days after the disintegration during flight of a Rolls-Royce engine on one of its planes triggered the most serious safety scare yet for the world's largest and newest jetliner.

The blowout caused serious damage to some of the plane's flight systems, and pilots scrambled for an hour to deal with a cascade of warnings before making a safe landing in Singapore. Qantas grounded its superjumbo fleet within hours.

Joyce said two A380s would be flown without passengers to Sydney from Los Angeles, where engines had been replaced or modified and exhaustive checks carried out, and then they would resume passenger flights from Sydney to London via Singapore.

"After those extensive checks with Airbus and Rolls-Royce we are completely comfortable with the operation of the aircraft," Joyce told a news conference in Sydney on Tuesday. "We believe it is appropriate to start the services this week."

A380s would be kept off routes to Los Angeles from Australia's two largest cities — Sydney and Melbourne — while Qantas conducts tests on the engines of those planes returned to service. Joyce said Qantas wants to be sure the airline is not using too much power to get the giant aircraft off the ground.

The Australia-U.S. flights are among the longest non-stop commercial flights in the world, and the A380s must load up on more fuel than on other flights to make them. That means the planes heavier when they take off, and need more thrust from the A380's four engines.

Joyce said the thrust settings Qantas uses is within the certified specifications of the Trent 900 engine fitted to its planes, and the manufacturers and the European regulator responsible for ensuring safety standards consider it is safe to fly those routes.

"We're imposing this restriction on ourselves until we're comfortable with the history and the information we'll get from the continued operation over the next few weeks," Joyce said. "Once we're comfortable with that we'll operate them again."

"We want to make sure that we're 100 percent sure of the aircraft before we put them on the LA route, which requires this extra use of thrust. So it's very precautionary on behalf of Qantas," he said.

Investigators say leaking oil caught fire in the Qantas engine on Nov. 4 and heated metal parts, causing them to disintegrate before the jetliner returned safely to Singapore with 466 people aboard. Experts say chunks of flying metal cut hydraulics and an engine-control line in the wing of the A380, causing a cascade of problems including the loss of control of a second engine and some braking power, fuel leaks and more than 50 on-board warnings.

It was the most serious safety scare for the world's largest and newest jetliners, and prompted Qantas to ground its fleet. Other airlines using the Trent 900 engine aboard A380s, Singapore Airlines and Germany's Lufthansa, also briefly grounded some planes while safety checks were carried out.

Between them, the three airlines fly 20 A380s with four Trent 900 engines apiece.

Some experts say the extra stress caused by the thrust Qantas uses on its longest-haul flights may have contributed to the problem showing up first in the Australian airline's A380s. A detailed investigation by air safety authorities is still underway and has not released any conclusions yet.

Peter Marosszeky, a jetliner maintenance expert at the University of New South Wales, said the decision to keep the A380s suspended from the long flights suggests Qantas is being extra careful rather than raising concerns there may still be a problem with the engines or with Qantas' thrust settings.

"They are trying to limit stressing the engine to its maximum, to prevent any aggressive deterioration or anything like that," Marosszeky said. "It is a very wise decision. You don't want to operate something at its maximum thrust capability then find out that you are prematurely creating a problem."

In addition to the two A380s from the existing fleet returning to the skies, Qantas would take delivery of two new superjumbos from Airbus that the airline will put immediately into service, Joyce said. Qantas expects to by flying four A380s by the time the busy Christmas holiday period starts.

The rest of Qantas' fleet will remain on the ground as it waits for new or modified engines to be supplied. So far, 16 engines on Qantas planes have been removed, Joyce said. The damaged plane will remain in Singapore for repairs.

Qantas has declined to say publicly what its thrust settings for its A380s are. The airline reportedly uses a maximum thrust setting of 72,000 pounds for A380s taking off on the two Australia-United States routes. That's below the engine's certified maximum of 80,000 pounds, according to Rolls-Royce documents.