415 ILCS 125/315
A person that is required to pay the fee imposed by this Law shall pay the fee to the Department by return showing all fuel purchased, acquired, or received and sold, distributed or used during the preceding calendar month, including losses of fuel as the result of evaporation or shrinkage due to temperature variations, and such other reasonable information as the Department may require. Losses of fuel as the result of evaporation or shrinkage due to temperature variations may not exceed 1% of the total gallons in storage at the beginning of the month, plus the receipts of gallonage during the month, minus the gallonage remaining in storage at the end of the month. Any loss reported that is in excess of this amount shall be subject to the fee imposed by Section 310 of this Law. On and after July 1, 2001, for each 6-month period January through June, net losses of fuel (for each category of fuel that is required to be reported on a return) as the result of evaporation or shrinkage due to temperature variations may not exceed 1% of the total gallons in storage at the beginning of each January, plus the receipts of gallonage each January through June, minus the gallonage remaining in storage at the end of each June. On and after July 1, 2001, for each 6-month period July through December, net losses of fuel (for each category of fuel that is required to be reported on a return) as the result of evaporation or shrinkage due to temperature variations may not exceed 1% of the total gallons in storage at the beginning of each July, plus the receipts of gallonage each July through December, minus the gallonage remaining in storage at the end of each December. Any net loss reported that is in excess of this amount shall be subject to the fee imposed by Section 310 of this Law. For purposes of this Section, "net loss" means the number of gallons gained through temperature variations minus the number of gallons lost through temperature variations or evaporation for each of the respective 6-month periods.
The return shall be prescribed by the Department and shall be filed between the 1st and 20th days of each calendar month. The Department may, in its discretion, combine the return filed under this Law with the return filed under Section 2b of the Motor Fuel Tax Law. If the return is timely filed, the receiver may take a discount of 2% through June 30, 2003 and 1.75% thereafter to reimburse himself for the expenses incurred in keeping records, preparing and filing returns, collecting and remitting the fee, and supplying data to the Department on request. However, the discount applies only to the amount of the fee payment that accompanies a return that is timely filed in accordance with this Section. The discount is not permitted on fees paid on aviation fuel sold or used on and after December 1, 2019 and through December 31, 2020.
Beginning with returns due on January 20, 2019 and ending with returns due on January 20, 2021 , each retailer required or authorized to collect the fee imposed by this Act on aviation fuel at retail in this State during the preceding calendar month shall, instead of reporting and paying tax on aviation fuel as otherwise required by this Section, report and pay such tax on a separate aviation fuel tax return, or on a separate line on the return . The requirements related to the return shall be as otherwise provided in this Section. Notwithstanding any other provisions of this Act to the contrary, retailers collecting fees on aviation fuel shall file all aviation fuel tax returns and shall make all aviation fuel fee payments by electronic means in the manner and form required by the Department. For purposes of this paragraph, "aviation fuel" means jet fuel and aviation gasoline .
If any payment provided for in this Section exceeds the receiver's liabilities under this Act, as shown on an original return, the Department may authorize the receiver to credit such excess payment against liability subsequently to be remitted to the Department under this Act, in accordance with reasonable rules adopted by the Department. If the Department subsequently determines that all or any part of the credit taken was not actually due to the receiver, the receiver's discount shall be reduced by an amount equal to the difference between the discount as applied to the credit taken and that actually due, and that receiver shall be liable for penalties and interest on such difference.
415 ILCS 125/315