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

Walking the four paragraphs reveals the whole architecture of how ATC and pilots share the airspace.

(a) Clearances. When ATC issues a clearance, the pilot flies it. Period. The only ways out are three: the pilot gets an amended clearance, an emergency exists, or the pilot is responding to a TCAS resolution advisory. Notice what is not on that list — preference and a more scenic route. Compliance is the default. Deviation is the exception, and the exception has to be defensible.

Tucked inside (a) is a line worth committing to memory: "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 unclear — the altitude, the fix, the frequency, the runway, the hold-short instruction — the pilot asks. Pretending to have 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, the pilot flies the instruction unless an emergency says otherwise.

(c) Tell them. If the pilot deviates — emergency or TCAS RA — ATC is notified as soon as possible. Not after landing. Not after the debrief. As soon as the cockpit workload permits keying the mic.

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

One more point — the TCAS RA precedence. When the box says climb and the controller says descend, the pilot flies the RA. That precedence is reinforced in AC 120‑55, and it is the correct answer every time. ATC is then notified of the maneuver.

What an Examiner Asks About § 91.123

This regulation appears on almost every oral — private, instrument, commercial, CFI. Common questions include:

• "What do you do if ATC gives you a clearance you can't comply with?" — Say unable, and offer what the aircraft 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 priority in an emergency and asks for a report, submit a detailed written report to the facility manager within 48 hours (paragraph d).

The trick question examiners favor: "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 demonstrates that the applicant actually read the rule.

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

Consider a scenario a pilot may encounter. Flying VFR with flight following, the pilot picks up an IFR clearance airborne, and the controller hands over a routing that descends straight into a layer the AIRMET reports as full of moderate ice — in a Cessna 172 with no anti‑ice. Or the vector points into a TFR just heard on the ATIS. Or the descent hits a minimum vectoring altitude that the sectional shows is below ridge level.

The correct response is one word: "Unable."

It is followed by 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. It is not an argument and not an apology. It tells the controller what the airplane and the conditions will allow.

A common misconception among new pilots is that ATC is the boss. ATC is a service. Controllers are very good at their job — and their job does not include knowing the aircraft's fuel state, equipment limitations, pilot currency, or cloud clearance. Those are the pilot's responsibility, and § 91.3 places final authority squarely on the pilot in command.

Examiners watch how the applicant pushes back. They want to see professional, calm, reasoned communication — 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 is the standard.