Australia's aviation authority believes it has come up with a test to determine which of the 5,000 light aircraft grounded in the country's worst case of fuel contamination need repair work. The Civil Aviation Safety Authority (CASA) said if the test were approved by an independent expert in an interim report due today, some of the piston engined aircraft could be cleared later this week. Tbe fuel, produced at Mobil's Altona refinery, was contaminated by an increase in the dosage of ethylene diamine and was sold throughout south-eastern Australia.