If by cargo plane you mean the MASSIVE boeing 747 lookalikes? You CAN enter them. Go to Los Santos International Airport.
Once your past the gate, be quick, you'll get a wanted level for tresspassing. (I guess they think you're a terrorist.) Run up to the big cargo plane.
However, You'd EXPECT to stand next to it and press traingle where the "Doors" seem to be. That does not work. You have to stand DIRECTLY under it.
You'll kinda "Warp inside" of the cockpit. There is no animation, you just teleport inside the plane.
Edit: Nevermind, that plane is the Jet. Sorry. However not alot of players know the big jets are actually enterable because when first seeing them, they try to enter from the side and upon discovering nothing happens assume its not enterable.