(3) Antennas. The active Mode S transponder shall have a top and bottom omni-directional antenna. The TCAS II shall have a top directional antenna and a bottom omni-directional or directional antenna.
i) Directional antennas. For an aircraft installation, the ICAS lI directional antenna should be located on the top forward fuselage as close to the centerline as possible. If more than one directional antenna is provided, locate the second antenna in a similar manner on the lower fuselage.
The TCAS II antennas should be mounted on the aircraft with at least 20 db.
isolation from other L-band frequency antennas. Since the antenna diameter offset resuiting in an angular offset of up to 5 degrees is acceptable. some structural considerations may be necessary and a centerline 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, anti-icing provisions should be considered. Section 3 of Volume I of RTCA document DO-185 provides antenna selection and performance criteria. For propeller driven aircraft, the location and performance of the directional antenna must be investigated for minimum blockage and to ensure that the propellers do not interfere with system operation.
(ii) Omni-directional antennas. The TCAS II antennas should be mounted on the aircraft with at least 20 db. isolation from other L-band frequency antennas. The Mode S transponder antennas shall be mounted at locations chosen for adequate isolation and signal coverage. These antennas may be standard ATCRBS transponder antennas.
(iii) Structural analysis. A structural analysis of the antenna installations showing compliance with the applicable FAR should be submitted to the FAA. This includes the structural provisions for a beam steering unit (if installed) if it is not mounted in a standard avionics rack.
(4) The TCAS II Processor. The TCAS II processor unit computes information to identify and to display potential and predicted collision threats, and to issue resolution advisories to avoid the threat aircraft.
TCAS II processor unit must comply with the environmental requirements and minimum performance standards specified in TSO-C119a. A manufacturer of TSO equipment may obtain authorization to produce equipment which deviates from the detailed criteria of the TSO. The FAA aircraft certification office which is 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 document DO-185, Volumes I and II, Changes 1 through 6, Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment.
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(5) Equipment Compatibility. An evaluation should be made to show that the TCAS II system will communicate with other approved TCAS II systems made by other manufacturers. This evaluation should include a TCAS II to TCAS II coordination demonstration, or equivalent, with at least one other manufacturer's approved TCAS II system. 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.
(6) Aircraft Performance.
(i) This paragraph, along with Table I, provides guidance on considerations and analysis to evaluate the need to inhibit TCAS II CLIMB and/or INCREASE CLIMB RAs because of inadequate aircraft climb performance.
The collision avoidance maneuvers posted as RAs by TCAS Il assume an the capastitthe sifer, then aS II must know beforehan set can If it is likely they are beyond change its strategy and issue an alternative RA. These performance limits shall be provided to TCAS II from the aircraft interface and discretes relative to altitude and/or configuration. However, the need to inhibit TCAS II CLIMB or INCREASE CLIMB RAs should be carefully considered since the alternative RAs will 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) increasing the probability that an RA may be thwarted by the intruder maneuvering, (D) 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.
(ii) 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 only, and no switch exists to sense that position, an additional switch should be installed in lieu of simply using one that may exist at lesser flap angles.
(iii) Because of the limited number of inputs to TCAS II for airplane performance inhibits, in some instances where inhibiting RAs would be appropriate it is not possible to do so. In these cases, TCAS II may command maneuvers which may significantly reduce stall margins or result in stall warning. Conditions where this may occur include bank angles greater than 15°, weight/altitude/temperature combinations outside the envelope shown in Table I, initial speeds below those shown in Table 1, one engine inoperative, leaving configuration fixed for climb RA on landing transition to go-around, and abnormal configurations such as landing gear not retractable.
Airplane Flight Manual (AFM) or Airplane Flight Manual Supplement (AFMS) should provide information concerning this aspect of TCAS so that flightcrews may take appropriate action.
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(iv) An aircraft's low altitude climb capability during takeoff, approach, or landing is significantly affected by the aircraft's configuration, initial climb true airspeed available to safely trade if needed for climb rate, and the initial airspeed margin from the current stall speed.
Table I, Conditions 1-3 apply for the takeoff and initial climb configuration analysis. Conditions 4-6 apply for the approach flap configuration analysis when operating in the terminal area with the flaps set at less than the landing flaps. Conditions 7-9 apply for the landing flight regime analysis.
To be consistent with normal operation, the AFM or AFMS should indicate that when a climb RA occurs with the aircraft in the landing configuration, the pilot should initiate the normal go-around procedure when complying with the TCAS II RA. Therefore, it may be assumed that the flaps are being retracted from the landing position to the go-around position when evaluating Conditions
(v) To prevent very unlikely combinations of events, e.g., weight/altitude/temperature limiting conditions, in conjunction with low carsing, hidabits he generally the aircrats permance is more than adequate, the entry and exit conditions and RAs in Table I are structured into two classes of encounters. Maneuvers A and B represent reasonably severe combinations of entry conditions and RAs, and restricts the exit conditions to conditions and as represer this very kely event may require flying near stall warning conditions through the recovery. Airspeeds between 1.2Vs1 and stall warning represent a range of usable airspeed that maybe traded for climb performance (as is currently recommended for windshear recovery) for evaluation of this low probability event. The altitude/temperature envelope represents a range of values which exist at busy airports in the continental USA. Operation routinely outside this envelope may require special crew procedures it the normal ArM weight, altitude, temperature, and contiguration limitations are not sufficiently compensating, e.g., operation at Mexico City.
For those airplanes that may routinely operate at low climb airspeeds during the clean configuration en route phase of flight, e.g., propeller commuter airplanes, consideration should be given to providing a discrete to the TCAS II based on airspeed. Such an input, derived trom a ICAS II interface system, would provide for climb or increase climb RA inhibits when the airplane is in the clean configuration and operating below a certain airspeed. Such a scheme would be considered appropriate in lieu of an acrossthe-board inhibit for the clean configuration regardless of flight regime, which is not considered to provide the best overall level of safety as previously discussed.
(vii) An aircraft's climb capability when operating at or near its maximum approved operating altitude is also affected by excess thrust and true airspeed, which may be available to safely trade if needed for climb rate. Climb RAs should not be inhibited if the aircraft has adequate performance available or because it may exceed its maximum certificated altitude by several hundred feet during an RA. The configurations that should be evaluated in this flight regime are shown in Table I, Conditions 10 and 1l.
If the aircraft is approved for significant alternative configurations, then the initial airspeed used for the analysis should be appropriate for them;
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e.g., spare engine pod, gear down operation, etc. In the analysis of the aircraft's ability to accelerate and return to the initial speed and altitude following the RA, an undershoot of approximately 200 feet is permissible.
(vili) In icing conditions, the FAR limited performance weights must be debited and sometimes the operating speeds increased to account for the capability to perform the TCAS II maneuvers remains essentially unchanged, icing system bleeds and residual ice on the unprotected surfaces.
eliminating the need to provide additional RA inhibits under these circumstances. However, if a particular aircraft design shows marginal capability to operate in the icing environment, additional RA inhibits enabled by icing system activation should be considered.
lable 1, then the CLIMB RA should be inhibited. If Maneuver B or C cause airspeeds below the minimum in Table I, then the INCREASED CLIMB RA should be inhibited. However, early recovery of 1 to 2 seconds is of little or no consequence on the collision avoidance maneuver and a higher overall level of safety will be achieved if inhibits are not provided under these
(x) System Inhibits. A summary of the inhibits (limitations) that have been programmed into the TCAS computer are given in Table II.
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00 TABLE I AC 20-131A
AIRSPEED REGIME THRUST FLAPS GEAR INITIAL MIN.
1. Takeoff EAR 25 Limit Makeof Rated All Takeoff UP V½+20г 1.2 Vs1 4 thru RA
2. = B AFM All-Engine 15°_ Stal bank to arning" C Takeoff Speed thru Recoverı
4. Approach spin up to max. _ess than 1.6 VS1 1.2 'si thru RA Go-Around Thrust _anding During Maneuver From. Thrusht for _eve 1 B = 5° bank to Warning C thru Recovery
7. Spin up to мах. Transition DA to VREF+10 1.2 V51 " thru RA frein Thrust Be Go-Around Maneuver Thrust lap to Landing GO - Around Flap TOT Guide Slope
8.
9. Vert Addining Stal Panartir C Warning' Procedures from irainino thru Recovery
10. En Route critical Wt/Alt. Giv- Enrust for Leve Increased UP UP Long-Range Cruise Higher of 1.2 Buffet gnset Na Require ous Vs' if Defined or Buffet Onset
11. 3/29/93
1. Weight = Lesser of climb limit or structural; Airport Pressure Altitude = S.L. to 5300 ft;
Temperature = ISA +/- 50°F(10°C); Conditions 1-3 evaluated 700 ft above airport; Conditions 4-9 evaluated 1700 ft above airport.
2. Vy + 10 for nontransport category aircraft without a defined V2.
3. Yol Vs for those at planes where the power-on stating speed is significantly reduced from the power-off stalling speed.
5. Vs1 for those airplanes where the power-on stalling speed is significantly reduced from the power-off stalling speed.
TABLE I MANEUVERS A. This maneuver is to evaluate the TCAS II CLIMB RA. From the initial steady state condition, after an expected 3-second pilot reaction time delay, rotate the aircraft at 1.25g to attain +1500 feet per minute climb. Hold until the total duration of the RA of 25 seconds has elapsed. Recover to attain the initial trim airspeed.
This maneuver is to evaluate the TCAS II INCREASE CLIMB RA following a CLIMB RA. From the initial steady state condition, after an expected 3- second pilot reaction time delay, rotate the aircraft at 1.25g to attain +1500 feet per minute climb. Hold until 15 seconds has elapsed from when the CLIMB RA was issued. Then, after an expected 1-second pilot reaction time delay to the INCREASE CLIMB RA, rotate the aircraft again at 1.25g to attain +2500 feet per minute climb and hold until the total duration of the RA of 25 seconds has elapsed. Recover to attain the initial trim airspeed.
C. This maneuver is to evaluate a maximum duration TCAS II INCREASE CLIMB RA following a minimum duration CLIMB RA. From the initial steady state condition, after an expected 3-second pilot reaction time delay, rotate the aircraft at 1.25g targeting +1500 feet per minute climb until 6 seconds has elapsed from when the CLIMB RA was issued. Then, after an expected 1-second pilot reaction time delay to the INCREASE CLIMB RA, rotate the aircraft again at 1.25g to attain +2500 feet per minute climb and hold until the total duration of the RA of 24 seconds has elapsed.
Delay recovery l second to account for expected pilot reaction time delay to the end of the encounter. Recover to attain the initial trim airspeed.
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TABLE II SYSTEM INHIBITS A summary of the inhibits (limitations) that have been programmed into the TCAS computer.
INHIBIT PARAMETERS Increase Descent RA Inhibited below 1450 ft AGL.
Descend RA RA & TA Voice Messages Close-Range Surveillance Self-Test Inhibited below 1200 ft AGL while climbing & inhibited below 1000 ft AGL while descending.
Inhibited below 400 ft AGL while descending & inhibited below 600 ft AGL while climbing. (TCAS automatically reverts to TA only).
May not function at separations of less than approximately 900 ft.
Can be inhibited when airborne.
Can revert to TA ONLY when higher Advisory Priority priority advisories (e.g., GPWS, Windshear) occur.
Climb RA 1500 fpm can be inhibited, based upon aircraft performance capability.
2500 fpm can be inhibited, based Increase Climb RA upon aircraft performance capability.
Altitude Limit for Climb RA Can be inhibited, based upon aircraft performance capability.
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(7) Aircraft Interfaces.
(i) Pressure altitude information. The pressure altitude data should be obtained from the most accurate source available in the aircraft and shall correspond to that being transmitted by the associated Mode S transponder. The accuracy of the altitude data shall be at least that specified in Appendix A of RTCA document DO-185. It shall be shown that the resolution of the altimetry source is compatible with TCAS II. The altitude source with the finest compatible resolution should be used. when available, the resolution should be in increments of 10 feet or less. Information should also be provided to indicate when the pressure altitude information is invalid.
(ii) Radio altitude information. Radio altitude information shall be provided to TCAS II to inhibit DESCEND resolution advisories below 1000 feet AGL, INCREASE DESCENT resolution advisories below 1450 feet AGL, all resolution advisories and aural traffic advisories below 400 feet AGL, to allow automatic sensitivity level selection when close to the ground, and to determine that individual targets are on the ground. Information shall also be provided to indicate when the radio altitude information is invalid.
(iii) Discrete information from aircraft configuration sensors such as flaps, slat, landing gear, etc., should be used to ensure that TCAS II appropriately inhibits climb and increased climb RAs to the airplane performance limits as described in paragraph 7c(6).
(iv) Aircraft identification. Discrete information shall be provided to the Mode S transponder for the unique aircraft Mode S identification code and its maximum airspeed capability.
(v) Attitude. Information of pitch and roll attitude may be provided to assist with stabilization of the directional antenna function to assure surveillance and TA display data remain unaffected by aircraft normal maneuvers. If attitude information is used by TCAS II, the information shall also be provided to indicate when the attitude data are invalid.
(vi) Heading. Information of aircraft heading may be provided for use of surveillance reference and for the TA display reference presentation. Information shall also be provided to indicate when the heading data are invalid.
(vii) System failure display. An indication shall be provided to indicate when resolution advisories are not possible due to failure of the TCAS II equipment or any of its sensors or displays.
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(8) Traffic Display.
(i) Purpose. The primary purpose of the traffic display is to aid the flightcrew in the visual acquisition of transponder equipped aircraft.
This is accomplished by displaying the intruder aircraft's horizontal and, if altitude information is available, vertical position relative to the TCAS equipped aircraft. The TCAS II systems will 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 provide the flightcrew with confidence in proper system operation and to give them time to prepare to maneuver the aircraft in the event TCAS II issues a resolution advisory.
(ii) Description. The traffic display may take several forms.
Trattic displays may be independent, stand-alone, integrated and time-shared with digital color radar, integrated with the flightcrew's Instantaneous ertical Speed indicators (ivsi), or integrated with other displays such a lectronic Horizontal Situation Indicators (EHSI), navigation, or othel multi-function displays. If the traffic display uses a multi-function display that is shared with other services such as ACARS, the traffic display function shall be immediately available for display by a single selection accessible to both pilots.
(iii) Symbology/Feature Criteria. The FAA has worked closely with the ATA, NASA, and both the SAE S-7 and G-10 Committees in an effort to standardize TCAS II symbology and features. A consensus has been reached for TCAS II symbols and they are provided in this AC. Other symbology and features may be used provided the applicant uses human factors technology to demonstrate that a clear and substantial benefit can be derived by its use.
Otherwise, the traffic display must depict or provide the following symbology, features, or information:
(A) Own Aircraft Symbol. This shall be an aircraft-like symbol. For fixed range displays, it should be located so as to provide a minimum of 2.5NM range to six o'clock position (indicating an aircraft is
2.5NM behind the TCAS II equipped aircraft) and a minimum of 6NM range to the twelve o'clock position (indicating an aircraft is 6NM in front of the TCAS II equipped aircraft). The symbol color should be white or cyan and should be the same color as the range ring.
(B) The Intruder Aircraft at the Traffic Advisory (TA) Level. This symbol shall be shaped as a circle and should be solid amber in color. This symbol must be positioned to depict its relative bearing and distance to the TCAS II aircraft. TAs and RAs must be displayed whenever the appropriate CAS Logic is satisfied, regardless of display/surveillance switch positions (above/below) intended for other traffic.
(C) The Threat Aircraft at the Resolution Advisory (KA) Level. This symbol shall be shaped as a square and it should be solid red in color. Since red is of low conspicuity, the use of other graphical techniques to enhance this symbol (such as outlining) should be evaluated, and research in this area is encouraged.
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(D) Proximate Traffic. When a TA or RA is present, a symbol shall be depicted for transponder equipped aircraft within 1200 feet relative altitude and 6NM (or the minimum range, whichever is less). This symbol shall be shaped like a diamond and colored solid white or cyan but not the same color as own aircraft symbol.
traffic only when the requirements to display a TA or an RA exist. The display of proximate traffic under these circumstances will assist the pilot in the detection of the intruder aircraft. Proximate traffic may also be (E) Other Traffic. Other transponder equipped traffic within +/- 9900 feet of ownship may be displayed. This symbol should be shaped as an open diamond and should be the same color as proximate traffic. The ability to display other traffic has been determined by service experience to be a significant aid to the flightcrew's situational awareness and incorporation of this feature is highly recommended.
NOTE: All transponder equipped aircraft without altitude reporting will be considered co-altitude by TCAS. Therefore, it will become a solid diamond the same color as proximate traffic when within 6NM and may represent a serious threat that cannot be resolved by TCAS.
(F) Relative Altitude Display. Relative altitude is the difference between the other aircraft's altitude and the TCAS II equipped aircraft's altitude. This should be presented in a digital format rounded off to the nearest one hundred feet. This digital information should be centered on the intruder aircraft symbol and positioned above or below the intruder symbol when the intruder aircraft is above or below the TCAS II aircraft respectively. The digital data should include a leading zero and a plus or minus sign as appropriate. For example, -05 positioned below the threat symbol (solid red square) would indicate the threat aircraft is 500 feet below the TCAS II aircraft. This digital data should conform to the color of the threat aircraft symbol. Co-altitude situations should be depicted as "00" (without the + or - symbol). The "00" symbol may be placed above the traffic symbol if the other aircraft closed from above, and vice versa.
(G) Displayed Traffic Range. The range of the displayed traftic is horizontal (slant range corrected) when altitude is available.
Slant range is displayed when altitude information is not available.
(H) The Range "Ring". There should be markings surrounding the own aircraft symbol at each of the twelve o'clock positions.
These markings may be small dots, small lines or indices, or asterisks.
Enhancement of the 3, 6, 9, and 12 o'clock positions is permissible. A solid range ring is not recommended due to clutter. A range ring of 2NM radius is required for small fixed range displays such as 3 ATI (ARINC Specification 408A for 3 inch Air Transport Indicator) displays and the entire range ring must be depicted. A range ring radius of 3NM has been found acceptable for all other displays. The radius must remain scaled to the range selected for variable range displays. When the traffic display is integrated with an EHSI or a Navigation Display (ND), a partial range ring is permissible for Arc or Expanded display modes. Full or partial range rings are acceptable when integrated with weather radar displays. The color should be the same as own Par 3 13
aircraft symbol. The 2 or 3NM range ring is not required when the range of the display exceeds 20NM.
(I) Off Scale Symbology. RA or TA traffic which is offscale shall be indicated by placing one half of the appropriate symbol at the edge of the active display area in the direction of the measured bearing of the traffic for fixed range displays. Data tags and appropriate vertical trend arrows shall remain fixed in the normal design position relative to the traffic symbol even if a portion of the information is masked by the edge of the active display area. An alternative method for variable range displays would be to annunciate a message "TRAFFIC OFF-SCALE", "OFF-SCALE", or "TA(RA if appropriate) OFF-SCALE".
(V) No Bearing Display. When the TCAS II system uses a bottom omni-directional antenna and the upper directional antenna loses track on the intruder aircraft, the threat aircraft can no longer be depicted on the traffic or light tel forehe bear altitude made as be selected) tual range should be digitally presented. This information must be colored to conform to the threat level (amber for TAs and red for RAs). There are several acceptable formats that may be displayed depending upon the type of altitude information being displayed. Some examples are as follows:
(1) "TA 1.2NM/+11u" depicting a TA intruder at 1.2NM and 1100 feet above ownship and descending at a rate greater than 500 fpm.
(2) "RA 4.6NM/FL230" depicting an RA threat at 4.6NM at Flight Level 230.
(3) "TA 8.8NM/00" depicting a co-altitude TA intruder at 8.8NM.
(4) "RA 3.3NM/165 " depicting an RA threat at 3.3NM at a barometric altitude of 16,500 feet MSL.
(5) "TA 5.5NM" depicting a mode A (no altitude reporting) TA intruder at 5.5NM.
NOTE: The TA or RA which precedes the range information in the above examples may be omitted if display space is limited.
(K) Mode A Transponder Equipped Aircraft. A data tag shall not be associated with the symbol for targets which are not reporting altitude. Note that an RA will not be generated for targets not reporting altitude.
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(L) Other aircraft Vertical Speed. A vertical arrow should be located immediately to the right of the other aircraft symbol display if the vertical speed of the other aircraft is equal to or greater than 500 feet per minute with the arrow pointing up for climbing traffic and down for descending traffic. The color of the tag should be the same as the symbol.
(M) TA Only Mode. The ability to inhibit the RA mode of operation should be provided to prevent the display of unwanted RAs.
feature should be pilot selectable, and suitably annunciated. In this mode of operation, all TAs (and threat aircraft that would ordinarily be displayed as RAs) will be displayed as TAs.
(iv) Optional Features/Symbology.
(A) Compass Heading. Aircraft heading and/or a compass rose may be included in the traffic display, and it should be colored white.
(B) Actual Intruder Altitude. A switch may be provided to select the display of the intruders actual barometric altitude in place of relative altitude. The display of actual barometric altitude should be displayed as a three digit number representing hundreds of feet above MSL;
e.g., 007 for 700 feet MSL and 210 for 21000 feet MSL. Clear and unambiguous indication must be provided on the display that the actual intruder altitude mode has been selected. Actual altitude tags shall be positioned above or below the traffic symbol in a manner consistent with relative altitude data The display of actual barometric altitude below the transition altitude (18,000 MSL in the U.S.) requires a barometric pressure correction from ownship. If barometric pressure corrections are not incorporated, the display of actual barometric altitudes below the transition altitude is considered potentially ambiguous and may be displayed by pilot selection only for approximately 15 seconds per selection.
(C) Range Selection/Reversion. Traffic displays may incorporate a variable range feature, and there must be a means to manually select each range. A feature may be provided to automatically select the optimum range when an RA/TA occurs. In addition, if a dedicated TCAS mode is provided within a multifunction display, the selection of that mode may automatically change the range of the display to a predetermined value.
(D) Maximum Number of Displayed Targets. There currently are no requirements to establish a maximum number of displayed targets when TA or RA conditions exist. (ror 3 All displays, a maximum of 12 targets has been found acceptable. Overlapping traffic symbols should be displayed with the appropriate information overlapped. If display technology requires prioritization of symbols, this shall be accomplished to allow the maximum display of data.
(E) Traffic Display Switch. A switch may be provided which will display all traffic within the range of the display and within 2700 feet vertically of own aircraft. This switch may be either momentary or a lock-on type of switch.
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