This is from memory, so bear with me....

The glitch isn't exclusive to the 360, but the regular XBox and PC too.
It occurs when you get a pilot's license from flying a plane around and
boosting CJ's skill. Note: This does NOT happen with helicopters, only
planes.

I can't remember the exact reason (Someone did post it here) but the
game won't spawn any more passengers for you in the Taxi Driver
mission, even when you have progressed far enough in the game to get a
pilot's license legitimately. The bad news: Unless you've dropped off
50 passengers before you get the illicit license, you can't reach 100%
as you'll be missing the 0.33% needed that the TD missions bring.

So, sorry but you'll have to start afresh, or at least load an earlier
save. If you get the proper pilot's license (Which is when the game
legally lets you into airports) the TD mission is OK.

I'll see if I can find the glitch discussion on GTAforums, it goes in
to much more technical description.

Cheers,
Rob

"Do sugar cubes have dots or is that a dice I just put in my tea?"

To the world you are somebody, but to somebody you are the world.