Definition
The lowest altitude that may be used under emergency conditions which will provide a minimum clearance of 300 meters (approximately 1,000 feet) above all obstacles located in an area contained within a sector of a circle of 46 kilometers (25 nautical miles) radius centered on a radio aid to navigation associated with an instrument approach procedure.
Plain English
It is the lowest height that still keeps an aircraft safely above all obstacles within a 25-nautical-mile slice of airspace around a navigation aid at an airport. It is published for use in an emergency, not for normal approach flying.
Context Anchor
Seen on instrument approach charts, especially as a quick reference for obstacle clearance near an airport or navigation point.
Derivation
The term is built from three plain words: 'minimum' (the lowest), 'sector' (a pie-slice section of the circle around a navaid), and 'altitude' (height above sea level). Knowing 'sector' refers to a slice of the surrounding area helps explain why different parts of the circle can have different minimum altitudes — terrain and obstacles vary by direction.
Why Pilots Care
Gives pilots a published, reliable floor that guarantees obstacle clearance during instrument navigation when terrain or obstacles may not be visible.
Analogy
Think of a pizza cut into slices. Each slice can have its own safe height written on it, based on the tallest obstacles in that slice.
Intuition Check
Minimum Sector Altitude does not mean the lowest altitude you are cleared to fly by air traffic control. It means the lowest published altitude that gives obstacle clearance within that charted sector.
Example Sentence 1
After losing communications during the approach, the pilot climbed to the Minimum Sector Altitude shown on the chart for that sector.
Example Sentence 2
Before descending for the arrival, the pilot confirmed the minimum sector altitude for the sector containing the initial fix.