Warning Systems - TCAS

Amendment: ACAS Bulletin 11 included.

Traffic Alert and Collision Avoidance System (TCAS)

Airborne Collision and Avoidance System (ACAS) Version 7.0

General

TCAS Envelope [Honeywell - Understanding ACAS/TCAS II]

TCAS alerts the crew to conflicting traffic. The system identifies a three-dimensional airspace around the airplane where a high likelihood of air traffic conflicts exist. These dimensions depend upon closure rates between the airplane and potentially conflicting traffic.

TCAS interrogates operating transponders in other aircraft, analyzes the replies, predicts flight paths and designates possible conflicting traffic as a "traffic aircraft."

FAA TCAS II is designed to provide collision avoidance protection in the case of any two aircraft that are closing horizontally at any rate up to 1200 knots and vertically up to 10,000 feet per minute.

When the system designates a traffic aircraft, TCAS provides the flight crew with a situational display. Additionally, TCAS may provide an aural annunciation and flight path guidance.

Note: Other aircraft that do not have an operating transponder can not initiate situational displays, aural annunciations or flight path guidance.

Note: TCAS is independent of ground-based air traffic control.

During normal operations, when TCAS designates a traffic aircraft, the system provides the following advisories and displays:

 

Advisory Triggers (Range and Altitude Test)

Normal Operations

TCAS is enabled from the Transponder Panel. The system is normally operated with the TCAS Mode Selector in the TA/RA mode.

VSI The TA ONLY mode may be used:

The TA mode may be used:

EuroControl Each symbol is displayed on the screen according to its relative position to own aircraft. The display accuracy depends upon the selected scale. When the 10 nm scale is in use the positional accuracy is ± 1 nm and approximately ±10° in bearing. Targets are artificially smoothed to prevent jitter.

Resolution Advisories (RA) and Displays

A Resolution Advisory (RA) is an immediate-threat prediction that traffic aircraft will enter the TCAS collision airspace within approximately 20 to 30 seconds. If altitude data from the traffic aircraft’s transponder is not available, no RA can be provided.

When TCAS issues a RA:

Voice Alert

When TCAS issues a RA, voice alerts will sound. These voice alters aurally elaborate on the displayed Vertical Guidance and are described in this Chapter under:

Vertical Guidance

Vertical guidance is displayed for a traffic-avoidance maneuver. Traffic avoidance is ensured by adjusting or maintaining:

Note: If the traffic aircraft also has TCAS and an operating mode S transponder, vertical guidance is coordinated with the traffic aircraft.

TCAS computes the most appropriate vertical manoeuvre in order to achieve a target vertical miss distance of 600 ft.

Display Symbology

Resolution Advisory (RA).

The RA traffic symbol is a filled red square with an accompanying data tag when the traffic aircraft is providing altitude information.

The data tag appears in red and contains the following information about the traffic aircraft:

The RA is displayed as follows:

VSI
  • When the traffic aircraft is within the display range, the traffic symbol’s relative position is displayed.
  • Off Scale Advisory
    When the RA is outside the display range, the red RA symbol appears as an half-filled red square and is positioned relative to the airplane on the outer edge of the display.
  • No Bearing Advisory
    When TCAS is unable to track the traffic aircraft’s bearing, a No Bearing Annunciation is displayed in the lower center of the VSI.
Tcas Resolution Advisory Eadi.
HSI
  • When the red TRAFFIC message appears and the traffic aircraft is within the selected display range, the traffic symbol’s relative position is displayed.
  • Off Scale Advisory
    When the traffic aircraft is outside the selected range, the red OFFSCALE message appears.
  • No Bearing Advisory
    When TCAS is unable to track the traffic aircraft’s bearing, the red RA symbol is displayed below the TRAFFIC message.

Traffic Advisories (TA) and Displays

A Traffic Advisory (TA) is a prediction that traffic aircraft will enter the TCAS collision airspace within approximately 35 to 40 seconds. TAs are intended to assist the crew in establishing visual contact with the traffic aircraft.

When TCAS issues a TA:

Voice Alert

When TCAS issues a TA, the Voice Annunciation, TRAFFIC, TRAFFIC will sound to aurally alert the crew.sound clip

Display Symbology

Traffic Advisory (TA).

The TA traffic symbol is a filled amber circle with an accompanying data tag when the traffic aircraft is providing altitude information.

The data tag appears in amber and contains the following information about the traffic aircraft:

The TA is displayed as follows:

VSI
  • When the traffic aircraft is within the display range, the traffic symbol’s relative position is displayed.
  • Off Scale Advisory
    When the TA is outside the display range, the amber TA symbol appears as an half-filled amber square and is positioned relative to the airplane on the outer edge of the display.
  • No Bearing Advisory
    When TCAS is unable to track the traffic aircraft’s bearing, a No Bearing Annunciation is displayed in the lower center of the VSI.
HSI Displays HSI
  • When the amber TRAFFIC message appears and the traffic aircraft is within the selected display range, the traffic symbol’s relative position is displayed.
  • No Bearing Advisory
    When TCAS is unable to track the traffic aircraft’s bearing, the amber TA symbol is displayed below the TRAFFIC message.
  • Off Scale Advisory
    When the traffic aircraft is outside the selected range, the amber OFFSCALE message appears.

Automatic TA and RA Display

TCAS automatically displays RA and TA symbols on the HSI when:

  • a RA or TA occurs, and
  • neither pilot has pushed the EFIS Traffic (TFC) Switch, and
  • the HSI Mode Selector is in the VOR, APP, or MAP mode, and
  • the TCAS Mode Selector is in TA/RA or TA

Proximate Traffic and Other Traffic Displays

Proximate Traffic

Proximate Traffic

Proximate Traffic is a traffic aircraft that is neither a RA nor a TA but is within:

Other Traffic

Other Traffic

Other Traffic is a traffic aircraft that is neither a RA, TA, or Proximate Traffic.

When TCAS identifies Proximate Traffic or Other Traffic:

If the other airplane is not providing altitude information, other traffic becomes Proximate Traffic automatically when within six miles.

Display Symbology

The Proximate Traffic symbol is a filled diamond and the Other Traffic symbol is a hollow diamond. Both Proximate Traffic and Other Traffic symbols are displayed with an accompanying data tag when the traffic aircraft is providing altitude information.

The data tag contains the following information about the traffic aircraft:

Proximate Traffic and Other Traffic are displayed as follows:

VSI
  • When Proximate Traffic is within the display range, the traffic aircraft’s relative position is displayed as a filled cyan diamond.
  • When Other Traffic is within the display range, the traffic aircraft’s relative position is displayed as an unfilled cyan diamond.
HSI
  • When Proximate Traffic is within the selected display range, the traffic aircraft’s relative position is displayed as a filled white diamond.
  • When Other Traffic is within the selected display range, the traffic aircraft’s relative position is displayed as an unfilled white diamond.

TCAS Voice Annunciations

Voice Annunciations for VSI / ADI Guidance

Voice Alert Condition Response
TRAFFIC, TRAFFIC TCAS has issued a TA Attempt to visually locate the traffic.
Voice Alert Condition Response
CLIMB, CLIMB

CLIMB

VSI Present vertical speed is within the red RA regions

ADI Present ADI pitch attitude is within the red RA regions

Adjust vertical speed to remain outside the red RA regions

Adjust ADI pitch attitude to remain outside the red RA regions

DESCEND, DESCEND

DESCEND

Present vertical speed is within the red RA regions

Present ADI pitch attitude is within the red RA regions

Adjust vertical speed to remain outside the red RA regions

Adjust ADI pitch attitude to remain outside the red RA regions

ADJUST VERTICAL SPEED,
ADJUST

TCAS requires change in pitch attitude

Present vertical speed is within the red RA regions

Present ADI pitch attitude is within the red RA regions

Adjust vertical speed to remain outside the red RA regions

Adjust ADI pitch attitude to remain outside the red RA regions

MONITOR VERTICAL SPEED

Present vertical speed is outside the red RA regions

Present ADI pitch attitude is outside the red RA regions

Continue to keep vertical speed outside the red RA regions

Continue to keep ADI pitch attitude outside the red RA regions

MAINTAIN
VERTICAL SPEED,
MAINTAIN

Present vertical speed is outside the red RA regions

Present ADI pitch attitude is outside the red RA regions

Continue to keep vertical speed outside the red RA regions

Continue to keep ADI pitch attitude outside the red RA regions

CLIMB, CROSSING CLIMB,
CLIMB, CROSSING CLIMB

Present vertical speed is within the red RA regions

Present ADI pitch attitude is within the red RA regions

Airplane will climb through the traffic aircraft’s altitude

Adjust vertical speed to remain outside the red RA regions

Adjust ADI pitch attitude to remain outside the red RA regions

MAINTAIN
VERTICAL SPEED,
CROSSING,
MAINTAIN

Present vertical speed is outside the red RA regions

Present ADI pitch attitude is outside the red RA regions

Airplane will pass through the traffic aircraft’s altitude

Continue to keep vertical speed outside the red RA regions

Continue to keep ADI pitch attitude outside the red RA regions

DESCEND, CROSSING DESCEND,
DESCEND, CROSSING DESCEND

Present vertical speed is within the red RA regions

Present ADI pitch attitude is within the red RA regions

Airplane will descend through the traffic aircraft’s altitude

Adjust vertical speed to remain outside the red RA regions

Adjust ADI pitch attitude to remain outside the red RA regions

INCREASE CLIMB,
INCREASE CLIMB

TCAS requires change in pitch attitude

Present vertical speed is within the red RA regions

Present ADI pitch attitude is within the red RA regions

Adjust vertical speed to remain outside the red RA regions

Adjust ADI pitch attitude to remain outside the red RA regions

INCREASE DESCENT,
INCREASE DESCENT
CLIMB, CLIMB NOW,
CLIMB, CLIMB NOW

Previous vertical guidance was to descend

Present vertical speed is within the red RA regions

Present ADI pitch attitude is within the red RA regions

Climb and adjust vertical speed to remain outside the red RA regions

Climb and adjust ADI pitch attitude to remain outside the red RA regions

DESCEND, DESCEND NOW,
DESCEND, DESCEND NOW

Previous vertical guidance was to climb

Present vertical speed is within the red RA regions

Present ADI pitch attitude is within the red RA regions

Descend and adjust vertical speed to remain outside the red RA regions

Descend and adjust ADI pitch attitude to remain outside the red RA regions

ADJUST VERTICAL SPEED,
ADJUST

Minimum vertical speed required to ensure separation has decreased

Present vertical speed is outside the red RA regions

Minimum ADI pitch attitude required to ensure separation has decreased

Present ADI pitch attitude is outside the red RA regions

Adjust vertical speed

Continue to keep vertical speed outside the red RA regions

Adjust ADI pitch attitude

Continue to keep ADI pitch attitude outside the red RA regions

Voice Alert Condition Response
CLEAR OF CONFLICT

Vertical guidance is no longer displayed and traffic symbology changes to TA

Separation between the airplane and the traffic aircraft is increasing

CLEAR OF CONFLICT will not sound if TCAS can no longer predict the track of the traffic aircraft

Attempt to visually locate the traffic aircraft

TCAS II Version 7 replaced both REDUCE CLIMB, REDUCE CLIMB and REDUCE DESCENT, REDUCE DESCENT with ADJUST VERTICAL SPEED ADJUST. The objective is to solve a predicted risk of collision by a reduction of the current vertical speed to either 2000, 1000, 500, or 0 feet per minute.

Inhibits

INCREASE DESCENT RAs are inhibited below approximately 1,450 feet Radio Altitude.

DESCEND RAs are inhibited below approximately 1,100 feet Radio Altitude (1,000 feet climbing / 1,200 feet descending).

VSI All TCAS voice annunciations and all RAs are inhibited below approximately 1,000 feet Radio Altitude (1,100 feet climbing / 900 feet descending). Below approximately 1,000 feet when the TA/RA mode is selected on the transponder panel, TA only mode is enabled automatically.

HSI RAs are inhibited below approximately 1,000 feet Radio Altitude (1,100 feet climbing / 900 feet descending). Below approximately 1,000 feet when the TA/RA mode is selected on the transponder panel, TA only mode is enabled automatically. All TCAS voice annunciations are inhibited below approximately 500 feet Radio Altitude.

Note: GPWS Immediate-Alert and Windshear Immediate-Alert annunciations inhibit all TCAS alerts.

Honeywell TCAS II does not display aircraft which are on the ground. TCAS considers an intruder to be on the ground when own aircraft is below 1700 ft AGL and the intruder is estimated to be within 400 ft of the ground.

Non-Normal Operations

VSI Messages

When the VSI message:

  • RA is displayed the RA symbology, vertical guidance and voice annunciations are unreliable.
  • TCAS is displayed, the TCAS system is not reliable.

HSI Messages

When the HSI message:

  • TCAS OFF is displayed, neither TA nor TA/RA is selected with the TCAS Mode Selector. The system cannot display symbology or vertical guidance. Voice Annunciations will not occur.
  • TCAS FAIL is displayed, the system cannot display symbology or vertical guidance. Voice Annunciations will not occur.

Note: TCAS OFF does not display if TCAS FAIL is annunciated.

EICAS Messages

When the EICAS advisory message:

  • TCAS is displayed, the system cannot display symbology or vertical guidance. Voice Annunciations will not occur.
  • TCAS OFF is displayed, neither TA nor TA/RA is selected with the TCAS Mode Selector. The system cannot display symbology or vertical guidance. Voice Annunciations will not occur.

Supplemental Notes

Honeywell The following notes are from the Honeywell document Understanding ACAS/TCAS II

Reaction Times

Resolution advisories which require a climb or descent are based on a rate of change of 1500 fpm, taking into account a standard reaction time of 5 seconds and vertical acceleration of 0.25 G.

If a threat maneuvers such that TCAS must either increase its vertical rate from 1500 to 2500 fpm or reverse the maneuvering sense, such changes are predicated on a standard reaction time of 2.5 seconds and vertical acceleration of 0.35 G.

Closest Point of Approach

It is possible to see an aircraft flying the same course and direction as your own aircraft and be in close proximity to your aircraft, yet TCAS II may not consider it a threat. TCAS II calculates the closure rate of the intruder, and derives the time to the closest point of approach (CPA). If there is no closure rate, no advisory will be issued, unless the intruder is very close (within approximately 0.25 mile).

Interference Limiting and Reduced Surveillance

To ensure that TCAS operations do not affect ground-based radars, TCAS must reduce output power and processor interrogations when a significant number of TCAS units are in the vicinity. Although the display is affected, the collision avoidance function remains unaffected. In very dense traffic environments the surveillance range may fall from the nominal 14 miles to as low as 5 miles.

In dense traffic areas at altitudes below FL180, the rate of interrogation, usually once per second, drops to once per 5 seconds for intruders considered to be non-threatening and at least 3 nm from own aircraft and which would not trigger an advisory for 60 seconds.

EICAS Messages