25. ACAS & TCAS

Question 1

Condense

Which aircraft must carry (and use) ACAS (TCAS II Version 7.1)?

ICAO ACAS 2 SARPs Mandate

Answer 1

Unless otherwise authorised, turbine-powered aeroplanes with a maximum certified take-off mass (MCTOM) of more than 5700 kg or;
- maximum operational passenger seating configuration (MOPSC) of more than 19 shall be equipped with ACAS II.

THE ONLY EQUIPMENT CURRENTLY AVAILABLE
TO MEET ACAS II REQUIREMENTS IS TCAS II.

Question 2

What is ACAS?

Answer 2

Airborne Collision Avoidance System (ACAS)

“An aircraft system based on SSR transponder
signals which operates independently of ground
based equipment to provide advice to the pilot
on potential conflicting traffic that are equipped
with SSR transponders”

Question 3

What is TCAS?

Answer 3

Traffic Alert and Collision Avoidance System
(TCAS)
is an implementation of the ICAO
Airborne Collision Avoidance System (ACAS)
standard.

TCAS II uses range and altitude data obtained by
interrogating other aircraft transponders to
determine the possibility of collision.
TCAS II is also able to present bearing
information of aircraft intruding into your
airspace, but uses this information on the
display only to assist in visual acquisition of an
intruder.

When range and altitude of an intruder are
computed and a collision risk is predicted, TCAS
II will declare the intruder a “threat” and give
manoeuvre advice in visual and aural form.
Avoidance manoeuvres using TCAS II are in the
vertical plane only. Bearing information
produced by TCAS II does not possess the
accuracy necessary for determining horizontal
avoidance manoeuvres.

Question 4

Types of transponder?

Answer 4

TCAS depends entirely on SSR.

Mode A/C Transponders
Give both Traffic Advisory and Resolution Advisory, but only if Mode C available but any manoeuvre may not be in the logical sense.
Mode S Transponders
‘communicate’ with each other and ‘agree’ a course of action.

NO TRANSPONDER – NO TCAS ALERT

Question 5

What are the key components of TCAS? (5)

Answer 5

• Antennae
• TCAS Processor
• Transponder Control Panel
• Vertical Speed Indicator – Visual Display
• Voice Warning System “Traffic Traffic”

Question 6

How does the TCAS/ACAS system operate?

Answer 6

The size of the protected volume depends on the altitude, speed, and heading of the aircraft involved in the encounter.

Question 7

Traffic display symbology

Answer 7

Hollow cyan (light blue) or white diamond – for other traffic.
Solid cyan (light blue) or white diamond – for proximate traffic.
Solid yellow or amber circle – for intruders (i.e. aircraft which trigger a TA).
Solid red square – for threats (i.e. aircraft which trigger an RA)

Question 8

System operation
Proximity traffic

Answer 8

Any target within 6nm and within 1200ft.

Question 9

System operation
TA

Answer 9

Traffic Advisory

Traffic assumes “threat” status.
20 – 48 secs before Closest Point of Approach (CPA).
“Traffic Traffic”

• PILOTS ARE ADVISED NOT TO TAKE AVOIDING ACTION ON THE BASIS OF A ‘TA’
• MAY REQUEST TRAFFIC INFORMATION FROM ATC.
• BUT DON’T ROUTINELY PASS.

Question 10

System Operation
RA

Answer 10

Resolution Advisory
Starts the conflict resolution process up to 35 secs before CPA
“Climb, Climb Now”

PILOTS SHALL RESPOND IMMEDIATELY (Disengage autopilot and respond within 5 seconds, unless doing so would jeopardise the aircraft)
BUT HAVE BEEN INSTRUCTED TO RESTRICT
MANOEUVRES TO THE MINIMUM NECESSARY TO
RESOLVE CONFLICTION, ADVISE ATC ASAP AND
RETURN TO ORIGINAL FLIGHT PATH AS SOON AS
SAFE TO DO SO.

Question 11

What are TCAS Limitations?

Answer 11

• Enhanced RA “Increase Descent” is inhibited below
  1450ft AGL.
• A Descend RA is inhibited below 1100ft AGL.
• All RAs inhibited below 1000ft AGL.
• All spoken messages inhibited below 500ft AGL.
• RAs also inhibited when some terrain avoidance
  systems or windshear warnings are active.

Question 12

Why can RA’s and TA’s occur even when standard separation exists?

Answer 12

• HIGH VERTICAL SPEED – PARTICULARLY AT LOW
  LEVEL AND IN CONGESTED AIRSPACE.
• GROUND TESTING OF TRANSPONDERS.

Question 13

What should ATC do when they’re notified of an RA?

Answer 13

“ROGER”
- Do Not Issue Instructions to the Aircraft.
- ATC responsibility ceases for provision of
standard separation from affected traffic.
- Controllers should not routinely pass traffic
information to aircraft conducting RA
manoeuvres, or other aircraft affected by such
manoeuvres.

The controller shall resume responsibility for
providing separation for all the aircraft affected
when:
1 - The controller acknowledges a report from the
flight crew that the aircraft has resumed the
current clearance; or
2 - The controller acknowledges a report from the
flight crew that the aircraft is resuming the current
clearance and issues an alternative clearance which
is acknowledged by the flight crew.

Question 14

ATC reporting of TCAS events

Answer 14

All ACAS RAs are required to be reported under
the MOR scheme. If there is good reason to
believe that the appropriate details of the
occurrence have already been, or will be,
reported under the MOR scheme by someone
else (e.g. airline or pilot) there is no need to file
a duplicate report.
Note: This does not absolve a controller from
reporting an AIRPROX where appropriate.