This is episode 22 and we’re going to hear more about an accident in the skies over India that was the final push in the drive to deploy traffic collision avoidance systems known as TCAS.

Initially we need to go back to the days days of commercial aviation in 1922.

Unfortunately the first collision between aircraft took place almost immediately as commercial aviation launched in the same year as earlier aviators were ignorant about each other’s plans, altitude and track. They also spoke many different languages which also didn’t help.

It took another 70 years before a system was introduced to ensure separation that was automated and computerised. Before then systems were developed that relied on accurate flying using the semi-circular rules.

The basic tenet is that when flying a track between 0° and 179° or generally speaking, easterly, your flight level or altitude must be odd such as 030 or 050 and so on, but when you are on a track between 180° and 359° generally speaking .. westerly, your flight level or altitude must be even – for example 18000, 16000 feet and so on.

There are other rules associated with this and whether you’re flying IVR or on Instruments.

That is supposed to reduce the chances of planes colliding but it only works if the pilots are flying their planes at those levels. Sometimes pilots do break the levels – and at other times their instruments are faulty.

On 12 November 1996 a Saudi Arabian Airlines Boeing 747 which had just taken off from Delhi in India en route to Dhahran collided with a Kazakhstan Airlines Ilyushin Il-76 en route from Chimkent to Delhi.

The crash killed all 349 people on board both planes, making it the world's deadliest mid-air collision of all time and the deadliest aviation accident to take place in India.

This was an example of a system that was used in a confusing way.