M1, S1, C10 - Airborne Collision Avoidance System

Question 1

Airborne Collision Avoidance System (ACAS) is an aircraft system based on…

Answer 1

…SSR transponder signals, which operates independently of ground- based equipment to provide advice to the pilot on potential conflicting aircraft that are equipped with SSR transponders

Question 2

All civil turbine-engine aircraft having a maximum take-off mass (MTOM) exceeding 5700 kg or a maximum approved seating configuration of more than 19 are mandated to carry and operate…

Answer 2

…ICAO SARPs-compliant Airborne Collision Avoidance System (ACAS) equipment. The only equipment currently able to meet the ACAS II mandate requirements is the Traffic Alert and Collision Avoidance System II (TCAS II) software version 7. Certain military transport-type aircraft may also be TCAS II equipped. Civil registered historical and ex-military turbine-engine aircraft are exempted from this requirement

Question 3

Aircraft Operators experiencing ACAS equipment failure may request permission to operate in UK airspace for up to…

Answer 3

…10 days in accordance with current TCAS II Minimum Equipment List provisions.

Question 4

Due to the safety benefits arising from TCAS operations and the collaborative way in which it arrives at collision avoidance solutions, any aeroplane with an unserviceable transponder as well as an unserviceable TCAS…

Answer 4

…will not be permitted in UK airspace for which mandatory carriage of a transponder is required.

Question 5

The carriage and use of TCAS II acts only as a…

Answer 5

…safety net, and does not in any way alter the respective responsibilities of pilots and controllers for the safe operation of aircraft

Question 6

TCAS II encounters may take place within all classifications of airspace. TCAS II equipment reacts to…

Answer 6

TCAS II encounters may take place within all classifications of airspace. TCAS II equipment reacts to the transponders of other aircraft to determine whether or not there is a potential risk of collision. The warning, based on the time to an assumed collision, enables the pilot to identify the conflicting traffic and, if necessary, take avoiding action

Question 7

Warnings for aircraft equipped with TCAS II are given in two steps:

Answer 7

Question 8

If a conflicting aircraft is not reporting altitude…

Answer 8

If a conflicting aircraft is not reporting altitude, a TA may be given, but not an RA.

Question 9

RAs will be presented only if the conflicting aircraft is…

Answer 9

RAs will be presented only if the conflicting aircraft is transponding on Mode C or Mode S. Where both aircraft in an encounter are fitted with TCAS II, the transponders will communicate with each other to agree complementary Resolution Advisories. If the conflicting aircraft is not transponding, no warnings are given

Question 10

TCAS II is programmed to take account of…

Answer 10

…the host aircraft’s performance characteristics and level when issuing RAs.

Question 11

The procedures to be applied for the provision of air traffic services to aircraft equipped with ACAS shall be identical to those applicable to non-ACAS equipped aircraft.

In particular, the prevention of collisions, the establishment of appropriate separation, and the information, which might be provided in relation to conflicting traffic and to possible avoiding action, shall conform with…

Answer 11

The procedures to be applied for the provision of air traffic services to aircraft equipped with ACAS shall be identical to those applicable to non-ACAS equipped aircraft. In particular, the prevention of collisions, the establishment of appropriate separation, and the information, which might be provided in relation to conflicting traffic and to possible avoiding action, shall conform with the normal ATS procedures and shall exclude consideration of aircraft capabilities dependent on ACAS equipment

Question 12

TAs and RAs may occur even though…

Answer 12

TAs and RAs may occur even though standard separation exists. Therefore, a controller should not immediately assume that separation has been lost, or that he is at fault, when a pilot reports they are manoeuvring in response to an RA

Question 13

One cause of an unnecessary RA is…

Answer 13

…high vertical speed, particularly at low level and in congested airspace.

Pilots have been asked to avoid these manoeuvres, particularly in the final stages of climb or descent, unless they are justified, for example, a request to expedite a climb, emergency descent etc.

It has been reported that ground testing of transponder equipment may generate advisories in TCAS II-equipped aircraft.

Question 14

The pilot in command of an aircraft is permitted to depart from an ATC clearance for the purposes of…

Answer 14

…avoiding immediate danger.

The pilot in command is required to notify ATC as soon as possible and submit a written report within 10 days. The completion of an Operator’s Air Safety Report satisfies the last requirement

Question 15

When a pilot reports a TCAS RA, controllers shall not…

Answer 15

…attempt to modify the aircraft’s flight path or reiterate previously issued instructions, until the pilot reports “Clear of Conflict

Question 16

Once an aircraft departs from an ATC clearance in compliance with an RA, or a pilot reports an RA, the controller ceases to be responsible for providing separation between that aircraft and any other aircraft affected as a direct consequence of the manoeuvre induced by the RA.

The controller shall resume responsibility for providing separation for all the aircraft affected when

Answer 16

(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 17

The passing of traffic information by controllers to aircraft conducting, or affected by a TCAS RA, is not proscribed, but such information has, if provided inappropriately, the potential to be misheard or to distract flight crews during a period of very high workload.

Consequently, controllers should not routinely pass traffic information to aircraft conducting RA manoeuvres, or other aircraft affected by such manoeuvres.

Nevertheless, there may be circumstances where the passing of traffic information is justified; consequently, controllers may provide traffic information under the following circumstances:

Answer 17

(1) To aircraft conducting an RA manoeuvre if it is considered essential for flight safety (e.g. information on aircraft which are known to be in close proximity that are not transponding Mode C information).
(2) To other aircraft affected by an RA manoeuvre if judged necessary by the controller (e.g. in airspace where the carriage and operation of TCAS and/or SSR transponders is not mandatory).

Question 18

Specific ICAO TCAS phraseology has been adopted.

The phraseology is clear, simple, concise and to the maximum extent devoid of ambiguity and potential to cause operational confusion. This phraseology provides:

Answer 18

(1) the means to ensure that the pilot and controller have, wherever possible, a clear mutual understanding of the progression of an RA manoeuvre; and
(2) the means to delineate the point at which the responsibility for the separation of aircraft directly affected by the manoeuvre is transferred from the controller to the pilot and, at the completion of the manoeuvre, from the pilot back to the controller.

Question 19

There is no requirement for the pilot to notify the controller prior to…

Answer 19

…responding to an RA.

Question 20

There are four cases for which communication is necessary between pilot and controller; the phraseology to be used is reproduced below

Answer 20

Notification after a flight crew starts to deviate from any ATC clearance or instruction to comply with a RA:

Pilot: (C/S) TCAS RA

Controller: (C/S) ROGER

Notification after the RA response is completed and a return to the ATC clearance or instruction is initiated:

Pilot: (C/S) CLEAR OF CONFLICT RETURNING TO (assigned clearance)

Controller: (C/S) ROGER (or alternative instructions)

Notification after the response to a RA is completed and the assigned ATC clearance or instruction has been resumed:

Pilot: (C/S) CLEAR OF CONFLICT (assigned clearance) RESUMED

Controller: (C/S) ROGER (or alternative instructions)

Notification after an ATC clearance or instruction contradictory to the ACAS RA is received, the flight crew will follow the RA and inform ATC directly:

Pilot: (C/S) UNABLE, TCAS RA

Controller: (C/S) ROGER

Question 21

A holder of an Air Traffic Controller’s licence or Flight Information Service Officer’s licence is to report, within 72 hours, any occurrence…

Answer 21

…which has, or if not corrected would have, endangered an aircraft, its occupants, or any other person. The ANO and CAP 382 contain guidance on what is a reportable occurrence but ultimately the individual officer involved will have to use his own judgment to assess whether the incident as a whole meets the requirements for mandatory reporting

Question 22

All ACAS RAs are required to be…

Answer 22

…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. UK airline or pilot), there is no need to file a duplicate report.

Question 23

The filing of the report does not…

Answer 23

…absolve the controller from additionally reporting an AIRPROX or other report about the same incident, according to the circumstances