2-8 strength is selected to achieve the required vertical miss distance while minimizing ATC clearance deviations.
2.8.5.2 ACAS Xa logic considers imperfect sensor information when deciding if an RA is to be generated. ACAS Xa uses a probabilistic distribution to account for uncertainty in the position and velocity of other aircraft.
2.8.5.3 ACAS Xa logic considers the uncertainty of what the pilot of the other aircraft will do to balance safety (i.e., vertical miss distance at CPA) with operational considerations (e.g., mitigation of ATC clearance deviation).
2.8.5.4 ACAS Xa RA generation decision making is completed by combining the results from a precomputed lookup table (offline costs) with algorithms running on board (online costs).
2.8.6 S ubsequent Advisories . After an initial RA, subsequent advisories will be generated to strengthen, reverse, weaken, or cancel the initial advisory.
2.8.6.1 S trengthened Advisories. ACAS II may generate a strengthened RA that increases the vertical speed required to comply with the RA (i.e., from “Climb, Climb” to “Increase Climb, Increase Climb”) to achieve a satisfactory vertical separation at CPA. Strengthened advisories may be the result of an RA response by own-ship flightcrew that is delayed, weak, or noncompliant.
A strengthened advisory may also result from a threat aircraft maneuvering vertically in a manner that thwarts the effectiveness of the initial RA.
2.8.6.2 Reversal Advisories. ACAS II may generate a reversal RA that reverses the direction of the previous RA (i.e., from “Climb, Climb” to “Descend, Descend NOW, Descend, Descend NOW”) to achieve a satisfactory vertical separation at CPA. Reversal advisories may be the result of an RA response by own-ship flightcrew that is delayed, weak, or noncompliant. A reversal advisory may also result from a threat aircraft maneuvering vertically in a manner that thwarts the effectiveness of the initial RA. Flightcrews are advised to respond to reversals immediately, as the reversal RA is not likely to reverse again.
2.8.6.3 Weakened RAs. During an RA, if ACAS II determines that the response to an RA has provided the desired vertical separation prior to CPA, the initial RA will be weakened to minimize the displacement from the original altitude
(i.e., “Level Off, Level Off”).
2.8.6.3.1 Incidents have occurred where a flightcrew misinterpreted the “Adjust Vertical Speed, Adjust” RA of TCAS II version 7.0 and exacerbated the situation by increasing the vertical speed. These incorrect reactions have caused altitude deviations and losses of separation. The “Adjust Vertical Speed, Adjust” RAs require a reduction of the vertical speed. “Adjust Vertical Speed, Adjust” RAs are not present in TCAS II version 7.1/ACAS Xa, and are replaced with a “Level Off, Level Off” RA. The response to the “Level Off, Level Off” RA is to reduce the vertical rate to zero feet per minute (fpm). The
2-9 level off must be achieved promptly, not at the next Flight Level (FL).
Flightcrews are advised after a “Level Off” RA to stay in level flight until a “Clear of Conflict” message or another RA is generated.
2.8.6.4 Clear of Conflict. After CPA is passed and the range between own-ship and threat aircraft begins to increase, or if ACAS II determines prior to CPA that there will be sufficient Horizontal Miss Distance (HMD) and/or Vertical Miss Distance (VMD), all RAs are canceled. ACAS will annunciate “Clear of Conflict,” and all visual RA guidance is removed.
2.8.6.4.1 If ACAS II switches from TA/RA mode to TA-ONLY either automatically
(e.g., low altitude inhibit or higher priority alert) or manually during an RA, the RA will be removed without the “Clear of Conflict” annunciation. In this case, the removal of the RA does not indicate a sufficient vertical or lateral separation has been achieved.
Table 2-4. Resolution Advisories, TCAS II Version 7.1 and ACAS Xa Advisory Aural TA Traffic, Traffic Climb Climb, Climb Descend Descend, Descend Crossing climb Climb, Crossing Climb — Climb, Crossing Climb Crossing descend Descend, Crossing Descend — Descend, Crossing Descend Level off Level Off, Level Off Reversal climb Climb, Climb NOW — Climb, Climb NOW Reversal descend Descend, Descend NOW — Descend, Descend NOW Increase climb Increase Climb — Increase Climb Increase descent Increase Descent — Increase Descent Maintain rate Maintain Vertical Speed, Maintain (TCAS v 7.1) Descend, Descend or Climb, Climb (ACAS Xa) Crossing, maintain rate Maintain Vertical Speed, Crossing Maintain (TCAS v 7.1) Descend, Crossing Descend/Climb, Crossing Climb (ACAS Xa) Preventive (no change in vertical speed required) Monitor Vertical Speed RA removed Clear of Conflict
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2.9 Traffic Display. Traffic information may be depicted on a dedicated display, shared display, or a part-time (popup) display. Refer to the AFM/AFMS for details. Traffic display information is intended to assist flightcrews in the search for, and visual acquisition of, airborne aircraft to avoid collisions. ACAS serves as a backup to visual collision avoidance, the application of right-of-way rules, and ATC separation services.
The nomenclature “advisory” notwithstanding, the FAA considers that TAs generally require immediate flightcrew awareness and potentially a subsequent flightcrew response.
RAs generally require immediate flightcrew awareness and immediate flightcrew response by complying with the RA in a timely manner. In accordance with 14 CFR § 25.1322, TAs are displayed in a yellow or amber color and correspond to a Caution-level alert. RAs are displayed in a red color and correspond to a Warning-level alert. See Table 2-5, Traffic Display Symbology Examples.
2.9.1 Traffic Display Symbology.
2.9.1.1 Own-ship is displayed as a white or cyan aircraft-like symbol.
2.9.1.2 The Other Traffic symbol is displayed for an aircraft that is more than 6 NM laterally or more than 1,200 feet vertically from own-ship. Other Traffic is displayed as an open diamond (outline only) in white or cyan. ACAS may not display all other aircraft operating a transponder in areas of high-density traffic.
2.9.1.3 The Proximate Traffic symbol is displayed for an aircraft within 6 NM and within 1,200 feet relative altitude. Proximate Traffic is displayed as a filled diamond in white or cyan. ACAS may not display all proximate aircraft operating a transponder in areas of high-density traffic.
2.9.1.4 The Traffic Advisory (TA) symbol is displayed for an intruder aircraft that has crossed the time to CPA or fixed distance threshold for a TA. TAs will be displayed as a yellow or amber filled circle. ACAS Xa optional AOTO may be displayed differently than a nominal TA to inform the flightcrew that the AOTO TA will not progress to an RA.
2.9.1.5 The Resolution Advisory (RA) symbol is displayed for a threat aircraft that has crossed the time to CPA or fixed distance threshold for an RA. RAs will be displayed as a red filled square.
2.9.2 Data Tag.
2.9.2.1 Relative altitude is shown using two digits with a prefix of a plus sign (+) or a minus sign (-). The prefix depicts whether the relative altitude is above with a plus sign or below with a minus sign. The two digits are the altitude difference in 100-foot increments (e.g., +09 depicts an aircraft 900 feet above the own-ship). Coaltitude (i.e., aircraft at the same altitude as own-ship) is identified as 00 without a prefix. It is possible to display traffic up to 9,900 feet away in relative altitude dependent on display settings.
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2.9.2.1.1 Though the data tag indicates the altitude difference in 100-foot increments, aircraft operating Mode S transponders, including own-ship, provide ACAS with pressure altitude information in 25-foot increments. The altitude data provided to ACAS is more current and more defined than the traffic information displayed to ATC.
2.9.2.1.2 Some ACAS provide the option to change the data tag from displaying relative altitude of an aircraft from own-ship to the reported pressure altitude of displayed aircraft, also known as the FL altitude. If this display mode is selected, the data tag shows the pressure altitude information of displayed aircraft in hundreds of feet using three digits (e.g., 125 for 12,500 feet pressure altitude).
2.9.2.2 The trend arrow is a vertical arrow that points up or down next to the displayed aircraft symbol. A trend arrow pointing up depicts a threat aircraft climbing at a vertical speed more than 500 fpm. A trend arrow pointing down depicts a threat descending at a vertical speed more than 500 fpm.
2.9.2.3 If an aircraft is non-altitude reporting, the data tag is omitted, and the aircraft will be displayed as either other traffic, proximate traffic, or a TA when own-ship altitude is below a pressure altitude of 15,500 feet. RAs are not generated from non-altitude reporting aircraft. When own-ship is above a pressure altitude of 15,500 feet, there is no ACAS protection for a non-altitude reporting aircraft, and this traffic will not be displayed.
2.9.2.4 No-Bearing TAs or RAs may be generated when bearing information is not available. The No-Bearing advisory is displayed on the traffic display via a text string. The text string presents the TA or RA, the range in NM, the relative altitude in hundreds of feet, and the vertical speed arrow
(e.g., TA 5.2 -06↑ represents an intruder generating a TA at 5.2 NM with a relative altitude of 600 feet below and climbing at a vertical speed of more than 500 fpm). The text is written in yellow or amber for a TA and red for an RA.
2.9.2.5 Certain Electronic Flight Information Systems (EFIS) ACAS installations operating in conjunction with track up or north up mode may require the flightcrew to make allowances for the difference between the aircraft heading and track when attempting to visually acquire nearby aircraft.
Table 2-5. Traffic Display Symbology Examples ACAS II Directional Information ACAS II w/ ADS-B In Own-Ship. Airplane-like symbol. Symbol and color vary.
Other Traffic. Nonthreat (at the time displayed).
+11 Proximate Traffic. Traffic is +11 within 6 NM and within A+ 1,200 feet from own-ship.
Traffic Advisory (TA).
Threshold for TA has been crossed. Refer to the AFM for
- 09 AOTO TA symbology, if - 09 equipped.
Resolution Advisory (RA).
Threshold for RA has been
- 05 crossed. - 05 RA 2.3 +0.31 RA 2.3 +0.31 TA 5.2 -031 No-Bearing Advisories TA 5.2 -031
2.9.3 RA Displays. The RA display provides guidance to attain or maintain the minimum vertical separation from a threat aircraft. The RA display depicts guidance for the vertical speed and/or pitch angle to be flown to comply with an RA, as well as the range of vertical speeds or pitch angles to be avoided during the RA.
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2.9.3.1 RA/Vertical Speed Indicator (VSI) (round dial VSI). This implementation indicates the vertical speed to be flown to comply with an RA in a green arc on the VSI. Vertical speeds to be avoided during an RA are shown in red arcs on the VSI. See Figure 2-1, Resolution Advisory/Vertical Speed Indicator (round dial vertical speed indicator).
2.9.3.2 RA/VSI, Integrated Tape VSI on a Primary Flight Display (PFD). This implementation indicates the vertical speed to be flown to comply with an RA in a green zone on the vertical speed tape of the PFD. Vertical speeds to be avoided during an RA are shown in red zones on the vertical speed tape of the PFD. See Figure 2-2, Integrated Vertical Speed Indicator Tape on a Primary Flight Display.
2.9.3.3 Pitch Cues on the PFD. This implementation indicates the pitch angles to be avoided while responding to an RA. A red trapezoid, or another similar geometric shape, displayed on the PFD attitude indicator displays the range of pitch angles that are to be avoided to maintain or attain the VMD from one or multiple threat aircraft. The red trapezoid remains displayed for the entire duration of the RA. The trapezoid changes as appropriate when an RA is strengthened or weakened. See Figure 2-3, Pitch Cues on the Primary Flight Display.
2.9.3.4 Head-Up Display (HUD)/Helmet-Mounted Display (HMD). This implementation indicates the vertical flight path to be flown and avoided using a unique display symbology on the HUD/HMD similar to the pitch cues on the PFD.
2.9.3.5 Visual Advisories. Some implementations of ACAS II will have advisories displayed as textual messages in the primary field of view of each pilot. The corresponding message to an RA (e.g., DESCEND NOW) may be depicted on the PFD or similar display.
Figure 2-1. Resolution Advisory/Vertical Speed Indicator (round dial vertical speed indicator) RA Required VS Range indication 1 2 RNG 5 Traffic Advisory Proximate Traffic -05 03 4 Resolution Advisory RA VS to be avoided Figure 2-2. Integrated Vertical Speed Indicator Tape on a Primary Flight Display SPEED L-NAV AP1 ATHR FL 310 JOU 315 340 10 320 31040 300 - 10 -10 305 - 280
- 818 STD 09 10 2-14
2-15 Figure 2-3. Pitch Cues on the Primary Flight Display
2.10 Examples of ACAS II Controls. Many ACAS II installations are combined with the transponder and ADS-B Out controls. Refer to the AFM/AFMS for details. ACAS II controls provide the following basic control functions (see Figure 2-4, Examples of ACAS II Controls):
2.10.1 ACAS T est. ACAS tests vary by model of ACAS implementation. Refer to the AFM/AFMS for details on how to perform an ACAS test and how a successful or failed ACAS test is annunciated.
2.10.2 ADS-B F light Identification (FLT ID) and Transponder Code. Verify ADS-B Out is set to the FLT ID that matches exactly with the filed flight plan, and the transponder code is set appropriately.
2.10.2.1 F lightcrews are advised to operate with the transponder in the altitude reporting mode with ADS-B Out transmissions enabled at all airports any time the aircraft is positioned on any portion of the airport movement area.
2.10.2.2 Flightcrews are advised to operate ADS-B Out with the equipment turned on at all times in order to comply with 14 CFR §§ 91.225 and 91.227.
2.10.3 Transponder Selector. Aircraft with multiple transponders have a control to select the transponder in use (e.g., ATC 1, ATC 2 or ATC L, ATC R). The transponders are provided with a specific altitude information source (e.g., ATC 1 provided altitude information from the left side pilot).
2.10.3.1 F lightcrews are advised to use the altitude data source in use by the Pilot Flying (PF) to provide altitude information to ACAS and the ATC transponders. Using a common altitude source precludes the possible generation of unnecessary RAs due to differences between pressure altitude data sources.
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2.10.4 S tandby (STBY) Mode. Power is applied to the ACAS processor and the Mode S transponder, but ACAS and the transponder do not transmit.
2.10.4.1 T here is an increased hazard for collision of ACAS-equipped aircraft when aircraft are not operating transponders or not operating the altitude reporting function of the transponder. Pilots are advised not to select the transponder to STBY or OFF in flight, especially in a nonradar environment in accordance with 14 CFR § 91.215.
2.10.5 Transponder (XPDR) Mode. The Mode S transponder is fully operational and will reply to all appropriate ground and ACAS interrogations. ACAS remains in standby.
2.10.6 TA-ONLY M ode. The Mode S transponder is fully operational. ACAS II will issue the appropriate interrogations and perform all tracking functions. ACAS II will only generate TAs. RAs are inhibited in TA-ONLY mode.
2.10.7 TA/RA or Automatic Mode. The Mode S transponder is fully operational. ACAS II will operate normally, issuing appropriate interrogations and performing all tracking functions. ACAS II will generate TAs and RAs when appropriate. ACAS II will coordinate RAs with other ACAS II-equipped aircraft operating in TA/RA mode.
2.10.8 Traffic (TFC) Display Mode. On ACAS equipment with the TFC display mode, the TA/RA mode provides only an ACAS popup function. Proximate traffic and other traffic are not displayed unless a TA or an RA is being generated. To display proximate and other traffic without a TA or an RA generated, the flightcrew selects the TFC display mode. The operation of ACAS II in TFC display mode or TA/RA mode is the same except for the display of proximate and other traffic. Refer to the AFM/AFMS for details regarding the limitations and advantages of each of the available mode selections.
Figure 2-4. E xamples of ACAS II Controls
2.10.9 Display Range. Various systems will implement traffic information display ranges differently. Aural advisories and RA maneuver guidance will occur without regard to the display range. Flightcrew response is aided by situational awareness.
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2.10.9.1 The display range selection does not affect the surveillance volume of ACAS.
2.10.9.2 Lower display ranges are advised for the terminal environment. Higher display ranges are better suited for the en route environment.
2.10.9.3 Some systems will show off-scale traffic as half the symbol at the limit of the range scale, at the respective bearing, while others will display a “Traffic Off Scale” or similar message.
2.10.9.4 It is recommended to select a lower range on the traffic display when an advisory is generated.
2.10.10 Altitude Range Selector. The Altitude Range Selector is a mode selection for the traffic display that filters displayed traffic based on relative altitude, and includes normal, above, and below modes. If an aircraft that was otherwise filtered from the traffic display due to a flightcrew setting generates a TA or an RA, the aircraft that generated the advisory will be displayed. It is recommended to select above when climbing and below when descending for situational awareness.
2.10.10.1 Normal mode or the normal altitude band filters the displayed traffic to within 2,700 feet of coaltitude.
2.10.10.2 Above mode or the above altitude band filters the displayed traffic to 9,900 feet above to 2,700 feet below coaltitude.
2.10.10.3 Below mode or the below altitude band filters the displayed traffic to 2,700 feet above to 9,900 feet below coaltitude.
2.10.10.4 Some display implementations have a selection of above and below and display traffic within 9,900 feet of coaltitude.
2.10.11 Popup Displays. Some ACAS manufacturers provide an optional display setting where the proximate traffic and other traffic are not displayed unless a TA or an RA is being generated. Refer to the AFM/AFMS for details regarding the limitations and advantages of each of the available mode selections.
2.10.12 R eported Altitude Selector. As an option, the data tag may indicate the pressure altitude of the displayed aircraft instead of the relative altitude to own-ship. If this option is implemented, a switch is provided to permit a flightcrew to select this type of altitude data tag. This mode, also known as Flight Level (FL) mode, is nominally temporary, and the display will switch back to relative altitude.
2.10.12.1 ACAS and transponders transmit and receive pressure altitude information (altimeter barometric scale setting 29.92 inHg/1013.25 mb), also known as QNE, from the selected pressure source (e.g., ATC L transponder selection for left side pilot pressure altitude source). Generation of TAs and RAs are not affected by the altimeter barometric scale setting. ACAS II TA and RA generation will function correctly without regard to the setting of the reported
2-18 altitude selector or whether the aircraft involved in a TA or an RA have mismatched altimeter barometric scale settings (e.g., the flightcrew of one aircraft has the altimeter barometric scale setting set to the barometric pressure as reported by a particular station (QNH), and the altimeter of the other aircraft is set to the atmospheric pressure at aerodrome elevation (or at runway threshold) (QFE)).
2.10.12.2 The reported altitude selector only changes the display of the traffic altitude and not the logic of ACAS. To alleviate possible confusion when the altimeter is set to reference zero feet at field elevation (QFE), the flightcrew is advised to set the traffic display to relative altitude (not FL).
2.11 ACAS Limitations.
2.11.1 Aircraft that are not operating a transponder will not be tracked nor displayed by TCAS II. ACAS Xa will display aircraft operating ADS-B Out, and with the AOTO option will generate a TA on a 1090 MHz ADS-B. RAs are not generated on aircraft operating ADS-B Only.
2.11.2 ACAS will not generate an RA for non-altitude reporting aircraft.
2.11.3 ACAS may display erroneous indications when a transponder or barometric altimeter system malfunctions (e.g., displaying an incorrect relative altitude on the traffic display).
2.11.4 ACAS may not display all proximate aircraft operating a transponder in areas of high-density traffic.
2.11.5 ACAS does not display aircraft on the ground and may not display an aircraft when own-ship ACAS estimates that the other aircraft is below 380 feet Above Ground Level (AGL), unless the other aircraft is operating with a Mode S transponder that reports airborne status.
2.11.6 Design implementation may result in some short-term errors in the tracked vertical speed of another aircraft during periods of high vertical acceleration by the other aircraft.
2.11.7 The bearing displayed by ACAS is not sufficiently accurate to support the initiation of horizontal maneuvers based solely on the traffic display (e.g., when two aircraft are converging at a 90º angle, the displayed aircraft appears to be converging with own-ship at a 45º angle on the traffic display).
2.11.8 In some situations (mainly due to high vertical closure rates) TAs will not precede RAs for one or both aircraft in an encounter.
2.11.9 TCAS II is designed to provide collision avoidance protection in the case of any two aircraft that have a horizontal closure rate up to 1,200 NM per hour (knots) and a vertical closure rate up to 10,000 fpm. ACAS Xa does not have a vertical closure rate limit.
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2.12 ACAS Failures and Anomalies.
2.12.1 Operators are advised to ensure their flightcrews are aware of the specific failure cases and indications for the installed ACAS version and model. ACAS failure and status indications include the following (actual messages may vary; refer to the AFM/AFMS for more details). See Table 2-6.
Table 2-6. ACAS Status and Failure Indication Examples TCAS TEST TCAS Test (c yan or white). ACAS test is in process.
TCAS STBY TCAS STBY (c yan or white). ACAS is in standby mode.
TA ONLY TA Only (c yan or white ). ACAS II is operating in TA-ONLY mode.
TCAS FAIL TCAS Fail (yellow). ACAS has failed.
TD FAIL TD Fail (yellow). Traffic display has failed.
2.12.1.1 ACAS II will automatically be set to standby or fail if the transponder or radio altimeter fails, or if the input from the barometric altimeter is lost.
2.12.1.2 In some installations, the loss of information from other onboard systems, such as an Inertial Reference System (IRS) or Attitude and Heading Reference System (AHRS), may result in a failure.
2.12.1.3 For ACAS II with Hybrid Surveillance (HS), in case of a failure or loss of Global Navigation Satellite System (GNSS) source, ACAS II will switch to active surveillance.
2.12.1.4 In rare cases, an RA can be generated due to self-tracking (i.e., when an aircraft tracks itself as a threat). The pseudo-traffic is then displayed at the same altitude and same position as own-ship. ACAS II will not track Mode S aircraft whose 24-bit aircraft address is the same as own-ship, and although a suppression bus should prevent own transponder from replying to Mode C interrogations, failures may occasionally occur. Self-tracking RAs may be operationally disruptive, as the flightcrew may comply with these RAs not knowing that they result from a failure and may cause large deviations from ATC clearances or instructions.