On the first day of the second month following the end of an aircraft registration period, a penalty of five percent of the annual registration fee shall be added to a fee not paid by that date, and five percent of the annual registration fee shall be added to the fee on the first day of each following month that the fee remains unpaid; however, the penalty shall not be less than one dollar.
Iowa Code § 328.50
88 Acts, ch 1063, §11