Steal the fast helicopter, land it next to plane, get in plane. Alternatively, use one of the bits of landscape around Fort Zacundo to jump your car over the outer perimeter fence and into the 'security ditch' between the outer and inner perimeter fences. The guards can't shoot you in there and you can drive around the edge of the base without being killed, then make a break for it.
You're presuming I did it in online though, remember:
Steal the fast helicopter, land it next to plane, get in plane. Alternatively, use one of the bits of landscape around Fort Zacundo to jump your car over the outer perimeter fence and into the 'security ditch' between the outer and inner perimeter fences. The guards can't shoot you in there and you can drive around the edge of the base without being killed, then make a break for it.
You're presuming I did it in online though, remember:
I didn't use any glitches though.