Above what altitude must an aircraft file an IFR flight plan?

• A. 12,000 ft
• B. 18,000 ft
• C. 25,000 ft
• D. 30,000 ft

Answer: (B)

In the United States, airspace above 18,000 feet MSL is Class A. In Class A, all operations must be conducted under instrument flight rules, and you must have an IFR flight plan filed with ATC clearance to enter and fly there. VFR flight is not allowed in Class A, so filing an IFR plan is mandatory once you go above that altitude. The other altitudes aren’t the threshold because 12,000 feet is below Class A, where VFR is possible if weather minimums are met, while higher altitudes like 25,000 or 30,000 feet fall inside Class A as well and require IFR, but the point at which the IFR requirement begins is 18,000 feet.