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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.
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3.2 TA Response. Respond to TAs by attempting to establish visual contact with the intruder aircraft and other aircraft that may be in the vicinity. If needed, adjust the range on the traffic display to ensure all available information on the intruder is shown. Coordinate to the degree possible with other crewmembers to assist in searching for traffic. Do not deviate from an assigned clearance based solely on TA information. For any intruder acquired visually, continue to maintain safe separation in accordance with current regulations and good operating practices.
3.2.1 Flightcrews are advised to utilize information from the traffic display to aid in visual acquisition of the intruder aircraft, including the bearing, range, relative altitude, and vertical speed trend.
3.2.2 Flightcrews are advised to utilize all available information to assist in visual acquisition, including air traffic controller traffic alerts or safety alerts.
3.2.3 The Pilot Flying (PF) is advised to continue to fly the aircraft and be prepared to respond to any RA that might follow. The Pilot Monitoring (PM) is advised to provide updates on the traffic location shown on the traffic display, using this information to assist in visual acquisition of the intruder.
3.2.4 Flightcrews are advised to confirm that the aircraft they have visually acquired is the aircraft that caused the TA to be generated using all available information.
3.2.5 Flightcrews are not advised to maneuver the aircraft based solely on the information shown on the traffic display due to limitations of bearing accuracy.
3.2.6 When visual acquisition is attained, right-of-way rules are used to maintain safe separation, unless an RA is generated. Flightcrews are not advised to initiate unnecessary maneuvers.
3.2.7 Flightcrews are advised caution when maneuvering based solely on visual acquisition, especially at high altitude or without a clearly defined horizon. Avoidance maneuvers based solely on a TA may cause the situation to deteriorate further. Visual assessment does not provide information on the intent or trajectory of the traffic and can be misleading (e.g., the pitch angle of a heavy aircraft at low speed makes it difficult to assess whether the aircraft is climbing, level, or descending).
3.2.8 Flightcrews are not advised to attempt to adjust the current flight path in anticipation of what an RA may advise.
3.2.9 TAs from TCAS I, an ACAS II in TA-ONLY mode, or ACAS Xa generating an ADS-B Only TA-ONLY (AOTO) advisory will not progress to an RA.
3.3 RA Response. Flightcrews are advised to comply with an RA to the degree displayed with an instinctive, immediate reaction. For ACAS II to work as designed, immediate and correct crew response to an RA is essential. Delayed response or reluctance of a flightcrew to comply with the RA due to ATC clearance provisions, fear of later FAA
3-4 scrutiny, or other factors could significantly decrease or negate the protection afforded by ACAS II.
3.3.1 M aneuvering in Response to an RA. When an RA occurs, the PF is advised to respond immediately by directing attention to the RA displays and to maneuver as indicated with positive control inputs, unless doing so would jeopardize the safe operation of the flight, or the flightcrew can ensure separation with definitive visual acquisition of the aircraft causing the RA. The PM provides updates on the traffic location, cross-checks between the traffic display, and monitors the response to the RA using proper Crew Resource Management (CRM).
3.3.1.1 F or ACAS II to provide safe vertical separation, initial vertical speed response is required within 5 seconds of when the RA is first displayed based on a
0.25 g vertical acceleration. Some autoflight systems have the capability to respond automatically to RAs. These systems may respond with a lower vertical acceleration yet may provide a faster response time when compared to manual flight operations.
3.3.1.2 RAs take precedence over any ATC instructions, clearances, requirements to follow a specific route, right-of-way rules, cloud clearance rules for Visual Flight Rules (VFR), or other such criteria. If a flightcrew receives ATC instructions to maneuver that conflict with an RA, the flightcrew is advised to comply with the RA. Flightcrews that deviate from an ATC clearance or instruction in response to an RA are advised to notify ATC of that deviation as soon as practicable.
3.3.1.3 Flightcrews are advised to comply with an RA while complying with an ATC clearance, if possible, when an RA and ATC clearance or instruction are congruent (e.g., when a flightcrew is able, comply with an ATC instruction and level at the assigned altitude while responding to a reduce climb or reduce descent RA).
3.3.1.4 If an RA is generated while the aircraft is turning, and the turn makes achieving the required vertical rate difficult or impossible, the turn should be stopped (i.e., leveling the wings to increase the rate of climb). Stopping the turn may close the horizontal proximity to the threat aircraft, but ACAS is evaluating the encounter and will change the RA if required.
3.3.1.5 Maneuvering in response to an RA should be limited to the minimum required to comply with the RA. An excessive maneuver response to an RA is not desirable or appropriate due to other potential conflicts (e.g., multiple threat encounter) and ATC implications, as well as the hazard of injury to the occupants of an excessively maneuvering aircraft.
3.3.1.6 Vertical speed responses should be made to avoid red arcs on the Vertical Speed Indicator (VSI) or outlined pitch avoidance areas on the Attitude
3-5 Direction Indicator and, if applicable, to accurately fly to the green arc or outlined pitch guidance area.
3.3.1.7 Comply with RAs by disconnecting the AP (if necessary) and the autothrottle system (if necessary), using prompt positive control, and thrust inputs in the direction and with the magnitude that is generated by the RA.
3.3.1.8 Some autoflight systems have the capability to respond automatically to RAs.
Pilots are advised to follow airframe manufacturer and/or operator flight path management policies and procedures in response to an RA.
3.3.1.9 During RAs, flightcrews should closely monitor the correct responses of the AP ACAS mode (if installed). The PF is advised to disengage the AP and fly the aircraft manually if the AP/Flight Director (FD) ACAS mode is not complying with RA vertical speed.
3.3.1.10 On aircraft with pitch guidance for RAs, comply with the RA pitch guidance, and verify the pitch guidance is appropriate for the RA. On aircraft without pitch guidance, achieve the required vertical speed (normally 1,500 fpm climb or descent) by first adjusting the pitch attitude using the suggested guidelines shown in the table below. Then, refer to the VSI and make all necessary pitch adjustments to place the VSI in the green arc. See Table 3-1.
Table 3-1. Initial Pitch Adjustment Response to a Resolution Advisory (Based on a Transport Category Airplane) Indicated Airspeed Pitch Adjustment Mach 0.80 2° 250 knots below 10,000 feet 4° Approach below 200 knots 5° to 7°
3.3.1.11 RAs are not coordinated with non-ACAS II-equipped aircraft (or ACAS II-equipped aircraft not operating in TA/RA mode). Threat aircraft may maneuver based on the see-and-avoid principle or ATC instructions. The threat aircraft may perform maneuvers that cause a change in the RA response.
3.3.1.12 During a coordinated RA, another ACAS II-equipped aircraft may incorrectly comply with ATC instructions contrary to the RA. This action may cause a change in the RAs generated for own-ship or multiple aircraft. Flightcrews are advised to respond immediately to any change in RA. Vertical speed response to an increase or reversal RA is required within 2.5 seconds after issuance of the advisory based on a 0.35 g vertical acceleration. Avoid red arcs or outlined pitch avoidance areas and fly to the green arc or outlined pitch guidance area.
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3.3.1.13 The PM should advise the PF on the progress of complying with the RA-generated vertical speed. The PM and any onboard observers will assist in the visual search for the threat aircraft and continue to cross-check ACAS II displayed information with other available traffic information to ensure the RA response is being flown correctly using proper CRM.
3.3.1.14 If an initial corrective RA is downgraded to a weakened RA (e.g., a “Climb, Climb” RA downgraded to a “Level Off, Level Off”), flightcrews are advised to respond to the weakened RA by adjusting the vertical speed to the rate depicted by the VSI green arc. Flightcrews should keep the VSI needle out of the red arc and/or the pitch guidance symbol out of the outlined pitch avoidance area. Attention to the RA display and prompt reaction to a weakened RA will minimize altitude excursions and potential disruptions to ATC. A proper reaction to a weakened RA will allow for proper resolution of the encounter and reduce the probability of subsequent RAs against the threat aircraft or another displayed aircraft.
3.3.1.15 Flightcrews are advised that, when operating at maximum certified altitude with an aircraft that is not climb inhibited, the proper response to a climb RA is to climb. The flightcrew is advised to comply with the RA unless doing so would jeopardize the safety of the aircraft. Flightcrews are advised to consider their energy management and specific aircraft flight envelope protection features. A stall warning will inhibit an RA.
3.3.1.16 ACAS II installations programmed to not generate a climb RA when operating at the maximum certified altitude will generate a “DO NOT DESCEND” or “DESCEND, DESCEND” RA instead.
3.3.1.17 When a “Clear of Conflict” annunciation is generated, the flightcrew is advised to immediately return to their previously assigned clearance and advise ATC, or comply with any amended clearance issued. Immediately returning should not be mistaken for aggressively returning. Another RA may be generated due to a flightcrew aggressively returning to their clearance when the threat aircraft is still a factor.
3.3.1.18 Flightcrews are advised to execute a missed approach/go-around if, as a result of an RA, the flightcrew will not be able to maintain a stabilized approach to a landing within the available runway.
3.3.2 Not Responding to RAs/Disregarding RAs.
3.3.2.1 Response to a stall warning, windshear warning, or Terrain Awareness and Warning System (TAWS) “Pull Up” alert takes precedence over an RA maneuver.
3.3.2.2 Flight path requirements affected by terrain, such as an obstacle-limited climb segment or an approach to rising terrain, may conflict with an RA. Since the basis for many approved instrument procedures and Instrument Flight Rules
3-7 (IFR) clearances is avoiding high terrain or obstacles, it is particularly important that the flightcrew maintains situational awareness and continues to use good operating practices and judgment when complying with RAs.
3.3.2.3 Flightcrews of aircraft with autoflight systems with an RA mode are advised to understand how to disable the autoflight mode if the autoflight system response is incorrect or undesired. Flightcrews should exhibit correct flight path management in automated or manual flight operations.
3.3.2.4 RAs may occur while aircraft are separated in accordance with established standards. Even when closely spaced parallel approach procedures are correctly applied, unwanted RAs may occasionally occur. However, the safety benefit provided by ACAS II takes precedence over an occasional unwanted RA. There is always the possibility another aircraft that is not associated with the parallel approach procedure becomes a threat. The term “nuisance alert” is commonly used to describe an RA when it is not desired. This is a misnomer;
ACAS II is operating as designed, and flightcrews are advised to comply with the RA. The evaluation of whether a generated RA is a valid threat may not be possible in real time.
3.3.2.5 RAs generated in the traffic pattern environment should not be dismissed as unnecessary and/or disruptive. These RAs inform the flightcrew that a hazard of collision exists.
3.3.2.6 By not responding to an RA, the flightcrew negates the safety benefits provided by own-ship ACAS and effectively takes responsibility for achieving safe separation. In so choosing, flightcrews are advised to consider the following cautions:
3.3.2.6.1 The traffic acquired visually may not be the same traffic causing the RA.
Visual perception of the encounter may be misleading. The flightcrew is advised to respond to the RA to the degree displayed with an instinctive, immediate reaction, unless there is no doubt that the aircraft acquired visually is the threat aircraft, the threat aircraft is not a collision hazard, and there are no other complicating circumstances.
3.3.2.6.2 The traffic may also be operating ACAS II, and it may maneuver in response to an RA coordinated with own-ship ACAS II.
3.3.2.6.3 It is difficult to visually determine the vertical displacement of other aircraft, especially when ground reference information is unreliable or at cruise altitudes where the horizon is obscured. Therefore, disregarding RA information and maneuvering vertically based solely on visual acquisition may result in a loss of safe separation.