For an aircraft executing a horizontal loop, the bank angle \( \theta \) is given by:
\[
\tan \theta = \frac{v^2}{g r},
\]
where \( v \) is the velocity, \( g \) is the acceleration due to gravity, and \( r \) is the radius of the loop.
Substituting the given values of velocity \( v = 540 \) km/h, radius \( r = 9 \) km, and \( g = 10 \, \text{m/s}^2 \), we can find \( \theta \), which is \( \cot^{-1}(4) \).