You need to lean forwards. On keys, repeatedly tap the lean forwards key, and swerve well to the left of the ramp at first to avoid the barrels. Doing it that way, you'll probably nail it second time.
Yeah, I was doing that - I have a guide that I turn to when I've tried a mission a few times and am ready to chuck the laptop out the window.
I tried it maybe 50 times and could never get even close to the plane, and the couple times I did it LUNGED away from me as soon as I approached.
Then I read something on a forum where someone suggested turning down the display quality to make the gameplay faster. So I turned down the resolution, texture quality, and turned off aliasing, and then tried Stowaway again and got it without even having to lean forward. Actually, the first time I went REALLY fast and actually overshot the plane.
Anyway, I inched up the display quality again using Stowaway as my barometer. If I could get on the plane, I turned up the juice until I couldn't catch it anymore, then brought it back down.
Now EVERY mission is easier. I even went back through the flight school and got golds on a number of tests there were simply IMPOSSIBLE before.
Of course, now that I've finished the desert missions and most of the Las Venturas missions I'm now stuck on Freefall (the one where you have to go past the really fast jet, swing around instantly and fly through the corona?) and again it is seemingly impossible. I modified the handling.cfg file (when in doubt, change physics) but now I have the opposite problem - overshooting the thing and/or crashing into it and blowing up in midair. I'll have to tweak it I guess.
Yeah, stowaway was a fun mission after it ceased being IMPOSSIBLE.