Definition
A defined volume of airspace within which flight information service and alerting service are provided by a designated air traffic services authority. Each FIR has published lateral and vertical boundaries, and the responsible authority issues advisories, weather information, and search-and-rescue alerting for aircraft operating inside it. The world's airspace is divided, without gaps, into FIRs so that every flight is always inside one.
Plain English
A chunk of sky with set borders that one country or agency is responsible for watching over. Inside it, they pass along useful information to pilots and raise the alarm if an aircraft goes missing.
Context Anchor
Pilots most often see FIRs on international flight plans, enroute charts, and procedures for crossing from one air traffic authority’s area into another.
Derivation
From 'flight information' (the service of passing along useful information to pilots in flight) plus 'region' (a defined area). The name describes exactly what the airspace is: a region within which flight information service is provided.
Why Pilots Care
Pilots must identify the current FIR to select the correct radio frequency and know whether they will receive only advisory information or full air-traffic control.
Intuition Check
“Information” here does not mean casual advice or general facts. In an FIR, it means safety-related flight information provided by the responsible air traffic authority.
Example Sentence 1
After departing Anchorage and heading west, the flight crew checked in with the next controller as they crossed into the Magadan FIR.
Example Sentence 2
Oceanic routes are divided into large FIRs where pilots receive flight information service rather than radar vectors.