TCAS ("tee-cas") stands for Traffic Alert and Collision Avoidance System.  It is intended to prevent aircraft from colliding with each other mid-air.  A number of serious mid-air collisions in the mid-20th century prompted research into a system that could be fitted to aircraft to help prevent these mishaps.  TCAS was the result, and has been required aircraft fitting in one form or another since the early 1990s, today forming part of an equipment package the International Civil Aviation Organisation (ICAO) requires public transport aircraft to carry.

TCAS works off the back of equipment that is present in virtually every single aircraft flying today: the transponder. The transponder is part of the Secondary Surveillance Radar (SSR) system that gives air traffic controllers additional information about the aircraft on their radar.  See, the blips on a radar screen come from a Primary Surveillance Radar (PSR) system, which is generally what the large spinning radar antennae you see near airports (but sometimes elsewhere) are.  The problem is these give nothing but a screen full of blips, none of which are identifiable on their face.

Now, air traffic controllers do have ways of identifying so-called 'primary-only' aircraft so something useful can be done with them, but that's another node (I hear drooling noders...or perhaps melting brains).  Air traffic control actually used to work this way, and a frightening prospect it is.  Screens full of glowing green blobs with no identifying information, with just a controller's working memory preserving their identity. That was fine when fewer aircraft were in the sky, but as traffic levels increased this became less workable, as it became difficult for controllers to maintain the identity of so many near-identical blips.

Secondary Radar then. This system works together with the Primary Radar system to produce a much more informative radar picture.  It has a spinning antenna just like Primary Radar, sending out thousands of pulses per second just like Primary Radar.  The difference is that where PSR is a passive system which receives signals reflected off aircraft in the sky, suitably-equipped aircraft actively respond to pulses - 'interrogations' - from SSR.  Those responses contain information that may be used, amongst other things, to identify that aircraft.  The responses from primary and secondary radar are combined, so that the controller sees not just blips on their screen, but bits of information tagged to them.  The aircraft system that provides this extra information is the transponder.

Modern transponders are pretty sophisticated, and can relay a raft of information about what an aircraft is doing.  At a minimum, a transponder returns a four-digit code that can be used to identify the aircraft.  This is called 'Mode A'. There is a selector in the cockpit that pilots can use to set specific codes, and those codes will in turn appear on the controller's screen next to the appropriate blip. Some of you will probably be thinking four digits isn't very much (it's actually even more restrictive than you think, since each digit can only be 0-7), and it isn't, but we're on it.

Codes can be assigned by a controller to help in identifying an aircraft (to confirm that the blip on their radar screen, that they think is a certain aircraft, is in fact that aircraft), or may be selected arbitrarily by the pilot to indicate some general information about their flight.  Code 7000, for example, is the code pilots select when they are pottering around outside controlled airspace, not under control of any air traffic unit, and is called the 'conspicuity' code.  7700 is the code a pilot selects when their flight is in distress.  Radar systems frequently have routines that automatically raise the visibility of such aircraft to the controller; they may flash red and alternate the callsign with 'SOS'.

Further usefulness can be found in the code/callsign conversion system, which uses databases of stored flight plans to correlate an aircraft's four-digit code - referred to as its 'squawk' code - with its callsign.  Local flights may be assigned a squawk code particular to the air traffic unit that is controlling them at that time, but regional and international flights frequently keep the same squawk code for the duration of their trip.  This squawk can thus be linked to the aircraft's callsign and at suitably-equipped air traffic units, the callsign will be displayed next to the appropriate blip on the radar display.  Very handy.

Moving on, another capability of transponders which is increasingly required for flight in controlled airspace, is to report the level of their aircraft.  This is called 'Mode C', and appears on the controller's radar display as a two or three-digit number attached to the aircraft callsign or squawk. This indicates the aircraft's altitude or flight level, depending on the circumstances.  It is this information that TCAS uses to try to stop planes from banging into each other. And air traffic controllers, for that matter.

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Okay, we're back where we started. TCAS is a system designed to meet the ICAO specification for an Airborne Collision Avoidance System (ACAS); indeed, it is currently the only system in existence that does so.  It is required fitting for all turbine-powered aircraft that either weigh more than 5700kg, or have an authorised capacity of more than 19 passengers.

TCAS works on the basis of transponder signals. TCAS equipment fitted to an aircraft transmits interrogation signals, much like those of Secondary Radar equipment on the ground, and transponders on board aircraft in the vicinity send replies, much like they do when they receive an interrogation from Secondary Radar equipment.  These responses include information about the altitude of the aircraft that sent the reply; since TCAS 'knows' the altitude of its own aircraft, and triangulates the position of the 'other' aircraft (TCAS uses several antennae to receive responses from aircraft) it can make a nominal judgement of whether any aircraft in the vicinity are 'intruders', 'threats' or just 'other' traffic.

This information is presented to the pilot on a cockpit instrument, which displays a symbolic plan-view representation of the aircraft's situation.  A symbol for the aircraft appears near the centre, and around it are blips representing the positions of aircraft within a certain distance, with a symbol indicating its considered status* and a two-digit figure indicating the level of that aircraft.  '-08' means a particular aircraft was 800ft below.  '+10' means the aircraft is 1000ft above. An appropriate arrow next to the symbol indicates if the aircraft is climbing or descending.  Not all of these blips represent threats, but are there for the pilot's information.  If the system judges a risk of collision exists the symbol and colour of the appropriate blip changes, and an audible warning is given to the pilot.

Such a warning is called a 'Traffic Advisory' (TA), and is given when the collision is judged to be 45 seconds away. A spoken warning is heard - "Traffic, Traffic" - which is the pilot's prompt to look out of the window, and on the TCAS display if the aircraft has one.  This is the limit of TCAS I's abilities. TCAS II does this too, but also gives a 'Resolution Advisory' (RA) in the vertical domain when the collision is judged to be 30 seconds away.  If both of the aircraft involved are fitted with TCAS II, the two systems will communicate to agree a course of action - i.e. one aircraft climbs and the other descends - this is a 'coordinated resolution'.  It is worth pointing out here that TCAS is only meant to stop collisions, not to provide standard separation (usually five miles horizontally or 1,000ft vertically); it aims for a miss distance of 500-700ft.

If one aircraft is fitted with TCAS and the other is not, TCAS will only give an RA if the other aircraft is fitted with a Mode C or Mode S-capable transponder. Obviously there is no way of ensuring the action of the other aircraft compliments that of the TCAS-equipped aircraft. If the other aircraft only has Mode A-capable transponder, TCAS will just issue a Traffic Advisory.

During an RA the system issues an instruction to climb or descend and the pilot must comply; it is one of the few circumstances under which pilots are permitted to deviate from their air traffic control clearance.  The pilot must tell ATC as soon as possible, that they are climbing or descending on TCAS' instructions.  While this is going on, ATC are not responsible for separating that aircraft.

A further item on the TCAS instrument that is used during an RA is the vertical speed gauge; this is a circular gauge, graded from -6 to +6, representing thousands of feet per minute.  An arrow on the gauge indicates the aircraft's current rate of climb or descent; if it is pointing at zero, the aircraft is in level flight. This gauge changes colour in sections, to indicate the required rate of climb or descent to avoid a collision.  If, for example, TCAS has determined that the aircraft must climb at a rate of 4,000ft per minute or more to avoid a collision, all of the gauge from -6 through +4 will be red, and all of the gauge above +4 will be green.  The aim here would be for the pilot to climb at a sufficient rate to get the arrow into the green area, and assuming the other pilot has similarly complied with his TCAS system's RA, everything should be hunky dory.  If any further changes to the rate of climb or descent are needed these are again communicated verbally - "Increase descent, increase descent" for example.

Mutual compliance with Resolution Advisories is where TCAS saw some problems. It was initially somewhat murky as to whether compliance with a TCAS RA was compulsory (despite an appeal by Japan to ICAO following the Japan Airlines Flight 907 incident in 2001). It was this uncertainty, in part, that resulted in the mid-air collision between a DHL Boeing 757 and a Bashkirian Airlines Tupolev Tu-154 over Lake Constance, Germany in 2002.  Both aircraft were on a collision course, both were fitted with TCAS, both systems operated correctly, but the DHL crew followed the instructions of TCAS while the Russian crew followed the instructions of a controller who was unaware that TCAS had issued instructions to either aircraft (neither pilot reported their TCAS systems had gone off).  The result was that the two aircraft intersected almost perfectly with each other at right angles in descending arcs, with one aircraft descending on TCAS' instructions but the other following an ATC instruction to descend, in contradiction with his TCAS system's instruction to climb. The 757 passed slightly below the Tu-154, its vertical stabiliser slicing the Russian aircraft in two. The Tupolev exploded almost immediately and the Boeing, minus most of its tail, crashed into a wooded area about four miles later.

Today ICAO mandates that pilots follow a TCAS RA, and controllers are not permitted to issue instructions to an aircraft that reports doing so. The best they can do is to provide both pilots with information on the position of the other aircraft.

TCAS can also cause problems when it misinterprets the actions of aircraft. If, for example, an aircraft with TCAS is steady at 20,000ft and an aircraft is climbing up underneath it to 19,000ft, the system has no way of knowing that that aircraft has been cleared to 19,000ft and may issue a 'nuisance RA' in the mistaken belief the other aircraft will continue climbing underneath.  TCAS RAs may trigger other losses of separation and further RAs in other aircraft, which can 'chain' and take some time for controllers to sort out.  The consequences of TCAS going off in a holding stack are pretty unpleasant to contemplate; at least, from the perspective of 'cleaning up' afterwards.  Imagine dozens of aircraft in a holding stack, circling around a particular point separated vertically by 1000ft.  If an unknown aircraft blunders into the airspace at low altitude and sets off the TCAS of the aircraft at the bottom of the stack and causes it to climb.. well, you work it out.  The controller gets to sit back powerless as, one by one, TCAS instructs every aircraft in the stack to climb to avoid the aircraft beneath it.

Thankfully controllers are not responsible for the separation of aircraft in the process of a TCAS climb or descent, and just tell the pilots to "report back under my control" when it happens.  Although to be honest, if TCAS has gone off in an aircraft, it's at least even odds that the controller concerned will be in trouble in the near future anyway.

The reliance on transponders is the other problem with TCAS.  It needs transponder signals to determine the location of potential threats to itself, so if an aircraft is not carrying a transponder or has it switched off, TCAS is blind to its existence.  Nonetheless, it has successfully averted a number of potential mid-air collisions since its introduction.

Up until recently further work was running to develop TCAS III, which would have been capable of providing horizontal, as well as vertical RAs.  The current view seems to be that vertical resolutions are adequate and that a system providing horizontal RAs is too complex to implement, as TCAS III development is presently suspended indefinitely.

* a red square is a threat, a yellow circle an intruder, a filled blue diamond is proximate traffic and a hollow diamond is 'other' traffic.