Definition
A phrase used by ATC to inform a pilot that the requested IFR clearance has not yet been received from the controlling facility, but has been requested and will be issued as soon as it becomes available.
Plain English
It means: 'We've asked for your clearance, but we don't have it yet. Stand by — we'll give it to you as soon as it comes through.'
Context Anchor
Heard from a tower or ground controller before an IFR departure, often while the pilot is waiting for the route and departure instructions.
Derivation
Clearance comes from clear, meaning open or free from obstruction. In aviation, it means official permission from ATC to do something. On request means it has been asked for, not that it has already been approved.
Why Pilots Care
Failing to request the clearance can delay departure and create confusion with ATC during busy operations.
Intuition Check
Do not hear clearance on request and think, "I am cleared." It means the clearance has been requested, but it has not been issued yet.
Example Sentence 1
After calling clearance delivery, the pilot was told, 'Cessna 12345, your clearance is on request, stand by.'
Example Sentence 2
After engine start the pilot called ground and requested the IFR clearance because the tower had announced clearance on request.