Answer
The forward momentum of the plane decreases.
Work Step by Step
According to the conservation of total linear momentum, $$\vec{p_{you}}+\vec{p_{plane}}=\vec{p_0}$$
The total linear momentum of the plane before you get up is conserved and always the same as long as there is no external force affecting its motion.
Therefore, as your momentum, or $p_{you}$, increases, it follows that $p_{plane}$ must decrease to balance the rise. So the forward momentum of the plane decreases.
