- 09 Feb 2023
- Ken Elliott
- Avionics - BizAv
Ken Elliott reviews the area of Airborne Collision Avoidance Systems (including ACAS-X and its family of current and future systems)...Back to Articles
Independent of Air Traffic Control, Airborne Collision Avoidance Systems (ACAS) provide the flight crew with visual and aural traffic advisories that could potentially avoid a collision. The technology has an added capability of a visual resolution advisory (RA) for vertical avoidance which is available in ACAS X (TCAS II) versions.
The International Civil Aviation Organization (ICAO) associates the term ACAS with a technical standard and requirement, whereas its use of Traffic Collision Avoidance System (TCAS) is used with respect to the implementation of the standard.
When the US Congress ordered the FAA to develop a collision avoidance system in response to several fatal airline accidents, it required the industry to develop a TCAS. Traditionally identified as Traffic Alert & Collision Avoidance System, TCAS is found in three versions:
Note: There are also Traffic Advisory Systems (TAS) for light aircraft that monitor and display target aircraft by interrogating Mode C transponders. TAS comes in several versions spanning a wide range of price points.
For expediency, only the operation of the latest version of ACAS II – TCAS II version 7.1 (v7.1) – will be covered here...
Surveillance via Interrogation: TCAS relies on Mode C or S transponders that interrogate and then receive replies from other aircraft. While Mode C provides altitude information, Mode S, with its unique identifier, provides range, altitude and heading to other aircraft and ATC. This data is used by the TCAS II processor.
Mode S transponders have been followed by systems updated for Automatic Dependent Surveillance - Broadcast (ADS-B). This added more aircraft parameters for the transponder to rebroadcast to all aircraft within range, including GPS position. Newer hybrid surveillance TCAS II systems use ADS-B In data for more accurate monitoring.
TCAS II monitors the rate of closure to a closest point of approach (CPA). As the time (and distance) decreases the TCAS processor increases its monitoring rate and level of critical advisory.
Figure 1: The Theoretical Protected Volume Around Each TCAS II-equipped Aircraft
Based on replies, the TCAS II can track the slant range, altitude, and relative bearing of surrounding traffic. TCAS then calculates the time it will take to reach the CPA for each potential intruder. The CPA is calculated by dividing slant range by closure rate. Slant range considers vertical and horizontal (or height and bearing) deviation between aircraft.
Figure 1 (above) shows a TCAS II ‘theoretical area of protected volume’. This protected volume encompasses target aircraft that have become more than just an object of interest and require the attention of the flight crew.
The protected volume has three levels of increasing criticality, both in horizontal and vertical airspace:
Table A: Traffic Advisory (TA) and Vertical Resolution Advisory (RA) Based on Own & Target Aircraft Equipage
Alerting the crew: Aircraft (targets) within TCAS II range are of interest and displayed in the cockpit. Once a target enters the area of protected volume it will be monitored with increasing intensity as it gets nearer. The symbol of each aircraft on the cockpit display will change in appearance, and an audio alert will be issued as the target becomes a greater collision threat.
TCAS II must be able to process up to 30 aircraft for display, at a nominal range of 14nm, within a one second cycle.
Not all aircraft have TCAS II and yet they can still be a threat. Table A (above) shows how a TCAS I or TCAS II equipped aircraft responds to all other threat aircraft.
A TA does not require the crew to take immediate action whereas an RA does. TCAS II RAs are a vertical climb-or descend-rate advisory command, issued on the premise that using the rate will ensure clearance from the opposing traffic.
Figure 2: Examples of TCAS II Traffic Display Symbology & RA Vertical Speed Commands
The visual RA is displayed on dual Instantaneous Vertical Speed Indicators (IVSI) and respective Attitude Director Indicators (ADI). Figure 2 (above) shows red regions on the Vertical Speed display as prohibited, and green regions as the required vertical speed range to use. The IVSIs also display a 360-degree view of all monitored traffic.
Correspondingly, TCAS traffic information, overlayed on navigation data, is displayed on the Primary Flight Displays (PFD) and secondarily on cockpit Multifunction Displays (MFD).
Figure 2 also shows traffic symbols that differ, based on the level of concern for each aircraft. These symbols indicate whether the target aircraft is proximate, close-in TA or close-in RA.
Want to know how ACAS is developing in Business Aviation? Continue reading this article in the AvBuyer April digital edition, or online by clicking the Page 2 button below.