Definition
Subdivisions of an instrument approach chart's safe-altitude depiction that show a lower minimum safe altitude for a specific portion of the area surrounding the navigation facility, used when terrain or obstacles in part of the area allow a lower altitude than the rest. Each sector is bounded by bearings from the reference facility and labeled with its own minimum altitude, providing obstacle clearance within that slice of airspace within 25 NM of the facility.
Plain English
When the area around a navigation aid has uneven terrain, the chart can split the surrounding area into pie-slice sectors, each with its own safe altitude. This lets you fly lower in the slices where the ground is lower, instead of using one high altitude for the whole area.
Context Anchor
Seen on instrument approach charts that use sector-style arrival areas to guide aircraft from the arrival portion of flight toward the approach.
Derivation
"Intermediate" comes from Latin inter- (between) + medius (middle) -- literally "in the middle." Here it means altitudes that sit between the highest and lowest sector values around the facility, rather than a single uniform altitude for the whole circle.
Why Pilots Care
If you need to descend in an emergency or lose situational awareness near the approach environment, knowing the sector you're in lets you safely descend to a lower altitude than the overall area minimum. Picking the wrong sector or ignoring the bearings that define it can put you below safe terrain clearance.
Grounding Statement
Picture the chart as dividing the arrival area into slices, with each slice showing how low you may safely descend there.
Intuition Check
Do not read intermediate altitude sectors as just “middle-height areas.” In this context, they are published chart areas with specific minimum altitudes that must match the aircraft’s position.
Example Sentence 1
The approach chart showed intermediate altitude sectors around the VOR, with 3,400 feet to the north and 4,800 feet to the south where the ridgeline rises.
Example Sentence 2
Charts show different minimum altitudes for each intermediate altitude sector to ensure terrain clearance.