SYDNEY, Australia (AP) _ From Wagga Wagga to Wee Waa, Rockhampton to Lightning Ridge, towns in the vast Australian Outback depend on small planes to deliver the mail, dust crops, round up cattle, shuttle schoolchildren, bring in doctors and evacuate medical emergencies.

For more than a week, half the nation's light aircraft have been grounded because of contaminated aviation fuel that thickens when it contacts copper and brass engine parts, raising the risk of clogged fuel lines and motors stalling in flight.

Nobody knows how many planes actually carry the bad fuel. The source of the contamination _ Mobil Oil Australia Ltd., a subsidiary of U.S.-based Exxon Mobil Corp. _ has no test to find out, and there is no known method to clean contaminated systems.

``This is a contamination crisis of a magnitude that has never been seen before anywhere in the world,'' said Mick Toller, safety director for the Civil Aviation Safety Authority, which ordered the planes grounded.

Scientists are working on a three-stage field diagnosis they hope will identify which aircraft have tainted fuel. But pending further tests, grounded planes would not be in the air before Thursday, Toller said Sunday.

The fuel contaminant affects piston-driven engines that turn propellers in aircraft ranging from single-engine Cessnas up to planes that carry a dozen or so passengers.

Bigger turboprop aircraft are not affected, so there is no disruption to major airlines. But hundreds of charter services and flight training schools have shut down. In Victoria state, wildfire-fighting crews scaled back operations, as did the national Royal Flying Doctor Service.

The air force grounded three of its 14 Caribou transport planes, including one in East Timor, where Australia is leading the peacekeeping force, until engine parts were replaced.

Just before Christmas, pilots started reporting discoloration in some fuel. The aviation authority told the pilots not to fly until they had checked and, if necessary, cleaned their fuel systems.

Then, on Jan. 10, the authority grounded all aircraft that had filled their tanks from batches of Avgas 100/130 produced at Mobil's plant in Altona, a suburb of Melbourne, from Nov. 21 to Dec. 23. The affected fuel had been distributed throughout Australia's eastern states. Scientists had found the tainted fuel could set off a chemical chain reaction, producing a viscous material that can clog fuel lines and cause motors to stall.

``What dreadful gunk,'' Bill Hamilton, president of the Aircraft Owners' and Pilots' Association, said as he examined the substance last week. ``You wouldn't put that in your lawn mower, would you?''

Mobil says it inadvertently altered the refining cycle for Avgas 100/130 at the plant Nov. 21 and added corrosion retardants out of the proper sequence, which left traces of ethyl diamine in the finished product. The thick contaminant forms when the ethyl diamine reacts with copper and brass engine components, Mobil said.

The fuel crisis is crippling Australia's network of small airports, flight schools, charter services and bush pilots, an industry worth $330 million a year.

In hangars and on runways across eastern Australia, planes stand idle and owners are canceling orders and laying off pilots and mechanics. Operators say weekly losses are running at $8 million.

On Monday, lawyers for two aircraft operators lodged class-action lawsuits against Mobil to recover losses. Airport owners have threatened to do the same.

Besides potential compensation claims, Mobil faces a huge cleanup bill and a government investigation into the cause of two helicopter crashes during flight tests.

On Tuesday, Mobil announced a $10 million financial-assistance plan for operators who ``face immediate financial hardship'' as a result of the crisis. But the company said it was not admitting liability, and measures to fix clients' business losses were yet to be decided.

The federal Transport and Safety Bureau says it is looking into reports that the contamination occurred before Nov. 21, and that it contributed to the two helicopter accidents, both of which occurred before that date. The flight tests were near ground level, and no one was injured.

Mobil has said it will pay cleanup costs where contamination is found. At a meeting with pilots last Thursday, however, the company refused to discuss compensation, said Hamilton, the head of the pilots and owners group.

Acting Prime Minister John Anderson said the government would defer fees for air traffic control services and has urged banks not to foreclose on small air operators who miss loan repayments.