CONTENTS
Amendment: ACAS Bulletin 12 included.
Airborne Collision and Avoidance System (ACAS) Version 7.0
From the Eurocontrol website:
http://www.eurocontrol.int/acas/
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)
TCAS II triggers an advisory when a range test and altitude test are both satisfied. These tests are performed on each altitude-reporting target every second.
The principle of the range test is to compute the time-to-go to the Closest Point of Approach (CPA) by dividing the distance between the aircraft by the closure rate. The range test is satisfied if this time is lower than a threshold value, typically 20 to 35 seconds for RAs and 35 to 48 seconds for TAs.
To address the specific case of very low closure rates, a protection distance has been introduced to avoid an intruder coming very close in range without triggering any advisories. For this type of encounter, the range test is satisfied if the distance between aircraft is lower than this protection distance of 1.1 nm for RAs and 1.3 nm for TAs.
The principle of the altitude test is to compute the time to reach co-altitude by dividing the relative altitude between aircraft by the relative vertical speed. The altitude test is satisfied if this time is lower than a threshold value, typically 20 to 35 seconds for RAs and 35 to 48 seconds for TAs. If the own aircraft is level, the time threshold value for RAs is reduced to 25 seconds to detect a possible level-off manoeuvre of the intruder and thus to avoid the triggering of the RA.
To address the specific case of level aircraft or aircraft with a very low relative vertical speed, the altitude test is satisfied if the relative altitude between aircraft is lower than a fixed vertical distance threshold of 850 ft for TAs and 700 ft for RAs.
On early aircraft, TCAS is displayed on the VSI.
On later aircraft, with improved Symbol Generators, TCAS is displayed on the EHSI.
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.
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:
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 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.
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:
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:
When TCAS issues a TA, the Voice Annunciation, TRAFFIC, TRAFFIC will sound to aurally alert the crew.
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:
TCAS automatically displays RA and TA symbols on the HSI when:
Proximate Traffic is a traffic aircraft that is neither a RA nor a TA but is within:
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.
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:
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
Present vertical speed is within the red RA regions
Present ADI pitch attitude is within the red RA regions
TCAS requires change in pitch attitude
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
Airplane will climb through the traffic aircraft’s altitude
Airplane will pass through the traffic aircraft’s altitude
Airplane will descend through the traffic aircraft’s altitude
Previous vertical guidance was to descend
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
Previous vertical guidance was to climb
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
Minimum vertical speed required to ensure separation has decreased
Minimum ADI pitch attitude required to ensure separation has decreased
Adjust vertical speed
Adjust ADI pitch attitude
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. With TCAS 7.1, ADJUST VERTICAL SPEED ADJUST is replaced by the instruction LEVEL OFF, LEVEL OFF.
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
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.
When the VSI message:
When the HSI message:
Note: TCAS OFF does not display if TCAS FAIL is annunciated.
When the EICAS advisory message:
Honeywell The following notes are from the Honeywell document Understanding ACAS/TCAS II
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.
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).
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