§ 91.123 Compliance with ATC clearances and instructions.

Section 91.123 governs compliance with ATC clearances and instructions. It is the regulatory framework for the controlled-airspace operating environment.

Paragraph (a) — Compliance is the default: When an ATC clearance has been obtained, no PIC may deviate from that clearance, except in an emergency, unless the PIC obtains an amended clearance. ATC controls the operation in controlled airspace; the pilot complies. This is the basic rule of operating under ATC.

Paragraph (a) — The two exceptions to compliance: (1) emergency authority under § 91.3(b); AND (2) when the pilot has obtained an amended clearance. There is no third exception. Pilot 'preference' is not a basis for deviation.

Paragraph (a) — TCAS resolution advisories: Operating a TCAS-equipped aircraft must comply with TCAS RAs even when the RA conflicts with an ATC instruction. The Administrator has interpreted that complying with a valid TCAS RA constitutes an 'emergency' for § 91.3 purposes — but the RA must be a genuine RA, not a TA. The pilot must promptly notify ATC of the RA-driven maneuver.

Paragraph (b) — Acknowledgment: The PIC must, when so requested by ATC, promptly read back the ATC clearance to verify that the pilot understands. This is the readback/hearback verification system — it is operationally essential and ATC training emphasizes it.

Paragraph (c) — Each pilot is responsible for separation, even when ATC clearance has been received: The PIC remains responsible for collision avoidance. ATC clearances do not absolve the pilot of see-and-avoid in VMC. This is the legal counterpart to § 91.113(a).

Paragraph (d) — Unable: If a pilot is given a clearance they cannot comply with — whether due to aircraft capability, fuel, weather, or any operational limitation — the pilot must say 'unable' and request an alternative. ATC will work the problem. The pilot must NOT accept a clearance they cannot fly.

'Roger' is NOT compliance acknowledgment: 'Roger' means 'I have received and understood your last transmission.' It is NOT an acknowledgment of a clearance. A clearance acknowledgment is a readback that includes the controllable items (altitude, frequency, heading, route, etc.). 'Roger' alone in response to a clearance is an FAA-recognized communication failure.

Reference: AIM 4-4 on ATC clearances and pilot/controller responsibilities; FAA Order 7110.65 (ATC) for the controller-side framework.

Compliance, Authority, and the Duty to Ask — § 91.123

Walk the four paragraphs and you'll see the whole architecture of how ATC and pilots actually share the airspace.

(a) Clearances. When ATC gives you a clearance, you fly it. Period. The only ways out are three: you get an amended clearance, an emergency exists, or you're responding to a TCAS resolution advisory. Notice what's not on that list — "I didn't feel like it" and "the other route was prettier." Compliance is the default. Deviation is the exception, and the exception has to be defensible.

Tucked inside (a) is a line that should be tattooed on every student's kneeboard: "When a pilot is uncertain of an ATC clearance, that pilot shall immediately request clarification from ATC." The duty to ask is a regulation, not a courtesy. If the clearance is fuzzy — the altitude, the fix, the frequency, the runway, the hold-short instruction — you ask. Pretending you heard it and guessing wrong is how pilots end up in a deviation file.

(b) Instructions. Same idea, broader scope. Anywhere ATC is exercising control, you fly the instruction unless an emergency says otherwise.

(c) Tell them. If you deviate — emergency or TCAS RA — you notify ATC as soon as possible. Not after landing. Not after the debrief. As soon as the airplane lets you key the mic.

(d) Put it in writing. If ATC gave you priority handling in an emergency and then asks for a report, you have 48 hours to submit a detailed written report to the manager of that ATC facility. Treat that timer as a hard wall.

One more thing — the TCAS RA precedence. When the box says climb and the controller says descend, you fly the RA. That precedence is reinforced in AC 120‑55, and it's the right answer every time. Then you tell ATC what you did.

What an Examiner Asks About § 91.123

This one shows up on almost every oral — private, instrument, commercial, CFI. Be ready for these:

• "What do you do if ATC gives you a clearance you can't comply with?" — You say unable, and you offer what you can do.
• "If TCAS says climb but ATC says descend, what do you fly?" — The RA. Every time. Then notify ATC.
• "What's the deviation reporting requirement?" — Notify ATC as soon as possible (paragraph c).
• "What's the 48‑hour rule?" — If ATC gave you priority in an emergency and asks for a report, you submit a detailed written report to the facility manager within 48 hours (paragraph d).

The trick question examiners love: "What's the difference between a clearance and an instruction?" A clearance is authorization to proceed under specified conditions (route, altitude, approach). An instruction is a directive for an immediate action (turn left heading 270, descend and maintain, contact tower). Paragraph (a) governs clearances; paragraph (b) governs instructions. Same compliance duty, slightly different language — and knowing the difference shows you actually read the rule.

Saying "Unable" — A Real ATC Pushback, Under § 91.123

Here's a scenario you'll fly someday. You're VFR getting flight following, you pick up your IFR clearance airborne, and the controller hands you a routing that drops you straight into a layer the AIRMET says is full of moderate ice. You're in a 172 with no anti‑ice. Or maybe the vector points you into a TFR you just heard about on the ATIS. Or the descent hits a minimum vectoring altitude that your sectional says is below ridge level.

You say one word: "Unable."

Then give them a reason and an alternative. "N2423U unable that altitude due to icing — request 8,000 or a turn back to the south." Short, professional, specific. You're not arguing. You're not apologizing. You're telling the controller what the airplane and the conditions will allow.

Here's what new pilots get wrong: they think ATC is the boss. ATC is a service. Controllers are very good at their job — and their job does not include knowing your fuel state, your equipment limitations, your currency, or your cloud clearance. That's your job, and § 91.3 puts the final authority squarely on the pilot in command.

Examiners watch how you push back. They want to see professional, calm, reasoned — not panicked, not combative. "Unable" with a reason is the language of a pilot who knows the rule, knows the airplane, and respects the controller enough to be honest. That's the standard.

Compliance, Authority, and the Duty to Ask — § 91.123

Walk the four paragraphs and you'll see the whole architecture of how ATC and pilots actually share the airspace.

(a) Clearances. When ATC gives you a clearance, you fly it. Period. The only ways out are three: you get an amended clearance, an emergency exists, or you're responding to a TCAS resolution advisory. Notice what's not on that list — "I didn't feel like it" and "the other route was prettier." Compliance is the default. Deviation is the exception, and the exception has to be defensible.

Tucked inside (a) is a line that should be tattooed on every student's kneeboard: "When a pilot is uncertain of an ATC clearance, that pilot shall immediately request clarification from ATC." The duty to ask is a regulation, not a courtesy. If the clearance is fuzzy — the altitude, the fix, the frequency, the runway, the hold-short instruction — you ask. Pretending you heard it and guessing wrong is how pilots end up in a deviation file.

(b) Instructions. Same idea, broader scope. Anywhere ATC is exercising control, you fly the instruction unless an emergency says otherwise.

(c) Tell them. If you deviate — emergency or TCAS RA — you notify ATC as soon as possible. Not after landing. Not after the debrief. As soon as the airplane lets you key the mic.

(d) Put it in writing. If ATC gave you priority handling in an emergency and then asks for a report, you have 48 hours to submit a detailed written report to the manager of that ATC facility. Treat that timer as a hard wall.

One more thing — the TCAS RA precedence. When the box says climb and the controller says descend, you fly the RA. That precedence is reinforced in AC 120‑55, and it's the right answer every time. Then you tell ATC what you did.

What an Examiner Asks About § 91.123

This one shows up on almost every oral — private, instrument, commercial, CFI. Be ready for these:

• "What do you do if ATC gives you a clearance you can't comply with?" — You say unable, and you offer what you can do.
• "If TCAS says climb but ATC says descend, what do you fly?" — The RA. Every time. Then notify ATC.
• "What's the deviation reporting requirement?" — Notify ATC as soon as possible (paragraph c).
• "What's the 48‑hour rule?" — If ATC gave you priority in an emergency and asks for a report, you submit a detailed written report to the facility manager within 48 hours (paragraph d).

The trick question examiners love: "What's the difference between a clearance and an instruction?" A clearance is authorization to proceed under specified conditions (route, altitude, approach). An instruction is a directive for an immediate action (turn left heading 270, descend and maintain, contact tower). Paragraph (a) governs clearances; paragraph (b) governs instructions. Same compliance duty, slightly different language — and knowing the difference shows you actually read the rule.

Saying "Unable" — A Real ATC Pushback, Under § 91.123

Here's a scenario you'll fly someday. You're VFR getting flight following, you pick up your IFR clearance airborne, and the controller hands you a routing that drops you straight into a layer the AIRMET says is full of moderate ice. You're in a 172 with no anti‑ice. Or maybe the vector points you into a TFR you just heard about on the ATIS. Or the descent hits a minimum vectoring altitude that your sectional says is below ridge level.

You say one word: "Unable."

Then give them a reason and an alternative. "N2423U unable that altitude due to icing — request 8,000 or a turn back to the south." Short, professional, specific. You're not arguing. You're not apologizing. You're telling the controller what the airplane and the conditions will allow.

Here's what new pilots get wrong: they think ATC is the boss. ATC is a service. Controllers are very good at their job — and their job does not include knowing your fuel state, your equipment limitations, your currency, or your cloud clearance. That's your job, and § 91.3 puts the final authority squarely on the pilot in command.

Examiners watch how you push back. They want to see professional, calm, reasoned — not panicked, not combative. "Unable" with a reason is the language of a pilot who knows the rule, knows the airplane, and respects the controller enough to be honest. That's the standard.