AC 20-151A

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: 9/25/09 Initiated by: AIR-130 AC No: 20-151A Change:

FOREWORD

This advisory circular (AC) provides applicants guidance for obtaining an airworthiness approval for traffic alert collision avoidance systems (TCAS II). It also provides guidance for certification of a stand-alone Mode S transponder system. This AC includes the TCAS II version 7.0 plus the new version 7.1 and performance standards for the optional functionality of TCAS II hybrid surveillance.

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TABLE OF CONTENTS

CHAPTER 1. GENERAL INFORMATION1

1-1. Purpose of This Advisory Circular1

1-2. Who this AC is for1

1-3. Cancellation1

1-4. Significant Changes1

1-5. Scope1

1-6. Recent TCAS Developments2

CHAPTER 2. TCAS II SYSTEM DESCRIPTION, EQUIPMENT AND AIRWORTHINESS

CONSIDERATIONS3

A. System Description3

2-1. What a TCAS System Is3

2-2. System Advisories3

2-3. System Flight Deck Displays3

2-4. Mode S Transponder3

2-5. Hybrid Surveillance4

B. System Components And Requirements4

2-6. Mode S Transponder4

2-7. Pilot Control5

2-8. Antennas6

2-9. The TCAS Processor6

2-10. Traffic Display7

2-11. Resolution Advisory Display7

2-12. Optional Caution/Warning Lights8

2-13. Aural Alert Requirements8

C. Airworthiness Considerations10

2-14. Certification Program...................................................................................................1 0

2-15. Equipment Compatibility Requirements11

2-16. Aircraft Performance Considerations11

2-17. Evaluating Aircraft Performance12

2-18. How Aircraft Systems Should Interface with TCAS II18

2-19. Verifying and Validating TCAS II Software20

2-20. Testing for Failures System Safety Analysis21

CHAPTER 3. TEST AND EVALUATION (INITIAL APPROVAL)22

3-1. General22

3-2 Create a Test Plan. .......................................................................................................2 2

3-3. Basic Ground Tests22

3-4. Basic Flight Tests24

3-5. Planned Encounter Flight Tests25

3-6. Mode S Transponder Tests26

CHAPTER 4. FOLLOW-ON APPROVALS (STCS OR AMENDED STC)30

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4-1. The Need for Tests and a Plan30

4-2. Ground Tests and Evaluations30

4-3. Flight Tests32

4-4. Upgrading an Existing TCAS II Installation34

CHAPTER 5. AIRPLANE FLIGHT MANUAL SUPPLEMENT35

5-1. What to Include in Your Ai rplane Flight Manual Supplement35

5-2. TCAS V7.1 AFMS Example35

APPENDICIES

APPENDIX A. EXAMPLE TCAS II V7.1 AFMSA-1

APPENDIX B. TRANSPONDER GROUND TESTS & EVALUATIONSB-1

APPENDIX C. BACKGROUNDC-1

APPENDIX D. RELATED DOCUMENTSD-1

APPENDIX E. DEFINITIONS AND ACRONYMSE-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-8

Figure A-5. TCAS II Examples – Preventive RAsA-10

Figure A-6. TCAS II Examples – Initial Corrective RAsA-11

Figure A-7. TCAS II Examples – Modifications to Initial Corrective RAsA-12

TABLE OF TABLES

Table 1. Maneuvers15

Table 2. System Inhibits18

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

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

Table A-3A. Paddle Switch REL ALT/FL SwitchA-6

Table A-3B. Above/Norm/Below SwitchA-6

Table A-4. Instantaneous Vertical Speed IndicatorA-9

Table E-1. AcronymsE-3

1 Chapter 1. General Information.

1-1. Purpose of this Advisory Circular.

a. We’ve written this advisory circular (AC) to guide applicants seeking airworthiness approval for traffic alert and collision avoidance systems (TCAS II) version 7.1 that are certified to technical standard order (TSO)-C119c, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment TCAS II and associated Mode S transponders. Guidance is also provided for those applicants seeking airworthiness approval for stand-alone Mode S transponders that are certified to technical standard order (TSO)-C112c, Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment. The guidance presented in this AC can also be used for those seeking airworthiness approval for traffic alert and collision avoidance systems (TCAS II) version 7.0 that are certified to TSO-C119b, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment TCAS II and associated Mode S transponders.

b. This AC is not mandatory and does not constitute a regulation. In it, we describe an acceptable means, though it is 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, you must follow it in all important respects.

1-2. Who does this AC apply to? Applicants seeking a type certificate (TC), amended type certificate (ATC), or supplemental type certificate (STC) under Title 14 of the Code of Federal Regulation (14 CFR) part 25 for initial approval and follow-on approvals of TCAS II equipment or stand alone Mode S transponder equipment. 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 when the equivalent Advisory Circulars and sections of 14 CFR part 23 are referenced.

1-3. Cancellation. This revision cancels AC-20-151.

1-4. Significant Changes. This AC has been revised to accommodate the changes of TCAS version 7.1, incorporate lessons learned since the original AC, and add policy for optional hybrid surveillance.

1-5. Scope.

a. In this AC, we cover design aspects, characteristics, mechanization, testing, and the criticality of system failure cases for TCAS II Versions 7.0 and 7.1 and associated Mode S transponders. Our guidance is directed at systems that provide traffic advisories (TAs) and resolution advisories (RAs) in the vertical sense only (TCAS II), and where operational

2 performance standards are defined in technical documents developed by a joint air transport industry-government group (the RTCA, Inc. Special Committee SC-147).

b. We also cover optional functionality, such as TCAS II hybrid surveillance, that can be included in a TCAS II system.

c. We do not cover traffic symbology for traffic displays in which TCAS and ASAS (Airborne Separation Assistance System) are integrated.

d. We do not cover Mode S Extended Squitter or Mode S Elementary/Enhanced surveillance.

(1) For guidance on Mode S Elementary surveillance, refer to JAA Temporary Guidance Leaflet (TGL) 13 Revision 1, Certification of Mode S Transponder Systems for Elementary Surveillance.

(2) For guidance on Mode S Enhanced surveillance, refer to AMC 20-13, Certification of Mode S Transponder Systems for Enhanced Surveillance.

(3) AC guidance for Mode S extended Squitter, also known as automatic dependant surveillance – broadcast (ADS-B), is being developed. Until such time as this guidance material is available, contact the cognizant Aircraft Certification Office (ACO) for more information.

1-6. Recent TCAS Developments.

a. Since the early 1990s, an operational evaluation known as the TCAS Transition Program (TTP) collected and analyzed data on the performance and use of TCAS II in both the U.S. National Airspace System (NAS) and in other airspace worldwide. As a result of these analyses, changes to TCAS II were developed, tested, and implemented in the early to mid 1990s. These changes, collectively known as TCAS II version 7.0, were implemented by industry in the late 1990s. TCAS II version 7.0 complies with the International Civil Aviation Organization (ICAO) Standards and Recommended Practices (SARP) for ACAS II. TCAS II V7.0 is mandated for carriage in Australia, India and certain European countries and was mandated for carriage in 2003 by ICAO. In addition, if you operate an aircraft that is equipped with TCAS II in RVSM airspace, it must be a TCAS II that meets TSO-C119b (version 7.0) or a later version.

b. Investigation of operational problems resulted in changes to TCAS II Version 7.0.

The latest changes, known as V7.1, are now reflected in TSO-C119c.

c. While developing corrective action for the operational problems, the Federal Aviation Administration (FAA), along with industry representatives, decided to develop performance standards for TCAS II Hybrid Surveillance. That work resulted in creation of RTCA/DO-300, Minimum Operational Performance Standards (MOPS) for Traffic Alert and Collision Avoidance System II (TCAS II) Hybrid Surveillance.

3 Chapter 2. TCAS II System.

A. System Description.

2-1. What a TCAS System Is. TCAS II is an airborne traffic alert and collision avoidance system that interrogates 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.

2-2. System Advisories. The system provides two types of advisories.

a. Traffic advisories (TAs) indicate the relative positions of intruding aircraft that are approximately 40 seconds from the closest point of approach and may a short time later require a resolution advisory. TAs also give the flight crew the opportunity to visually acquire the intruding aircraft.

b. RAs provide a vertical avoidance maneuver to increase separation when the computer predicts the threat aircraft is within approximately 25 seconds from the closest point of approach.

2-3. System Flight Deck Displays. The system provides two types of flight deck displays:

a. A traffic display indicates the relative position of ATC transponder-equipped aircraft.

b. A resolution display for each pilot indicates the appropriate vertical maneuver to avoid a threat.

2-4. Mode S Transponder.

a. 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.

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

c. 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 air traffic control radar beacon system (ATCRBS) transponder, regardless of its ability to provide information on the intruder aircraft’s altitude. We view TCAS II equipment as a supplement to the pilot who has the primary responsibility for avoiding midair collisions. The TCAS II system provides no indication of aircraft without operative transponders.

2-5. Hybrid Surveillance. Hybrid surveillance is an optional function of TCAS that is 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 data broadcast from other aircraft. Passive surveillance data is provided by an on-board navigation source, such as a global positioning system (GPS). The passive surveillance data is broadcast and received through the use of Mode S extended squitter, that is, 1090 megahertz (MHz) automatic dependent surveillance-broadcast (ADS-B). Hybrid surveillance does not degrade the performance of TCAS active surveillance. This is a requirement and is tested by ensuring that active surveillance performs as specified by the TCAS II performance standards.

B. System Components And Requirements.

2-6. Mode S Transponder.

a. A Mode S transponder is required for TCAS II operation. It is an enhanced version of ATCRBS transponders that is interoperable and compatible with the current 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 that the resolution advisory displayed in one TCAS II-equipped aircraft is compatible with the maneuver displayed in the other TCAS II equipped aircraft. It has the capability to provide a data link between the equipped aircraft and the ground, and performs all the functions of current ATCRBS transponders. A Mode S transponder may be installed independently or with a TCAS II installation. The performance standard for Mode S installed independently of TCAS is provided in technical standard order TSO-C112C, Requirements, paragraph 3.

b. 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 each aircraft in which a Mode S transponder is installed. 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.

5 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 be caused when a change in registration numbers occurs.

When an airplane’s registration number is changed, the operator should verify that the ICAO 24-bit aircraft address and the new registration number have a one-to-one correspondence.

2-7. Pilot Control. A pilot control for the TCAS equipment shall be provided.

a. Provide a means to select the following:

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

(2) Operation of TCAS II in the TA mode and Mode S transponder simultaneously.

(3) Operation of TCAS II in the standby mode.

b. Also, provide the following additional features:

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

(2) A means to initiate the transponder “IDENT” function.

(3) A means to initiate the TCAS II self-test.

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

c. The following optional controls may be provided:

(1) Selection of the weather radar only.

(2) Control to select the display of traffic within selected altitude bands.

(3) Selection of the weather radar and traffic display simultaneously.

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

(5) Selection of TCAS traffic information on multi-function displays.

2-8. Antennas. The Mode S transponder shall have a top and bottom omni-directional antenna (also know as diversity antennas). The TCAS II shall have a top directional antenna and a bottom omni-directional or directional antenna.

a. Directional antennas.

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

(2) If more than one directional antenna is provided, locate the second antenna in a similar manner on the lower fuselage.

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

(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.

(5) The maximum height of the directional antenna is expected to be approximately 1 inch, and therefore is not considered susceptible to icing effects in the general area of the proposed installation. Otherwise, consider anti-icing provisions. RTCA/DO-185, Section 3, Volume I, provides antenna selection and performance criteria.

(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.

b. 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 frequency antennas. Mount the Mode S transponder antennas at locations chosen for adequate isolation and signal coverage. These antennas may be standard ATCRBS transponder antennas.

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

2-9. 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 requirements and minimum performance standards specified in TSO-C119b or TSO-C119c as applicable. 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 may include hybrid surveillance as optional functionality.

2-10. Traffic Display.

a. Purpose. The primary purpose of the traffic display is to aid the flight crew in the 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 transponder-equipped 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.

b. Description. Traffic displays may take several forms. They may be independent, stand alone, integrated and time-shared with digital color radar, integrated with instantaneous vertical speed indicators (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.

c. Symbology/Feature Criteria. The FAA worked closely with the Air Transport Association (ATA), 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 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.

2-11. Resolution Advisory Display .

a. Purpose. The purpose of the resolution advisory 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 aircraft) and a threat aircraft.

b. Description. The resolution advisory display may be integrated with the two primary IVSIs on the flight deck, integrated into the primary flight display (PFD), or incorporated 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 display implementations.

c. Symbology. We worked closely with the ATA, 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 provided 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

8 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.

d. Failures. Provide indications for TCAS II failures (for example, TCAS II unable to generate RAs). Provide annunciation for the TA only mode. Electrical IVSI failures must also be annunciated.

2-12. Optional Caution/Warning Lights.

a. Discrete caution and/or warning lights may be installed that are separate from the traffic display. The purpose of these additional indicators is to annunciate the presence of potentially threatening intruder aircraft at times when the pilot’s attention may be diverted from the primary TCAS display. Two different discrete TCAS II annunciators have been used:

(1) A discrete amber caution annunciator, which indicates the presence of a TCAS II TA. Installation of this discrete caution annunciator is optional. When installed, it must be located in each pilot’s primary field of view and be inhibited below 400 feet above ground level (AGL).

(2) A discrete red warning annunciator that indicates the presence of a TCAS II RA.

This red warning must be located in each pilot’s primary field of view and be inhibited below 900 feet AGL. An IVSI with a lighted red arc or an alphanumeric message on the electronic attitude display indicator (EADI) is acceptable instead of this discrete warning annunciator.

b. Because of the number of TCAS II advisories expected in service, the basic aircraft master caution and warning system should not be interfaced with these TCAS II caution/warning discretes. Overuse of the primary aircraft caution and warning system tends to reduce its effectiveness in annunciating non-TCAS II system failures.

c. Discrete visual alerts should remain on until canceled by the pilot or until the aircraft is no longer considered an intruder or a threat by TCAS II.

2-13. Aural Alert Requirements.

a. Annunciate each TCAS II aural alert by a dedicated voice message over a cockpit speaker at a volume adequate for clear understanding at high cockpit noise levels, but not excessively loud at low noise levels. The evaluation includes the case where a flight crew member is wearing a headset, covering the outboard ear when appropriate. In turbo-prop aircraft where the aircrew utilizes headsets via the aircraft audio distribution panel, the aural messages should hold the same acceptable volume and intelligibility during both low and high cockpit noise levels.

b. Annunciate TCAS II TAs by the voice message “TRAFFIC, TRAFFIC” stated once for each TA.

c. Annunciate TCAS II RAs by the following voice messages, as appropriate:

(1) “CLIMB, CLIMB”-- climb at the rate depicted by the green (fly to) arc or line on the IVSI or other suitable indicator.

(2) “DESCEND, DESCEND”-- descend at the rate depicted by the green (fly to) arc or line on the IVSI or other suitable indicator.

(3) “MONITOR VERTICAL SPEED”-- ensure that vertical speed is out of the illuminated IVSI red arc or line, or other suitable indication.

(4) “ADJUST VERTICAL SPEED, ADJUST”-- modify the vertical speed to a value within the illuminated green arc or line, or outside the prohibited area on other suitable indications. This aural is annunciated only by V7.0 TCAS II units.

(5) ”LEVEL OFF, LEVEL OFF” -- reduce vertical speed to zero feet per minute. A green arc or line will be illuminated beginning at zero feet per minute. This can be issued as the initial RA or as a subsequent RA. This aural is annunciated only by V7.1 TCAS II units.

(6) “CLEAR OF CONFLICT”-- range is increasing, and separation is adequate;

expeditiously return to the applicable clearance, unless otherwise directed by ATC.

(7) “CLIMB, CROSSING CLIMB, CLIMB, CROSSING CLIMB”-- climb at the rate depicted by the green (fly to) arc or line on the IVSI or other suitable indicator. Safe separation will best be achieved by climbing through the threat’s flight path.

(8) “DESCEND, CROSSING DESCEND, DESCEND, CROSSING DESCEND”-- descend at the rate depicted by the green (fly to) arc or line on the IVSI or other suitable indicator. Safe separation will best be achieved by descending through the threat’s flight path.

(9) “MAINTAIN VERTICAL SPEED, MAINTAIN”-- maintain the existing climb or descent rate as depicted by the green (fly to) arc or line on the IVSI or other suitable indicator.

Safe separation is best achieved by not altering the existing vertical speed.

(10) “MAINTAIN VERTICAL SPEED, CROSSING MAINTAIN”-- maintain the existing climb or descent rate as depicted by the green (fly to) arc or line on the IVSI or other suitable indicator. Safe separation will best be achieved by not altering the existing vertical speed and climbing or descending through the threat’s flight path.

d. The following voice messages are required to annunciate enhanced TCAS II maneuvers when the initial RA does not provide sufficient vertical separation. The tone and inflection must connote increased urgency.

(1) “INCREASE CLIMB, INCREASE CLIMB”--climb at the rate depicted by the green (fly to) arc or line on the IVSI or other suitable indicator. Received after “CLIMB”