That should be either Dornoch Firth or Moray Firth. It's probably Moray Firth.
8 seconds sound clip from the Bon Voyage, Charlie Brown movie soundboard.
You can hear this line at 00:14:29 in the Blu-ray version of the movie.
Quote context
[...]
- So long, Chuck. I'm going to the center section.
- I'll leave the kid stuff for the likes of you.
- That should be either Dornoch Firth or Moray Firth. It's probably Moray Firth.
- I think that's Inverness Bridge!
- See? We are just approaching the Firth of Forth, and there in the distance, you can see Glasgow.
- That's Scotland we're flying over, the way I figure it.
- We should be landing in London within the hour.
[...]