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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.
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2.12.1.5 In rare situations, ACAS II may generate a false RA due to an ACAS II malfunction, surveillance, or tracking anomalies when a threat is not present.
For instance, ACAS may sense another aircraft in the vicinity much closer than it is, and generate an RA. During an encounter, flightcrews may not have the time to determine if the RA is false. Flightcrews are advised to report any suspicious RAs so that the RA may be investigated to determine whether there is an underlying problem with the ACAS II equipment or design. See Chapter 4, ACAS Reports.
2.13 ACAS II Inhibits. Below is a listing of conditions where specific ACAS II functions are inhibited. Altitudes below have hysteresis values of ±100 feet to ensure that the ACAS II inhibit state does not oscillate rapidly.
2.13.1 When a stall warning, TAWS “Pull Up” alert, or windshear warning occurs, TA and RA aural annunciations will be inhibited, and ACAS II will automatically switch to the TA-ONLY mode of operation. ACAS II will remain in TA-ONLY mode for 10 seconds after the stall warning, TAWS “Pull Up” alert, or windshear warning is removed.
2.13.2 RAs are inhibited if ACAS II is set to TA-ONLY mode.
2.13.3 “CLIMB” and “INCREASE CLIMB” RAs may be inhibited when operating at the certified ceiling or in some flap and landing configurations. ACAS Xa does not inhibit climb RAs at the maximum certified ceiling.
2.13.4 “INCREASE DESCENT” RAs are inhibited below 1,450 feet AGL.
2.13.5 “DESCEND” RAs are inhibited below 1,100 feet AGL.
2.13.6 RAs are inhibited below 1,000 feet AGL. ACAS II switches to TA-ONLY mode automatically. If an RA was active at the time of mode switch, “Clear of Conflict” will not be annunciated.
2.13.7 TA aural annunciations are inhibited below 500 feet AGL. Display symbology of a TA may remain below 500 feet AGL for situational awareness.
3-1 CHAPTER 3. FLIGHTCREW RESPONSIBILITIES
3.1 Flightcrew Considerations. ACAS alone does not ensure safe separation in every case.
ACAS does not alter nor diminish the responsibility or authority of a flightcrew to ensure a safe flight. The following actions optimize the safety functions of ACAS.
3.1.1 Flightcrews are advised to make frequent outside visual scans while using see-and-avoid techniques. Communication with Air Traffic Control (ATC) should be initiated as necessary.
3.1.2 Flightcrews are advised to operate ACAS while in flight in all airspace, including oceanic, international, and foreign airspace.
3.1.3 Flightcrews are advised, in the absence of a Resolution Advisory (RA), to comply with ATC clearances and instructions unless an amended clearance is obtained, or an emergency exists. If unable to comply with ATC clearances and instructions, flightcrews shall inform ATC as soon as possible.
3.1.4 Flightcrews are advised, in the absence of an RA, that complying with ATC turn instructions will not adversely affect vertical maneuvers required by an ensuing RA.
3.1.5 Flightcrews are advised to use the traffic display to increase their situational awareness of nearby traffic and to assist in establishing visual contact with other aircraft.
3.1.6 Flightcrews are advised not to report “TCAS contact,” “we have it on TCAS,” or other nonstandard colloquialisms after traffic information is received from ATC. Such a report provides no benefit to ATC. Correct responses to traffic information include “traffic in sight,” or “negative contact.”
3.1.7 Flightcrews are advised to use a range selection on the traffic display appropriate for the phase of flight (e.g., lower range settings for the terminal area, and higher range settings for climb/descent and cruise, as appropriate).
3.1.8 Flightcrews are advised to attempt to limit vertical speed to less than 1,500 feet per minute (fpm) when within 1,000 feet of assigned altitudes. This procedure will reduce the frequency of RAs and be in conformance with the Aeronautical Information Manual (AIM) and International Civil Aviation Organization (ICAO) guidance. One of the most common cases that cause unwanted RAs is when one or multiple aircraft are leveling off 1,000 feet from an aircraft operating ACAS II, and an RA is triggered due to high vertical rates when approaching the cleared altitude. Some aircraft may have Autopilot (AP) modes installed that are able to limit the vertical speed when approaching the selected altitude. Refer to the Aircraft Flight Manual (AFM)/Aircraft Flight Manual Supplement (AFMS) for more details on this feature.
3.1.9 Flightcrews are advised, prior to positioning on any airport movement area, to verify the Automatic Dependent Surveillance-Broadcast (ADS-B) Out is set to the Flight Identification (FLT ID) that matches the flight plan exactly, and the transponder code is set appropriately, prior to transmitting with ADS-B Out and transponder with altitude
3-2 reporting. Flightcrews are advised to keep ADS-B Out and transponder with altitude reporting enabled throughout the flight (i.e., from completion of the before starting engines or pushback checklist until the shutdown checklist).
3.1.10 Flightcrews are advised not to activate ACAS II (Traffic Advisory (TA)/RA mode or TA-ONLY mode) until just prior to taking the active runway for departure, and ACAS II should be deactivated immediately after clearing the runway after landing.
3.1.10.1 Routine usage of ACAS II on the ground unnecessarily contributes to 1030/1090-megahertz (MHz) frequency congestion and can degrade the performance of ATC radars and surveillance performed by ACAS of airborne aircraft.
3.1.10.2 Flightcrews may use TA-ONLY or TA/RA mode for a short period of time before crossing an active runway to aid in visual acquisition of airborne aircraft in the vicinity (some aircraft will not display when ACAS computes that the aircraft is below 380 feet Above Ground Level (AGL)). Traffic symbols may take several seconds to display after selecting TA-ONLY or TA/RA mode.
3.1.10.3 Refer to the AFM/AFMS for details regarding the use of an automatic mode switching selection.
3.1.11 Flightcrews are advised to operate ACAS II in TA/RA mode to the maximum extent possible while airborne. Any decision to operate ACAS II in TA-ONLY mode is to be carefully considered due to the degradation of ACAS II safety functions. If a flightcrew elects to use TA-ONLY mode, the flightcrew should ensure that ACAS II is returned to the TA/RA mode as soon as possible. The following are examples that may warrant the use of TA-ONLY mode in the National Airspace System (NAS):
3.1.11.1 In the event of a Minimum Equipment List (MEL) deferral requiring the use of TA-ONLY. Refer to FAA Master Minimum Equipment List (MMEL) Policy Letter 32 (PL-32), Traffic Alert and Collision Avoidance System (TCAS).
3.1.11.2 I n the event of non-normal, abnormal, or emergency procedure scenarios;
particular in-flight failures; or performance limiting conditions, as specified by the AFM or operator.
3.1.11.3 During takeoff toward known nearby traffic that is in visual contact and that could cause an unwanted RA during initial climb, such as a visually identified helicopter passing near the departure end of the runway.
3.1.11.4 At certain airports, during particular procedures, or in circumstances identified by the operator as having a significant potential for unwanted or inappropriate RAs.