No other piece of technology has made a greater contribution to flight safety than TCAS. Every airliner operating in the known world must be fitted with this marvellous bit of kit, although the MEL (Minimum Equipment List, remember?) does allow for flight with an unserviceable system!

Each aircraft’s TCAS unit transmits coded information which is received and decoded by all other close-by aircraft. The information sent includes the aircraft’s altitude, its heading, and whether it is climbing or descending. The rate of climb or descent is also transmitted. Should a proximate aircraft’s TCAS unit believe that the other aircraft is too close for comfort then one of three conditions will be triggered.


The on-board display, usually on the navigation display screen, will show a hollow white diamond representing other aircraft within a set distant depending on the particular equipment fitted. It’s a bit like Harry Potter’s Marauder’s Map. We can see where they are, their relative heights and whether they are climbing or descending. The white diamond becomes solid should the aircraft pass close by, but not pose a threat of collision.

Traffic Advisory

The on-board display changes to a solid amber circle, and some computerised bloke says, “traffic, traffic” in an American accent. This happens if the TCAS system computes that a collision could occur within 40 seconds. Harry Potter’s magic wand would be useful in this situation.

Resolution Advisor

The amber circle becomes a solid red square and the American bloke gives instructions to manoeuvre out of the way of the offending aircraft. Typically you are told to climb or descend. This condition is triggered when a collision risk could occur within 20 seconds.

The Airbus A320 is essentially a French plane. Airbus headquarters are in Toulouse and that’s where most of the planes are assembled. There are several international partners in the Airbus consortium, including BAe Systems in the UK, but no one doubts the Frenchness of the aircraft’s philosophy. So it’s a wonder that someone was brave enough to suggest using an American voice for the warning announcements. Perhaps the shoulder shrugging, disinterested mumblings of a typically nonchalant Frenchman would not have the same sense of urgency and authority.

Crucially, both TCAS systems are communicating with each other to ensure that pilots of the proximate aircraft are not given the same avoidance instructions. I mean, that would be stupid and negate the whole point of the technology. One aircraft will be told to climb, the other to descend. Makes sense. It also raises an important point: TCAS must be obeyed immediately, regardless of any instructions from ATC.

An appalling tragedy occurred in Swiss airspace in 2002 when a Boeing 757 cargo carrier operated by DHL collided with a Bashkirian Airlines aircraft destined for Barcelona. The Swiss controller had given emergency avoidance instructions to the Russian aircraft which contradicted the instruction from his TCAS. He chose to obey ATC and that put him back into conflict with the freighter which had correctly taken avoiding action according to his own TCAS. There was total loss of life.