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According to the FAA, every airplane is required to undergo an annual inspection : "no person may operate an aircraft unless, within the preceding 12 calendar months, it has had an annual inspection and has been approved for return to service by a person authorized by FAR 43.7 ." A period of 12 calendar months extends from any day of a month to the last day of the same month the following year.
To meet the above requirement every 12 months each aircraft owner is required to ensure that his airplane is inspected.