14 CFR 91.173 — ATC Clearance and Flight Plan Required (Explained)

What does 14 CFR 91.173 actually say?

14 CFR 91.173 contains two requirements, stated in full:

No person may operate an aircraft in controlled airspace under IFR unless that person has — (a) Filed an IFR flight plan; and (b) Received an appropriate ATC clearance.

Both requirements must be satisfied simultaneously. Filing a flight plan without receiving a clearance is insufficient, and operating on a clearance without a filed plan is impermissible. The rule is deliberately brief — it says nothing about weather conditions, aircraft equipment, or the method of obtaining the clearance. Those requirements come from adjacent sections of Subpart B and the AIM. The key phrase is "in controlled airspace under IFR": both conditions must be true for the regulation to trigger.

What does "controlled airspace" mean for this rule?

"Controlled airspace" as used in 91.173 refers to all airspace classes except Class G. Specifically: Class A (18,000 feet MSL and above), Class B (around major terminals), Class C (around busy regional airports), Class D (around towered airports), and Class E (most of the low-altitude IFR structure in the continental U.S.).

The practical significance: Class E airspace begins at 1,200 feet AGL over most of the U.S. (or at 700 feet AGL in transition areas depicted by fuzzy magenta), so departing from an uncontrolled airport quickly brings you into Class E — and the 91.173 requirements apply the moment you enter it.

Can you fly IFR in Class G airspace without a clearance?

Yes. Because 91.173 applies only to controlled airspace, there is no regulatory prohibition on operating IFR in Class G without an ATC clearance. Class G is uncontrolled, so ATC has no separation responsibility there.

In practice, however, a departure from an uncontrolled airport almost always requires coordination with ATC before or immediately after departure, because Class E begins at low altitudes. You may legally lift off in Class G without a clearance in hand, but you need one before you enter Class E. The AIM provides departure procedures for exactly this scenario (see the clearance void time discussion below).

How do you get an IFR clearance from an uncontrolled airport?

Departing IFR from an uncontrolled airport (one without an operating control tower) requires obtaining your clearance from the controlling ATC facility before entering controlled airspace. Per AIM 5-2-6 , common methods include:

1. 1
   Call the controlling ARTCC or TRACON by phone before engine start and receive a "clearance on request" with a release time.
2. 2
   Contact the FSS by radio while still on the ground (Remote Communications Outlet or UNICOM with phone relay), if the airport has one.
3. 3
   Contact ATC airborne in Class G airspace before penetrating Class E — this requires a specific void time or release authorization from the controlling facility.
4. 4
   Receive the clearance in the air via radio contact after departure from Class G, prior to Class E entry.

When ATC issues a clearance for an uncontrolled-field departure, they typically assign a clearance void time — a specific clock time by which you must be airborne. Per AIM 5-2-6 , if you do not depart before the void time, the clearance is cancelled and you must advise ATC of your intentions no later than 30 minutes after the void time. ATC may initiate search and rescue if they do not hear from you within that window.

What is a pop-up IFR clearance, and when would you request one?

A pop-up clearance is an IFR clearance requested in flight by a pilot who is currently VFR but needs IFR protection — typically because of deteriorating weather. ATC may issue a pop-up clearance when workload permits. There is no regulatory prohibition on requesting one, but ATC is not required to issue it.

Per AIM 5-1-8 , if a pilot in VFR-on-top conditions cannot maintain VFR and cannot immediately obtain a clearance, declaring an emergency under 14 CFR 91.185 preserves full regulatory protection. Key points about pop-up clearances:

• Contact the nearest ATC facility on 121.5 MHz or any approach/center frequency.
• State your position, altitude, aircraft type, and that you are requesting an IFR clearance.
• ATC will issue a clearance if workload permits; you may be held in VFR conditions until the clearance is worked into the traffic flow.
• If IMC entry is imminent with no clearance received, declare an emergency — you are then protected by 91.185 procedures.

What happens after you receive a clearance — what does 91.183 require?

Once operating under IFR in controlled airspace, the ongoing communication requirements are governed by 14 CFR 91.183 . Unless otherwise authorized by ATC, the PIC must:

• Maintain a continuous watch on the appropriate frequency.
• Report the time and altitude of passing each designated reporting point (or as specifically requested by ATC when under radar control).
• Report any unforecast weather conditions encountered.
• Report any other information relating to the safety of flight.

These requirements run in parallel with 91.173 — once you have the clearance, 91.183 governs how you communicate while operating under it.

What if you lose communications after receiving a clearance?

If two-way radio communications fail after you have an IFR clearance, 14 CFR 91.185 takes over. In VFR conditions, continue VFR and land as soon as practicable. In IFR conditions:

1. 1
   Route: Fly the last assigned route, then the expected route, then the filed route — in that priority order.
2. 2
   Altitude: Fly the highest of (1) the last assigned altitude, (2) the MEA, or (3) the altitude ATC told you to expect — for each route segment.
3. 3
   Leave the clearance limit: Begin descent at the expect-further-clearance time if one was given; otherwise, begin the approach at your estimated arrival time.

Practice Questions

1. You are departing from a non-towered airport. The closest controlled airspace is Class E beginning at 700 feet AGL. At what point does 14 CFR 91.173 require you to have an IFR clearance?

2. ATC issues you a clearance void time of 1430Z. You are not ready to depart until 1435Z. What are your legal obligations?

3. You are VFR on top at 8,500 feet MSL and encounter an unexpected cloud layer that prevents you from maintaining VFR. You contact Approach and request an IFR clearance. ATC says they are unable to issue a clearance due to traffic saturation. What are your options?

4. A friend tells you that you can legally fly an IFR practice approach in VMC without an IFR flight plan because the weather is VFR. Is this correct? What regulation applies?

5. You are IFR in controlled airspace and your radio fails. The weather is VFR. What does 14 CFR 91.185 require you to do?

Frequently Asked Questions

Q: Does 14 CFR 91.173 require a clearance before you depart IFR?

Yes — you must have both a filed IFR flight plan and a received ATC clearance before operating in controlled airspace under IFR. You can legally depart from an uncontrolled airport in Class G airspace without a clearance, but you need one before entering any controlled airspace.

Q: Can you fly IFR in Class G airspace without an ATC clearance?

Yes. 14 CFR 91.173 applies only to controlled airspace. Class G is uncontrolled, so the regulation does not require a clearance there. However, you still need an IFR flight plan on file and a clearance before entering Class E, D, C, or B airspace above.

Q: What is a pop-up IFR clearance?

A pop-up clearance is an ATC IFR clearance requested in the air by a VFR pilot who encounters IMC or needs IFR protection. ATC issues it when workload permits. Per AIM 5-1-8, you should declare an emergency if IMC is imminent and no clearance has been received.

Q: What is a clearance void time?

A void time is the specific time ATC assigns when you depart an uncontrolled airport — you must depart by that time or the clearance is cancelled. Per AIM 5-2-6, if you do not depart before the void time, you must advise ATC no later than 30 minutes after the void time.

Q: What happens if a clearance void time passes and you have not departed?

Your IFR clearance is cancelled. Per AIM 5-2-6, you must contact ATC within 30 minutes of the void time to advise your intentions. ATC may initiate search and rescue if they do not hear from you within that window.

Q: Do you need an IFR flight plan to practice instrument approaches under IFR in VMC?

Yes. If you are operating under IFR in controlled airspace — even in VMC — 14 CFR 91.173 requires both a filed IFR flight plan and a received ATC clearance. The weather conditions do not change the legal requirement.

Q: What CFR section covers IFR communications requirements once you have a clearance?

14 CFR 91.183 covers IFR communications. It requires a continuous watch on the assigned frequency and mandatory reporting of: passing each designated reporting point, any unforecast weather conditions, and any other information relating to the safety of flight.

Q: What regulation governs IFR flight if you lose radio communications after receiving a clearance?

14 CFR 91.185 governs lost-comm procedures. In VFR conditions, continue VFR and land as soon as practicable. In IFR conditions, follow the assigned route, fly the highest applicable altitude, and initiate descent at the expect-further-clearance time or ETA.

Sources

• 14 CFR 91.173 — ATC Clearance and Flight Plan Required (Cornell LII)
• 14 CFR 91.173 — eCFR
• 14 CFR 91.183 — IFR Communications (eCFR)
• 14 CFR 91.185 — IFR Operations: Two-Way Radio Communications Failure (eCFR)
• AIM Chapter 5, Section 1 — Preflight, Clearance Procedures (FAA)
• AIM 5-2-6 — Departure Procedures: Clearance Void Times (FAA)
• FAA Instrument Flying Handbook (FAA-H-8083-15B)
• FAA Instrument Rating ACS (FAA-S-ACS-8C)

This article was researched from FAA primary sources (14 CFR Part 91, AIM, Instrument Flying Handbook) and citing current regulatory text — drafted by MockDPE. Last updated: May 2026. If you spot an inaccuracy, email corrections@mockdpe.org.