Frequently Asked Questions

For more information about ACAS II refer to EUROCONTROL ACAS II Guide

Are there any requirements to reduce the vertical speed in the last 1000 feet before level-off?

ICAO Annex 6 contains the following provision:

Unless otherwise specified in an air traffic control instruction, to avoid unnecessary airborne collision avoidance system (ACAS II) resolution advisories in aircraft at or approaching adjacent altitudes or flight levels, operators should specify procedures by which an aeroplane climbing or descending to an assigned altitude or flight level, especially with an autopilot engaged, may do so at a rate less than 8 m/sec or 1 500 ft/min (depending on the instrumentation available) throughout the last 300 m (1 000 ft) of climb or descent to the assigned level when the pilot is made aware of another aircraft at or approaching an adjacent altitude or flight level.

Can aircraft equipped only with Mode A transponders be detected by ACAS II?

Aircraft equipped with only Mode A transponders are not tracked nor detected by ACAS II because ACAS II does not use Mode A interrogations. However, if the aircraft is equipped with Mode A/C transponder but does not provide altitude information (Mode C) this aircraft will be tracked as a non-altitude reporting target using range and bearing information and it will be shown on ACAS II traffic display.

Neither a data tag nor a trend arrow will be shown for traffic symbol for an intruder that is not reporting altitude. TAs will be generated against non-altitude reporting aircraft when the range test for TA generation is satisfied. Non-altitude reporting aircraft are deemed to be at the same altitude as own aircraft.

Can an RA be generated against an aircraft that is not ACAS II equipped?

An RA can be generated against all aircraft equipped with an altitude reporting transponder (Mode S or Mode A/C). The intruder does not need to be fitted with ACAS II. However, RAs are coordinated only between ACAS II equipped aircraft.

Experience shows that in the majority of cases only one aircraft will receive an RA (regardless of whether the intruder is ACAS II aircraft or not).

Do I require ACAS II to operate in Reduced Vertical Separation Minima (RVSM) airspace?

The carriage and operation of ACAS II is not an RVSM requirement in itself. For the ACAS equipage mandate see questions above.

How does ACAS II work if one aircraft is using the standard altimeter setting and the other QNH?

ACAS II always utilises pressure altitude information which relates to the standard pressure (altimeter setting 1013.25 hPa). ACAS II operation are not affected if aircraft are flying Flight Levels on the standard pressure setting, altitude on QNH, or height on QFE as the same pressure setting (i.e. standard) is always used.

The pressure selection by the flight crew does not affect the ACAS II system at all. Additionally, below 1750 feet ACAS II also uses radar altimeter data.

Is version 7.1 compatible with earlier TCAS versions?

Yes, version 7.1 is compatible with all existing versions being operated today; i.e. version 7.0 as well as version 6.04a (which is still in use by a small population of aircraft, mainly outside Europe).

What are the equipage requirements for military (state) aircraft?

For information regarding State (military) equipage requirements, please refer to the web page of the EUROCONTROL Civil-Military ATM Coordination webpage

What is the difference between TCAS and ACAS?

TCAS (Traffic Alert and Collision Avoidance System) is a specific implementation of the ACAS (Airborne Collision voidance System) concept. TCAS II version 7.0 and 7.1 are currently the only available equipment that is fully compliant with the ACAS II Standards and Recommended Practices (SARPs).

ACAS II provides "Resolution Advisories" (RA's) in the vertical sense (direction) telling the pilot how to regulate or adjust his vertical speed so as to avoid a collision.

TCAS II Minimum Operational Performance Specification (MOPS) have been published by RTCA (DO-185B) and EUROCAE (ED-143).

What is the European ACAS/TCAS II version 7.1 mandate?

An Implementing Rule published by the European Commission mandates the carriage of ACAS II version 7.1 within European Union airspace by all civil aeroplanes with a maximum certified take-off mass exceeding 5,700 kg or authorised to carry more than 19 passengers as of 1 December 2015.

What is the ICAO ACAS/TCAS version 7.1 mandate?

Amendment 85 to ICAO Annex 10 volume IV, published in October 2010, introduced a provision stating that all new ACAS installations after 1 January 2014 shall be compliant with version 7.1 and all ACAS units shall be compliant with version 7.1 after 1 January 2017.