Answer
a) There is no net impulse acting on the plane during this time.
b) The impulse of the thrust is equal in magnitude but opposite in direction with the impulse of the air resistance force.
Work Step by Step
a) The impulse-momentum theorem states that $$\vec{J}=\vec{p_f}-\vec{p_0}$$
When the plane is flying with a constant momentum, $\vec{p_f}-\vec{p_0}=0$, there is no net impulse acting on the plane during this time.
b) We call the thrust force $T$ and air resistance force $R$. We have $$\sum \vec{F}=\vec{T}+\vec{R}$$ $$\sum \vec{\overline F}\Delta t=\vec{\overline T}\Delta t+\vec{\overline R}\Delta t$$ $$\vec{J_T}+\vec{J_R}=\vec{J}=0$$ $$\vec{J_T}=-\vec{J_R}$$
The impulse of the thrust is equal in magnitude but opposite in direction with the impulse of the air resistance force.
