Subject: AIR CARRIER OPERATIONAL Date: 10/22/01 AC No: 120-55B

APPROVAL AND USE OF TCAS II Initiated by: AFS-400 Change:

1. PURPOSE. This advisory circular (AC) provides an acceptable, but not the only, means to

address Traffic Alert and Collision Avoidance System (TCAS) issues related to installation and use

of TCAS II regarding compliance with Title 14 of the Code of Federal Regulations (14 CFR) Parts

121, 125, and 129 requirements for air carriers.

2. CANCELLATION. AC 120-55A, Air Carrier Operational Approval and Use of TCAS II, dated

August 27, 1993, is hereby canceled.

3. APPLICABILITY. This AC applies to air carriers operating under Part 121, other organizations

conducting training approved in accordance with Part 121 (for example, training centers or aircraft

manufacturers), operators under Part 125, and foreign air carriers conducting operations in U.S.

airspace under Part 129. This AC describes the TCAS operational approval process, acceptable

methods for TCAS training, acceptable programs for maintenance, operational policies for TCAS

use, appropriate actions in the event of a TCAS occurrence, and criteria for foreign operator use of

TCAS in U.S. airspace. In addition, this AC addresses commonly asked TCAS questions in order to

facilitate timely and consistent application of 14 CFR §§ 121.356, 125.224, and 129.18. The

principles described may also be applied to those air carriers operating under 14 CFR Part 135.180,

as well as any other aircraft operating under 14 CFR Part 91, where the operator chooses to install a

TCAS II system meeting Technical Standard Order (TSO)-C119A or TSO-C119B, Traffic Alert and

Collision Avoidance System (TCAS) Airborne Equipment, TCAS II. It should be noted, however,

the international standard for TCAS II, referred to as Airborne Collision Avoidance System (ACAS),

includes the latest software version (v 7.0), as detailed by the International Civil Aviation

Organization (ICAO).

4. RELATED REFERENCES.

a. Title 14 CFR §§ 1.1, 1.2, 71.9, 71.33, 91.3, 91.123, 91.213, 91.215, 91.217, 91.219, 91.221,

91.413, 119.7, 119.49, 121.345, 121.356, 125.224, 129.11, 129.14, and 129.18.

b. FAA ACs and Technical Standard Orders (TSO). ACs and TSOs may be obtained on the

FAA Website or from the U.S. Department of Transportation, Subsequent Distribution Center,

SVC-121.23, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, Maryland 20785.

(1) AC 20-131, Airworthiness and Operational Approval of Traffic Alert and Collision

Avoidance Systems (TCAS II) and Mode S Transponders, current edition;

(2) TSO-C112, Air Traffic Control Radar Beacon System (ATCRBS/Mode S) Airborne

Equipment; and
AC 120-55B 10/22/01

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(3) TSO-C119A and TSO-C119B, Traffic Alert and Collision Avoidance System (TCAS)

Airborne Equipment, TCAS II.

NOTE: Both TSO-C119A and TSO-C119B are in effect.

c. RTCA Technical Standards Documents. These documents may be obtained on-line at

www.rtca.org or from RTCA Secretariat, 1828 L Street, NW, Suite 807, Washington, D.C. 20036.

(1) DO-181C, Minimum Operational Performance Standards for ATCRBS/Mode S Airborne

Equipment; and

(2) DO-185A, Minimum Operational Performance Standards for TCAS II Airborne

Equipment, Volume II, Consolidated Edition (for TCAS Logic Version 7.0).

5. BACKGROUND. TCAS systems are now implemented in Part 121 operations and other

applications. Approval of TCAS for FAA Type Certification (TC) or Supplemental Type

Certification (STC) is comprehensively addressed in AC 20-131, Airworthiness and Operational

Approval of Traffic Alert and Collision Avoidance Systems (TCAS II) and Mode S Transponders, as

amended. This AC provides information for U.S. air carriers, aircraft and TCAS manufacturers,

various inspectors, foreign air carriers operating in U.S. airspace, and other aviation organizations

regarding standard means acceptable to the FAA to establish and ensure continued compliance with

14 CFR as related to TCAS. This information is intended to promote timely and comprehensive

program implementation, to encourage development of standard practices for the application of

TCAS, and to provide for suitable follow-up to TCAS events.

6. DEFINITIONS. See Appendix 7. For convenience, some definitions are repeated from other

pertinent FAA references. Other definitions are unique to this AC and their application is limited to

use with TCAS.

7. APPROVAL TO USE TCAS IN 14 CFR PART 121 OPERATIONS.

a. General.

(1) The use of TCAS in Part 121 operations requires both FAA airworthiness certification

and operational approval. Airworthiness certification of TCAS refers to an FAA approval of changes

in an aircraft’s type design by amendment to a TC or issuance of an STC. Operational approval

pertains to changes to training and maintenance programs, manuals, operational procedures,

Minimum Equipment Lists (MEL), and other areas necessary for safe and effective TCAS use and

the qualification of aircrews through the approved training programs. An airworthiness TC/STC of a

TCAS system alone does not constitute operational approval for use of TCAS under provisions of

Part 121.

(2) Responsibilities of Various FAA Offices Regarding TCAS.

(a) FAA Aircraft Certification Offices (ACO) approve changes to a type design or

issuance of STCs.

(b) FAA Aircraft Evaluation Groups (AEG) formulate operational criteria for specific

aircraft types related to training, checking, maintenance, Master Minimum Equipment Lists

(MMEL), or other operational issues, as necessary.
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(c) FAA Flight Standards District Offices (FSDO) use information developed by AEG to

review a particular operator’s programs. FSDOs approve a particular operator’s training and

maintenance programs, operational procedures, MELs, etc., provided they are consistent with criteria

specified in the MEL, Flight Standardization Board (FSB), Maintenance Review Board (MRB)

reports, and policy guidance from the Flight Standards Service, Air Transportation Division,

AFS-200, and the Continuous Airworthiness Maintenance Division, AFS-300.

b. Airworthiness Approval. Criteria for TCAS airworthiness approval (TC or STC) are

addressed by the current version of AC 20-131. TCAS installations are to be made in accordance

with a type design change, an aircraft manufacturer’s FAA-approved service bulletin (SB), or by an

STC.

c. Operational Approval.

(1) Approval Criteria. Operational approvals are based on criteria specified and, if

applicable, training, maintenance, MMEL, or other operationally related criteria formulated by

AEGs. If the criteria for training or checking are other than as specified, the criteria may be found in

FSB reports applicable to a particular aircraft type. Provisions for dispatch with inoperative

equipment are specified by the MMEL for each aircraft type. Maintenance requirements are as

identified herein, except as otherwise described by an MRB report for a specific aircraft type, or in

FAA-approved maintenance instructions identified in conjunction with an STC or manufacturer’s

SB.

(2) Approval Methods. TCAS operational approval is accomplished through FAA approval

of pertinent training programs, checklists, operations manuals, training manuals, maintenance

programs, MELs, or other pertinent documents or document revisions applicable to the particular air

carrier. An operator’s TCAS programs are usually approved for each specific aircraft type.

However, programs common to one or more types may be approved if TCAS program elements are

common to different aircraft types, such as the same TCAS or procedures.

(3) Approval Procedures.

(a) Approval to use TCAS is issued by an FAA Principal Inspector (PI). Operators

should make early contact with their respective PIs to ensure timely FAA response. Usually such

contact is initiated at the time preparations are being made for TCAS selection or purchase, and

generally not later than TCAS TC/STC application.

(b) Operations Specifications (OpSpecs) issuance is normally not required for U.S.

operators. Installations, training, maintenance programs, MELs, and other TCAS program elements

are reviewed and accepted or approved by the FAA in accordance with 14 CFR § 121.356.

8. PART 121 FLIGHTCREW QUALIFICATION FOR USE OF TCAS.

a. General.

(1) TCAS Qualification. Air carriers must ensure appropriate flightcrew TCAS

qualification. The flightcrew must demonstrate proficiency in the following:

(a) Knowledge of TCAS concepts, systems, and procedures; and
AC 120-55B 10/22/01

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(b) Cognitive, procedural, and motor skills necessary to properly respond to TCAS

advisories.

(2) Acceptable Qualification Methods. First-time TCAS qualification must be

accomplished for each airplane type. Qualification may be accomplished during initial, transition, or

upgrade ground and flight training programs with appropriate differences. By this method, TCAS

information will be integrated with other curriculum elements and modules. First-time TCAS

qualification may also be accomplished as a stand-alone module of ground and flight training.

Recurrent TCAS qualification will be accomplished during recurrent ground and flight training.

Recurrent ground training will be a stand-alone module. However, TCAS will be fully integrated

with the recurrent flight training during proficiency training (PT) or line-oriented flight training

(LOFT). For first-time and recurrent TCAS qualification, an instructor will accomplish evaluation of

TCAS objectives during training. There are no formal TCAS evaluation requirements for flight

testing or checking. However, routine TCAS operations will be included in all evaluation

environments and check airmen/examiners should include TCAS as a routine discussion item.

(3) Principal Operations Inspectors (POI) may give an operator credit when training is

conducted by another operator or training center stand-alone TCAS program for first-time

qualification if that program has been approved by the FAA and if aircraft, TCAS system,

procedures, and other relevant factors or circumstances are the same or equivalent to those of the

operator seeking credit. The POI should consult with the appropriate division of AFS, National

Simulator Evaluation Team (NSET), or the assigned AEG about the suitability of a proposed

program for a particular version of TCAS or aircraft type.

b. TCAS Training Program Requirements.

(1) Curriculum. Appendix 6 contains the required areas of instruction for ground training

for the first-time TCAS qualification. For subsequent type airplanes and for recurrent training, only

the new, revised, or emphasized items need to be addressed. Appendix 6 also contains the objectives

and related simulation characteristics required for flight training.

(2) Evaluation of TCAS Knowledge and Skills (Maneuvers).

(a) TCAS knowledge must be evaluated with written, oral, or computer-based

instructional tests. Combinations of these methods may be used if the required body of knowledge is

completely covered. For any of these methods, a passing grade of 90% must be achieved. First-time

qualification in any type airplane must include evaluation of all knowledge areas. For recurrent

training, complete coverage of the knowledge requirements must be completed every 36 months.

(b) TCAS skills (maneuvers) must be evaluated by an instructor for first-time TCAS

qualification in each type airplane. This evaluation may be accomplished by an instructor in a

qualified Flight Training Device (FTD), Simulator, or Computer-Based Instructional System (CBI)

approved for each maneuver. For recurrent training, all maneuvers must be provided during training

in any 36 month period. Recurrent training is desirable in an FTD or Simulator approved for the

maneuvers.

(3) Line-Oriented Flight Training (LOFT). LOFT programs using simulators equipped

with TCAS should be enhanced by an interactive TCAS. In addition, LOFT programs should

consider proper crew vigilance for aircraft that may not be transponder or Mode C equipped.

Advisories accomplished during LOFT are creditable toward first-time or recurrent qualification.
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c. Training Center Approval. Training centers conducting training, such as contract training or

aircraft manufacturers, may have TCAS elements of those programs approved if curriculum

requirements (ground and flight) provided in Appendix 6 are met.

d. Initial Evaluation of TCAS Knowledge and Skills. Individual crewmember TCAS

knowledge and skills must be evaluated prior to TCAS use. Acceptable means of initial assessment

include the evaluation by an authorized instructor or check airman using written, computer-based, or

oral tests, and a simulator, FTD, or CBI system capable of depicting traffic encounters.

e. TCAS Recurrent Training. TCAS recurrent training should be integrated into and/or

conducted in conjunction with other established recurrent training programs. Recurrent training for

TCAS must include both ground and flight (maneuver) and should address any significant issues

identified by line operating experience (OE), system changes, procedural changes, or unique

characteristics such as the introduction of new aircraft/display systems or operations in airspace

where high numbers of Traffic Advisories (TA) and Resolution Advisories (RA) have been reported.

f. TCAS Recurrent Evaluation. Recurrent TCAS checking should be incorporated as an

element of routine proficiency training.

g. Line Checks and Route Checks. When TCAS-equipped aircraft are used during line or

route checks, check airmen should routinely incorporate proper TCAS use as a discussion item.

h. LOFT. LOFT programs using simulators equipped with TCAS should be enhanced by

interaction with TCAS. In addition, LOFT programs should consider proper crew vigilance for

aircraft which may not be transponder or Mode C equipped.

i. Crew Resource Management (CRM). CRM programs should address effective teamwork in

responding to TCAS events with emphasis on the following areas:

(1) The crew should conduct preflight briefings on how TCAS advisories will be handled;

and

(2) The proper reaction to a TA by the pilot flying (PF) and the pilot-not-flying (PNF).

9. OTHER OPERATIONAL ISSUES.

a. Manuals and Other Publications. Airplane flight manuals, operating manuals, maintenance

manuals, general policy manuals, other manuals, publications, or written material (such as operating

bulletins that may relate to TCAS use) must be appropriately amended to describe TCAS equipment,

procedures, and operational policies according to the appropriate regulation.

b. MMEL/MEL.

(1) General. Operators formulate necessary TCAS revisions to their MELs for each

particular fleet (for example, B727, DC10). MEL revisions must be consistent with the FAA MMEL

established for each aircraft type. A summary of the process for addressing the necessary changes to

MEL items as well as examples of MMEL and acceptable MEL provisions for TCAS are provided in

Appendix 2.

(2) The MMEL was changed April 6, 2000, and the relief category was changed from “C” to

“B” for the entire system.
AC 120-55B 10/22/01

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c. Aircraft with TCAS Differences. Operators who have aircraft with TCAS differences in

displays, controls, procedures, or involved with interchange operations must account for those TCAS

differences. This is accomplished as part of an approved differences training program in accordance

with Part 121, or as otherwise specified in applicable FAA FSB reports concerning crew qualification

pertaining to a particular airplane type.

d. Issues Unique to a Particular Operator. Operators should address any TCAS issues that

may be unique to their particular route environment, aircraft, procedures, or TCAS display and

control features. Examples include the following:

(1) Route Environment Issue. Air carriers having takeoffs or landings outside of the

reference TCAS performance envelope (for example, airport elevations outside of the range between

sea level and 5,300 feet mean sea level (MSL), or temperatures outside the range of International

Standard Atmosphere ((ISA) ±50° F) should advise crews of appropriate procedures and precautions

regarding RA compliance. To ensure proper response to TCAS in limiting performance conditions

(for example, TCAS RA during takeoff climb or in final landing configuration at high altitude

airports such as Mexico City and La Paz), specific procedures or training may be needed, unless

these situations can be adequately addressed by bulletin or manual information.

(2) A Procedural Issue. Air carriers should describe the use of TA-only mode of operation

when required on certain aircraft with an engine failure.

(3) A Unique TCAS Issue. TCAS flight level or absolute display of traffic altitude on a

traffic display should not be used during operations when the altimeter is set to zero relative to the

intended field of landing field elevation (QFE).

e. Response to TCAS RA for Aircraft at Maximum Certified Altitude.

(1) Aircraft not inhibited to climb when at maximum certified altitude are expected to climb

in response to TCAS climb RAs.

(2) Aircraft that are climb inhibited from TCAS climbs at maximum certified altitude will be

issued a “DO NOT DESCEND” RA.

(3) In no case should a pilot maneuver opposite to a TCAS RA.

10. MAINTENANCE.

a. General. Maintenance procedures for TCAS are approved or accepted as part of an

operator’s initial maintenance manual approval or as a revision to that manual. TCAS maintenance

procedures should be consistent with the TCAS manufacturer’s maintenance procedures and/or

aircraft manufacturer’s maintenance procedures for TCAS.

b. Maintenance Training. An operator must provide adequate TCAS maintenance training in

accordance with the appropriate regulation to ensure that its maintenance personnel or contract

maintenance personnel at facilities not staffed by the operator are able to properly implement TCAS

related maintenance programs. This includes, but is not limited to, addressing installation,

modification, correction of reported system discrepancies, use of test equipment, procedures, MEL

relief, and return-to-service authorizations. The training procedures should address testing installed

transponders and automatic pressure altitude reporting equipment on the ground in such a way that
10/22/01 AC 120-55B

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false targets are not generated in airborne TCAS systems. A method for eliminating potential

interference with TCAS-equipped aircraft during transponder testing would be the use of shields for

the transponder antennas. AC 20-131 provides more detailed information and guidance concerning

the proper procedures to follow when conducting operational testing of TCAS II or transponders on

the ground.

c. TCAS Software Updates. When necessary, operators should ensure that appropriate TCAS

software updates are incorporated. The current appropriate software for TCAS II is version 7.0. To

ensure compatibility with international standards, the software should be installed as soon as

practical. ICAO requirements for ACAS specify TCAS II with software version 7.0 reduces

nuisance RAs in high speed climb to level-off situations.

11. TCAS OPERATIONAL USE.

a. General. Operationally, those skills addressed and the guidance provided on TCAS training

in paragraph 9 should be followed and implemented by each operator electing to use TCAS II and

apply the appropriate 14 CFR.

b. Pilot Responsibilities. TCAS is intended to serve as a backup to visual collision avoidance,

application of right-of-way rules, and air traffic separation service. For TCAS to work as designed,

immediate and correct crew response to TCAS advisories is essential. Delayed crew response or

reluctance of a flightcrew to adjust the aircraft’s flight path as advised by TCAS due to Air Traffic

Control (ATC) clearance provisions, fear of later FAA scrutiny, or other factors could significantly

decrease or negate the protection afforded by TCAS. Flightcrews are expected to respond to TCAS

in accordance with the following guidelines when responding to alerts:

(1) Respond to TAs by attempting to establish visual contact with the intruder aircraft and

other aircraft which may be in the vicinity. Coordinate to the degree possible with other

crewmembers to assist in searching for traffic. Do not deviate from an assigned clearance based only

on TA information. For any traffic that is acquired visually, continue to maintain or attain safe

separation in accordance with current regulations and good operating practices.

(2) When an RA occurs, the PF should respond immediately by direct attention to RA

displays and maneuver as indicated, unless doing so would jeopardize the safe operation of the flight

or the flightcrew can assure separation with the help of definitive visual acquisition of the aircraft

causing the RA. By not responding to an RA, the flightcrew effectively takes responsibility for

achieving safe separation. In so choosing, the following cautions should be considered:

(a) The traffic may also be equipped with TCAS and it may maneuver in response to an

RA that has been coordinated with your own TCAS.

(b) The traffic acquired visually may not be the same traffic causing the RA.

(c) Visual perception of the encounter may be misleading. Unless it is unequivocally

clear that the target acquired visually is the one generating the RA and there are no complicating

circumstances, the pilot’s instinctive reaction should always be to respond to RAs in the direction

and to the degree displayed.
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(3) Satisfy RAs by disconnecting the autopilot, if necessary, using prompt, positive control

inputs in the direction and with the magnitude TCAS advises. To achieve the required vertical rate

(normally 1,500 feet per minute (fpm) climb or descent), first adjust the aircraft’s pitch using the

suggested guidelines shown in the table below. Then refer to the vertical speed indicator (VSI) and

make all necessary pitch adjustments to place the VSI in the green arc.

 SPEED PITCH ADJUSTMENT

.80 MACH 2°

250 KIAS below 10,000 feet 4°

APPROACH below 200 KIAS 5° to 7°

(a) On aircraft with pitch guidance for TCAS RA displays, follow the RA pitch

command for initial, increase, and weakening RAs.

(b) For TCAS to provide safe vertical separation, initial vertical speed response is

expected within 5 seconds of when the RA is first displayed. Excursions from assigned altitude,

when responding to an RA, typically should be no more than 300 to 500 feet to satisfy the conflict.

Vertical speed responses should be made to avoid red arcs or outlined pitch avoidance areas and, if

applicable, to accurately fly to the green arc or outlined pitch guidance area.

(4) Respond immediately to any “increase” or “reversal” RA maneuver advisories. Initial

vertical speed response to an increase or reversal RA is expected by TCAS within 2 1/2 seconds after

issuance of the advisory. Again, avoid red arcs or outlined pitch avoidance areas and fly to the green

arc or outlined pitch guidance area.

(5) The PNF should advise the PF on the progress of achieving the vertical rates commanded

by TCAS. The PNF and any on-board observers will assist in the visual search for the intruder and

continue to cross-check the TCAS displayed information with other available traffic information to

ensure the RA response is being flown correctly.

(6) If an initial corrective RA is downgraded or weakened, such as a “climb” RA

downgraded to a “do not descend” RA, pilots should respond to the weakening RA and adjust the

aircraft’s vertical speed accordingly, but still keep the needle or pitch guidance symbol out of the red

arc or outlined pitch avoidance area. Pilots are reminded that attention to the RA display and prompt

reaction to the weakened RA will minimize altitude excursions and potential disruptions to ATC.

This will allow for proper TCAS-to-TCAS resolution of encounters and reduce the probability of

additional RAs against the intruder or other traffic.

(7) Excessive responses to TCAS RAs are inappropriate and may increase interference with

other traffic and result in additional RAs.

(8) In some instances it may not be possible to respond to a TCAS RA and continue to

satisfy a clearance at the same time. Even if a TCAS RA maneuver is inconsistent with the current

clearance, respond appropriately to the RA. Since TCAS tracks all transponder equipped aircraft in

the vicinity, responding to an RA for an intruder assures a safe avoidance maneuver from that

intruder and from other Mode C equipped aircraft. Guidance in this paragraph does not conflict with

that in paragraph 12b(2). Exceptions noted in paragraph 12b(2) are meant to hold down obvious

nuisance excursions and unsafe handling of the aircraft.
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(9) If a TCAS RA requires maneuvering contrary to “right-of-way” rules, “cloud clearance”

rules for visual flight rules (VFR), instrument flight rules (IFR), or other such criteria, pilots are

expected to follow the TCAS RAs to resolve the immediate traffic conflict. Deviations from rules or

clearances should be kept to the minimum necessary to satisfy a TCAS RA.

(10) If a TCAS RA response requires deviation from an ATC clearance, expeditiously return

to the current ATC clearance when the traffic conflict is resolved, the TCAS “clear of conflict”

message is heard, or follow any subsequent change to clearance as advised by ATC. In responding to

a TCAS RA that directs a deviation from assigned altitude, communicate with ATC as soon as

practicable after responding to the RA. When the RA is cleared, the flightcrew should advise ATC

that they are returning to their previously assigned clearance or should acknowledge any amended

clearance issued. In addition, the flightcrew’s discretionary use of other types of reports may be

desired. See Appendix 5 for suggested phraseology.

(11) If a TCAS RA maneuver is contrary to other critical cockpit warnings, then those other

critical warnings are respected as defined by TCAS certification and training (that is, responses to

stall warning, windshear, and Ground Proximity Warning System (GPWS) take precedence over a

TCAS RA, particularly when the aircraft is less than 2,500 feet Above Ground Level (AGL)).

(12) Pilots should use TCAS traffic information displays to increase their awareness of

nearby traffic and to assist in establishing visual contact with other aircraft. Certain Electronic Flight

Information System (EFIS) TCAS installations operating in conjunction with “track up” mode may

require the pilot to make allowances for the difference between the aircraft heading and track when

visually sighting nearby aircraft.

(13) Unless approved by the Administrator, pilots are expected to operate TCAS while inflight in all airspace, including oceanic, international, and foreign airspace.

(14) When feasible, flightcrews should use the same altitude data source that is being used

by the PF to provide altitude information to TCAS and the ATC transponders. Using a common

altitude source precludes unnecessary RAs due to differences between altitude data sources.

(15) Note and accurately report TCAS encounters and system anomalies in accordance with

operator policies so that remedial improvements to TCAS or the National Airspace System (NAS)

may be initiated.

(16) TCAS does not alter or diminish the pilot’s basic authority and responsibility to ensure

safe flight. Since TCAS does not respond to aircraft which are not transponder equipped or aircraft

with a transponder failure, TCAS alone does not ensure safe separation in every case. TCAS RAs

may, in some cases, conflict with flight path requirements due to terrain, such as an obstacle-limited

climb segment or an approach to rising terrain. Since many approved instrument procedures and IFR

clearances are predicated on avoiding high terrain or obstacles, it is particularly important that pilots

maintain situational awareness and continue to use good operating practices and judgment when

following TCAS RAs. Maintain frequent outside visual scan, “see-and-avoid” vigilance, and

continue to communicate as needed and as appropriate with ATC.

c. The potential consequences of improperly maneuvering the aircraft in response to an RA

include:

(1) An aircraft seen visually may not necessarily be the aircraft causing the RA or may not

be the only aircraft to which TCAS is responding.
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(2) It is difficult to visually determine the vertical displacement of other aircraft especially

when ground reference information is unreliable or at cruise altitudes where the earth’s horizon is

obscured. Therefore, disregarding RA information and maneuvering vertically based solely on visual

acquisition may result in a loss of safe separation.

(3) ATC may not know when TCAS issues RAs. It is possible for ATC to unknowingly

issue instructions that are contrary to the TCAS RA indications. Safe vertical separation may be lost

during TCAS coordination when one aircraft maneuvers opposite the vertical direction indicated by

TCAS and the other aircraft maneuvers as indicated by TCAS. As a result, both aircraft may

experience excessive altitude excursions in “vertical chase” scenarios due to the aircraft maneuvering

in the same vertical direction. Accordingly, during an RA, do not maneuver contrary to the RA

based solely upon ATC instructions.

(4) ATC may not be providing separation service or be communicating with the aircraft

causing the RA.

(5) Failure to maneuver during a coordinated encounter with another TCAS-equipped

aircraft can result in loss of safe separation.

d. TCAS Good Operating Practices. The following TCAS good operating practices have been

identified:

(1) To preclude unnecessary transponder interrogations and possible interference with

ground radar surveillance systems, TCAS should not be activated (TA-only or TA/RA mode) until

taking the active runway for departure. Standby mode for a Mode S transponder is adequate in order

for ATC to “see” the aircraft while taxiing on the airport surface.

(2) Following landing and clearing of the runway, TCAS should be selected to the “standby”

mode as specified by the air carrier’s operating procedures or local airport surveillance requirements.

(3) During flight, TCAS displays should be used to enhance situational awareness. Displays,

which have a range selection capability, should be used in an appropriate range setting for the phase

of flight. For example, use minimum range settings in the terminal and longer ranges for

climb/descent and cruise as appropriate.

(4) It is appropriate to operate TCAS in the TA-only mode in circumstances where

unnecessary RAs frequently occur and where such RAs are disruptive to the operation of the aircraft.

These circumstances may include:

(a) During takeoff towards known nearby traffic which is in visual contact and which

could cause an unwanted RA during initial climb, such as a visually identified helicopter passing

near the departure end of the runway. The TA/RA mode should be selected after the potential for an

unwanted RA ceases to exist, such as after climbing above a known VFR corridor.

(b) In instrument or visual conditions during approaches to closely-spaced parallel

runways.

(c) In visual conditions, when flying in close proximity to other aircraft.
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(d) At certain airports, during particular procedures, or in circumstances identified by the

operator as having a significant potential for unwanted or inappropriate RAs.

(e) In the event of particular in-flight failures, such as engine failure, as specified by the

Airplane Flight Manual (AFM) or operator.

(f) During takeoffs or landings outside of the nominal TCAS reference performance

envelope for RAs, as designated by the AFM or operator. TCAS reference performance for RAs is

typically attainable during takeoffs and landings at airports within the envelope of ISA ±50° F, sea

level to 5,300 feet MSL. When takeoffs or landings are outside of this envelope, use of “TA only”

may be appropriate during the limited time period when TCAS reference performance cannot be

achieved. This typically occurs when the aircraft is at low speed in specified limiting configurations

during takeoff or landing at “hot day” high altitude airports, such as Mexico City or La Paz.

(g) When participating in Parallel Runway Monitoring (PRM) Operations.

(5) When safe, practical, and in accordance with the carrier’s approved operating procedures,

pilots should limit vertical speeds to 1,500 fpm or less when within 2,000 feet of assigned altitudes.

This procedure will reduce the frequency of unnecessary RAs and be in conformance with the

Aeronautical Information Manual (AIM) and ICAO guidance.

e. Operator Responsibilities. Operators have the following general responsibilities regarding

TCAS:

(1) Assure follow-up and evaluation of unusual TCAS events; and

(2) Periodically assess TCAS training, checking, and maintenance programs to ensure their

correctness, pertinence, timeliness, and effectiveness.

f. ATC Responsibilities. ATC responsibilities relating to TCAS are highlighted below.

(1) Controllers will not knowingly issue instructions that are contrary to RA guidance when

they are aware that a TCAS maneuver is in progress. When an aircraft deviates from its clearance in

response to an RA, ATC is still responsible for providing assistance to the deviating aircraft as

requested until:

(a) The pilot informs ATC that the RA conflict is clear; and

(b) The aircraft has returned to the previously assigned altitude; or

(c) Alternate ATC instructions have been issued and acknowledged.

NOTE: See Appendix 5 for suggested phraseology.

(2) Workload permitting, controllers may continue to provide pertinent traffic information in

accordance with FAA Order 7110.65, Air Traffic Control.

(3) Maintain awareness of TCAS programs and program changes.

(4) Train ATC specialists on TCAS and expected flightcrew responses to TCAS advisories

and provide familiarization flights for specialists on TCAS-equipped aircraft to the extent possible.
AC 120-55B 10/22/01

Page 12 Par 11

(5) When requested by the flightcrew, and if appropriate, provide separation from TCAS

observed traffic and assistance in returning to the assigned clearance. Issue additional clearance

instructions when the situation so requires.

(6) Advise pertinent FAA offices, such as a FSDO, via TCAS questionnaires about airspace

or airports where excessive numbers of RAs occur. This facilitates initiation of corrective actions

related to TCAS enhancements, TCAS procedures, and airspace adjustments. The information

should then be forwarded to AFS-400 in Washington, D.C. for further disposition.

(7) Advise FAA if you become or are aware of other hazardous conditions, situations, or

events which may be related to TCAS.

12. TCAS EVENT REPORTING.

a. General. Operators and manufacturers are encouraged to develop procedures to ensure

effective identification, tracking, and follow-up of significant TCAS-related events, as appropriate.

Such procedures should focus on providing useful information to:

(1) Properly assess the importance of TCAS events;

(2) Follow up on information related to specific TCAS events, as necessary; and

(3) Keep the industry and FAA informed of the performance of TCAS in the NAS and in

international operations.

b. Pilot Reports.

(1) TCAS-Specific Reports. Pilots should make the following reports for TCAS TAs and

RAs, as necessary.

(a) Upon query from ATC, or after a deviation from an ATC clearance, make radio

communications as appropriate to report a response to a TCAS advisory. Refer to section 4 of the

AIM ATC Clearance/Separations for guidance, and Appendix 5 of this AC for recommended

phraseology.

(b) Reports, as specified by the operator, concerning TCAS anomalies, procedural

difficulties, or system failures typically are made by pilots through one or more of the following

methods:

i. Pilot/observer questionnaire;

ii. Logbook entry and Aircraft Communications Addressing and Reporting System

(ACARS), etc.; or

iii. Other record used by that operator, such as a captain’s report. An example of a

typical reporting form for TCAS event information is shown in Appendix 1.
10/22/01 AC 120-55B

Par 12 Page 13

(2) Other Reports Incidental to TCAS.

(a) Near Midair Collision (NMAC) Reports. Flightcrews should continue to submit

NMAC reports in accordance with existing policies and procedures. Crews should be aware that

there is no requirement to submit an NMAC report solely due to a TCAS event and that a TCAS

report does not constitute an NMAC report.

(b) Title 14 CFR § 91.123, Compliance with ATC Clearances and Instructions Reports.

Unless required due to other circumstances, reports in compliance with § 91.123 regarding

emergency deviation from an ATC clearance are not necessary solely as a result of a TCAS

maneuver.

(c) Aviation Safety Reporting System (ASRS) Reports. ASRS reports may be filed at

the discretion of the flightcrew.

c. Maintenance Personnel Reports. Maintenance personnel should report TCAS problems that

relate to system performance, manufacturers, and/or vendors to the appropriate Principal Avionics

Inspector (PAI).

d. FAA ATC. Report the following:

(1) TCAS events to FSDOs, as necessary; and

(2) Any significant TCAS-related events regarding NAS performance to the Air Traffic

Procedures Division, ATP-100, Washington, D.C.

e. TCAS Manufacturer Reports. TCAS avionics manufacturers report problems found with

specific TCAS systems in accordance with established service difficulty report (SDR) procedures.

Generic problems, such as those that may relate to the definition of collision avoidance system

algorithms as defined by RTCA/DO-185, should be reported to the Aircraft Engineering Division,

AIR-100, Washington, D.C.

13. FAA RESPONSE TO TCAS EVENTS. Regulatory compliance issues. The FAA will not

initiate enforcement action solely on the basis of a TCAS event. Letters of investigation will not be

sent to pilots involved in a TCAS-related deviation provided:

a. The aircraft was equipped with TCAS, the system was operable, and the equipment was

turned on at the time of the event;

b. The pilots have properly operated their aircraft in compliance with ATC clearances prior to

the TCAS-related deviation;

c. The pilots have successfully completed their air carrier’s FAA-approved TCAS training

program; and

d. The pilots have otherwise complied with the requirements of § 91.123.
AC 120-55B 10/22/01

Page 14 Par 14

14. FOREIGN AIR CARRIERS.

a. General. Section 129.18 requires TCAS II installation and use by certain foreign air carriers

when operating in U.S. airspace. Foreign air carriers are not required to install and use TCAS for any

aircraft or operations taking place outside of the U.S. 12 nm territorial limit (14 CFR § 71.9), even

though separation services may be provided by a U.S. ATC facility (for example, in oceanic

airspace). Various states may abide by ICAO guidance found in ACAS Standards and

Recommended Practices (SARPS).

(1) TCAS Approval for Foreign Air Carriers. FAA does not approve TCAS installation,

training programs, MELs, or maintenance programs for foreign operators operating non-U.S.

registered aircraft. Such authorizations are addressed as specified by the state of the operator or by

the ICAO. However, since compatibility of TCAS and Mode S transponders with other aircraft and

NAS facilities within U.S. airspace is essential, 14 CFR Part 129 operations guidelines for TCAS are

issued by FAA to all foreign air carrier operations within U.S. airspace. Compliance with these

TCAS provisions ensures both TCAS system and procedural compatibility. The issuance of

OpSpecs, or an amendment to existing OpSpecs for TCAS, must take place before a foreign air

carrier operating a TCAS-equipped aircraft in U.S. airspace, in accordance with § 129.18. Standard

provisions for foreign air carriers for TCAS are shown in Appendix 3.

(2) Guidance. Guidance for foreign air carriers operating under Part 129 applies to nonU.S. registered aircraft as well as U.S.-registered aircraft flying in U.S. airspace. The “plan of State”

of registry for U.S.-registered aircraft operated by foreign operators is, for practical purposes, that of

the U.S. or the same as that for any U.S. operator in order to meet the requirements of § 129.14.

b. Application and Approval. Foreign air carriers should contact their FAA POI to obtain

application information for Part 129 TCAS OpSpecs. When a foreign air carrier submits the

necessary information to the respective PI showing that its aircraft is in compliance, the PI may

approve those OpSpecs or an amendment. Standard OpSpecs provisions regarding TCAS for foreign

air carriers are shown in Appendix 3. Although not mandatory, foreign operator compliance with the

provisions, or equivalent provisions specified by the state of the operator or specified by ICAO, is

expected.

/s/ Louis C. Cusimano (for)

Ava L. Mims

Acting Director, Flight Standards Service
10/22/01 AC 120-55B

Appendix 1

Page 1 (and 2)

APPENDIX 1.

SAMPLE TCAS EVENT REPORTING INFORMATION

TCAS TRANSITION PROGRAM

PILOT EVENT QUESTIONNAIRE (LOGIC VERSION 7.0)

The FAA and the airline industry have established the TCAS Transition Program to assess the operational acceptability of TCAS in the NAS. Your participation in the

program is essential to the success of the program. Please complete and return this questionnaire as quickly as possible. The information contained herein will be used only

by the Program participants to assess the operation of TCAS and will not be released to the public in a manner which allows the identification of you or the airline. The

information WILL NOT be used to initiate or pursue enforcement action against you or the crew. Once the data is entered into the TCAS Transition Programming (TTP)

databases the questionnaire will be maintained for no longer than 60 days, at which time it will be destroyed. Please fill out the form in full and check all that apply.

DATE: _____/_____/_____ TIME: ______________UTC AIRLINE :_______________FLIGHT #: _____________

A/C TYPE: _________________ TAIL NUMBER: ___________ ORIGIN: _________ DESTINATION: _________

OWN ALTITUDE:___________FT OWN POSITION:__________/__________/__________

VOR RADIAL DME

PHASE OF FLIGHT: DEPARTURE (TAKEOFF TO 10,000 FT) CLIMB (10,000 FT TO TOC) CRUISE DESCENT (TOC TO

10,000 FT) APPROACH (BELOW 10,000 FT)

TCAS DISPLAY RANGE SETTING: 5 NM 10 NM 20 NM 30 NM OTHER __________ NM

INTRUDER AIRCRAFT

ID (IF KNOWN):________________ TCAS-EQUIPPED (IF KNOWN)? YES NO

RELATIVE ALTITUDE AT TA:____________FT CLOCK POSITION:____________ EST. RANGE AT TA:____________NM

 WERE MULTIPLE TAS (AURALS) ISSUED ON THE SAME AIRCRAFT? YES NO IF YES HOW MANY? __________

RELATIVE ALTITUDE AT RA:____________FT CLOCK POSITION:____________ EST. RANGE AT RA:____________NM

WERE MULTIPLE RAS ISSUED ON THE SAME AIRCRAFT? YES NO IF YES HOW MANY? __________

ESTIMATED CLOSEST PROXIMITY : RANGE ____________ NM ALTITUDE ____________ NM

INITIAL RA ISSUED: DESCEND, DESCEND CLIMB, CLIMB MONITOR VERTICAL SPEED

 ADJUST VERTICAL SPEED, ADJUST CLIMB, CROSSING CLIMB DESCEND, CROSSING DESCEND

 MAINTAIN VERTICAL SPEED, MAINTAIN MAINTAIN VERTICAL SPEED, CROSSING MAINTAIN

DID THE INITIAL RA CHANGE ANY OF THE FOLLOWING?

 ADJUST VERTICAL SPEED, ADJUST INCREASE CLIMB INCREASE DESCENT

 CLIMB, CLIMB NOW DESCEND, DESCEND NOW

TYPE OF RA DISPLAY IN YOUR AIRCRAFT: IVSI PITCH CUES VERTICAL SPEED TAPE

FOR THE ENCOUNTER, PLEASE INDICATE THE SEQUENCE OF EVENTS (e.g., ATC ADVISORY 1, VISUAL CONTACT 2, TA 3, RA 4)

ATC ADVISORY:__________ TA:__________ RA:_________ VISUAL CONTACT:__________

WAS THE RA:

FOLLOWED: YES NO NECESSARY FOR THE SITUATION: YES NO

CONFLICT WITH ATC INSTRUCTIONS: YES NO RESULT IN A CLEARANCE DEVIATION: YES NO

IF YES, WHAT WAS THE DEVIATION? ___________________FT

REMARKS: (PLEASE PROVIDE COMMENTS/CONCERNS ON THIS ENCOUNTER)

_______________________________________________________________________________________________________________

_______________________________________________________________________________________________________________

_______________________________________________________________________________________________________________

10/22/01 AC 120-55B

Appendix 2

Page 1

APPENDIX 2.

TCAS MINIMUM EQUIPMENT LIST (MEL) AND MASTER MINIMUM

EQUIPMENT LIST (MMEL) PROVISIONS

1. Each operator with authority to dispatch an aircraft with a TCAS system or component

temporarily inoperative must do so in accordance with provisions of a MEL. MELs are approved for

each operator and type of aircraft, within provisions of the FAA MMEL for that type. When

proposed MEL provisions are consistent with the FAA MMEL, POIs may approve the MEL. If a

less restrictive MEL or different MEL provisions are requested, a proposal for consideration of an

FAA MMEL change must be forwarded to the AEG assigned for that aircraft type. No relief will be

granted for the voice command portion of the TCAS system when functioning in the TA-only mode.

The audio will be provided via a speaker, which may also service windshear and ground proximity

equipment. Enhanced features (those above and beyond the basic TCAS system) may be inoperative

provided that the inoperative features do not degrade the system (for example, Flight Level traffic

altitude selection feature for a traffic display).

2. The provisos and repair category intervals were intended to grant the operator sufficient relief,

especially during the early stage of the TCAS transition, in order to promote the installation process,

as well as support the use of a practical system. Both equipment reliability and operational

experience dictated if any revision to this MMEL relief was to be considered.

3. On April 6, 2000, PL-32, Revision 5 was issued as updated guidance to the Flight Operations

Evaluation Board (FOEB) for standardized MMEL relief for the TCAS. Policy for TCAS II relief

was outlined in the original PL-32 in the automated MMEL system. The most significant change in

Revision 5 is to change the relief category from “C” to “B” for the entire system when required by

14 CFR.

The following list is the standard set of provisos for all aircraft for which relief is granted for the

TCAS I and II systems:
AC 120-55B 10/22/01

Appendix 2

Page 2

APPENDIX 2. (continued)

TCAS MINIMUM EQUIPMENT LIST (MEL) AND MASTER

MINIMUM EQUIPMENT LIST (MMEL) PROVISIONS

Traffic Alert and B | - | O | (M) (O) May be inoperative provided:

Collision Avoidance a) System is deactivated and secured; and

System (TCAS I) b) Enroute or approach procedures do not 

require its use.

C | - | O | (M) (O) May be inoperative provided:

a) Not required by 14 CFR;

b) System is deactivated and secured; and

c) Enroute or approach procedures do not 

require its use.

Traffic Alert and B | - | O | (M) May be inoperative provided:

Collision Avoidance a) System is deactivated and

System (TCAS II) secured; and

b) Enroute or approach procedures do not

require its use.

C | - | O | (M) (O) May be inoperative provided:

a) Not required by 14 CFR;

b) System is deactivated and

secured; and

c) Enroute or approach procedures

do not require its use.

1) Combined Traffic C | 2 | 1 | May be inoperative on the non-flying pilot

Alert (TA) and side provided:

Resolution Advisory (RA) a) TA and RA visual display is operative on

Dual Display System(s) the flying pilot side; and

b) TA and RA audio function is operative

on the flying pilot side.

2) Resolution Advisory (RA) C | 2 | 1 | May be inoperative on non-flying

Display System(s) pilot side.

C | - | O | (O) May be inoperative provided:

a) TA visual display and

audio functions are operative;

b) TA-only mode is selected by the crew;

and

c) Enroute or approach procedures do not

require its use.
10/22/01 AC 120-55B

Appendix 2

Page 3 (and 4)

APPENDIX 2. (continued)

TCAS MINIMUM EQUIPMENT LIST (MEL) AND MASTER MINIMUM

EQUIPMENT LIST (MMEL) PROVISIONS

3) Traffic Alert C | - | O | (O) May be inoperative provided:

Display System(s) a) RA visual display and audio functions

are operative; and

b) Enroute or approach procedures do not

require its use.
10/22/01 AC 120-55B

Appendix 3

Page 1 (and 2)

APPENDIX 3.

14 CFR 129 PROVISIONS FOR USE OF TCAS IN U.S. AIRSPACE

1. An appropriate Mode S transponder must be installed and operated on a suitable code specified by

ATC during flight in U.S. airspace. In addition, a valid unique aircraft-specific Mode S address must

be assigned to the airplane, and the Mode S transponder must be set to this address. Valid addresses

are those consistent with the ICAO Mode S address allocation plan contained in appendix C, part I,

ICAO Annex 10, and plan of the State of registry for the specific aircraft. The unique address, when

properly set, may not be altered, set to a duplicated address, or set to an address that potentially

interferes with ATC or TCAS safety functions (for example, must not be set to all “ones” or all

“zeros,” or the country address must not be set without the unique aircraft specific address). This

guidance is appropriate for non-U.S. registered or U.S.-registered aircraft operated by a foreign air

carrier in U.S. airspace.

2. A TCAS II System capable of coordinating with TCAS units meeting TSO-C119A or

TSO-C119B must be installed. Except as provided for by MEL provisions acceptable to the State of

the operator, the TCAS system must be operated in an appropriate TCAS mode during flight in U.S.

airspace.

3. Training and procedures for use of TCAS as specified by ICAO, this AC, or other equivalent

criteria acceptable to FAA must be used when operating in U.S. airspace.

4. Unsafe conditions or performance related to TCAS operation which potentially could affect

continued safe operations in the U.S. NAS must be reported to the FAA POI within 10 business days

of the time that such a hazard is identified.

10/22/01 AC 120-55B

Appendix 4

Page 1

APPENDIX 4.

ACRONYMS AND ABBREVIATIONS

14 CFR Title 14 of the Code of Federal Regulations

ACARS Aircraft Communications Addressing and Reporting System

ACAS Airborne Collision Avoidance System (with Change 7.0)

AC Advisory Circular

ACO Aircraft Certification Office

AD Airworthiness Directives

AEG Aircraft Evaluation Group

AFM Airplane Flight Manual

AFS Flight Standards Service (FAA)

AGL Above Ground Level

AHRS Attitude Heading Reference System

AIM Aeronautical Information Manual

AIR Aircraft Certification Service (FAA)

ALIM Altitude Limit

ASRS Aviation Safety Reporting System

ATCRBS ATC Radar Beacon System

ATC Air Traffic Control

ATP Air Traffic Procedures Division (FAA)

CBI Computer-Based Instructional System

CHDO Certificate-Holding District Office

CPA Closest Point of Approach

CRM Crew Resource Management

EFIS Electronic Flight Instrument System

FAA Federal Aviation Administration

FOEB Flight Operation Evaluation Board

FPM Feet Per Minute

FSB Flight Standards Board

FSDO Flight Standards District Office

FTD Flight Training Device

GPWS Ground Proximity Warning System

ICAO International Civil Aviation Organization

IFR Instrument Flight Rules

IRS Inertial Reference System

ISA International Standard Atmosphere

LOFT Line-Oriented Flight Training

MEL Minimum Equipment List

MMEL Master Minimum Equipment List

MRB Maintenance Review Board

MSL Mean Sea Level

NAS National Airspace System

nm Nautical Mile

NMAC Near Midair Collision

NSET National Simulator Evaluation Team

OE Operating Experience

AC 120-55B 10/22/01

Appendix 4

Page 2

OPSPECS Operations Specifications

PAI Principal Avionics Inspector

PF Pilot Flying

PI Principal Inspector

PMI Principal Maintenance Inspector

PNF Pilot-not-Flying

POI Principal Operations Inspector

PRM Parallel Runway Monitor

PT Proficiency Training

QFE Altimeter Reference to Zero Feet at Field Elevation

RA Resolution Advisory

RTCA Requirements for Technical Concepts for Aviation

SARPS Standards and Recommended Practices (ICAO)

SB Service Bulletin

SDR Service Difficulty Report

STC Supplemental Type Certificate

TA Traffic Advisory

TC Type Certificate

TCAS Traffic Alert and Collision Avoidance System

TTP TCAS Transition Programming

TSO Technical Standard Order

VFR Visual Flight Rules

VSI Vertical Speed Indicator

ZTHR Altitude Threshold
10/22/01 AC 120-55B

Appendix 5

Page 1 (and 2)

APPENDIX 5.

PHRASEOLOGY FOR TCAS EVENTS

In order to keep everyone informed during a TCAS maneuver, radio communication should be in

terminology common to all parties on the frequency regarding a TCAS RA. The following

phraseology is suggested and should contain: (1) name of the ATC facility, (2) aircraft identification

(ID), and (3) nature of the TCAS deviation.

When a flightcrew receives a TCAS RA to either climb or descend from their assigned altitude,

or the RA otherwise affects their ATC clearance or their pending maneuver or maneuver in progress,

the crew should inform ATC when beginning the excursion from clearance or as soon as workload

allows in the following manner:

“XYZ Center, (Aircraft ID), TCAS Climb/Descent”

EXAMPLE:

“New York Center, Quantum 321, TCAS Climb”

“Cleveland Center, Universal 602, TCAS Descent”

Following such a communication, the designated air traffic facility is not required to provide

approved standard separation to the TCAS maneuvering aircraft until the TCAS encounter is cleared

and standard ATC separation is achieved. If workload permits, traffic information should be

provided in accordance with FAA Order 7110.65M.

When the RA is clear, the flightcrew should advise ATC that they are returning to their

previously assigned clearance or subsequent amended clearance in the following manner:

“ABC Center, (Aircraft ID), clear of conflict, returning to assigned altitude.”

EXAMPLE:

“Boston Center, Northern 429, clear of conflict, returning to assigned altitude.”

When the deviating aircraft has renegotiated its clearance with ATC, the designated air traffic

facility is expected to resume providing appropriate separation services in accordance with FAA

Order 7110.65M.

NOTE: Communication is not required if the pilot is able to satisfy the RA

guidance and maintain the appropriate ATC clearance.

10/22/01 AC 120-55B

Appendix 6

Page 1

APPENDIX 6.

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

Introduction.

This appendix provides a set of training standards for TCAS II pilot training. The information

contained in this appendix related to TAs is also applicable to TCAS I users. Training standards are

divided into the areas of instruction required for ground training (academics) and the performance

objectives for the maneuvers required in flight training.

1. TCAS GROUND TRAINING. Ground training must cover the following areas:

a. General Concepts of TCAS Operation. TCAS ground training should cover, in general terms,

TCAS theory to the extent appropriate to ensure proper operational use. Aircrews should understand

basic concepts of TCAS logic, CPA, tau, altitude separation thresholds for the issuance of RAs, as well

as the relationship between displayed traffic information and issuance of TAs and RAs. The ground

training program should address the following:

(1) The meaning of TAs and preventive versus corrective RAs;

(2) Increase, reversal, crossing, and weakened RAs;

(3) That TCAS II assures separation from Mode C equipped aircraft;

(4) The detection and protection provided by TCAS against altitude reporting and nonaltitude

reporting intruders;

(5) That the system detects multiple aircraft;

(6) TCAS-to-TCAS coordination;

(7) The potential impact of not following RAs;

(8) TCAS surveillance range versus display range;

(9) When an intruder will not be displayed;

(10) TCAS on ground performance; and

(11) The continued applicability of the see-and-avoid concept.

b. Expected Flightcrew Response and Level of Protection Provided by TCAS. Academic

training should explain the normal, expected pilot response to TAs, RAs, use of displayed traffic

information to establish visual contact, and constraints on maneuvering based solely on TAs.

c. TCAS General Limitation. There are several system, operational, and/or performance

limitations which should be understood that apply to all aircraft types. System limitations include the

inability of TCAS to detect nontransponder equipped aircraft, no RAs issued for traffic without an
AC 120-55B 10/22/01

Appendix 6

Page 2

APPENDIX 6. (continued)

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

altitude reporting transponder, etc. Operational limitations include some RA inhibit altitudes, certain

RAs being inhibited by aircraft performance constraints, the inability to comply with an RA due to

aircraft performance limitations after an engine failure, and appropriate response to RAs in limiting

performance conditions, such as during heavy weight takeoff or while en route at maximum altitude

for a particular weight.

d. ATC Communication and Coordination. Training should discuss communication and

coordination with ATC related to or following a TCAS event, when to contact ATC, and accepted

TCAS phraseology.

e. TCAS Equipment Components Controls, Displays, Audio Alerts, and Annunciations.

Academic training should include a discussion of TCAS terminology, symbology, operation, and

optional controls and display features, including any items particular to an air carrier’s

implementation or unique to its system.

f. Interfaces and Compatibility with Other Aircraft Systems. Training should discuss the

role of the Mode S transponder with a correct, discreet address installed, radar altimeter inputs to

TCAS, and weather radar/EFIS interfaces, including any items particular to an air carrier’s

implementation or unique to its system.

g. Aircraft Flight Manual (AFM) Information. AFM provisions should be addressed,

including information on TCAS modes of operation; normal and atypical flightcrew operating

procedures; and response to TAs, RAs, and any AFM limitations.

h. MEL operating provisions.

i. Appropriate pilot response to TCAS RAs and TAs, ATC clearance compliance, nuisance

alerts, and other such issues.

j. The air carrier’s TCAS event reporting policies for flightcrews.

k. Flightcrew procedures for reporting TCAS malfunctions or irregularities, if not otherwise

addressed by routine maintenance procedures of that operator.

2. Classroom Training. An understanding of TCAS operation and the criteria used for issuing TAs

and RAs may be assessed using the following: Objectives and Criteria. This training should address

the following topics:

a. System Operation.

(1) Objective: Demonstrate knowledge of how TCAS functions.

(2) Criteria: The pilot must demonstrate an understanding of the following functions:
10/22/01 AC 120-55B

Appendix 6

Page 3

APPENDIX 6. (continued)

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

(a) Surveillance:

i. TCAS interrogates other transponder-equipped aircraft within a nominal range of

14 nautical miles (nm).

ii. TCAS surveillance range can be reduced in geographic areas with a large number

of ground interrogators and/or TCAS II equipped aircraft.

(b) Collision Avoidance:

i. TAs can be issued against any transponder-equipped aircraft which responds to the

ICAO Mode C interrogations, even if the aircraft does not have altitude reporting capability.

ii. RAs can be issued only against aircraft that are reporting altitude and in the

vertical plane only.

iii. RAs issued against a TCAS-equipped intruder are coordinated to ensure

complementary RAs are issued.

b. Advisory Thresholds.

(1) Objective: Demonstrate knowledge of the criteria for issuing TAs and RAs.

(2) Criteria: The pilot must be able to demonstrate an understanding of the methodology

used by TCAS to issue TAs and RAs and the general criteria for the issuance of these advisories to

include:

(a) TCAS advisories are based on time to closest point of approach (CPA) rather than

distance. The time must be short and vertical separation must be small, or projected to be small,

before an advisory can be issued. The separation standards provided by Air Traffic Services are

different from the miss distances against which TCAS issues an alert.

(b) Thresholds for issuing a TA or RA vary with altitude. The thresholds are larger at

higher altitudes.

(c) The TA tau threshold (trigger point) varies from 15 to 48 seconds before the

projected CPA and the RA tau threshold varies from 15 to 35 seconds.

(d) RAs are chosen to provide the desired vertical miss distance at CPA. As a result,

RAs can instruct a climb or descent through the intruder aircraft’s altitude.

c. TCAS Limitations.

(1) Objective: To verify the pilot is aware of the limitations of TCAS.

AC 120-55B 10/22/01

Appendix 6

Page 4

APPENDIX 6. (continued)

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

(2) Criteria: The pilot must demonstrate a knowledge and understanding of the TCAS

limitations including:

(a) TCAS will neither track nor display non-transponder equipped aircraft, nor aircraft

not responding to TCAS Mode C interrogations.

(b) TCAS will automatically fail if the input from the aircraft’s barometric altimeter,

radio altimeter, or transponder is lost.

NOTE: In some installations, the loss of information from other onboard

systems such as an Inertial Reference System (IRS) or Attitude Heading

Reference System (AHRS) may result in a TCAS failure. Individual operators

should ensure their pilots are aware of what types of failures will result in a

TCAS failure.

(c) An intruder aircraft within 380 feet AGL (nominal value) may or may not be

displayed by your TCAS (i.e., declared to be airborne or on the ground, respectively) depending upon

whether the intruder is Mode S or ATCRBS Mode C equipped and whether your TCAS-equipped

aircraft is airborne or on the ground.

(d) TCAS may not display all proximate transponder-equipped aircraft in areas of high

density traffic.

(e) Because of design limitations, the bearing displayed by TCAS is not sufficiently

accurate to support the initiation of horizontal maneuvers based solely on the traffic display.

(f) Because of design limitations, TCAS will not track intruders with a vertical speed in

excess of 10,000 fpm. In addition, the design implementation may result in some short term errors in

the tracked vertical speed of an intruder during periods of high vertical acceleration by the intruder.

(g) Ground Proximity Warning System (GPWS) warnings and windshear warnings take

precedence over TCAS advisories. When either a GPWS or windshear warning is active, TCAS

aural annunciations will be inhibited.

d. TCAS Inhibits.

(1) Objective: To verify the pilot is aware of the conditions under which certain functions of

TCAS are inhibited.

(2) Criteria: The pilot must demonstrate a knowledge and understanding of the various

TCAS inhibits including:

(a) Increase Descent RAs are inhibited below 1,450 (±100) feet AGL.

(b) Descend RAs are inhibited below 1,100 (±100) feet AGL.
10/22/01 AC 120-55B

Appendix 6

Page 5

APPENDIX 6. (continued)

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

(c) All RAs are inhibited below 1,000 (±100) feet.

(d) All TCAS aural annunciations are inhibited below 500 (±100) feet AGL. This

includes the aural annunciation for TAs.

(e) Altitude and configuration under which Climb and Increase Climb RAs are inhibited.

Know if your aircraft type issues Climb and Increase Climb RAs when operating at the aircraft’s

certified ceiling. If your aircraft type provides RA Climb and Increase Climb commands at certified

ceiling, the commands are to be followed.

NOTE: In some aircraft types, Climb or Increase Climb RAs are never

inhibited.

e. Use of Controls.

(1) Objective: To verify the pilot can properly operate all TCAS and display controls.

(2) Criteria: Demonstrate the proper use of controls including:

(a) Aircraft configuration required to initiate a Self Test.

(b) Steps required to initiate a Self Test.

(c) Recognizing when the Self Test was successful and when it was unsuccessful. When

the Self Test is unsuccessful, recognizing the reason for the failure, and if possible, correcting the

problem.

(d) Recommended usage of range selection. Low ranges are used in the terminal area

and the higher display ranges are used in the en route environment and in the transition between the

terminal and en route environment.

(e) If available, recommended usage of the Above/Below mode selector. Above mode

should be used during climb and the Below mode should be used during descent.

(f) Recognition that the configuration of the display does not affect the TCAS

surveillance volume.

(g) Selection of lower ranges when an advisory is issued to increase display resolution.

(h) If available, selection of the display of absolute altitude instead of relative altitude

and the limitations of using this display if a barometric correction is not provided to TCAS.

(i) Proper configuration to display the appropriate TCAS information without

eliminating the display of other needed information.
AC 120-55B 10/22/01

Appendix 6

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APPENDIX 6. (continued)

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

NOTE: The wide variety of display implementations make it difficult to

establish more definitive criteria. When the training program is developed, this

general criteria should be expanded to cover specific details for an operator’s

specific display implementation.

f. Display Interpretation.

(1) Objective: To verify a pilot understands the meaning of all information that can be

displayed by TCAS.

(2) Criteria: The pilot must demonstrate the ability to properly interpret information

displayed by TCAS including:

(a) Other traffic, i.e., traffic within the selected display range that is not proximate

traffic, or causing a TA or RA to be issued.

(b) Proximate traffic, i.e., traffic that is within 6 nm and ±1200 feet.

(c) Non-altitude reporting traffic.

(d) No bearing TAs and RAs.

(e) Off-scale TAs and RAs. The selected range should be changed to ensure that all

available information on the intruder is displayed.

(f) Traffic advisories. The minimum available display range which allows the traffic to

be displayed should be selected to provide the maximum display resolution.

(g) Resolution advisories (Traffic Display). The minimum available display range of the

traffic display which allows the traffic to be displayed should be selected to provide the maximum

display resolution.

(h) Resolution advisories (RA Display). Pilots should demonstrate knowledge of the

meaning of the red and green areas displayed on the RA display and when the green areas will and

will not be displayed. Pilots should also demonstrate an understanding of the RA display limitations

(i.e., if a vertical speed tape is used and the range of the tape is less than 2,500 fpm, an Increase Rate

RA cannot be properly displayed).

(i) If appropriate, awareness that Navigation Displays oriented on Track-Up may require

a pilot to make a mental adjustment for drift angle when assessing the bearing of proximate traffic.

NOTE: The wide variety of display implementations will require the tailoring of

some criteria. When the training program is developed, these criteria should be

expanded to cover details for an operator’s specific display implementation.
10/22/01 AC 120-55B

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APPENDIX 6. (continued)

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

g. Use of the TA-Only Mode.

(1) Objective: To verify that a pilot understands the appropriate times to select the TA-only

mode of operation and the limitations associated with using this mode.

(2) Criteria: The pilot must demonstrate the following:

(a) Knowledge of the operator’s guidance for the use of TA-only.

(b) Reasons for using this mode and situations in which its use may be desirable. If

TA-only is not selected when an airport is conducting simultaneous operations from parallel runways

separated by less than 1,200 feet, and to some intersecting runways, RAs can be expected.

(c) The TA aural annunciation is inhibited below 500 feet AGL. As a result, TAs issued

below 500 feet AGL may not be noticed unless the TA display is included in the routine instrument

scan.

(d) When this mode is selected, TAs will be issued at the time an RA is normally issued.

h. Crew Coordination.

(1) Objective: To verify the pilot adequately briefs other crew members on how TCAS

advisories will be handled.

(2) Criteria: The pilot must demonstrate their preflight briefing addresses the procedures

that will be used in responding to TAs and RAs including:

(a) Division of duties between PF and PNF.

(b) Expected call-outs.

(c) Communications with ATC.

(d) Conditions under which an RA may not be followed and who will make this

decision.

NOTES:

1. Different operators have different procedures for conducting preflight

briefings and for responding to TCAS advisories. These factors should be taken

into consideration when implementing the training program.

2. The operator must specify the conditions under which an RA need not be

followed, reflecting advice published by the State Civil Aviation Authority. This

should not be an item left to the discretion of a crew.
AC 120-55B 10/22/01

Appendix 6

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APPENDIX 6. (continued)

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

3. This portion of the training may be combined with other training such as

CRM.

i. Reporting Requirements.

(1) Objective: To verify the pilot is aware of the requirements for reporting RAs to the

controller and other authorities.

(2) Criteria: The pilot must demonstrate the following:

(a) The use of the phraseology contained in PANS-RAC (ICAO DOC. 4444).

(b) Where information can be obtained regarding the need for making written reports to

various states when an RA is issued. Various states have different reporting requirements and the

material available to the pilot should be tailored to the airline’s operating environment.

j. TCAS Flight Training (Maneuver). The scenarios included in the maneuver training should

include corrective RAs, initial preventive RAs, maintain rate RAs, altitude crossing RAs, increase

rate RAs, RA reversals, weakening RAs, and multi-aircraft encounters. Training must provide pilots

the opportunity to reach the TCAS proficiency indicated in the following TA and RA response

objectives. This proficiency may be assessed and certified by a TCAS qualified instructor.

k. TA Responses.

(1) Objective: To verify the pilot properly interprets and responds to TAs.

(2) Criteria: The pilot must demonstrate the following:

(a) Proper division of responsibilities between the PF and PNF. The PF should continue

to fly the airplane, and be prepared to respond to any RA that might follow. The PNF should provide

updates on the traffic location shown on the TCAS display, using this information to help visually

acquire the intruder.

(b) Proper interpretation of the displayed information. Both pilots confirm that the

aircraft they have visually acquired is that which has caused the TA to be issued. Use should be

made of all information shown on the display, note being taken of the bearing and range of the

intruder (amber circle), whether it is above or below (data tag), and its vertical speed direction (trend

arrow).

(c) Other available information is used to assist in visual acquisition. This includes ATC

party-line information, traffic flow in use, etc.

(d) Because of the limitations that may exist with various display systems, the PF should

not maneuver the aircraft based solely on the information shown on the TCAS display. No attempt

should be made to adjust the current flight path in anticipation of what an RA would advise.
10/22/01 AC 120-55B

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APPENDIX 6. (continued)

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

(e) When visual acquisition is attained, right-of-way rules are used to maintain or attain

safe separation. No unnecessary maneuvers are initiated. The limitations of making maneuvers

based solely on visual acquisition, especially at high altitude or without a definite horizon, are

understood.

l. RA Responses.

(1) Objective: To verify the pilot properly interprets and responds to RAs.

(2) Criteria: The pilot must demonstrate the following:

(a) Proper division of responsibilities between the PF and PNF. The PF responds to the

RA with positive control inputs, when required, while the PNF provides updates on the traffic

location and cross-checks between the traffic display and monitors the response to the RA. Proper

CRM should be used.

(b) Proper interpretation of the displayed information. The pilot recognizes the intruder

causing the RA to be issued (red square on display). Pilot responds appropriately.

(c) For corrective RAs, the response is initiated in the proper direction within 5 seconds

of the RA being displayed.

(d) Recognition of the initially displayed RA being modified. Response to the modified

RA is properly accomplished.

i. For Increase Rate RAs, the vertical speed is increased within 2 1/2 seconds of the

RA being displayed.

ii. For RA reversals, the vertical speed is reversed within 2 1/2 seconds of the RA

being displayed.

iii. For RA weakenings, the vertical speed is modified to initiate a return towards the

original clearance within 2 1/2 seconds of the RA being displayed.

iv. For RAs which strengthen, the vertical speed is modified to comply with the

revised RA within 2 1/2 seconds of the RA being displayed.

(e) Recognition of altitude crossing encounters and the proper response to these RAs.

(f) For preventive RAs, the vertical speed needle remains outside the red area on the RA

display.

(g) For Maintain Rate RAs, the vertical speed is not reduced. Pilots should recognize

that a Maintain Rate RA may result in crossing through the intruder’s altitude.
AC 120-55B 10/22/01

Appendix 6

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APPENDIX 6. (continued)

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

(h) If a decision is made to not follow an RA, no changes in the existing vertical speed

are made in a direction opposite to the sense of the displayed RA. Pilots should be aware that if the

intruder is also TCAS-equipped, the decision to not follow an RA may result in a decrease in

separation at CPA because of the intruder’s RA response.

(i) When the RA weakens, pilot initiates a return towards the original clearance, and

when Clear of Conflict is annunciated, pilot completes the return to the original clearance.

(j) The controller is informed of the RA as soon as time and workload permit, using the

standard phraseology.

(k) When possible, an ATC clearance is complied with while responding to an RA. For

example, if the aircraft can level at the assigned altitude while responding to a Reduce Climb or

Reduce Descent RA, it should be done.

(l) If pilots simultaneously receive instructions to maneuver from ATC and an RA which

are in conflict, the pilot should follow the RA.

(m) Knowledge of the TCAS multi-aircraft logic and its limitations. For example,

TCAS only considers intruders which it believes to be a threat when selecting an RA. As such, it is

possible for TCAS to issue an RA against one intruder which results in a maneuver towards another

intruder that is not classified as a threat. If the second intruder becomes a threat, the RA will be

modified to provide separation from that intruder.

(n) The consequences of both responding to, and not responding to, an RA.

m. Characteristics of Training Equipment Suitable for Maneuver Training.

(1) Acceptable Characteristics. Flight training devices, simulators, and CBIs must have

certain characteristics to be effective. This is due to the interactive nature of TCAS, the variety of

encounter scenarios possible, the immediate and standardized pilot response required, and the instant

and correct display interpretation that is necessary. Thus, training equipment used for TCAS flight

training should have the following characteristics:

(a) The ability to functionally represent TCAS displays, controls, indications, and

annunciations;

(b) Ability to depict selected traffic encounter scenarios, including TCAS display and

audio advisories;

(c) Ability to show proper TCAS reaction to depicted scenarios and advisories; and

(d) Ability to interactively respond to pilot inputs regarding TCAS advisories, including

responses to RAs displayed on relevant vertical speed and pitch indicators.
10/22/01 AC 120-55B

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APPENDIX 6. (continued)

TCAS II GROUND AND FLIGHT TRAINING REQUIREMENTS

(2) Simulator and TCAS Fidelity. For a particular TCAS, maneuver training may be

accomplished in simulators or training devices that represent the specific aircraft or an aircraft that

has similar characteristics. For the purposes of TCAS maneuver training, simulators or training

devices may use simplified TCAS algorithms or displays and do not require TCAS logic or a TCAS

processor. TCAS displays do not have to be identical, but must be functionally equivalent to the air

carrier operator’s specific aircraft in use.

(3) Training Device or Simulator Approval. Training devices or simulators meeting FAA

criteria are qualified by the NSET and approved for use by the POI. Any one or combination of the

following devices or simulators which meet characteristics of paragraph m(1) may be used:

(a) Level A through D simulators;

(b) Level 2 through 7 FTDs; or

(c) Dedicated TCAS training devices acceptable to the FAA, including those devices

described in FAA Order 8400.10, Air Transportation Operations Inspector’s Handbook, volume 3,

paragraph 443, Aircraft Systems Integration Training, which are shown to be suitable for TCAS

training and approved by the POI.

NOTE: Flight Training Devices and Simulators are as defined and qualified in

accordance with AC 120-45 and approved by the POI.
10/22/01 AC 120-55B

Appendix 7

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APPENDIX 7. DEFINITIONS

1. Aircraft Certification Office (ACO). FAA offices responsible for determining an aircraft’s

airworthiness regarding issuance of TCs, STCs, issues related to 14 CFR Parts 21, 23, 25, 33, and

other similar airworthiness rules. ACOs are also responsible for technical assessment of service

difficulties including issuance of Airworthiness Directives (AD).

2. Aircraft Evaluation Group (AEG). FAA offices responsible for operational aspects of

newly-certificated, modified, or in-service aircraft. AEGs establish FAA criteria for pilot

qualification, MELs, initial airworthiness, other such requirements as they relate to 14 CFR Parts 43,

61, 91, 121, 135, and other operationally related regulations.

3. Altitude Limit (ALIM). ALIM is the specified projected amount of vertical separation that

TCAS is designed to provide between aircraft. It is also the altitude threshold (ZTHR) for the

issuance of a corrective RA. The value of ALIM varies with aircraft barometric altitude.

4. Altitude Threshold (ZTHR). The projected minimum ZTHR for the issuance of a preventive

RA. The ZTHR varies with aircraft altitude.

5. Certificate-Holding District Office (CHDO). A Flight Standards office that is responsible for

administration of a Parts 121 and 135 operating certificate for a particular operator.

6. Closest Point of Approach (CPA). The occurrence of minimum range between your own TCAS

aircraft and the intruder.

7. Coordination (as related to TCAS). The process by which TCAS units in conflicting aircraft

communicate with one another to select complementary RAs to resolve an encounter (for example,

one unit selects a “climb” command and the other a “descend” command).

8. Flight Standards District Office (FSDO). An FAA field office serving an assigned

geographical area and staffed with Flight Standards personnel who serve the aviation industry and

the general public on matters relating to the certification and operation of air carriers and general

aviation aircraft.

9. Flight Standardization Board (FSB). The FAA board responsible for establishing or revising

crew qualification requirements (for example, training, checking, currency, and type rating(s)) for

specific aircraft. FSBs are established for each large turbojet, turboprop, and special

14 CFR Part 41 airplane type used in air transportation, as well as other 14 CFR Part 25 airplanes,

transport category multiengine helicopters, and large multiengine piston airplanes.

10. Follow-on STC (As related to TCAS). A TCAS STC other than as described in paragraph 8.

The following examples are considered to be follow-on STCs:

a. A previously approved TCAS II installation, which is installed in a subsequent type or model

aircraft; and

b. Changes of display configuration (weather radar/TCAS display), supporting system (Radar

Altimeter), or other aircraft interface (GPWS, etc.).
AC 120-55B 10/22/01

Appendix 7

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APPENDIX 7. DEFINITIONS (continued)

11. Initial TC/STC (As related to TCAS). The first FAA TCAS airworthiness approval in

accordance with a TC or STC of any one or combination of the following components: a TCAS

processor (computer), TCAS directional antenna, and/or Mode S transponder.

NOTE: For previously approved TCAS systems, if the part number of any of

the above components changes due to a significant modification, the system

change requires an initial TC/STC.

12. Intruder. An aircraft which has satisfied the TCAS traffic advisory detection criteria.

13. Master Minimum Equipment List (MMEL). An FAA document listing stipulations in

accordance with 14 CFR §§ 121.628 and 135.180 that provides authorization for the continuation of

flight beyond a terminal point with certain inoperative equipment. AEGs develop MMELs in

conjunction with an FOEB that is established for each aircraft type. FAA MMELs serve as the basis

for an operator to develop specific MELs applicable to its particular aircraft and operational

requirements.

14. Maintenance Review Board (MRB). An FAA board responsible for establishing maintenance

requirements for a specific aircraft type. MRB requirements are usually formulated in conjunction

with information provided by the manufacturer and prospective operators through industry working

groups. FAA CHDOs apply MRB requirements in reviewing and approving each carrier’s proposed

maintenance program.

15. National Airspace System (NAS). The common network of U.S. airspace; air navigation

facilities, equipment and services, airports or landing areas; aeronautical charts, information, and

services; rules, regulations, and procedures; technical information, manpower, and material; and

system components shared jointly with the military.

16. National Simulator Evaluation Team (NSET). Team of FAA operations specialists

responsible for evaluating flight simulators to aid POI in approving those simulators in accordance

with 14 CFR requirements. For example, NSET evaluations support POI approval of a particular

simulator for use in a specific Part 121 program.

17. Other Traffic. Aircraft ± 1,200 feet vertical and 6 nm from own aircraft, which are neither an

RA nor a TA.

18. Principal Inspector (PI). Refers to one of the following three FAA inspectors who is selected

as primary for TCAS for an air carrier (normally the POI):

a. Principal Avionics Inspector (PAI). The FAA inspector responsible for overseeing all

avionics issues relative to a specific operator, to include input on training programs, OpSpecs, MEL

requests, etc.

b. Principal Operations Inspector (POI). The FAA inspector responsible for overseeing all

operational issues relative to a specific operator, including training programs, OpSpecs approval,

MEL approval requests, etc.
10/22/01 AC 120-55B

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APPENDIX 7. DEFINITIONS (continued)

c. Principal Maintenance Inspector (PMI). The FAA inspector responsible for overseeing all

maintenance issues relative to a specific operator, to include input on training programs, OpSpecs,

MEL requests, etc.

19. Proximate Traffic. An aircraft which is within 6 nm in range and ± 1,200 feet vertically, but

does not meet the TCAS thresholds of a TA or RA.

20. Resolution Advisory (RA). Aural voice and display information provided by TCAS to a

flightcrew, advising that a particular maneuver should or should not be performed to attain or

maintain minimum safe vertical separation from an intruder.

a. Corrective RA. An RA that advises the pilot to either deviate from current vertical speed,

such as climb when the aircraft is level or to maintain an existing climb or descent rate.

b. Altitude Crossing RA. An RA that directs a pilot to cross through the intruder aircraft’s

altitude to achieve safe vertical separation.

c. Coordinated Crossing RA. Initial RAs that are coordinated between TCAS-equipped aircraft

and that direct the aircraft to cross in altitude.

d. Increase Rate RA. An RA that is issued after an initial climb or descend RA, which indicates

that additional climb or descent rate is required to achieve safe vertical separation. An increase rate

RA requires a climb or descent rate of 2,500 fpm.

e. Preventive RA. An RA that requires a pilot to avoid certain deviations from current vertical

rate, such as a Don’t Climb RA when the aircraft is level.

f. Reversal RA. An RA that indicates a change in the direction previously issued by TCAS and

is required to achieve safe vertical separation. For example, an Initial Descend RA that reverses to a

climb RA.

g. Turn Around RA. An initial corrective RA issued to a TCAS aircraft with an established

vertical rate that directs the aircraft in the opposite direction. For example, a -1,500 fpm Descend

RA issued while climbing at +2,000 fpm.

h. Weakened RA. An initial corrective RA that changes to an RA requiring less vertical speed

because TCAS has determined that safe vertical separation is projected. For example, a Climb RA

that weakens to a Do-Not-Descend advisory.

21. Significant TCAS Events. Significant TCAS events are those which meet any one of the

following guidelines:

a. There is a loss of standard ATC separation resulting from compliance with a TCAS-generated

RA;
AC 120-55B 10/22/01

Appendix 7

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APPENDIX 7. DEFINITIONS (continued)

b. TCAS is suspected of not performing as designed;

c. TCAS issues a descent RA when own aircraft is below 1,000 feet AGL;

d. There is an altitude excursion of more than 1,000 feet from an assigned level altitude;

e. TCAS issues a turn around RA; or

f. TCAS issues a coordinated crossing RA.

22. STC. An FAA certificate attesting to the fact that modifications to the respective aircraft,

engines, or other components meet 14 CFR airworthiness requirements.

23. Tau. Greek symbol representing the time to CPA.

24. Threat. An intruder which has satisfied the threat detection criteria and thus requires an RA.

25. Traffic. Aircraft with an operating transponder capable of being tracked and displayed by a

TCAS-equipped aircraft.

26. Traffic Advisory (TA). Aural voice and display information provided by TCAS to a flightcrew,

identifying the location of nearby traffic that meets certain minimum separation criteria.

27. Traffic Alert and Collision Avoidance System (TCAS). A family of airborne devices meeting

specified TSO and RTCA/DO-185 requirements, as amended, which function independently of the

ground-based ATC system to provide collision avoidance information.

a. TCAS I. A TCAS providing only TAs to assist pilots in the visual acquisition of intruder

traffic.

b. TCAS II. A TCAS which provides TAs and RAs in the vertical plane, such as climb or

descend.

28. TCAS Academic Training (as applied herein). Part of ground training which exclusively

addresses acquiring the required TCAS concepts, systems, limitations, or procedures knowledge

(rather than skills), and demonstration of that knowledge. TCAS academic training generally is

accomplished using a combination of classroom methods (standup instruction, slide/tapes, computer

based instruction, tutorial, etc.), flight manual information bulletins, or self-study. See paragraph 9b

for an expanded explanation of what should be taught.

29. TCAS Event. For the purpose of this AC, one or more of the following occurrences or

situations are related to TCAS:

a. Issuance of any TCAS RA or TA as specified by a flightcrew, regardless of whether it is

valid, unwarranted, or a nuisance;
10/22/01 AC 120-55B

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APPENDIX 7. DEFINITIONS (continued)

b. Other TCAS-related in-flight traffic conflicts or potential conflicts as determined by a

flightcrew;

c. Near midair collisions (NMAC) involving a TCAS-equipped aircraft;

d. TCAS system failures related to a traffic conflict, potential traffic conflict situation, or TCAS

general system performance;

e. ATC system error involving a TCAS-equipped aircraft; or

f. Other TCAS occurrences or situations related to potential TCAS or ATC system safety

performance.

30. TCAS Maneuver Training. Part of flight training that includes the integration of TCAS

knowledge with the specific skills required to demonstrate satisfactory performance of a particular

TCAS procedure or maneuver, or series of procedures or maneuvers.

31. TCAS Transition Program (TTP). A phased TCAS implementation and evaluation program

established to assure timely and effective TCAS installation and use for U.S. air carrier aircraft and

to assist in the integration of a large number of TCAS units in the NAS.

32. Type Certificate (TC). An FAA certificate attesting to the fact that the respective aircraft,

engines, or other components meet 14 CFR airworthiness requirements.