That's a tilt switch, positioned to detect a side bump to the cabinet.
.................David Marston

AKA slam switch or slam tilt switch.

I never heard of Airport until I saw this post, looks like a cool machine! It would look nice sitting next to my Bon Voyage.

If you look around, you may find two more. One positioned to detect a bump from the front (maybe on the coin door), and another to detect a bump from above/below (maybe on the bottom panel) as if you lifted the front of the machine and dropped it. Since they can only really detect a bump in one direction, you need one for each of the three axes or directions (side to side, front to back, top to bottom).

/Mark

Let's get more technical. Tilt switches are normally open. Slam switches, at least on Gottlieb EMs of this era, are normally closed. You will see them in different places on the schematic. Their effect on the game is different, especially for multi-player games like Airport, where a regular tilt only kills the current ball in play.
.................David Marston