Which airspace class would IFR be the sole flight rule?

• A. Class A
• B. Class C
• C. Class D
• D. Class E

Answer: (A)

In high-altitude airspace, the traffic environment is managed so every flight operates under instrument flight rules. In the airspace designated as Class A, all operations must be IFR, and VFR is not permitted. This arrangement, typically from 18,000 feet MSL up to the upper limits, ensures ATC can provide precise separation for all aircraft relying on instrument navigation and continuous communication. In the other classes, both IFR and VFR can operate, with VFR determined by weather minima and visibility, so IFR is not the sole flight rule there.