Smoke System. How does it work?

I was wondering, how does the smoke system work in FS2004? Does it turn on automatically on landing or do I always have to manually turn it on to see effects. Aerodesigns A340 has smoke system and it always turns on landing and the thing is that I can't turn it off.
It goes on and off by it self. I was wondering if that is how it is suppose to be and whether or not it happens on all aircraft that have smoke system because I only want to manually turn it on and off as I don't want to see any effects while on landing. I prefer them when my aircraft is cruising at FL300+. Please let me know if any of you know. Thanks alot
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