Definition
14 CFR §91.123 is the Federal Aviation Regulation titled 'Compliance 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 to comply with ATC instructions in areas where ATC is exercised, to notify ATC if a clearance cannot be complied with, and — when a deviation occurs due to an emergency — to notify ATC as soon as possible and submit a written report if requested.
Plain English
This is the rule that says once you accept a clearance from air traffic control, you have to fly it as given. You can only deviate if ATC gives you a new clearance, you have an emergency, or your collision-avoidance system tells you to. If you can't do what ATC asked, you have to tell them.
Context Anchor
Seen in FAA handbook discussions of arrival procedures, ATC clearances, assigned altitudes, and pilot responsibility during instrument operations.
Derivation
The number comes from the federal aviation rule system. “91” refers to Part 91, the general operating rules for aircraft, and “123” identifies the specific section about following ATC clearances and instructions. The dash before it in the source text is just punctuation leading into the citation.
Why Pilots Care
Noncompliance can result in loss of separation from other traffic or regulatory enforcement action.
Intuition Check
Do not read 91.123 as a radio frequency, chart code, or procedure step. It is a regulation citation pointing to the FAA rule about complying with ATC instructions.
Example Sentence 1
After being cleared direct to the fix, the crew flew the routing as issued in accordance with 91.123 rather than shortcutting on their own.
Example Sentence 2
When the controller issued a new heading, the crew complied immediately under the requirements of 91.123.