ATC Clearance Compliance

FAR 91.123 — ATC Clearance Compliance

FAR 91.123 explains when pilots must follow ATC clearances and instructions, when deviation is allowed, and reporting duties after emergencies or TCAS RAs.

In Plain English

FAR 91.123 governs how pilots must respond to ATC clearances and instructions. Once you've accepted a clearance, you can't deviate from it unless one of three things happens:

You obtain an amended clearance
An emergency exists
You're responding to a TCAS resolution advisory (RA)

If you're ever uncertain about a clearance, you must immediately ask ATC for clarification — never guess. Outside Class A airspace, you can cancel an IFR flight plan any time you're in VFR conditions.

In areas where ATC is exercised, you can't operate contrary to an ATC instruction unless it's an emergency. If you do deviate (for an emergency or a TCAS RA), you must notify ATC as soon as possible. If ATC gave you priority in an emergency, you may be asked to submit a detailed written report within 48 hours to the facility manager.

Finally, you can't act on a clearance issued to another aircraft — a common trap when call signs sound similar on a busy frequency. This rule keeps the IFR system predictable and safe.

Regulation Text

14 CFR § 91.123

§ 91.123 Compliance with ATC clearances and instructions. (a) When an ATC clearance has been obtained, no pilot in command may deviate from that clearance unless an amended clearance is obtained, an emergency exists, or the deviation is in response to a traffic alert and collision avoidance system resolution advisory. However, except in Class A airspace, a pilot may cancel an IFR flight plan if the operation is being conducted in VFR weather conditions. When a pilot is uncertain of an ATC clearance, that pilot shall immediately request clarification from ATC. (b) Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised. (c) Each pilot in command who, in an emergency, or in response to a traffic alert and collision avoidance system resolution advisory, deviates from an ATC clearance or instruction shall notify ATC of that deviation as soon as possible. (d) Each pilot in command who (though not deviating from a rule of this subpart) is given priority by ATC in an emergency, shall submit a detailed report of that emergency within 48 hours to the manager of that ATC facility, if requested by ATC. (e) Unless otherwise authorized by ATC, no person operating an aircraft may operate that aircraft according to any clearance or instruction that has been issued to the pilot of another aircraft for radar air traffic control purposes. [Docket 18834, 54 FR 34294, Aug. 18, 1989, as amended by Amdt. 91-227, 56 FR 65658, Dec. 17, 1991; Amdt. 91-244, 60 FR 50679, Sept. 29, 1995] (Approved by the Office of Management and Budget under control number 2120-0005)

Oral Exam Questions a DPE Might Ask

Q1Under what conditions may a pilot in command deviate from an ATC clearance?

Per FAR 91.123(a), a PIC may deviate from an ATC clearance only when an amended clearance is obtained, an emergency exists, or the deviation is in response to a TCAS resolution advisory.

Q2If you deviate from a clearance due to an emergency or TCAS RA, what are you required to do?

FAR 91.123(c) requires the PIC to notify ATC of the deviation as soon as possible, and under 91.123(d), submit a detailed written report within 48 hours to the ATC facility manager if requested.

Q3What should you do if you don't fully understand an ATC clearance?

FAR 91.123(a) states that when a pilot is uncertain of an ATC clearance, the pilot must immediately request clarification from ATC rather than act on assumptions.

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Related Sections in Part 91

§ 91.1

Part 91 Applicability