Traffic collision avoidance system - TCAS[PDF]
- 1 Introduction
- 2 ICAO regulation
- 3 TCAS instrument onboard
- 4 Technical description
- 5 TCAS in IVAO
- 6 TCAS representation
- 7 Use of TCAS
- 8 See also
- 9 Reference
- 10 Author
Definition and rules
Types of ACAS
- ACAS I gives Traffic Advisories (TAs) but does not recommend any manoeuvres. The only implementation of the ACAS I concept is TCAS I. These equipments are limited to interoperability and interference issues with ACAS II.
- ACAS II gives Traffic Advisories (TAs) and Resolution Advisories (RAs) in the vertical sense (direction). The only implementations of the ACAS II concept are TCAS II Version 7.0 and Version 7.1.
- ACAS III gives TAs and RAs in vertical and/or horizontal directions. ICAO SARPs for ACAS III have not been developed. Currently, there are no plans to proceed with such a development
TCAS I is mandated in the United States for certain smaller aircraft. TCAS II Versions 7.0 and 7.1 are mandated in Europe and elsewhere Collision avoidance systems can be passive, like for example the PCAS or Portable Collision Avoidance System which only monitors the surrounding aircraft without emitting any signal. This portable system is often used within General Aviation.Some gliders are now equipted with an avoidance system named FLARM.
ACAS II as a standard
ACAS II is an aircraft system based on Secondary Surveillance Radar (SSR) transponder signals. ACAS II interrogates the Mode C and Mode S transponders of nearby aircraft (‘intruders’) and from the replies tracks their altitude and range and issues alerts to the pilots, as appropriate. ACAS II works independently of the aircraft navigation, flight management systems, and Air Traffic Control (ATC) ground systems.
TCAS instrument onboard
The TCAS system can be implemented as:
- a dedicated instrument
- a combined instrument with the vertical speed indicator
- a combined instrument with the Navigation Display or the Electronic Horizontal Situation Indicator
TCAS can be currently operated in the following modes:
- Standby: TCAS does not issue any interrogations and the transponder only replies to discrete interrogations. This mode is used on the ground, outside of the runway.
- Transponder: the transponder replies to all appropriate ground and TCAS interrogations and TCAS remains in stand-by. This is a passive mode which is the minimum mandatory mode to be set by any airborne aircraft (typically in General Aviation)
- Traffic Advisory (TA): TCAS issues the appropriate interrogations and perform all tracking functions. However, TCAS will only provide traffic advisories (TA) and the resolution advisories (RA) are inhibited
- Automatic (TA/RA): TCAS provides traffic advisories (TA) and resolution advisories (RA) when appropriate. This mode is mandatory for all Commercial Aviation aircrafts.
- Monitors the airspace around the aircraft and communicates with all traffic equipped with a corresponding active transponder
- Queries all surrounding aircraft on the frequency 1030 MHz and each aircraft transponder replies on the frequency 1090MHz
- Warns pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air collision (MAC)
These are defined as follows:
- Resolution advisory (RA) is an indication given to the flight crew recommending:
- a manoeuvre intended to provide separation from all threats, or
- a manoeuvre restriction intended to maintain existing separation.
- Traffic advisory (TA) is an indication given to the flight crew that a certain intruder is a potential threat.
A protected volume of airspace surrounds each ACAS II equipped aircraft. The size of the protected volume depends on :
- the altitude of the aircraft involved in the encounter.
- the speed of the aircraft involved in the encounter.
- the heading of the aircraft involved in the encounter.
The TCAS system builds a three dimensional map (Protection Volume) of the airspace around the aircraft. The map range depends on the TCAS configuration.
The map is filled with all intruder aircraft depending on:
- their distance (deduced from the interrogation and response round trip time)
- their altitude (as reported by the interrogated aircraft)
- their bearing (by the directional antenna from the response)
Traffic is represented by small geometrical symbols with associated numbers indicating the vertical separation with respect to your airplane, expressed in hundreds of feet.
- -012 = 1200 feet below
- 121 = 12100 feet above
- -02↓ = 200 feet below and descending
- +07 = 700 feet above
- -01↑= 100 feet below and climbing
Finally, by extrapolating the current range and altitude difference to anticipated future values, the system determines whether a potential collision threat exists.
TCAS is also an active system since it reports to the pilot the position of all aircraft inside the protected volume and warns the pilot about any risk of conflict.
Following the identification of potential collisions, the TCAS automatically negotiates a mutual avoidance manoeuvre (currently, manoeuvres are restricted to changes in altitude and modification of climb/sink rates) between the two (or more) conflicting aircraft. These avoidance manoeuvres are communicated to the pilot by a cockpit display and by synthesized voice instructions.
Traffic Advisory (TA)
In the case of a potential collision, a sounding alert is emitted by the “Traffic Advisory” (TA). This system warns the pilot about every intruder aircraft by a “traffic, traffic” vocal announcement. It does not provide any avoidance manoeuvre suggestion.
TAs are nominally generated 20–48 seconds prior to the predicted Closest Point of Approach which would be 10–13 seconds earlier than any RA, although shorter generation times are possible in some geometries – indeed, in certain cases an RA can occur without a preceding TA. The majority of TAs will not be followed by an RA because often the separation between the aircraft does not drop below the alert threshold for an RA. On average, only 1 in 10 TAs will be followed by an RA.
When a TA is issued, the pilot is meant to initiate a visual search for the traffic causing the TA. If the traffic is visually identified, the pilot has to maintain visual separation.
He is also supposed to avoid any horizontal manoeuvre based on the solely information shown on the traffic display. On the contrary, slight adjustments in vertical speed while climbing or descending, or slight adjustments in airspeed while still complying with the ATC clearance are acceptable.
Resolution Advisory (RA)
Whenever the conflict situation gets worst after a TA alert and the collision seems impending, an audio message and a visual alert are generated by the « Resolution Advisory » (RA). This alert indicates the concerned airplane and suggests an avoidance action to be immediately executed by the pilot.
The system is conceived in such a way that the TCAS of the other airplane suggests another action.
List of TCAS advisories
The list of TCAS advisories is presented below:
TCAS in IVAO
This means that it is always available regardless the aircraft model you choose, even for airplanes which are not equipped in real life. It is up to you to make use of it or not depending on the aircraft you fly with and on the degree of realism you wish to reach.
On IVAO, the TCAS function is useful when ATC is not present in order to prevent collision.
Use of TCAS
The symbol of your aircraft is placed in the middle of a circle in the TCAS window.
The radius of this circle can be increased or decreased by clicking on « + » or « - ». The range possible values are 3, 5, 10, 15, 20 or 40 NM.
- OFF: (inactive TCAS) with « TCAS OFF »
- TA: « Traffic Advisory » mode
- TA/RA: « Traffic Advisory/Resolution Advisory » mode
- NORM: the traffic at +/- 2700 ft is displayed
- ALL: all traffic within the TCAS range is displayed
- ABV: the traffic within the interval (-2700, +9000) ft is displayed
- BLW: the traffic within the interval (-9000, +2700) ft is displayed
Finally, the TCAS sound can be deactivated or activated by the right bottom button.
- A TCAS “Caution” is issued whenever an airplane is at ±1600 ft and/or at 7 NM from your position. The intruder aircraft is displayed in yellow.
- A TCAS “Alert" is issued whenever an airplane is at ±900 ft and/or at 3 NM from your position. The intruder aircraft is displayed in red.
- VID 150259 - Creation
- VID 450012 - Update
DATE OF SUBMISSION
- 02:16, 14 May 2021
- This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.
- The content of this documentation is intended for aviation simulation only and must not be used for real aviation operations.