Definition
The lowest altitude that may be used under emergency conditions which will provide a minimum clearance of 300 m (1,000 feet) above all obstacles located in an area contained within a sector of a circle of 46 km (25 NM) radius centered on a radio aid to navigation associated with a particular aerodrome.
Plain English
The lowest safe altitude you can drop to in an emergency within a 25 NM slice of airspace around an airport's navigation aid, and still be guaranteed at least 1,000 feet above anything on the ground.
Context Anchor
Seen on instrument approach charts, usually shown as sector altitudes around a radio navigation aid or other charted point.
Derivation
Sector comes from the Latin 'sectus,' meaning a cut or slice. The chart divides the area around the navaid into pie-slice sectors, each with its own minimum safe altitude based on the terrain and obstacles in that slice.
Why Pilots Care
Gives pilots a guaranteed terrain-safe altitude to descend to when normal navigation or approach options are lost.
Intuition Check
Do not read “minimum” as a normal target altitude for the whole procedure. It is an obstacle-clearance floor for that sector only, not a guarantee of radio reception, weather clearance, or permission to descend.
Example Sentence 1
After losing the approach in heavy weather, the crew climbed to the published minimum sector altitude until they could regroup and try again.
Example Sentence 2
The approach chart showed a Minimum Sector Altitude of 2,800 feet in the sector containing the initial approach fix.