General: Airborne Collision Avoidance System

Question 1

What is ACAS?

Answer 1

• Airborne Collision Avoidance System is an aircraft system based on SSR transponder signals, which operates independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft equipped with SSR transponders.

Question 2

What aircraft shall carry & operate ACAS?

Answer 2

• All civil turbine-engine aircraft having a maximum take-off mass EXCEEDING 5700kg / max approved seating configuration of more than 19.
• The only equipment currently able to meet the ACAS II mandate is the Traffic Alert & Collision Avoidance System II

Question 3

Can aircraft requiring TCAS operate in UK airspace if their TCAS has failed?

Answer 3

• They may request permission to operate in UK airspace for up to 10 days.
• Due to the safety benefits and it’s collaborative nature, any aeroplane with an unserviceable transponder as well as an unserviceable TCAS will not be permitted in UK airspace for which mandatory carriage of a transponder is required.

Question 4

What is a Traffic Advisory (TA) Warning?

Answer 4

• Typically 45 seconds before the assumed collision
• Pilots are advised not to take avoiding action on the basis of TA info alone, but may ask for traffic info.
• Pilot looks for conflict visually
• Does NOT manoeuvre

Question 5

What is a Resolution Advisory (RA) Warning?

Answer 5

• Typically 30 seconds before the assumed collision
• Pilot receives advice to climb, descend, maintain their existing flight profile.
• RoC / descent may also be given.
• Pilots are expected to respond immediately but have been instructed to restrict manoeuvres to the minimum necessary to resolve the confliction
• Advise ATC units asap thereafter & return to original flight path as soon as it is safe to do so.

Question 6

What are some additional quirks with TAs & RAs?

Answer 6

• If a conflicting aircraft is not reporting altitude, a TA may be given, but not an RA
• RAs will be presented only if the conflicting aircraft is transponding on Mode C / S. Where both aircraft in an encounter are fitted with TCAS, the transponders will commuinicate with each other to agree complementary RAs
• TCAS is programmed to take account of the host aircraft’s performance charistics & level when issuing RAs

Question 7

What are nuisance advisories?

Answer 7

• TAs & RAs may occur even though standard separation exists.
• ATCOs should not immediately assume that separation has been lost or that they are at fault, when a pilot reports they are manoeuvring in response to an RA
• One cause of an unnecessary RA is high vertical speed, particularly at low level / congested airspace. (Pilots have been asked to avoid these manoeuvres particularly in final stages of climb/descent.

Question 8

Are pilots allowed to depart from an ATC clearance?

Answer 8

• Yes, for the purposes of avoiding immediate danger.
• They are required to notify ATC asap and submit a written report within 10 days.

Question 9

What shall an ATCOs actions be when a pilot reports a TCAS RA?

Answer 9

• ATCOs shall not attempt to modify the aircraft’s flight path / reiterate previously issued instructions until the pilot reports “Clear of Conflict”.

Question 10

Who is responsible for separation once an aircraft departs from an ATC clearance in response to an RA?

Answer 10

• The ATCO ceases to be responsible for providing separation.
• The ATCO shall resume responsibility for providing separation for all the aircraft affected when;
  1. The ATCO acknowledges a report from the crew that the aircraft has resumed the current clearance
  2. The ATCO acknowledges a report from the flight crew that the aircraft is resuming the current clearance & issues an alternative which is acknowledged by the crew

Question 11

Should Traffic Info be passed during a TCAS RA?

Answer 11

• The passing of TI to aircraft affected by an RA is not proscribed.
• Such information has, if inappropriate, the potential to be mishears / to distract crews during periods of very high workload.

Question 12

What phraseology is used during a RA?

Answer 12

Pilot: C/S TCAS RA
ATCO: C/S ROGER

OR

Pilot: C/S Clear of Conflict, returning to (assigned clearance)
ATCO: C/S ROGER (or alternative instruction)

Question 13

How long does an ATCO have to file a report on a TCAS RA?

Answer 13

• All TCAS RAs are required to be reported under the MOR scheme.
• 72 hours