I demonstrate how the Ghostplane video was created. I use Apple Motion, any high-end compositing environment could have been used on 9/11.
A better quality version of the demonstration video may be downloaded here.
1. Stabilize source video.
2. Create airplane.
3. Add airplane, color correct, blur.
4. Establish motion path of airplane.
5. Add layer mask to disappear airplane.
6. Add shadows on tower face.
7. Add hole, with masking.
8. Add puffballs.
9. Unstabilize everything.