Traffic Alert and collision avoidance system (TCAS)
Traffic Alert and Collision
Arvind Kumar Singh
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CAS is known as TCAS and internationally as ACAS
(Airborne Collision Avoidance System)
Series of midair collisions led to airline and governmental
action to develop airborne devices that function
independent of air traffic control (ATC) for alerting and
Early attempts (late 50s-60s) for reliable systems proved
to be impractical
Mid 70s Beacon Collision Avoidance System (BCAS)
became the basis for TCAS interrogation and tracking
Aircraft must have operating transponder for TCAS to
provide collision avoidance protection
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Major Midair Accidents
1956 Grand Canyon, AZ — United DC-7 / TWA L-1049 (128 fatalities)
1960 New York, NY — United DC-8 / TWA L-1049 (134 fatalities)
1978 San Diego, CA — PSA B-727 / Cessna 172 (144 fatalities)
1986 Cerritos, CA — Aeromexico DC-9 / Piper PA-28 (82 fatalities)
1996 Charkhi Dadri, India — Saudi B-747 / Kazakhstan IL-76 (349 fatalities)
1997 Namibia (off-coast) — USAF C-141 / German AF Tu-154 (33 fatalities)
2002 Überlingen, Germany — Bashkirian Tu-154 / DHL B-757 (71 fatalities)
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• An airborne system that interrogates transponders in other
aircraft.(Independently of Ground based equipment)
• From the replies received, the system analyzes the
Aircraft’s altitude (if altitude reporting),
Projected flight path to predict a penetration
• Displays the other aircraft visually
To alert any potential threats of airspace conflict.
Report a resolution advisory (RA) only if the other
transponder is transmitting an altitude.
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•Closest Point of Approach (CPA):
Predicted point in space at which the intruder will be closest to aircraft.
An SSR transponder equipped aircraft within the surveillance range.
Recognizes as a potential threat
Indication given to the flight crew that a intruder is a potential threat
and entering protected airspace
20-48 seconds from CPA
Maneuver intended to provide separation from all threats
Maneuver restriction intended to maintain existing separation
15-35 seconds from CPA
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TCAS – System Description
TCAS uses on-board surveillance to
detect transponder-equipped traffic and
Traffic Display and Traffic Alerts (TA) for
situational awareness of close aircraft
Resolution Advisories (RA) with vertical
Alerts are based on both projected:
Time to Closest Point of Approach (CPA)
Miss distance less than
600 to 800’ depending on altitude
(vertical miss distance)
0.2 to 1.1 NM depending on altitude
(horizontal miss distance)
RAs are selected to achieve or maintain
adequate vertical distance (300 – 700’)
and minimize pilot response/vertical
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Standardized Symbology for
Own-aircraft. Airplane-like symbol, in
white or cyan.
Other Traffic, altitude unknown.
Unfilled diamond in white or cyan
Proximate Traffic, 1100 feet above and
descending. Filled diamond in white or
Traffic Advisory (TA), 900 feet below
and level. Filled yellow/amber circle.
Resolution Advisory (RA), 500 feet
below and climbing. Filled red square.
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Functions of TCAS II
Interrogations at 1030 MHz and respond at 1090 Mhz.
Reliable surveillance to a range of 14 NM and in traffic densities of up to
0.3 aircraft per square NM.
•Generation of TA
•Generation of RAs
•Communication with Ground Stations
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Salient Features of TCAS
Collision Avoidance System
Predicts time to and separation at the intruders CPA using
Altitude and vertical speed
If further close, will issue RA ( Five second crew reaction
time to achieve separation)
Display different traffic symbols , changes colour and shape
depends upon urgency
Can track 45 aircraft and display 30 of them
21 aircraft in a 5 NM radius, can provide advisory
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Alerting Threshold Values
TCAS alerts based on time to closest approach and time to being co-
All RAs are inhibited below 1,000’ AGL
Fixed distance alerting thresholds are also used in some situations
Many parallel runway operations
500’ IFR/VFR separation when both aircraft are level
TCAS does not consider IFR/VFR status or pilot intentions
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Transition from Passive to Active Surveillance
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TCAS Protection Volume
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CAS Logic Functions
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Ground Estimation Logic
• Aircraft is below 1750 ft AGL.
• Ground level estimate is
subtracted from the pressure
altitude received from each Mode
C equipped aircraft.
to determine each aircraft above
• If this difference is less than 360
feet, TCAS considers the reporting
aircraft to be on the ground.
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RA Sense Selection
Selection of Non-crossing
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Increase Rate RA
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Concept of Operation
• TCAS creates a “bubble” of protected airspace around aircraft.
• TCAS begins to “track” other transponder equipped aircraft.
• At approx 40 NM out other transponder equipped aircraft can be
displayed on the active PFD/MFD (or depends upon range).
• At 30 approx NM a data-link is established with other Mode S
transponders to begin conflict resolution planning.
• At 20 to 48 seconds prior to CPA a TA is issued.
• AT 15 to 35 seconds from CPA an RA is generated.
• If the RA is ignored, the crew response is too slow, or the conflict
traffic alters its course a corrective RA is issued.
• When the traffic is no longer a threat TCAS announces “CLEAR
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Carriage of Airborne Collision Avoidance
Unless otherwise authorized by DGCA , no person shall
operate in the Indian airspace , an aeroplane having a
maximum certified passenger seating configuration of
more then 30 or maximum payload capacity of more then
After 31st December 1998, if it is not equipped with an
approved TCAS II and
After 1st Jan 2003, if it is not equipped with an approved
TCAS II with change 7 (equivalent to ACAS`)
After 1st Jan. 2002 for the purpose of registration and
operation within, to and from India shall be fitted with
TCAS II with change 7
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Operational Circular 4 of 2002
All crew must reduce the aircraft rate of climb or
descent as applicable
500 ft per minute or less when the airplane is 2000 ft
to level off altitude.
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• If an aircraft responding to a TCAS RA
Do not issue control instructions that are contrary to the RA,
which crew are executing.
Provide safety alerts regarding terrain or obstructions and traffic
• Unless advised by other aircraft to responding to a TCAS RA,
Do not assume that the other aircraft in proximity involved in the RA
Continue to provide control instructions, safety alerts, and traffic
advisories as appropriate to such aircraft.
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• Issue traffic advisories
• Except more RAs, when working aircraft that climb descend quickly.
• Controller is not responsible for providing standard separation
between the aircraft that is responding to an RA and any other
aircraft, airspace, terrain, or obstructions.
• Responsibility for standard separation resumes when-
The responding aircraft has returned to its assigned altitude.
The flight crew informs that the TCAS maneuver is completed and
standard separation has been established.
The responding aircraft has executed an alternate clearance and
standard separation has been re-established.
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After a flight crew starts to deviate from any ATC
clearance or instruction to comply with an ACAS
resolution advisory (RA)
* TCAS RA;
After the response to an ACAS RA is completed
and a return to the ATC clearance or instruction
*CLEAR OF CONFLICT,
RETURNING TO (assigned
ROGER (or alternative
After the response to an ACAS RA is completed
and the assigned ATC clearance or instruction
has been resumed
* CLEAR OF CONFLICT
(assigned clearance) RESUMED;
ROGER (or alternative
After an ATC clearance or instruction
contradictory to the ACAS RA is received, the
flight crew will follow the RA and inform ATC
* UNABLE, TCAS RA;
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for your patience
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