Atc Clearance and Flight Plan Required

Section 91.173 — ATC clearance and flight plan required for IFR — establishes the basic gate: no IFR operations in controlled airspace without an ATC clearance.

Paragraph (a) — The requirements: No person may operate an aircraft in controlled airspace under IFR unless that person has:

• (1) Filed an IFR flight plan; AND
• (2) Received an appropriate ATC clearance.

Both conditions required: Just having a clearance without filing isn't enough. Just filing without clearance isn't enough. The clearance specifically authorizes the operation.

'Cleared as filed': This phrase from ATC means the aircraft is authorized to fly the route as filed in the flight plan — typically with altitude assigned separately. The pilot must read back the full clearance including any modifications.

Uncontrolled airspace exception: § 91.173 applies in CONTROLLED airspace only. IFR operations in uncontrolled (Class G) airspace do not require a clearance — though the pilot is still bound by all other IFR equipment, currency, and weather rules.

VFR-on-top: A VFR-on-top clearance allows the pilot to operate at any VFR altitude above the underlying cloud layer while operating under an IFR clearance. The pilot complies with VFR cloud clearance rules above the layer; ATC provides IFR-style separation services.

Reference: AIM 5-2 on Departure Procedures; AIM 5-4 on Arrival Procedures.