Assume a flat Earth, then the angle of elevation phi satisfies:

tan(phi) = height/range ... (1)

height = 9000 m.

d range/dt = 800000*60*60 m/s.

When the elevation is pi/4 rad, the range is 9000 m.

Use implicit differentiation on (1), and plug in what you know to find d phi/dt.

RonL