Within what radius is the highest obstacle considered when determining MSA?

Within what radius is the highest obstacle considered when determining MSA?

• A. 10 NM
• B. 5 NM
• C. 1 NM
• D. 15 NM

Answer: (B)

MSA is the minimum altitude that guarantees obstacle clearance within a defined area around a navigation aid. To set this altitude, you look at all obstacles within a specified radius of the facility and take the highest elevation there. That highest obstacle, plus the required clearance, determines the MSA for that area. The radius used for this calculation is five nautical miles. So you identify the tallest obstacle within 5 NM of the facility, then add the standard clearance (typically 1000 ft for non-mountainous terrain, or 2000 ft in mountainous areas) to obtain the MSA. The other radii don’t apply to the standard MSA calculation in this context, which is why the five-mile radius is the key factor here.

MSA is the minimum altitude that guarantees obstacle clearance within a defined area around a navigation aid. To set this altitude, you look at all obstacles within a specified radius of the facility and take the highest elevation there. That highest obstacle, plus the required clearance, determines the MSA for that area.

The radius used for this calculation is five nautical miles. So you identify the tallest obstacle within 5 NM of the facility, then add the standard clearance (typically 1000 ft for non-mountainous terrain, or 2000 ft in mountainous areas) to obtain the MSA. The other radii don’t apply to the standard MSA calculation in this context, which is why the five-mile radius is the key factor here.