Airborne Collision Avoidance System (ACAS)

Wg Cdr Rajagopal

Airborne Collision Avoidance System

Index

Introduction to ACAS

  • Airborne collision avoidance system is called Traffic collision avoidance system
  • TCAS II is mandatory for all commercial aircraft with 19 passengers or more
  • TCAS II detects, assesses and warns of collision risk
  • Warnings consist of Traffic and Resolution Advisories
  • Traffic Advisories provide proximity traffic information only.
  • Resolution Advisories offer maneuvering advice in the vertical plane.

Principle of ACAS

  • ACAS works on the principle of secondary radar in air to air mode
  • Minimum requirement for ACAS are Mode A SSR transponders
  • Frequencies used is 1030 to 1090 MHz just like SSR
  • TCAS creates two protective three-dimensional bubbles around aircraft
  • Bubbles represent the time to closest point of contact

Types of ACAS Advisories

  • ACAS provides advisories based on the type of transponder
    • Mode A does not transmit height therefore can only give Traffic Advisories.
    • Mode C transponders provide both traffic as well as resolution advisories
    • Mode S transponders allow mutually resolved avoidance maneuvers

Operation of ACAS-TCAS

  • TCAS measures range of an intruder by measuring the time lapse principle.
  • Interrogation signal is sent once a second and detection range of 30 NM.
  • Reduced surveillance mode is used in high traffic density areas
  • Interrogation signal is sent once in 5 seconds and detection range is 5 NM.
  • Altitude information is obtained directly from the Mode C transponder
  • Bearing of an intruder is determined by a directional antenna.
  • Resolution of bearings by ACAS is un reliable due to short wavelength
  • Collision avoidance commands are not given in the horizontal plane

Traffic Advisory

  • Traffic Advisories cautions when an intruder penetrates the outer bubble
    • TA means that the traffic is between 45 and 35 seconds to collision
    • TAs appear as solid yellow amber circles on the TCAS display
    • Synthetic voice announces “Traffic, Traffic”

Preventive Resolution Advisory

  • Resolution Advisories alerts if intruder penetrates the inner bubble
  • RA means traffic is between 35 and 15 seconds from the collision
  • RAs appear as solid red rectangles on the TCAS
  • Synthetic voice warnings are issued.
  • Preventative Advisories indicates no collision risk unless levels change
  • The synthetic voice advisory is “Monitor Vertical Speed”

Corrective Resolution Advisory

  • Corrective Resolution Advisories are situations where collision risk exists
  • Manoeuvre is necessary to avert collision.
  • The synthetic voice produces the appropriate command
    • Reduce Climb
    • Increase Climb
    • Descend
    • Reduce Descent

Mode S SSR Advisory

  • Mode S transponder uses unique identification
  • Resolution advisories with Mode S are coordinated between aircrafts
  • One of the traffic is asked to climb and the other advised to descend

Proximate Traffic

  • Proximate Traffic appears as a solid cyan diamond
  • Aircraft within 6 NM and within 1200 feet relative height.
  • Other Traffic appears as hollow cyan diamond
  • Aircraft within range of the display and within 2700 feet relative height
  • Climb or Descend switch is selected for climb and descend respectively
    • During climb scan is 8700 feet above and 2700 feet below
    • During descent scan is 2700 feet above and 8700 feet below
  • A + sign indicates the intruder is above, – sign below.
  • A trend arrow ↑ or ↓ appears if intruder’s vertical rate is 500 feet per minute

ACAS Display

  • TCAS information is displayed on one of the three displays
    • Electronic Flight Information System
    • Weather radar
    • Independent display

Restricted Advisories

  • TCAS and Radio altimeter modifies RAs in proximity to the ground.
    • No instructions given when the aircraft is below 400 ft AGL
    • No descent RAs are given below 1000 ft AGL
    • No increase rate of descent commands below 1450 ft AGL.
  • TCAS and Aircraft configuration modifies certain RAs
  • Climb RAs are avoided with gear and flaps down
  • TCAS and ADC modifies certain RAs
  • Climb RA are avoided close to its performance ceiling

Action on ACAS Advisories

  • Action on Traffic Advisories
    • Start visual search and identify the intruder.
    • Contact ATC if the intruder is not located.
  • Action on Resolution Advisories
    • Initiate required manoeuvres as advised
    • Look out for other aircraft and inform ATC
  • Resolution Advisories can be ignored only if conflicting traffic is visually located
  • If ATC and ACAS provide conflicting advise, ACAS advisory must be followed

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