They still use nitrogen in aircraft tyres. They also use nitrogen in other aviation related applications such as oleos (Special name for aircraft suspension), accumulators and other such devices.
There are a couple of reasons for this.
1. Nitrogen is sold in bottles under high pressure. This makes it easier to fill things that need to operate under very high pressure. The last accumulator that I saw filled was filled to 750PSI. Try doing that with your normal compressor.
2. Air, contains lots of nitrogen and also oxygen. Oxygen can be quite flammable, especially when under pressure or exposed to heat and oil. In the tyres on large aircraft, there may be oil or grease in the tyre. As such, after a landing (gas compressed and heated by compression and the nice carbon brakes) if the tyre was filled with plain old air, under high pressure, there is a danger of an explosion.
3. Oleo legs on aircraft contain oil and nitrogen. Again, the idea is that air may contain too much oxygen, and when compressed on landing, the mixture would be too dangerous and it could explode.
My take on this whole nitrogen in car tyres is that it is a con, plain and simple. Some garage owner deduced that he can get a bottle of nitrogen for $A and inflate X tyres for $Y and make $Z in profit.
Other people may suggest that this is just a stupid tax.
