An airline company acquires an aircraft engine with a five year guarantee. Based on their past experience it is estimated that the engine can only be flown for a maximum of 10,000 hours. The average annual flying hours are expected to be 2,000 and therefore the company plans to depreciate the engine over five years. In reality the number of flying hours is erratic. In a good year an aircraft can be flown for 3,000 hours. What is the appropriate depreciation method the entity should adopt?