Spoofs are derivative works but original works compared to remakes or sequels.

You could also argue it was spoofing the Airport films which were based on books.

There are a significant number of scenes in Airplane that are directly lifted from Zero Hour. I haven't checked recently, but I think it's almost a scene-for-scene remake in many respects.

Here's a video comparing the two:

https://www.youtube.com/watch?v=8-v2BHNBVCs

It really was a remake of a "serious" drama as a comedy.

I didn’t realize Hailey was behind both.