Definition
A specific clock time issued by ATC as part of a clearance, after which the clearance is no longer valid if the aircraft has not become airborne. Used primarily for IFR departures from uncontrolled airports, the void time defines the latest moment the pilot may take off under that clearance; if not off the ground by that time, the clearance is automatically cancelled and the pilot must contact ATC for a new one.
Plain English
A deadline for takeoff. ATC says, 'Your clearance is good only if you leave the ground by this time.' Miss the time, and the clearance no longer counts — you have to call back and get another one.
Context Anchor
You may hear this in a departure clearance, especially when leaving an airport without an operating control tower.
Derivation
“Void” comes from an older word meaning empty or without effect. In this clearance, it means the clearance becomes no longer valid after the stated time if you have not taken off.
Why Pilots Care
It keeps clearances current so ATC can maintain safe separation between aircraft and avoid conflicts with later traffic.
Intuition Check
Do not read this as a flexible target time. “Not off by” means not airborne by that time; if you are still on the ground when the time passes, the clearance is void.
Example Sentence 1
ATC issued the clearance with the instruction, 'Clearance void if not off by 1415,' giving the pilot eight minutes to taxi and depart.
Example Sentence 2
We taxied out promptly so we could be airborne before the clearance void time passed.