The floor of Class B airspace above Hex airport is 4000 feet MSL. Which of the following altitudes could be the floor directly over Hex?

The floor of Class B airspace above Hex airport is 4000 feet MSL. Which of the following altitudes could be the floor directly over Hex?

• A. 2000 feet MSL
• B. 4000 feet MSL
• C. 6000 feet MSL
• D. 8000 feet MSL

Answer: (B)

In Class B airspace the floor is the lower boundary of the shelf that covers a given location, and it is defined in feet MSL. If the floor above Hex is 4000 feet MSL, that means Hex lies in a shelf whose bottom boundary is at 4000 MSL. The floor cannot be lower than that for the area, because that would place Hex inside airspace that isn’t part of the shelf described. It also cannot be higher, like 6000 or 8000, because the stated floor is 4000, indicating the shelf directly over Hex begins at 4000 and not higher. So the only altitude that could be the floor directly over Hex is 4000 feet MSL.

In Class B airspace the floor is the lower boundary of the shelf that covers a given location, and it is defined in feet MSL. If the floor above Hex is 4000 feet MSL, that means Hex lies in a shelf whose bottom boundary is at 4000 MSL. The floor cannot be lower than that for the area, because that would place Hex inside airspace that isn’t part of the shelf described. It also cannot be higher, like 6000 or 8000, because the stated floor is 4000, indicating the shelf directly over Hex begins at 4000 and not higher. So the only altitude that could be the floor directly over Hex is 4000 feet MSL.