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 TCAS II version 7.1 (V7.1) that are certified to technical standard order (TSO) C119c or TSO-C119d, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, TCAS II with Hybrid Surveillance. Guidance is also provided for those applicants seeking airworthiness approval for stand-alone Mode S transponders that are certified to TSO C112d, 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 TCAS II version 7.0 (V7.0) 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 its entirety. The term “must” is used to indicate mandatory requirements when following the guidance in this AC. The terms “should” and “recommend” are used when following the guidance is recommended but not required to comply with this AC.
1-2. Who this AC appl ies to. Applicants seeking a type certificate (TC), amended type certificate, 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 ACs and sections of 14 CFR part 23 are referenced.
1-3. Cancellation . This revision cancels AC 20-151A.
1-4. Significant Changes. This AC has been revised to accommodate the changes of TCAS V7.1, incorporate lessons learned since the last revision to the AC, and add policy for the hybrid surveillance functionality. The most significant change associated with the latest iteration of the TCAS II TSO Standard, TSO-C119d, is the requirement to incorporate RTCA/DO-300A Hybrid Surveillance functionality. This requirement has been added principally as a means of reducing congestion on the 1090 Mhz frequency while airborne and when on the ground. From the pilot’s perspective, when the equipment is airborne, there will be no difference in operation of a TSO C119d traffic display verses the earlier variants. However, when the aircraft is on the ground and TCAS is placed in the TA or TA/RA mode, traffic advisories will not be annunciated. This change will mitigate unnecessarily burdening the 1090 Mhz frequency when TCAS is operated on the ground. While on the ground, a TSO-C119d TCAS unit will only perform passive (i.e., 1
automatic dependent surveillance-broadcast (ADS-B)) surveillance of aircraft transmitting extended squitter. This precludes annunciation of traffic advisories, i.e., display of the filled amber (or yellow) symbol(s), on any aircraft while own-ship is on the ground. Traffic in the vicinity of own-ship with operable transponders (transmitting qualified ADS-B transmissions) will still be tracked and displayed by TCAS. To ensure that hidden failures of hybrid surveillance are not resident in airborne TCAS II units for long periods of time, failures of TSO C119d hybrid surveillance must be annunciated to the flight crew or the continued airworthiness of that functionality must be assessed during periodic scheduled maintenance tasks. Depending on the level of avionics integration in the aircraft, different methods can be used to ensure the continued airworthiness of the 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 V7.0 and V7.1 and associated Mode S transponders. Our guidance is directed at systems that provide traffic advisories (TA) and resolution advisories (RA) in the vertical sense only (TCAS II), and where operational performance standards are defined in technical documents developed by a joint air transport industry-government group (the RTCA, Inc. Special Committee SC-147).
b. We also cover the TCAS II hybrid surveillance, which is now included in a TCAS II system.
c. We do not cover traffic symbology for traffic displays in which TCAS and Airborne Separation Assistance System (ASAS) 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 Joint Aviation Authorities (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 Acceptable Means of Compliance (AMC) 20-13, Certification of Mode S Transponder Systems for Enhanced Surveillance.
Chapter 2. TCAS II System.
Section A. System Description.
2-1. Description of a TCAS II System. TCAS II is an airborne traffic alert and collision avoidance system that interrogates air traffic control (ATC) transponders in nearby aircraft and uses computer processing to identify and display potential and predicted collision threats. The system is designed to protect a volume of airspace around the TCAS II equipped aircraft. The system will provide appropriate aural and visual advisories to the flight crew to take action to ensure adequate separation when the computer analysis of the intruding aircraft transponder replies predict a penetration of the protected airspace. The TCAS II system can only generate RAs for intruders equipped with responding Mode S or Mode C transponders, which provide information on the altitude of the threat aircraft. Traffic advisories can be generated for any aircraft equipped with an operative Mode S or an ATCRBS transponder, regardless of its ability to provide information on the intruder aircraft’s altitude. We view TCAS II 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. TCAS II does not alter or diminish the pilot’s basic authority and responsibility to ensure safe flight.
2-2. System Advisories . The system provides two types of advisories:
a. Traffic advisories indicate the relative positions of intruding aircraft that are approximately 20 - 48 seconds from the closest point of approach (CPA), depending on the sensitivity level (SL), and may a short time later require a RA. TAs also give the flight crew the opportunity to visually acquire the intruding aircraft out-the-window.
b. RAs provide a vertical avoidance maneuver to increase separation when the computer predicts the threat aircraft is between 15-35 seconds from the closest point of approach, depending on the SL.
2-3. Sys tem Flight Deck Displays. The system provides two types of flight deck displays:
a. A traffic display depicts the relative position of ATC transponder-equipped aircraft.
b. An RA display for each pilot indicates the appropriate vertical maneuver to avoid a threat. The RA display provides the pilot with information on the vertical speed or pitch angle to fly in order to avoid a threat. The RA display is typically implemented on an instantaneous vertical speed indicator (IVSI), a vertical speed tape that is part of a Primary Flight Display (PFD), or using pitch cues displayed on the PFD.
2-4. 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.
2-5. Hybrid Surveillance. Hybrid surveillance is a 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 ADS-B data broadcast from other aircraft. The passive surveillance data is broadcast and received through the use of Mode S Extended Squitter, that is, 1090 megahertz (MHz) ADS-B. Hybrid surveillance does not degrade the performance of TCAS active surveillance. This is a requirement and is tested by ensuring that active surveillance performs as specified by the TCAS II performance standards.
Section 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 RA 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 TSO-C112d, 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.
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 must 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.
NOTE: Guidance addressing integration of controls for TCAS and ADS-B In installations is provided in AC 20-172 ( ), Airworthiness Approval for ADS-B In Systems and Applications.
2-8. Antennas. The Mode S transponder must have a top and bottom omnidirectional antenna (also know as diversity antennas). The TCAS II must have a top directional antenna and a bottom omnidirectional 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, TSO-C119c or TSO-C119d 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 also includes hybrid surveillance functionality which only applies to TSO C119c/d (optional for TSO C119c, required for TSO-C119d)”.
2-10. Traffic Display.
a. Purpose. The primary purpose of the traffic display is to aid the flight crew in the out-the-window visual acquisition of transponder-equipped aircraft. This is accomplished by displaying the intruder aircraft’s horizontal and, if altitude information is available, vertical position relative to the TCAS II equipped aircraft. The TCAS II systems provide traffic information on Mode A (no altitude data available), Mode C, and Mode S 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 RA display is to give each pilot the information to readily correct the aircraft flight path or to prevent a maneuver that would significantly reduce the vertical separation between the pilot’s aircraft (own aircraft) and a threat aircraft.
b. Description. The RA display may be integrated with the two primary IVSIs on the flight deck, integrated into the 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 accuracy, over/under shoot tendencies, flight technical error, and potential confusion resulting 7
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. 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 (or yellow) 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” advisory, and indicates an additional climb rate is required to achieve safe vertical separation from a maneuvering threat aircraft.
(2) “INCREASE DESCENT, INCREASE DESCENT”--descend at the rate depicted by the green (fly to) arc or line on the IVSI or other suitable indicator. Received after “DESCEND” advisory, and indicates additional descent rate is required to achieve safe vertical separation from a maneuvering threat aircraft.
(3) “CLIMB - CLIMB NOW, CLIMB - CLIMB NOW”--climb at the rate depicted by the green (fly to) arc or line on the IVSI or other suitable indicator. Received after a “DESCEND” RA and indicates a reversal in direction is required to achieve safe vertical separation from a maneuvering threat aircraft.
(4) “DESCEND - DESCEND NOW, DESCEND - DESCEND NOW”--descend at the rate depicted by the green (fly to) arc or line on the IVSI or other suitable indicator.
Received after a “CLIMB” RA and indicates a reversal in direction is required to achieve safe vertical separation from a maneuvering threat aircraft.
e. All TCAS II aural alerts must be inhibited below 400 ft AGL while descending and inhibited below 600 ft AGL while climbing.
f. Both increases and decreases in the threat level must be aurally annunciated.
g. In general, other messages that are clear and unambiguous will be evaluated on an individual basis. Do not use messages that contain negatives (for example, “DON’T CLIMB”).
Section C. Airworthiness Considerations.
2-14. Certification Program.
a. This AC will guide your installation of TCAS II V7.0 or V7.1 equipment and Mode S transponders. TCAS II installation includes the TCAS, the Mode S transponder, antennas, control panels and display components. These components are all certified initially as a single installed system. Any change in any of the system part numbers requires either a new Initial Approval or a Follow-On Approval. The system displays information and provides advisories in a number of formats. The degree of system integration to perform these functions is extensive and as a result, your program must be directed toward airworthiness approval through the type certification or supplemental type certification process.
b. Certification plan. Develop a comprehensive certification plan. Include how you’ll comply with the applicable certification requirements, and list the substantiating data and necessary tests in your plan. Include a system description and an estimated time schedule. A well-developed certification plan will be of significant value both to you (the applicant) and the appropriate FAA certification office.
2-15. Equipment Compatibility Requirements. Make an evaluation to show that the TCAS II system communicates with other approved TCAS II systems made by other manufacturers.
Include a TCAS II to TCAS II coordination demonstration, or equivalent, with at least one other 10
manufacturer’s approved TCAS II system in your evaluation. If it can be shown for a specific design that communication link failures are no more hazardous than encountering a Mode C intruder, then these tests are not necessary. Also, after completing mature bench tests, future certification experience may show that these tests are no longer necessary. Submit evidence to show that you performed TCAS/transponder interoperability bench tests using the same TCAS/transponder pairing (the same part numbers) as the installation seeking certification.
Interoperability bench tests can be satisfied by either of the following:
a. Execute the following RTCA/DO-185A or RTCA/DO-185B tests as applicable, using the actual TCAS unit and Mode S transponder seeking certification:
(1) 2.4.2.2.3.1 and 2,
(2) 2.4.2.2.4.1,
(3) 2.4.2.2.4.2.1, 3 and 5-8, and,
(4) 2.4.2.2.5.
b. Execute the following tests in RTCA/DO-181E as applicable, Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment, using the actual TCAS unit and Mode S transponder seeking certification:
(1) Procedure 30,
(2) Procedure 31 a-d,
(3) Procedure 32a-c and d (1 & 2),
(4) Procedure 34,
(5) Procedure 36 a and c, and,
(6) Procedure 37 a and b.
c. In addition, execute the following RTCA/DO-181E tests as applicable, using a TCAS simulator coupled with the actual Mode S transponder:
(1) Procedure 31e,
(2) Procedure 32d (3-16),
(3) Procedure 33,
(4) Procedure 35, 11
(5) Procedure 36b,
(6) Procedure 37c and,
(7) Procedure 38.
2-16. Aircraft Performance Considerations. Use paragraphs 2-16 through 2-17 and Table 1 of this AC to help you evaluate the need to inhibit TCAS II CLIMB and/or INCREASE CLIMB RAs resulting from inadequate aircraft climb performance. The collision avoidance maneuvers posted as RAs by TCAS II assume an aircraft’s ability to safely achieve them. If it’s likely the required response to CLIMB and INCREASE CLIMB RAs are beyond the performance capability of the aircraft, then TCAS II must know beforehand so it can change strategy and issue an alternative RA. These performance limits must be provided to TCAS II from the aircraft interface and discrete settings relative to altitude and/or aircraft configuration. However, carefully consider the need to inhibit TCAS II CLIMB or INCREASE CLIMB RAs since the alternative RAs may not provide the optimum solution to the encounter. Inhibiting these RAs will increase the likelihood of TCAS II:
a. Issuing crossing maneuvers (crossing through an intruder’s altitude), thus increasing the probability that an RA may be thwarted by the intruder maneuvering,
b. Causing an increase in DESCEND RAs at low altitude, and
c. Providing no RAs if below the descend inhibit altitude of 1200 feet AGL during takeoff and 1000 feet AGL on approach.
2-17. Evaluating Aircraft Performance . The configuration interface may need switches or sensors, besides the basic airplane flap position switches, to prevent unnecessary TCAS II inhibits. For example, if CLIMB RAs need to be inhibited for the maximum takeoff flap setting only, and no switch exists to sense that position, install an additional switch instead of simply using one that may exist at lesser flap angle settings.
a. Because TCAS II can only accept a limited number of inputs related to airplane performance, it’s not possible to automatically inhibit CLIMB and INCREASE CLIMB RAs in all cases where it may be appropriate to inhibit such RAs. In these cases, TCAS II may command maneuvers that may significantly reduce stall margins or result in stall warnings.
Conditions where this may occur include bank angles greater than 15 degrees, weight/altitude/temperature combinations outside the envelope shown in Table 1, initial speeds below those shown in Table 1, one engine inoperative, leaving the aircraft configuration fixed for climb RAs on landing transition to go-around, and abnormal configurations such as landing gear not retractable. Provide information concerning this aspect of TCAS in the airplane flight manual (AFM) or airplane flight manual supplement (AFMS) so that flight crews may take appropriate action.