Regulation Text
(a) All airspace: U.S.-registered civil aircraft. Any traffic alert and collision avoidance system installed in a U.S.-registered civil aircraft must be approved by the Administrator.
(b) Traffic alert and collision avoidance system, operation required. Each person operating an aircraft equipped with an operable traffic alert and collision avoidance system shall have that system on and operating.
Research Notes
Section 91.221 — Traffic alert and collision avoidance system (TCAS) — establishes the operational requirements for TCAS-equipped aircraft. The rule has been refined as TCAS technology has matured from TCAS I (proximity advisories) to TCAS II (resolution advisories) to ACAS-X (the next-generation system entering service).
Paragraph (a) — General: No person may operate a civil aircraft equipped with TCAS unless the TCAS is operating per its installation specifications, including the inhibit logic and the proper integration with transponder modes.
Paragraph (b) — TCAS II RA compliance: When a TCAS resolution advisory (RA) is generated, the pilot must comply with the RA in accordance with the aircraft's training materials and ICAO standards, even if doing so requires deviation from an ATC clearance. Compliance with a valid TCAS RA is treated as authorized under § 91.3(b) emergency authority.
The TCAS RA/ATC interaction: The FAA Chief Counsel and ICAO standards are clear: TCAS RA wins over ATC instructions when they conflict. The pilot promptly notifies ATC of the RA-driven deviation. Several near-miss and midair incidents have been traced to pilots who attempted to follow ATC rather than the RA — TCAS RAs are now mandatory.
TCAS I vs TCAS II vs ACAS-X:
- TCAS I: Provides traffic advisories (TAs) only — no resolution advisories. Common on smaller regional jets and certain GA equipment.
- TCAS II (v7.1 the current standard): Provides both TAs and RAs (climb/descend instructions). Required for Part 121 turbine aircraft, most large turbines, and increasingly common on business jets.
- ACAS-X: Next-generation collision avoidance, optimized for higher traffic densities and ADS-B integration. Beginning deployment 2024-2026.
Equipment + transponder coupling: TCAS requires that the aircraft's transponder be operating in Mode S (or Mode A/C in some legacy installations) to query and respond to other aircraft. § 91.221 requires the transponder be on whenever TCAS is on.
Reference: AC 120-55C on TCAS II operations; ICAO PANS-OPS Volume I.
Angle of Attack is an aviation flight-training brand founded by Chris Palmer, a two-time Master Aviation Educator and Gold Seal CFI. We decode the FARs so pilots understand not just the words, but what they mean in the cockpit.
AOA's Decoded pages are plain-English interpretation for training and reference. They are not legal advice and do not replace the official regulation. Always confirm current requirements against the authoritative source before acting.