Definition
A circular symbol on the plan view of an instrument approach chart depicting the Minimum Safe Altitude (MSA) -- the lowest altitude that provides at least 1,000 feet of obstacle clearance within a defined radius (typically 25 NM) from a designated reference point such as a navaid or fix. The circle may be divided into sectors, each showing its own minimum altitude, and is intended for emergency use only.
Plain English
A circle drawn on the approach chart that tells you the lowest altitude you can safely fly within a set distance of a chosen reference point, with at least 1,000 feet of clearance above any obstacle in that area. It's there as a backup if something goes wrong.
Context Anchor
Seen in the plan view of an instrument approach chart, near the airport, fix, or navigation aid the circle is based on.
Derivation
MSA stands for Minimum Safe Altitude. The 'circle' refers to the literal ring drawn around a reference point on the chart -- everything inside that ring is covered by the published minimum altitude.
Why Pilots Care
Gives pilots a guaranteed safe altitude above terrain and obstacles when position is uncertain or an immediate descent is required in instrument conditions.
Analogy
Think of it like a safety floor drawn over the area. If things become uncertain, staying above that floor helps keep you away from the things sticking up from the ground.
Intuition Check
Do not assume the MSA circle is an assigned altitude or a normal descent altitude. It is a charted emergency reference for obstacle clearance in the area shown.
Example Sentence 1
After losing communications during the approach, the pilot climbed to the altitude shown inside the MSA circle until she could sort out the situation.
Example Sentence 2
Inside the MSA circle the minimum safe altitude was listed as 2,800 feet.