Definition
On instrument en route low altitude charts, a solid (filled-in) black triangle symbol used to identify a compulsory reporting point. At a compulsory reporting point, pilots operating on an IFR flight plan in a non-radar environment must report their position to ATC.
Plain English
A filled-in black triangle on an IFR chart that marks a spot where you must call ATC and report your position when radar coverage is not available.
Context Anchor
Seen on instrument en route charts when identifying intersections and reporting points.
Derivation
Solid comes from a word meaning firm or whole; here it means the triangle is filled in, not just outlined. Triangle means a three-sided shape. Together, the words describe the filled-in three-sided chart symbol.
Why Pilots Care
Failing to report at a compulsory point can create ATC workload issues and safety gaps in radar coverage areas.
Intuition Check
Do not read solid triangle as just a drawing detail. On an FAA chart, the filled-in triangle has a specific meaning: it marks a compulsory reporting point.
Example Sentence 1
The intersection ahead is marked with a solid triangle, so I'll need to give ATC a position report when we cross it.
Example Sentence 2
When planning the route the crew noted every solid triangle as a required radio call.