Definition
The minimum vertical separation between an aircraft operating under Instrument Flight Rules and terrain or obstacles, guaranteed by flying at or above published IFR altitudes such as MEAs, MOCAs, MVAs, OROCAs, or assigned ATC altitudes. These altitudes are designed to keep the aircraft a defined distance above the highest obstacle within a specified lateral area along the route or sector.
Plain English
It is the safe height the rules guarantee you will be above mountains, towers, and other obstacles when flying in the clouds, as long as you stay at or above the altitudes shown on your IFR charts or assigned by ATC.
Context Anchor
You encounter this term when reading IFR clearances, assigned altitudes, instrument departure instructions, and route planning guidance.
Derivation
“IFR” means instrument flight rules: the rules used when a pilot is flying mainly by instruments. “Obstruction” means something that blocks or sticks up into the flight path. “Clearance” can mean permission, but here it also means safe space between the aircraft and an obstacle.
Why Pilots Care
It prevents controlled flight into terrain during instrument flight by relying on ATC routing and minimum altitudes rather than pilot visual sighting.
Grounding Statement
If you are in cloud and cannot see the ground, IFR obstruction clearance is the built-in altitude buffer that keeps unseen obstacles below you.
Intuition Check
Do not read “clearance” here as only permission from ATC. IFR obstruction clearance is the physical safety margin above obstacles; if you leave the assigned route or descend below the protected altitude, that margin may no longer exist.
Example Sentence 1
The MEA on that segment provides IFR obstruction clearance of at least 1,000 feet above the highest obstacle, and 2,000 feet in mountainous terrain.
Example Sentence 2
Before accepting the clearance, the pilot confirmed it maintained IFR obstruction clearance to the destination airport.