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2.6.3.1 If own-ship is above a pressure altitude of 15,500 feet, the non-altitude reporting aircraft is filtered out. In this case, non-altitude reporting aircraft will neither be displayed nor generate an advisory.
2.6.3.2 If own-ship altitude reporting is switched off or fails, own-ship will not be fully protected and may not be depicted on the traffic display of other aircraft operating ACAS. If the pressure altitude data fed to the transponder is incorrect, the error will be propagated in other aircraft ACAS and ATC systems. Consequently, ATC will not know the actual altitude of the aircraft, and ACAS advisories may be generated incorrectly, or ACAS may not generate advisories when needed. Flightcrews are advised to verify that the altimeter and transponder system has been correctly maintained and inspected and to monitor transponder operation, including altitude reporting.
2.6.4 If both aircraft are equipped with ACAS II, and both aircraft are operating in TA/RA mode, both aircraft can generate a coordinated RA that generates complementary vertical maneuvers or lack of maneuvers (e.g., one ACAS II unit generates a climb RA and the other a descend RA).
2.6.5 ACAS II does not generate horizontal maneuvers. This functionality is planned for ACAS III systems.
Table 2-2. Levels of Protection Own-Ship (equipment and operating mode) Other Aircraft (equipment and operating mode) ACAS II (TA-ONLY) or
ACAS I ACAS II (TA/RA) No Transponder/ Transponder Off No ACAS protection No ACAS protection No Transponder, but Transmitting 1090 MHz ADS-B Out ADS-B Only TA-ONLY (ACAS Xa only) ADS-B Only TA-ONLY (ACAS Xa only) Non-Altitude Reporting TA (no protection above 15,500 feet pressure altitude) TA (no protection above 15,500 feet pressure altitude) Transponder with Altitude Reporting TA TA and RA ACAS II (TA-ONLY) or ACAS I TA TA and RA ACAS II in TA/RA Mode TA TA and coordinated RA
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2.7 Advisory Thresholds.
2.7.1 TCAS II primarily uses the measure of the time to the Closest Point of Approach (CPA) rather than distance to determine when to generate a TA or an RA.
2.7.2 Time to CPA is also indicated by the Greek symbol tau (τ). Tau thresholds (trigger points for the generation of an advisory) vary with the altitude of own-ship, with less time to CPA at lower altitudes. The TA tau threshold varies from 20 to 48 seconds before the projected CPA, and the RA tau threshold varies from 15 to 35 seconds.
2.7.3 At close ranges and slow closure rates, TCAS II uses a fixed horizontal distance threshold called Distance Modification (DMOD) at or less than the relative altitude (altitude difference between own-ship and another aircraft) threshold called Z-axis Threshold (ZTHR) to determine when to generate a TA or an RA. DMOD thresholds vary with altitude of own-ship, with less distance at lower altitudes.
2.7.3.1 Fixed distance advisories are more likely to occur during CSPO, especially at higher elevation airports, where aircraft are in a higher Sensitivity Level (SL) during approach.
2.7.4 SLs are based on the altitude of the aircraft. The higher the altitude of the aircraft operating TCAS II is, the greater the time to CPA and larger the fixed distance thresholds (horizontal and vertical) become. ACAS Xa does not use SLs. ACAS Xa was designed to provide advisory thresholds similar to TCAS II across these altitude bands. See Table 2-3.
Table 2-3. TCAS II Advisory Thresholds Seconds to CPA (tau) DMOD (NM/feet) ZTHR (feet) Own Altitude (feet) TA RA TA RA TA RA 0–500 AGL N/A N/A N/A N/A N/A N/A 500–1000 AGL 20 N/A 0.3/1822 N/A 850 N/A 1000–2350 AGL 25 15 0.33/2005 0.20/1215 850 600 2350–5000 SDP 30 20 0.48/2916 0.35/2126 850 600 5000–10000 SDP 40 25 0.75/4557 0.55/3341 850 600 10000–20000 SDP 45 30 1.00/6076 0.80/4861 850 600 20000–42000 SDP 48 35 1.30/7898 1.10/6684 850 700 Above 42000 SDP 48 35 1.30/7898 1.10/6684 1200 800
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2.7.5 ACAS Xa is designed to provide advisory thresholds that are like TCAS II, but ACAS Xa does not directly use time to CPA (tau), DMOD, or ZTHR. ACAS Xa uses an alternative risk-based approach for RA generation. ACAS Xa generates advisories based on perceived risk and filters out many potential RAs where the estimated risk is low.
2.7.5.1 ACAS Xo is an option on an ACAS Xa platform that has additional equipment and features intended for aircraft to desensitize RA thresholds in certain airport environments.
2.7.5.1.1 ACAS Xo provides a Designated No Alert (DNA) capability intended for use in visual separation procedures, such as visual approach procedures to closely spaced parallel runways that are likely to trigger unwanted RAs. TA and RA thresholds are suppressed for a single designated aircraft while full ACAS II TA/RA mode protection is provided against all other aircraft. The flightcrew visually acquires the traffic within 6 NM laterally before designating it as a DNA aircraft. If there is a multiple threat encounter involving the designated aircraft, DNA mode will suspend automatically. If the designated aircraft diverges from own aircraft by more than 6 NM, the aircraft will be undesignated.
2.7.5.1.2 The ACAS Xo CSPO-3000 function provides advisory threshold logic appropriate for use in Instrument Meteorological Conditions (IMC) during CSPO down to a runway centerline spacing of 3,000 feet and below a pressure altitude of 14,000 feet. When enabled, CSPO-3000 will still provide TAs and RAs on blundering designated traffic. CSPO-3000 mode is unavailable when own-ship is above a pressure altitude of 14,000 feet, and any time in TA-ONLY mode.
2.8 Types of ACAS RAs.
2.8.1 RA Generation. TCAS II uses projected separation at CPA based on tau, DMOD, and ZTHR to determine RA guidance. The RA is intended to provide adequate vertical miss distance from the threat aircraft while generally minimizing the disturbance to the existing flight path.
2.8.1.1 ACAS Xa threat logic estimates the potential collision risk based on the relative position and velocity of nearby aircraft and generates an RA when warranted. ACAS Xa provides the same advisories as TCAS II with some small variations in the aural annunciations (e.g., Maintain Vertical Speed RAs changed to the corresponding climb or descend advisory).
2.8.1.2 ACAS Xa is unchanged from TCAS II with respect to low altitude inhibits, handling of bearingless and non-altitude reporting aircraft, and interaction with higher priority alerts (e.g., stall warning, Terrain Awareness and Warning System (TAWS) “Pull Up” alert, or windshear warning).
2.8.1.3 ACAS Xa will generate RAs less often when compared to TCAS II while maintaining an equivalent airborne collision risk mitigation.
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2.8.2 Multiple Threat Encounters. ACAS II is designed to handle multiple threat encounters (encounters where more than one threat is active at the same time). ACAS II will attempt to resolve these types of encounters by selecting a single or composite (initial and subsequent) RA that will provide adequate vertical separation from each of the threat aircraft.
2.8.3 P reventive or Corrective . RAs can be classified as preventive or corrective. Corrective RAs require a change in vertical speed while preventive RAs do not. Preventive RAs instead generate restrictions of vertical speeds.
2.8.4 C oordinated RAs . ACAS II uses explicit coordination, meaning that in an encounter where two ACAS II-equipped aircraft operating in TA/RA mode declare each other a threat, ACAS II will generate RAs based on a real-time exchange of the sense (vertical direction) of the RA. In coordinated ACAS II encounters, nominally, one aircraft will declare the other aircraft to be a threat slightly earlier than the other. The first aircraft that declares the other to be a threat verifies that a coordination message has not been received from the other aircraft, generates an RA based on the encounter geometry, and transmits an intent message (vertical direction of the generated RA) through the Mode S transponder to the other aircraft. The second aircraft, having already received an intent message from the first aircraft, will generate a complementary RA. The complementary RA generated is transmitted back to the other aircraft. Occasionally, the two aircraft declare each other as threats simultaneously, and both aircraft will generate an RA independently based on the encounter geometry. There is a chance that noncomplementary RAs are generated. When this happens, as a tiebreaking function, the ACAS II in the aircraft with the higher 24-bit Mode S transponder address will detect the selection of a noncomplementary RA and generate an RA in the opposite direction.
ACAS II versions 7.0 and higher added the capability for ACAS II to issue RA reversals in coordinated encounters if the encounter geometry changes after the initial RA is issued.
2.8.5 In itial RAs . When ACAS II declares an aircraft a threat, ACAS II will check to see if a coordination message has been received from another ACAS II aircraft. If a coordination message is received, ACAS II will exclude noncomplementary RA possibilities. If a coordination message is not received, ACAS II logic is free to select an RA sense (vertical direction) and strength (the degree of restriction placed on the vertical flight path or the magnitude of the altitude rate change) that best fits the encounter. ACAS II is designed to generate the RA sense and strength that is the least disruptive to the existing flight path, while still providing the desired vertical miss distance (vertical separation).
ACAS II logic prefers to generate an RA with a vertical path that will not cross through the vertical path of the threat aircraft. If ACAS II computes that crossing the vertical path of the threat aircraft is required to obtain the desired vertical miss distance, the generated RA will announce, “Crossing” (e.g., “Climb, Crossing Climb”).
2.8.5.1 TCAS II utilizes the range and altitude tracks of the threat aircraft to model the expected threat aircraft flight path from its present position to CPA. Then, an RA sense is selected that maximizes vertical miss distance. The RA
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.