Section 1, Chapter 10 - Airborne Collision Avoidance System

Question 1

What is the Airborne Collision Avoidance System (ACAS), and what are the regulations for its use in civil turbine-engine aircraft?

Answer 1

ACAS Definition:

• An aircraft system based on SSR transponder signals that operates independently of ground-based equipment to advise pilots of potential conflicts with other SSR transponder-equipped aircraft.

Regulations for Civil Turbine-Engine Aircraft:

• All civil turbine-engine aircraft with an MTOM exceeding 5700 kg or more than 19 seats must carry and operate ICAO SARPs-compliant ACAS equipment.
• Currently, the only equipment meeting ACAS II requirements is TCAS II software version 7.
• Certain military transport-type aircraft may also be equipped with TCAS II.
• Civil registered historical and ex-military turbine-engine aircraft are exempt from this requirement.

ACAS Equipment Failure:

• Aircraft operators may request permission to operate in UK airspace for up to 10 days with ACAS equipment failure, according to current TCAS II Minimum Equipment List provisions.
• Aircraft with both an unserviceable transponder and TCAS will not be permitted in UK airspace where a transponder is mandatory.

Safety Net Role:

• The use of TCAS II does not alter the responsibilities of pilots and controllers for the safe operation of aircraft; it acts only as a safety net.

Question 2

What is the function of the Traffic Alert and Collision Avoidance System (TCAS II) and how do its warnings work?

Answer 2

TCAS II: Detects potential collision risks using other aircraft’s transponders.

Warnings:

• Traffic Advisory (TA): Typically 45 seconds before a potential collision. Advises pilots to visually locate the conflicting aircraft without taking immediate avoiding action.
• Resolution Advisory (RA): Typically 30 seconds before a potential collision. Provides climb, descend, or maintain instructions with expected immediate response and minimal maneuvering to resolve the conflict. Pilots should inform ATC and return to original path safely.

Notes:

• Times vary with altitude; given examples are for FL200.
• RA only occurs if conflicting aircraft uses Mode C or Mode S transponders.
• If the conflicting aircraft does not report altitude, only a TA is given.
• TCAS II accounts for the host aircraft’s performance characteristics when issuing RAs.

Question 3

How does the presence of ACAS equipment affect Air Traffic Control (ATC) operations?

Answer 3

ATC Procedures: The procedures for providing air traffic services (ATS) to ACAS-equipped aircraft are identical to those for non-ACAS-equipped aircraft.

Key Aspects:
* Collision Prevention: Maintains standard procedures for preventing collisions.
* Separation Establishment: Ensures appropriate separation without relying on ACAS capabilities.
* Traffic Information: Provides information on conflicting traffic and possible avoiding actions according to normal ATS procedures, excluding consideration of ACAS-dependent capabilities.

Question 4

What should ATC understand about Nuisance Advisories from TCAS II, and what causes them?

Answer 4

Nuisance Advisories:
* Traffic Advisories (TAs) and Resolution Advisories (RAs) may occur even with standard separation.
* ATC Response: Controllers should not assume loss of separation or fault when pilots report maneuvering due to an RA.

Causes of Unnecessary RAs:

• High Vertical Speed: Particularly problematic at low levels and in congested airspace. Pilots are advised to avoid high vertical speeds unless necessary (e.g., expedited climb, emergency descent).
• Ground Testing: Testing transponder equipment on the ground can generate advisories in TCAS II-equipped aircraft. Guidance on precautions for this is provided in CAP 562 Civil Aircraft Airworthiness Information and Procedures.

Question 5

What are the procedures and responsibilities regarding departure from ATC clearance due to a TCAS RA?

Answer 5

Pilot Actions:

• Departure from ATC Clearance: Permitted to avoid immediate danger.
• Notification: Must notify ATC ASAP and submit a written report within 10 days (ANO 2016, SERA.11014).
• Operator’s Air Safety Report: Satisfies the written report requirement.

Controller Actions:

• No Modifications: Do not attempt to modify the aircraft’s flight path or reiterate instructions until “Clear of Conflict” is reported by the pilot (SERA.11014(c)).
• Ceasing Responsibility: Controller ceases responsibility for separation during RA maneuver and resumes it once:
• Acknowledges pilot report of resuming clearance.
• Acknowledges and issues an alternative clearance.

Traffic Information:

• Not routinely provided during RA maneuvers to avoid distractions.
• Can be provided if essential for flight safety or judged necessary by the controller

Question 6

What is the ICAO TCAS phraseology and when should it be used?

Answer 6

ICAO TCAS Phraseology:

Purpose: Ensures clear communication between pilots and controllers during TCAS RA maneuvers, delineating responsibility for aircraft separation.

Cases for Communication:
* Notification of RA Maneuver Initiation:
* Pilot: (C/S) TCAS RA
* Controller: (C/S) ROGER
* After RA Response, Returning to ATC Clearance:
* Pilot: (C/S) CLEAR OF CONFLICT RETURNING TO (assigned clearance)
* Controller: (C/S) ROGER (or alternative instructions)
* After RA Response, Resuming ATC Clearance:
* Pilot: (C/S) CLEAR OF CONFLICT (assigned clearance) RESUMED
* Controller: (C/S) ROGER (or alternative instructions)
* Contradictory ATC Clearance During RA:
* Pilot: (C/S) UNABLE, TCAS RA
* Controller: (C/S) ROGER

**No Pre-Notification Required: **Pilots do not need to notify controllers before responding to an RA.

Question 7

What are the reporting requirements for controllers regarding ACAS RAs and other incidents?

Answer 7

Reporting Requirements:

• Mandatory Reporting: Controllers must report within 72 hours any occurrence that endangered or could have endangered an aircraft, its occupants, or any person.
• ACAS RAs: Must be reported under the Mandatory Occurrence Reporting (MOR) scheme.
• Duplicate Reports: No need to file if it’s known that the incident has already been or will be reported by another party (e.g., airline or pilot).
• Additional Reports: Filing an MOR does not absolve the need to report an AIRPROX or other relevant reports about the same incident.

Regulations: UK Reg (EU) 2015/1018 Annex 3 and UK Reg (EU) No. 376/2014 provide the guidelines for mandatory reporting.