AC 90-120

Advisory circular · Operational Use of Airborne Collision Avoidance Systems

Approved
November 20, 2024
Last reviewed
November 21, 2024
Family
AC 90-120 · 1 version
14 CFR parts
91, 121, 125, 129, 135

Guidance, not mandatory unless a rule or AD requires it. DRS record last changed November 22, 2025.

Show pages 30–41

5-4 simulator) and should address any significant issues identified by operating experience, system changes, procedural changes, or unique characteristics, such as the introduction of a new aircraft, display system, or operations in airspace where high numbers of TAs and RAs have been reported. The flightcrew should demonstrate knowledge of ACAS concepts, systems, and procedures. The flightcrew should demonstrate proficiency in cognitive, procedural, and motor skills necessary to properly respond to ACAS.

5.1.7.1 There are no formal ACAS flightcrew evaluation requirements for flight testing or checking. An instructor should accomplish an evaluation of ACAS objectives during training. Operators should include routine ACAS operations in all evaluation environments, and check pilot/examiners should include ACAS as a routine discussion item.

5.1.7.2 ACAS knowledge training is typically conducted in a classroom environment.

The knowledge demonstrations may be met by successfully completing written tests or providing correct responses to Computer-Based Training (CBT) questions. Combinations of these methods may be used if the required body of knowledge is completely covered.

5.1.7.3 ACAS maneuver training instructs flightcrews how to properly respond to ACAS displayed information. TAs and RAs are most effective when accomplished in a flight simulator equipped with a traffic display and controls similar in appearance and operation to those in the aircraft. Alternatively, the required maneuver can be carried out by means of an interactive computer-based trainer with a traffic display and controls similar in appearance and operation to those in the aircraft. This interactive computer-based trainer should depict scenarios where real-time responses are to be made. Line-Oriented Flight Training (LOFT) programs may be enhanced by interaction with ACAS. In addition, LOFT programs should consider proper crew vigilance for aircraft that may not be equipped with and operating a transponder or are non-altitude reporting aircraft. Advisories accomplished during LOFT are creditable toward first-time or recurrent qualification. Operators should ensure that their instructors could select all the required RA scenarios on their training devices at any applicable time

(e.g., own-ship above 1,000 feet for an RA) during a simulator session.

5.1.7.4 CRM programs should address effective teamwork, including conducting effective preflight briefings on how to react to ACAS advisories and the proper reaction to a TA and an RA by the Pilot Flying (PF) and the Pilot Monitoring (PM).

5.1.7.5 First-time ACAS training should be accomplished for each ACAS-equipped aircraft type that a pilot is assigned. An operator may accomplish first-time ACAS qualification as a standalone module of ground and flight training or combined with initial, transition, or upgrade ground and flight training programs with the appropriate differences training.

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5.1.7.6 An operator may accomplish recurrent ACAS training during recurrent ground and flight training. Recurrent ground training will be a standalone module. However, ACAS will be fully integrated with the recurrent flight training.

5.1.7.7 For recurrent training, it is recommended that flight training include responses to all RA types. Recurrent training is desirable in a Flight Training Device (FTD) or a simulator approved for the maneuvers. See Appendix B, Sample ACAS Ground and Flight Training Program, for a sample training program.

5.1.8 C ontinuing Operator Responsibilities . After operational approval has been obtained, operators have the following general responsibilities regarding ACAS:

5.1.8.1 Analyzing the implementation of ACAS, revisions to policies and procedures, and identifying hazards, changes in the operational environment, or ineffective risk controls.

5.1.8.2 Using a Safety Risk Management (SRM) program as part of a Safety Management System (SMS) to assess whether the risk is acceptable for permitting procedures such as the use of TA-ONLY mode during operations.

See Appendix C, Safety Management System (SMS) Considerations for ACAS II TA-ONLY Mode Operations.

5.1.8.3 Monitoring and auditing of the operational environment to detect changes needed to processes and procedures, including training, checking, and maintenance programs to ensure their correctness, pertinence, timeliness, and effectiveness.

5.1.8.4 Communicating to operator personnel, as appropriate, when hazards or changes to policies and procedures are identified.

5.1.8.5 Ensuring all ACAS events reported to the operator are assessed.

5.1.8.6 Ensuring all required maintenance reports are completed and assessed for hazards.

5.1.9 Operator ACAS Event Reporting.

5.1.9.1 Operators are encouraged to develop procedures to ensure effective identification, tracking, and followup of significant ACAS-related events, as appropriate. Such procedures should focus on providing useful information to:

• Properly assess the importance of ACAS events.

• Follow up on information related to specific ACAS events, as necessary.

• Keep the industry and the FAA informed of the performance of ACAS in the National Airspace System (NAS) and in international operations.

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5.1.10 Operator Maintenance Personnel Reports. Maintenance personnel are advised to report ACAS problems that relate to system performance, manufacturers, and/or vendors to the appropriate Principal Avionics Inspector (PAI).

6-1 CHAPTER 6. FOREIGN CARRIERS

6.1 General Requirements. Most foreign states mandate ACAS II equipage in compliance with ICAO requirements and guidance. The United States may require ACAS II equipage of U.S.-registered aircraft operating abroad, and foreign air carriers operating within the 12 NM territorial limit of the United States, its territories, or its possessions. The FAA cannot require the installation and use of ACAS II for any foreign aircraft operating outside of U.S. territorial limits.

6.2 ACAS Transponder Requirements. A foreign air carrier conducting operations within the United States, or foreign person operating U.S.-registered aircraft solely outside the United States in common carriage, should equip and operate ACAS transponders in U.S. airspace, as required by 14 CFR §§ 129.5, 129.14, and 129.18; ICAO Annex 6, Operation of Aircraft, Part I, International Commercial Air Transport—Aeroplanes;

ICAO Annex 8, Airworthiness of Aircraft; and the FAA-issued OpSpecs, as appropriate.

6.3 ACAS Approval for Foreign Air Carriers. The FAA does not approve ACAS installation, training programs, MELs, or maintenance programs of 14 CFR part 129 air carriers operating non-U.S.-registered aircraft. Such authorizations are addressed as specified by the State of Operator and by ICAO. Compatibility of ACAS II and Mode S transponders with other aircraft and NAS facilities within U.S. airspace is essential.

Compliance with these ACAS provisions ensures both ACAS II and procedural compatibility.

6.4 ACAS II RA Reporting Requirements. Civil aircraft of the United States operating anywhere, and foreign civil aircraft where the events occur within 12 NM of the United States, its territories, or its possessions, are also subject to 49 CFR § 830.5.

6.5 Standardization. Pilots are advised to use training and procedures for use of ACAS II, as specified by ICAO, this AC, or other equivalent criteria acceptable to the FAA, when operating in U.S. airspace.

Appendix A A-1 APPENDIX A. ADMINISTRATIVE INFORMATION A.1 Related Regulations.

1. Title 49 of the United States Code (49 U.S.C.) § 44716, Collision Avoidance Systems.

2. Title 14 of the Code of Federal Regulations (14 CFR):

• Sections 1.1 and 1.2.

• Section 61.156.

• Sections 91.3, 91.111, 91.113, 91.123, 91.180, 91.213, 91.215, 91.217, 91.219,

91.221, 91.225, 91.227, 91.413, 91.703, 91.1023, 91.1045, and 91.1065; and part 91 appendix G.

• Sections 119.7 and 119.49.

• Sections 121.135, 121.345, 121.356, and 121.628; and part 121 appendix E and appendix F.

• Sections 125.73, 125.201, 125.224, and 125.287.

• Sections 129.5, 129.14, and 129.18.

• Sections 135.23, 135.143, 135.179, 135.180, and 135.323.

3. Title 49 of the Code of Federal Regulations (49 CFR) §§ 830.5 and 830.15.

A.2 Requirements by 14 CFR Type.

A.2.1 Title 14 CFR part 121 air carrier requirements of equipage and operation of ACAS II for turbine aircraft are based upon maximum certificated takeoff weight of above 33,000 pounds. Some other aircraft operating under 14 CFR part 121 may require equipage and operation of ACAS I. Title 14 CFR § 121.356 also describes transponder requirements.

A.2.2 Title 14 CFR part 125 airplanes (large airplanes that are not conducting common carriage) have similar requirements to 14 CFR part 121 air carriers for equipage and operation of ACAS II. Title 14 CFR § 125.224 also describes transponder requirements.

A.2.3 Title 14 CFR part 135 air ca rrier ACAS equipage (approved ACAS I or ACAS II) requirements are based on passenger seat configuration for turbine aircraft. Title 14 CFR § 135.143 describes transponder requirements.

A.2.4 Title 14 CFR part 91 subpart K (part 91K) fractional ownership requirements of equipage and operation of ACAS depend on passenger seats or payload capacity. Airplanes with more than 30 passenger seats or a payload capacity of more than 7,500 pounds are subject to 14 CFR § 121.356. Airplanes with a lesser passenger and payload capacity are subject to 14 CFR § 135.180.

Appendix A A-2 A.2.5 Title 14 CFR part 129 foreign air carriers and foreign operators of U.S.-registered aircraft engaged in common carriage have similar requirements to 14 CFR part 121 air carriers for equipage and operation of ACAS II. Title 14 CFR § 129.18 describes transponder requirements.

A.2.6 Title 14 CFR part 91 flights may be optionally equipped with ACAS. If the aircraft is equipped with an operable ACAS, ACAS shall be on and operating. If ACAS II is installed, then Automatic Dependent Surveillance-Broadcast (ADS-B), if installed, must be provided with an indication that ACAS II is operating, and an indication if a Resolution Advisory (RA) is in effect. If an aircraft is authorized for Reduced Vertical Separation Minimum (RVSM) and has Traffic Alert and Collision Avoidance System (TCAS) II installed, TCAS II must be version 7.0 or a later version.

A.3 Related Reference Material (current editions).

1. International Civil Aviation Organization (ICAO) Guidance (available at https://www.icao.int/publications/Pages/default.aspx):

a. Annex 6, Operation of Aircraft:

• Part I, International Commercial Air Transport—Aeroplanes.

• Part II, International General Aviation—Aeroplanes.

b. Annex 10, Aeronautical Telecommunications, Volume IV, Surveillance and Collision Avoidance Systems.

c. Doc 4444, Procedures for Air Navigation Services—Air Traffic Management.

d. Doc 8168, Procedures for Air Navigation Services—Aircraft Operations.

e. Doc 9863, Airborne Collision Avoidance System (ACAS) Manual.

2. FAA Booklet HQ-111358, Introduction to TCAS II, Version 7.1 (Feb. 28, 2011).

3. FAA Orders and ACs:

• FAA Order JO 7110.65, Air Traffic Control.

• FAA Order 8900.1, Volume 3, Chapter 19, Section 8, Safety Assurance System:

Special Training.

• AC 20-151, Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II), Versions 7.0 and 7.1 and Associated Mode S Transponders.

• AC 20-165, Airworthiness Approval of Automatic Dependent Surveillance - Broadcast OUT Systems.

• AC 20-172, Airworthiness Approval for ADS-B In Systems and Applications.

• AC 43-6, Altitude Reporting Equipment and Transponder System Maintenance and Inspection Practices.

• AC 90-48, Pilots’ Role in Collision Avoidance.

• AC 90-114, Automatic Dependent Surveillance-Broadcast Operations.

Appendix A A-3

• AC 91-70, Oceanic and Remote Continental Airspace Operations.

• AC 91-85, Authorization of Aircraft and Operators for Flight in Reduced Vertical Separation Minimum (RVSM) Airspace.

• AC 120-92, Safety Management Systems for Aviation Service Providers.

• AC 120-123, Flightpath Management.

4. Technical Standard Orders (TSOs) (available at https://drs.faa.gov):

• TSO-C112f, Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment.

• TSO-C118a, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, TCAS I.

• TSO-C119e, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, TCAS II with Hybrid Surveillance.

• TSO-C219a, Airborne Collision Avoidance System (ACAS) Xa/Xo.

Note: ACAS installations may have been previously issued a TSO Authorization (TSOA) or Letter of Deviation Authority (LODA) under TSO-C119a–d and the corresponding ATCRBS/Mode S TSO.

5. RTCA, Inc. Technical Standards Documents (available for purchase at https://my.rtca.org/nc__store):

• RTCA DO-181F as modified by Change 1, Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment.

• RTCA DO-185B, Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System II (TCAS II).

• RTCA DO-197A, Minimum Operational Performance Standards for an Active Traffic Alert and Collision Avoidance System I (Active TCAS I).

• RTCA DO-300A, Minimum Operational Performance Standards (MOPS) for Traffic Alert and Collision Avoidance System II (TCAS II) Hybrid Surveillance.

• RTCA DO-385A, Minimum Operational Performance Standards for Airborne Collision Avoidance System X (ACAS X) (ACAS Xa and ACAS Xo).

A.4 Accident History Related to the Development and Evolution of ACAS. This paragraph provides links to online information on significant historical accidents that have played a role in the development and evolution of ACAS. This paragraph does not provide a detailed narrative of those events, but rather enables interested readers to conduct further research into accidents that have influenced the development of ACAS by providing easy access to relevant information. The intent of providing links to this information is to provide access to historical lessons learned related to ACAS and to preserve key safety objectives for the system and facilitate its continued refinement.

Appendix A A-4

1. Pacific Southwest Airlines Flight 182 Boeing 727 collision with a Cessna 172, near San Diego, on September 25, 1978 (NTSB Report AAR-79/05).

2. Aéromexico Airlines Flight 498 McDonnell Douglas DC-9 collision with a Piper PA-28-181, near Los Angeles, on August 31, 1986 (NTSB Report AAR-87/07).

3. Saudi Arabian Airlines Flight 763 Boeing 747 collision with Kazakhstan Airlines Flight 1907 Ilyushin IL-76, near Charkhi Dadri, India, on November 12, 1996.

4. German Air Force Tupolev TU-154 collision with a United States Air Force (USAF) Lockheed C-141, near the coast of Namibia, on September 13, 1997.

5. DHL Flight 611 Boeing 757 midair collision with Bashkirian Airlines Flight 2937 Tupolev Tu-154, near Überlingen, Germany, on July 1, 2002 (German Federal Bureau of Aircraft Accidents Investigation BFU Report AX001-1-2/02).

6. GOL Linhas Aéreas Flight 1907 Boeing 737-800 collision with an Embraer Legacy 600, near Peixoto de Azevedo, MT, Brazil, September 29, 2006 (Aeronautical Accident Investigation and Prevention Center CENIPA A-022/CENIPA/2008).

Appendix A A-5 A.5 Acronyms.

Acronym Term 14 CFR Title 14 of the Code of Federal Regulations 49 CFR Title 49 of the Code of Federal Regulations 49 U.S.C. Title 49 of the United States Code AAUP Attention All Users Page AC Advisory Circular ACARS Aircraft Communications Addressing and Reporting System ACAS Airborne Collision Avoidance System (also known as TCAS) ACAS X Airborne Collision Avoidance System X ACAS Xa ACAS X variant using “active” interrogations (used for fixed-wing aircraft) ACAS Xo ACAS X variant used for “operations” (tailors the guidance for designated aircraft to an operation) AD Airworthiness Directive ADS-B Automatic Dependent Surveillance-Broadcast ADS-R Automatic Dependent Surveillance-Rebroadcast AED Aircraft Evaluation Division AFM Aircraft Flight Manual AFMS Aircraft Flight Manual Supplement AGL Above Ground Level AHRS Attitude and Heading Reference System AIM Aeronautical Information Manual AIP Aeronautical Information Publication AIR Aircraft Certification Service AOTO ADS-B Only TA-ONLY AP Autopilot ASAP Aviation Safety Action Program ASRS Aviation Safety Reporting System ATAS ADS-B Traffic Advisory System ATC Air Traffic Control ATCRBS ATC Radar Beacon System CAA Civil Aviation Authority

Appendix A A-6 Acronym Term CAS Collision Avoidance System CBT Computer-Based Training CH Certificate Holder CMO Certificate Management Office CMT Certificate Management Team CPA Closest Point of Approach CRM Crew Resource Management CSPO Closely Spaced Parallel Operations DAA Detect and Avoid DMOD Distance Modification DNA Designated No Alert DRS Dynamic Regulatory System EFIS Electronic Flight Instrument Systems ERC Event Review Committee ES Extended Squitter FAA Federal Aviation Administration FD Flight Director FL Flight Level FLT ID Flight Identification FOEB Flight Operations Evaluation Board fpm Feet Per Minute FS Flight Standards FSB Flight Standardization Board FSBR Flight Standardization Board Report FSDO Flight Standards District Office FSS Flight Service Station FSTD Flight Simulation Training Device FTD Flight Training Device g Gravitational acceleration of 9.81 meters per second2 GA General Aviation GNSS Global Navigation Satellite System

Appendix A A-7 Acronym Term HMD Helmet-Mounted Display HMD Horizontal Miss Distance HS Hybrid Surveillance HUD Head-Up Display HURS High Update Rate Surveillance ICAO International Civil Aviation Organization IFR Instrument Flight Rules IMC Instrument Meteorological Conditions inHg Inches of Mercury IRS Inertial Reference System LOA Letter of Authorization LODA Letter of Deviation Authority LOFT Line-Oriented Flight Training mb Millibars MEL Minimum Equipment List MHz Megahertz MMEL Master Minimum Equipment List Mode S Mode Select MOPS Minimum Operational Performance Standards MOR Mandatory Occurrence Report MRB Maintenance Review Board MRBR Maintenance Review Board Report MSpec Management Specification NAS National Airspace System NASA National Aeronautics and Space Administration NM Nautical Mile NMAC Near Midair Collision NSP National Simulator Program NTSB National Transportation Safety Board OpSpec Operations Specification PAI Principal Avionics Inspector

Appendix A A-8 Acronym Term PF Pilot Flying PFD Primary Flight Display PI Principal Inspector PL Policy Letter PM Pilot Monitoring PMI Principal Maintenance Inspector POI Principal Operations Inspector PRM Precision Runway Monitor QFE The atmospheric pressure at aerodrome elevation (or at runway threshold) QNE The barometric pressure used for the standard altimeter setting (29.92 inHg) QNH The barometric pressure as reported by a particular station RA Resolution Advisory RBDM Risk-Based Decision Making RVSM Reduced Vertical Separation Minimum RWC Remain Well Clear SA Safety Assurance SAFO Safety Alert for Operators SDP Standard Datum Plane SDR Service Difficulty Report SL Sensitivity Level SMS Safety Management System SRM Safety Risk Management SSR Secondary Surveillance Radar STBY Standby STC Supplemental Type Certificate TA Traffic Advisory Tau (τ) Time to Closest Point of Approach TAWS Terrain Awareness and Warning System TC Type Certificate TCAS Traffic Alert and Collision Avoidance System (also known as ACAS) TFC Traffic