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\begin{align*}
    \Delta m_{s,z}=&  \sqrt{ \left[ \frac{\partial m_{s,z}}{\partial l} \Delta l\right) ^2 + \left[\frac{\partial m_{s,z}}{\partial L} \Delta L\right]^2 + \left[\frac{\partial m_{s,z}}{\partial u_e} \Delta u_e\right]^2 + \left[\frac{\partial m_{s,z}}{\partial T} \Delta T\right]^2 + \left[\frac{\partial m_{s,z}}{\partial \frac{\partial B}{\partial z}} \Delta \frac{\partial B}{\partial z}\right]^2 }\\\\
    =& \sqrt{\left[-\frac{6u_e kT L \frac{\partial B}{\partial z}}{(lL(1-\frac{L}{2l}) \frac{\partial B}{\prtial z})^{2}} \Delta l \right]^2 + \left[- \frac{6u_e kT \cdot (l \frac{\partial B}{\partial z} - \frac{L}{2l} \frac{\partial B}{\partial z})}{(lL(1-\frac{L}{2l})\frac{\partial B}{\partial z})^2} \Delta L\right]^2  + \left[\frac{6kT}{lL(1-\frac{L}{2l})\frac{\partial B}{\partial z}} \Delta u_e \right]^2 + \\ & \left[\frac{6u_e k}{lL(1-\frac{L}{2l})\frac{\partial B}{\partial z}} \Delta T\right]^2 + \left[- \frac{6u_e kT }{lL(1-\frac{L}{2l})(\frac{\partial B}{\partial z})^2   } \Delta\frac{\partial B}{\partial z}\right]^2}
\end{align*}

\documentclass{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\begin{document}
\begin{align*} 
 \Delta m_{s,z}=& \sqrt{a\hskip \textwidth minus \textwidth b}\\%verdeutlicht die lange Zeile unter der Wurzel
 &\overline{a\hskip .5\textwidth minus .5\textwidth b}\\%Fortsetzung der Wurzel mittels \overline{}
 \end{align*}
\end{document}