Definition
That segment of a North American Route between the inland navigation facility and the coastal fix along which all U.S. to Europe traffic and Europe to U.S. traffic flow.
Plain English
The shared piece of an oceanic route, between an inland point and a point near the coast, that all transatlantic flights between the U.S. and Europe use before fanning out across the ocean.
Context Anchor
Seen in FAA route-planning and air traffic route descriptions for North American Routes, especially where inland routes connect to coastal or oceanic routing.
Derivation
"Common" here means "shared by everyone" -- this segment is used in common by all eastbound and westbound transatlantic traffic. "Portion" simply means a part of the larger route. Together: the part of the route that everyone flies.
Why Pilots Care
Ensures correct FMS programming and prevents missing altitude or speed restrictions that apply to the shared segment.
Intuition Check
Do not read Common Route/Portion as a casual phrase meaning any route segment that is commonly used. In FAA use, it names a specific segment of a North American Route: the part between an inland navigation facility and a coastal fix.
Example Sentence 1
After crossing the inland navigation facility, the flight joined the common portion of the North American Route toward the coastal fix.
Example Sentence 2
The common route ends at the runway transition fix where each aircraft turns toward its assigned runway.