AC 20-151C

U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II), Versions 7.0 & 7.1 and Associated Mode S Transponders Date: 07/21/2017 Initiated by: AIR-100 AC No: 20-151C Change:

FOREWORD This AC provides guidance for applicants seeking airworthiness approval for TCAS II certified to technical standard order (TSO) C119c, TSO-C119d or TSO-C119e, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, TCAS II with Hybrid Surveillance.

If you have suggestions for improving this AC, you may use the form at the end of this document.

Susan J. M. Cabler Manager, Design, Manufacturing, & Airworthiness Division, Aircraft Certification Service

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TABLE OF CONTENTS

Chapter 1. General Information1-1

1.1 Purpose of this Advisory Circular1-1

1.2 Who this AC applies to1-1

1.3 Cancellation1-1

1.4 Significant Changes1-1

1.5 Scope1-2

Chapter 2. TCAS II SYSTEM2-1

2.1 System Description2-1

2.1.1 Description of a TCAS II System2-1

2.1.2 System Advisories2-1

2.1.3 System Flight Deck Displays2-1

2.1.4 Mode S Transponder2-2

2.1.5 Hybrid Surveillance2-2

2.2 System Components and Requirements2-2

2.2.1 Mode S Transponder2-2

2.2.2 Pilot Control2-3

2.2.3 Antennas2-4

2.2.4 Structural analysis2-5

2.2.5 The TCAS Processor2-5

2.2.6 Traffic Display2-5

2.2.7 Resolution Advisory Display2-6

2.2.8 Caution/Warning Lights2-7

2.2.9 Aural Alert Requirements2-7

2.2.10 Failures, TCAS and Transponder2-9

2.3 Airworthiness Considerations2-10

2.3.1 Certification Program2-10

2.3.2 Certification plan2-10

2.3.3 Equipment Compatibility Requirements2-10

2.3.4 Aircraft Performance Considerations2-12

2.3.5 Evaluating Aircraft Performance2-12

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2.3.6 How Aircraft Systems Should Interface with TCAS II2-18

2.3.7 Verifying and Validating TCAS II Software2-20

2.3.8 Failure Conditions System Safety Assessment and Design Assurance2-22

2.3.9 Maintenance Considerations for Hybrid Surveillance Functionality (TSO-C119d

and later)2-23

2.3.10 Populating Mode S Transponder registers 4016 and 6216 (if implemented)2-24

Chapter 3. Test and Evaluation (Initial Approval)3-1

3.1 General3-1

3.2 Create a Test Plan3-1

3.2.1 Basic Ground Tests3-1

3.2.2 Basic Flight Tests3-3

3.2.3 Planned Encounter Flight Tests3-5

3.2.4 Mode S Transponder Tests3-6

Chapter 4. Follow-on Approvals4-1

4.1 General4-1

4.2 Ground Tests and Evaluations4-1

4.3 Flight Tests4-3

4.4 Upgrading an Existing TCAS II Installation4-5

4.5 Upgrading an Existing Mode S Transponder Installation4-5

Chapter 5. Airplane Flight Manual Supplement5-1

5.1 Content of an AFMS5-1

5.2 TCAS V7.1 AFMS Example5-1

Appendix A. Example TCAS II V7.1 AFMSA-1

Appendix B. Transponder Ground Tests & EvaluationsB-1

Appendix C. Historical Information on Development of TCASC-1

Early TCASC-1

Recent TCAS DevelopmentsC-1

Accident history related to the development and evolution of TCASC-3

Appendix D. Related DocumentsD-1

14 CFR PartsD-1

FAA ACsD-2

FAA TSOsD-2

FAA OtherD-3

RTCA Inc. PublicationsD-3

SAE InternationalD-3

AERONAUTICAL RADIO, INC. (ARINC)D-4

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International Civil Aviation Organization (ICAO)D-4

JAAD-4

EASAD-4

Appendix E. Definitions and AcronymsE-1

Definitions. -E-1

AcronymsE-3

Appendix F. Interfacing the Transponder to Aircraft Systems to Support Transmission of Short-

Term IntentF-1

GeneralF-1

Populating Selected Vertical Intention Register BDS 4016F-2

Populating Target State and Status Information Register BDS 6216F-3

Appendix G. Advisory Circular Feedback InformationG-1

TABLE OF FIGURES

Figure A-1. TCAS II/Transponder Control Panel (Example)A-4

Figure A-2. TCAS II – Traffic Display (Example)A-5

Figure A-3. TCAS II Traffic Display (Example)A-6

Figure A-4. TCAS II Instantaneous Vertical Speed Indicator (Example)A-9

Figure A-5. TCAS II Examples – Preventive RAsA-11

Figure A-6. TCAS II Examples – Initial Corrective RAsA-12

Figure A-7. TCAS II Examples – Modifications to Initial Corrective RAsA-13

Figure F- 1: Aircraft Flight PathF-5

Figure F- 3. Format of BDS register 6216, Target State and Status Information (Subtype 1;

Compatible with ADS-B Version Number = 2)F-15

TABLE OF TABLES

Table 1. Maneuvers2-15

Table 2 System Inhibits2-18

Table A- 1. TCAS II/Transponder Function SelectorA-4

Table A- 2. Displayed Aircraft Symbols (Examples)A-5

Table A- 3. REL ALT/FL SwitchA-7

Table A- 4. Above/Norm/Below SwitchA-8

Table A- 5. Instantaneous Vertical Speed IndicatorA-9

Table F- 1. BDS 4016 ParametersF-6

Table F- 2: BDS 6216 ParametersF-6

1-1 CHAPTER 1. GENERAL INFORMATION.

1.1 Purpose of this Advisory Circular. We wrote this advisory circular (AC) to guide applicants seeking airworthiness approval for TCAS II version 7.1 (V7.1) certified to technical standard order (TSO) C119c, TSO-C119d or TSO-C119e, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, TCAS II with Hybrid Surveillance. This AC also provides guidance for applicants seeking airworthiness approval for stand-alone Mode S transponders certified to TSO-C112e, Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment.

You can also use the guidance in this AC for seeking airworthiness approval for TCAS II version 7.0 (V7.0) certified to TSO-C119b, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, TCAS II and associated Mode S transponders.

This AC is not mandatory and does not constitute a regulation. In it, we describe an acceptable means, but not the only means, to gain airworthiness approval of TCAS II versions 7.0 and 7.1 systems. However, if you use the means described in the AC, you must follow it in all important respects. The term “must” is used to indicate mandatory requirements when following the guidance in this AC. The terms “should” and “recommend” are used when following the guidance is recommended but not required to comply with this AC.

1.2 Who this AC applies to. Applicants seeking a type certificate (TC), amended type certificate, or supplemental type certificate (STC) under Title 14 of the Code of Federal Regulations (14 CFR) part 25 for initial approval and follow-on approvals of TCAS II systems or stand-alone Mode S transponder systems. References to 14 CFR part 25 are appropriate when TCAS II is installed on transport category airplanes. When TCAS II is to be certified for non-transport category airplanes, use the equivalents to the above 14 CFR part 25 sections in other parts of the regulations. Although this AC is intended for TCAS II installed on transport category airplanes, it provides useful guidance for part 23 installations if you refer to the equivalent ACs and sections of 14 CFR part 23.

1.3 Cancellation. This revision cancels AC 20-151B, dated March 18, 2014.

1.4 Significant Changes. This revision of the AC does the following:

1.4.1 Mitigates spurious RAs with installation of TSO-C119e TCAS II units.

1.4.2 Adds guidance for implementing select functionality associated with extended squitter, elementary and enhanced Mode S surveillance messaging.

1.4.3 Elaborates on and clarifies the failure condition classification of TCAS II systems, particularly with regard to missing RAs (failure to provide an RA, or providing one late, when a threat is present).

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1.4.4 Sets policy and provides guidance to annunciate TCAS and transponder failures in the primary field of view as an advisory level alert.

1.5 Scope. What this AC Covers.

1.5.1 In this AC, we cover design aspects, characteristics, mechanization, testing, and the criticality of system failure cases for TCAS II V7.0 and V7.1 and associated Mode S transponders. Our guidance focuses on systems that provide traffic advisories (TA) and resolution advisories (RA) in the vertical sense only (TCAS II), and where operational performance standards are defined in technical documents developed by a joint air transport industry-government group (the RTCA, Inc. Special Committee SC-147).

1.5.2 We also cover TCAS II hybrid surveillance, which is now included in a TCAS II system.

1.5.3 We do not cover traffic symbology for traffic displays in systems that integrate TCAS and Airborne Separation Applications System (ASAS). Guidance for airworthiness approvals of ADS-B Out and ADS-B In can be found in Advisory Circulars 20-165B and 20-172B respectively.

1.5.4 We also cover implementation of certain messaging capability of extended squitter and ELS/EHS Mode S Transponders, specifically, Selected Vertical Intention (BDS register 4016) and Target State and Status Information (BDS register 6216).

1.5.5 For more thorough guidance on Mode S Elementary surveillance, refer to Joint Aviation Authorities (JAA) Temporary Guidance Leaflet (TGL) 13 Revision 1, Certification of Mode S Transponder Systems for Elementary Surveillance, and EASA Certification Specification and Acceptable Means of Compliance – Airborne Communication Navigation and Surveillance, (CS-ACNS).

1.5.6 For more thorough guidance on Mode S Enhanced surveillance, refer to Acceptable Means of Compliance (AMC) 20-13, Certification of Mode S Transponder Systems for Enhanced Surveillance, and EASA CS-ACNS.

1.5.7 This advisory circular provides applicants with guidance for obtaining an airworthiness approval for TCAS II. Integration with flight guidance systems in an aircraft is out of scope for this document. For guidance on integration with an aircraft’s flight guidance system, refer to Advisory Circular 25.1329-1C, Approval of Flight Guidance Systems.

2-1 CHAPTER 2. TCAS II SYSTEM

2.1 System Description.

2.1.1 Description of a TCAS II System. TCAS II is an airborne traffic alert and collision avoidance system that interrogates air traffic control (ATC) transponders in nearby aircraft and uses computer processing to identify and display potential and predicted collision threats. The system is designed to protect a volume of airspace around the TCAS II equipped aircraft. The system will provide appropriate aural and visual advisories to the flight crew to take action to ensure adequate separation when the computer analysis of the intruding aircraft transponder replies predict a penetration of the protected airspace.

The TCAS II system can only generate RAs for intruders equipped with responding Mode S or Mode C transponders, which provide information on the altitude of the threat aircraft. Traffic advisories can be generated for any aircraft equipped with an operative Mode S or an ATCRBS transponder, regardless of its ability to provide information on the intruder aircraft’s altitude.

We view TCAS II systems as a supplement to the pilot who has the primary responsibility for avoiding midair collisions. The TCAS II system provides no protection from threat aircraft without operative transponders. TCAS II does not alter or diminish the pilot’s basic authority and responsibility to ensure safe flight.

Nevertheless, TCAS II is a safety-enhancing system that has been introduced and refined in response to accident experience, and it is required by operating rules for certain aircraft. Therefore, we consider it important for TCAS II systems to perform their intended function with adequate reliability, and for flight crews to be made aware promptly and conspicuously when TCAS failure or loss of function occurs. It is important to note that own-ship and intruder aircraft transponders are a vital part of the system.

2.1.2 System Advisories. The system provides two types of advisories:

2.1.2.1 TAs indicate the relative positions of intruding aircraft that are approximately 20 - 48 seconds from the closest point of approach (CPA), depending on the sensitivity level (SL), and may a short time later require a RA. TAs also give the flight crew the opportunity to visually acquire the intruding aircraft out-the-window.

2.1.2.2 RAs provide a vertical avoidance maneuver to increase separation when the computer predicts the threat aircraft is between 15-35 seconds from the closest point of approach, depending on the SL.

2.1.3 System Flight Deck Displays. The system provides two types of flight deck displays:

2.1.3.1 A traffic display depicts the relative position of ATC transponderequipped aircraft.

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2.1.3.2 An RA display for each pilot indicates the appropriate vertical maneuver to avoid a threat. The RA display provides the pilot with information on the vertical speed or pitch angle to fly in order to avoid a threat. The RA display is typically implemented on an instantaneous vertical speed indicator (IVSI), a vertical speed tape that is part of a Primary Flight Display (PFD), or using pitch cues displayed on the PFD.

2.1.4 Mode S Transponder.

2.1.4.1 The TCAS II aircraft must be equipped with a Mode S ATC transponder, which provides air-to-air communications for coordinating the resolution maneuvers between TCAS II equipped aircraft.

2.1.4.2 The Mode S transponder also provides discrete-address replies to interrogations from ground stations and other aircraft equipped with TCAS II.

2.1.5 Hybrid Surveillance. Hybrid surveillance is a function of TCAS used as a means to decrease Mode S interrogations. Aircraft may use passive surveillance instead of active surveillance to track intruders that meet validation criteria and are not projected to be near-term collision threats. Active surveillance uses the standard TCAS transponder interrogation that provides range, bearing and altitude to the intruder. Passive surveillance uses automatic dependent surveillance-broadcast (ADS-B) data broadcast from other aircraft. The passive surveillance data is broadcast and received through the use of Mode S Extended Squitter, that is, 1090 megahertz (MHz) ADS-B. Hybrid surveillance does not degrade the performance of TCAS active surveillance. This is a requirement and is tested by ensuring active surveillance performs as specified by the TCAS II performance standards.

2.2 System Components and Requirements.

2.2.1 Mode S Transponder.

2.2.1.1 A Mode S transponder is required for TCAS II operation. It is an enhanced version of the ATCRBS transponder design that is interoperable and compatible with ATCRBS. Each aircraft equipped with a Mode S transponder is assigned a discrete address code. Mode S also provides the air-to-air data link between TCAS II-equipped aircraft to coordinate resolution maneuvers. This ensures the RA displayed in one TCAS II- equipped aircraft is compatible with the maneuver displayed in the other TCAS II equipped aircraft. For example, two TCAS equipped aircraft involved in a conflict may coordinate with each other so that the aircraft at the higher altitude issues a “climb” command, while the aircraft at the lower altitude issues a “descend” command, to prevent the two aircraft’s flight paths from crossing. The Mode S transponder has the capability to provide a data link between the equipped aircraft and the ground, in addition to all of other transponder functions. A Mode S transponder may

2-3 be installed independently or with a TCAS II installation. The performance standard for Mode S installed independently of TCAS is provided in TSO-C112e, Requirements, paragraph 3.

2.2.1.2 For each aircraft in which a Mode S transponder is installed, the discrete aircraft address for the Mode S transponder must be obtained from the appropriate airworthiness authority of the country in which the aircraft is registered. For U.S. registered aircraft, obtain the discrete aircraft address from the Federal Aviation Administration, Mike Monroney Aeronautical Center, Aircraft Registration Information, AFS-750, PO Box 25504, Oklahoma City, OK 73125, Telephone: (405) 954-3116.

Note: The ICAO 24-bit aircraft address is a uniquely assigned aircraft identification that also identifies the country of registration. For civil aircraft registered in the U.S., the ICAO 24-bit aircraft address is established as a function of the aircraft’s registration number (for example, N1234A). There have been occurrences where two airplanes report identical ICAO 24-bit aircraft addresses. This problem can happen when a change in registration numbers occurs. When you change an airplane’s registration number, you should verify that the ICAO 24-bit aircraft address and the new registration number have a one-to-one correspondence.

2.2.2 Pilot Control. A pilot control for the TCAS and transponder equipment must be provided.

2.2.2.1 Provide a means to select the following:

2.2.2.1.1 Operation of TCAS II in the TA/RA mode and Mode S transponder simultaneously.

2.2.2.1.2 Operation of TCAS II in the TA mode and Mode S transponder simultaneously.

2.2.2.1.3 Operation of TCAS II in the standby mode.

2.2.2.2 Also, provide the following additional features:

2.2.2.2.1 A means to select the assigned ATCRBS (MODE A 4096) code.

2.2.2.2.2 A means to initiate the transponder “IDENT” function.

2.2.2.2.3 A means to initiate the TCAS II self-test.

2.2.2.2.4 A means to suppress transponder altitude (Mode C) reporting.

2.2.2.3 The following optional controls may be provided:

2.2.2.3.1 Selection of the weather radar only.

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2.2.2.3.2 Control to select the display of traffic within selected altitude bands.

2.2.2.3.3 Selection of the weather radar and traffic display simultaneously.

2.2.2.3.4 Selection of actual flight level (FL) or relative altitude of traffic.

2.2.2.3.5 Selection of TCAS traffic information on multi-function displays.

Note: AC 20-172 ( ), Airworthiness Approval for ADS-B In Systems and Applications, provides guidance addressing integration of controls for TCAS and ADS-B In installations.

2.2.3 Antennas. The Mode S transponder must have a top and bottom omnidirectional antenna (also known as diversity antennas) when installed in an aircraft with TCAS II.

The TCAS II must have a top directional antenna and a bottom omnidirectional or directional antenna.

2.2.3.1 Directional antennas.

2.2.3.1.1 For an aircraft installation, locate the TCAS II directional antenna on the top, forward fuselage as close to the centerline as possible.

2.2.3.1.2 If the TCAS II has more than one directional antenna, locate the second antenna in a similar manner on the lower fuselage.

2.2.3.1.3 Mount the TCAS II antennas on the aircraft with at least 20 decibel (dB) isolation from other L-band antennas.

2.2.3.1.4 Since the antenna diameter may be large, some structural considerations may be necessary and a centerline offset resulting in an angular offset of up to 5 degrees is acceptable.

2.2.3.1.5 The maximum height of the directional antenna is approximately 1 inch, and therefore should not be susceptible to icing effects in the general area of the proposed installation. If the antenna height is more than approximately 1 inch, consider anti-icing provisions. RTCA/DO-185, Section 3, Volume I, provides antenna selection and performance criteria.

2.2.3.1.6 For propeller-driven aircraft, investigate the location and performance of the directional antenna for minimum blockage and to ensure that the propellers do not interfere with system operation.

2.2.3.2 Omni-directional antennas.

Mount the TCAS II omni-directional antenna on the bottom of the aircraft fuselage as close to the centerline as possible with at least 20 dB isolation from other L-band antennas. Mount the Mode S transponder antennas at locations chosen for adequate isolation and signal coverage.

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2.2.4 Structural analysis. Submit a structural analysis of the antenna installations showing compliance with the applicable regulations of 14 CFR to the FAA.

2.2.5 The TCAS Processor. The TCAS II processor unit uses both transponder reply information and information from the aircraft to identify and to display potential and predicted collision threats, and to issue RAs to avoid the threat aircraft. The TCAS II processor unit must comply with the environmental qualification requirements and minimum performance standards specified in TSO-C119( ). A manufacturer of TSO equipment can obtain authorization to produce equipment that deviates from the detailed criteria of the TSO as provided for in 14 CFR § 21.609. The FAA ACO approving the initial installation of the TCAS II equipment must verify that the TCAS II processor design does not differ from the criteria specified in RTCA/DO-185A or RTCA/DO-185B as applicable. The TCAS II processor also includes hybrid surveillance functionality which only applies to TSO C119c/d/e (optional for TSO- C119c, required for TSO-C119d and TSO-C119e).

2.2.6 Traffic Display.

2.2.6.1 Purpose.

The primary purpose of the traffic display is to aid the flight crew in the out-the-window visual acquisition of transponder-equipped aircraft. This is accomplished by displaying the intruder aircraft’s horizontal and, if altitude information is available, vertical position relative to the TCAS II equipped aircraft. The TCAS II systems provide traffic information on Mode A (no altitude data available), Mode C, and Mode S transponderequipped aircraft. A secondary purpose of the traffic display is to give the flight crew confidence in proper system operation and time to prepare to maneuver the aircraft if TCAS II issues a RA.

2.2.6.2 Description.

Traffic displays may take several forms. They may be independent, stand alone, integrated and time-shared with digital color radar, integrated with IVSI, or integrated with other displays such as electronic horizontal situation indicators (EHSI), navigation, or other multifunction displays. If the traffic display uses a multifunction display shared with other services such as aircraft communications addressing and reporting system (ACARS), the traffic display function must be immediately available for display by a single selection accessible to both pilots.

2.2.6.3 Symbology/feature criteria.

The FAA worked closely with the Air Transport Association (ATA), now known as Airlines for America (A4A), National Aeronautical and Space Administration (NASA), and both the SAE S-7 and G-10 Committees to standardize TCAS II symbology and features. The consensus we reached for TCAS II symbols is provided in RTCA/DO-185(). You can use other symbology and features if you use human factors methodology to

2-6 demonstrate that a clear and substantial benefit can be derived. Otherwise, the traffic display must depict or provide the symbology, features, or information provided in RTCA/DO-185A or RTCA/DO-185B, Section

2.2.6 as applicable.

2.2.7 Resolution Advisory Display.

2.2.7.1 Purpose.

The purpose of the RA display is to give each pilot the information to readily correct the aircraft flight path or to prevent a maneuver that would significantly reduce the vertical separation between the pilot’s aircraft (own-ship) and a threat aircraft.

2.2.7.2 Description.

You may integrate the RA display with the two primary IVSIs on the flight deck, or integrate it into the PFD, or incorporate it into a head-up display (HUD). Refer to RTCA/DO-185A or RTCA/DO-185B Section

2.2.6 as applicable for descriptions and requirements for the various resolution advisory display implementations.

2.2.7.3 Symbology.

We worked closely with A4A, NASA, and both the SAE S-7 and G-10 Committees to standardize TCAS II RA features. The consensus we reached for TCAS II RA displays is in RTCA/DO-185(). You can use other symbology and features if you use human factors methodology to demonstrate that a clear and substantial benefit can be derived. Otherwise, the RA display must depict or provide the symbology, features, or information shown in RTCA/DO-185A or RTCA/DO-185B Section 2.2.6 as applicable. The use of new TCAS symbology will require testing throughout the flight envelope to determine accuracy, over/under shoot tendencies, flight technical error, and potential confusion resulting from the proposed symbology. If you are using new symbology, contact the appropriate FAA ACO early in the development cycle.

2.2.7.4 Failures.

2.2.7.4.1 Provide indications for TCAS II failures (specifically including TCAS II unable to generate RAs), as specified in Section 2.2.10 below. Provide annunciation for the TA only mode. Electrical IVSI failures must also be annunciated.

2.2.7.4.2 Design the TCAS II system to prevent incorrect RAs, missing RAs (including late issuance of an RA, i.e. not within nominal timing parameters for the CAS logic declaring and posting an RA), and false RAs, as defined in Appendix E, Section E.1, Definitions. Section 2.3.8