Definition
Minimum Safe/Sector Altitude (MSA) is an emergency-use altitude depicted on instrument approach charts that provides at least 1,000 feet of obstacle clearance within a defined radius (typically 25 nautical miles) of a specified navigation fix, usually the airport reference point or a primary navaid associated with the approach. MSAs are published in feet above mean sea level and may be divided into sectors when terrain or obstacles vary significantly around the reference point.
Plain English
An altitude shown on the approach chart that guarantees you'll clear all terrain and obstacles within a circle around the airport or navaid. It's there as a safety net if you get lost, disoriented, or have an emergency near the airport — fly at or above this altitude and you know you won't hit anything.
Context Anchor
Seen on instrument approach charts, usually in the plan view area, as a circle or sector diagram with altitudes printed inside it.
Derivation
Sector' is used because the protected area is sometimes split into pie slices, each with its own minimum altitude when terrain differs around the reference point. 'Safe' reflects its purpose — it's the altitude that keeps you safely above everything in that area.
Why Pilots Care
Gives pilots a reliable, charted floor during instrument flight that prevents controlled flight into terrain when weather or darkness hides obstacles.
Analogy
Think of the area around the airport as a round pie. If the pie is divided into slices, each slice may have its own lowest safe height because the terrain and obstacles are different in each direction.
Intuition Check
“Safe” does not mean safe for every purpose. Here it means obstacle clearance for emergency use; it does not mean the altitude is cleared for normal navigation, radio contact, or every part of the approach procedure.
Example Sentence 1
Before starting the approach, the pilot noted the MSA of 4,200 feet within 25 NM of the airport in case of an emergency climb.
Example Sentence 2
In the southwest sector the minimum safe altitude rises to 5,200 feet because of higher terrain.