The Airport Portal is a minor myth in Grand Theft Auto: San Andreas.
The myth revolves around a glitch that happens to vehicles in the tunnel under Los Santos International Airport. The glitch causes vehicles to vanish, launch into the air, or drive inverted while on the first corner of the tunnel.
It is unknown officially why either of them happen, but both seems to happen even when Frame 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 Frame Limiter being off, as many things do not work while it is off (e.g. Not being able to turn on sirens.)