SPOILERS
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The plane isn’t supposed to crash into a building. There is a poison gas bomb on the plane, and it has a barometric trigger. Once the plane goes below a certain altitude, the bomb goes off, spreading the gas. I’m sure that this wouldn’t really work in real life (try the goofs entry for the film on the IMDb).
Don’t ask me how I remember this. I saw the movie once, during its original theatrical run, although I noticed it was on TV last night–pretty weird that Halle Berry was in it, if you think about it now.
In “Mike Nelson’s Movie Megacheese” he calls this his favorite movie ever since they kill off Steven Seagal so fast.