Australia's aviation authorities said yesterday they were investigating whether contaminated fuel supplied by Exxon Mobil, the US oil group, might have contributed to air crashes in the past 18 months. The Civil Aviation Safety Authority this week grounded about 5,000 light aircraft that filled up with Mobil aviation gas between November 21 and December 23. The grounding was ordered after the contaminated fuel was found to be causing blockage in aircraft fuel systems.