Definition
The inner core of a Terminal Radar Service Area (TRSA) where the airspace immediately surrounding the primary airport is designated as Class D. This portion typically extends from the surface up to a specified altitude and requires two-way radio communication with the control tower before entering.
Plain English
The part of a TRSA right around the main airport that works as Class D airspace. To fly into it, you have to be talking to the tower on the radio first.
Context Anchor
Seen when studying TRSAs on aeronautical charts and when planning a flight near a tower-controlled airport inside a TRSA.
Why Pilots Care
Even though the surrounding TRSA participation is voluntary, the Class D portion is not. You must establish two-way radio contact with the tower before entering it, or you have busted controlled airspace.
Intuition Check
Do not assume the whole TRSA is Class D airspace. The Class D portion is only the tower-controlled part around the airport where Class D entry rules apply.
Example Sentence 1
Before entering the Class D portion of the TRSA, the pilot called the tower and received clearance to land.
Example Sentence 2
VFR traffic in the Class D portion receives sequencing and separation from the control tower.