The Airport Portal is a minor myth in Grand Theft Auto: San Andreas.
The myth revolves around a glitch that happens to cars in the tunnel under Los Santos International Airport. The glitch causes cars to vanish, launch into the air, or drive inverted while on the first corner of the tunnel. This is very similar to a similar glitch in the Bayside tunnel.
It is unknown officially why either happen, but both only seem to happen when FPS Limiter is turned on. For this particular glitch, it may be just an error in the level geography, as there is a small gap into Blue Hell between two pieces of road. When a vehicle's tire hits this area, the physics engine will be glitched resulting in the phenomena.
It most likely can be ruled out by the FPS Limiter being off, as many things do not work while it is on (e.g. Not being able to turn on sirens.)
Unlike its Bayside counterpart, it is significantly easier to spot, standing and waiting in a corner is all that is needed to see the vehicles seemingly automatically vanish.