Remember the airplane crash season? It started out with a shot of that teddy bear floating in the pool. Slowly, bit by bit they revealed more, but never WHAT have caused the explosion or accident that the hazmat team was cleaning up.
Vince Gilligan said they came up first with the teddy bear in the pool idea, then had to figure out WHAT happened. They thought a meth lab explosion would be too obvious, so someone mentioned a plane crash.
They thought just a random plane crash was too cheap, so THAt's how they came up with the girlfriend plot. She dies, the dad goes into a depression, he happens to be an air traffic controller, and he causes the crash.
Then entire show is based on cause and effect, chain of events.
Walter stands by and watches the girlfriend die in bed, the result is an airplane crash.
ALSO, if you put the episode titles in order it gives away the secret of the crash:
1 "Seven Thirty-Seven"
737 Down over ABQ. (ABQ is the airline code for Albuquerque, NM)