Last I checked, there was no time limit for this mission. (How stupid of me, there is) If there is, then that's what I meant in the other topic about this mission about it not being easy since you don't have the bulletproof Barracks OL to dodge the M16 bullets at the draw bridge. Isn't there a marker on the radar? It should point to one of the Terminals in Francis International.
If you're talking about the Porter Tunnel, then it's practically at the top of Staunton in Aspatria. It's really difficult because the Porter Tunnel from Staunton to Shoreside Vale drops off somewhere underground in Wichita Gardens (If I'm not mistaken) and it's a great drive to the airport.