Definition
In radio communications, a transmission that is unreadable or unintelligible because two or more stations transmitted on the same frequency at the same time, causing their signals to interfere with each other.
Plain English
Two people pressed their radio buttons at the same moment, so neither call came through clearly. Controllers and pilots use the word "blocked" to say the message was lost and needs to be repeated.
Context Anchor
Heard on aviation radios when two aircraft or a pilot and controller transmit at the same time.
Derivation
From the everyday sense of "blocked" meaning obstructed or prevented from getting through. In radio use, one transmission obstructs the other so neither reaches the listener cleanly.
Why Pilots Care
A blocked transmission means ATC may not have received a critical report or clearance readback, so the pilot must retransmit to maintain safety and compliance.
Analogy
It is like two people talking to you at the same time. You may hear noise, but you cannot clearly understand either message.
Intuition Check
“Blocked” does not mean a runway, route, or flight path is physically closed here. In this context, it means a radio message was covered up or made unreadable.
Example Sentence 1
The controller said, "Last transmission was blocked, say again," so the pilot repeated the position report.
Example Sentence 2
After hearing 'blocked' on the frequency, the pilot waited a few seconds before calling again.