Definition
The section of Title 14 of the Code of Federal Regulations (14 CFR Part 91) that establishes a pilot's legal obligation to comply with ATC clearances and instructions. It requires that, 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. It also requires pilots operating in controlled airspace to comply with ATC instructions, and obliges a pilot who deviates from a clearance or instruction in an emergency to notify ATC as soon as possible.
Plain English
The federal rule that says once you accept a clearance or instruction from ATC, you have to follow it. You can only deviate if ATC gives you a new clearance, you're in an emergency, or you're responding to a collision-avoidance alert. If you do deviate, you have to tell ATC as soon as you can.
Context Anchor
Seen in discussions of ATC instructions, clearances, readbacks, and what a pilot may or may not do after receiving instructions from a controller.
Derivation
A regulation “section” is one numbered part of a larger set of rules. Here, “91” refers to Part 91 of the federal aviation rules, which covers general operating and flight rules, and “.123” identifies the specific rule about ATC clearances and instructions.
Why Pilots Care
It establishes the legal obligation to follow ATC instructions while preserving the pilot’s authority to deviate for safety.
Grounding Statement
In normal operations, ATC instructions are not suggestions; they are directions the pilot must either follow, question, or have changed before doing something different.
Intuition Check
Do not read “section 91.123” as just a handbook paragraph or study reference. It points to an actual federal rule that applies to pilot conduct in flight.
Example Sentence 1
Under section 91.123, the pilot could not descend below the assigned altitude until ATC issued an amended clearance.
Example Sentence 2
When an engine began to run rough the pilot deviated from the assigned altitude as permitted by section 91.123 and notified ATC as soon as possible.