Q:

Airplane executing horizontal loop

The Problem:

An aircraft executes a horizontal loop at a speed of $720$ kmph (or $200$ m/s) with its wings banked at an angle of $15^{\circ}$. What is the radius of the loop.

My Confusion: Unlike the case of cars on banked roads executing a loop (which have the aid of friction and normal forces to provide the centripetal force), there seems to be no force (except air resistance) that aids the plane to execute a loop. So how does an aircraft maintain a loop horizontally? 

A:

$$\tan \left( \beta \right) =\dfrac {f_{z}}{f_g}=\dfrac {m\dfrac {v^{2}}{r}}{m\cdot g}\tag 1$$
Where $f_z$ Is the centrifugal forces ,$f_g$ Is the weight forces and $r$ is the radius of the loop.
$\begin{aligned}r=\dfrac {v^{2}}{\tan \left( \beta \right) \cdot g}=
\dfrac {200^2}{10\cdot \tan \left( \dfrac {15\cdot \pi }{180}\right) }\simeq 14.9 \end{aligned} \quad [km]$

