Definition
A defined portion of airspace within an instrument approach procedure where a lower minimum safe altitude is published, based on terrain and obstacle clearance for arrivals from a specific direction. Low altitude sectors appear on approach charts as wedge-shaped areas around the navaid or fix, each labeled with its own minimum altitude that pilots must maintain until established on a published segment of the approach.
Plain English
A slice of the sky around an approach, marked on the chart, that has its own safe minimum altitude. If you arrive from that direction, you can fly at that altitude instead of a higher one, because the chart designers have already checked it's clear of terrain and obstacles for that slice.
Context Anchor
Seen in instrument chart and air traffic control discussions when charted controller sectors and frequencies are described.
Derivation
"Sector" comes from Latin sector, meaning "a cutter" or "one who cuts," later used in geometry for a pie-slice portion of a circle. That geometric sense carries straight into aviation: the airspace is literally divided into pie-slice sections around a fix, each with its own altitude.
Why Pilots Care
Pilots must follow the boundaries and altitudes of these sectors to receive proper ATC instructions and avoid traffic conflicts during arrivals.
Intuition Check
Low altitude does not mean “dangerously close to the ground.” Here it means the lower part of the air traffic control altitude structure. Sector does not mean a random area; it means a defined controller responsibility area.
Example Sentence 1
Arriving from the northwest, the crew descended to the published low altitude sector minimum of 3,400 feet until established on the inbound course.
Example Sentence 2
On the arrival chart the low altitude sector begins at 8,000 feet and below.