Chapter 2 - Section 2.4 - Inertia and Equilibrium: Newton's First Law of Motion - Practice Problem - Page 41: 2.7

The net force on the plane is $0.5~kN$ downward.

Let north be the positive direction. We can find the net horizontal force on the plane: $\sum F_x = 1.2~kN+(-1.2~kN) = 0$ Let upward be the positive direction. We can find the net vertical force on the plane: $\sum F_y = 15.5~kN+(-16.0~kN) = -0.5~kN$ The net force on the plane is $0.5~kN$ downward.