FAR 91.173 — IFR Clearance Requirements
FAR 91.173 requires every IFR flight in controlled airspace to have a filed IFR flight plan and an ATC clearance. Here's what pilots need to know.
FAR 91.173 is short but critical: before you can legally fly IFR in controlled airspace, you must do two things.
- File an IFR flight plan — submit your route, altitudes, and aircraft details to ATC ahead of time (typically through a flight service or an electronic filing platform).
- Receive an appropriate ATC clearance — you cannot just file and go. ATC must issue a clearance authorizing you to operate IFR before you enter controlled airspace under instrument flight rules.
Why it matters operationally: IFR traffic is separated by ATC, and that separation only works if controllers know who is out there and have approved the route. Departing IFR without a clearance — even if a flight plan is on file — is a regulatory violation and a serious safety hazard. At non-towered airports, this is why pilots typically pick up their clearance via phone, RCO, or by calling Clearance Delivery before takeoff, often with a void time if departing into controlled airspace.
Note that this rule applies to controlled airspace. IFR operations in uncontrolled (Class G) airspace do not require a filed flight plan or ATC clearance under this section, though other rules still apply.