Definition
The procedure of submitting an IFR flight plan to ATC while already airborne, typically by radio to the nearest Flight Service Station or directly to the controlling ATC facility, rather than filing on the ground before departure.
Plain English
Asking for an IFR flight plan to be set up while you are already flying, instead of filing it before you took off.
Context Anchor
Seen in flight planning and instrument flying discussions when a pilot needs to start or change planned instrument flight after departure.
Derivation
“File” originally meant to place something in an official record or organized set of papers. In aviation, filing a flight plan means putting your planned flight on record with the air traffic system. “In flight” simply means this happens after takeoff.
Why Pilots Care
Gives pilots flexibility to depart VFR and activate an IFR plan later or adjust for changing conditions without returning to the ground.
Intuition Check
Do not read “filing” here as handling paperwork in the cockpit. It means submitting the flight plan information by radio so the flight can be recorded and handled by the air traffic system.
Example Sentence 1
The weather ahead looked marginal, so the pilot contacted Flight Service and began filing in flight to pick up an IFR clearance before entering the clouds.
Example Sentence 2
The crew chose to file in flight rather than delay departure while waiting for a ground filing confirmation.