align
My minimum working example:
\documentclass{report}
\usepackage{amsmath}
\begin{document}
\begin{align}
x_1 = \alpha,\, x_2 = q,\, u = \delta \\
f_1(x_1) &= C_1 \left [ \phi_{z1}(x_1) + \phi_{z2}(x_1)M \right ] \\
f_2(x_1) &= C_2 \left [ \phi_{m1}(x_1) + \phi_{m2}(x_1)M \right ] \\
g_1 = C_1 b_z,\, g_2 = C_2 b_m \\
C_1 = \frac{\bar{q}S}{mV_T},\, C_2 = \frac{\bar{q}Sd}{I_{yy}}
\end{align}
\end{document}
The result:
What I want:
Since you're not really using an array, I suggest another strategy:
\documentclass{article}
\usepackage{amsmath, array}
\makeatletter
\newcommand{\X}[1]{\ifmeasuring@ #1\else\fgnu@X{#1}\fi}
\newcommand{\Y}[1]{\ifmeasuring@ {}:#1\else\fgnu@Y{#1}\fi}
\def\fgnu@X#1{\hbox to \ifcase1\maxcolumn@widths\fi{$\displaystyle#1$\hfil}}
\def\fgnu@Y#1{\hbox to \ifcase2\maxcolumn@widths\fi{${}:{}$\hfil$\displaystyle#1$\hfil}}
\makeatother
\begin{document}
\begin{align*}
x^2 & = y^2 \\
z^2 &= a^2 \\
\X{aaaa} & \Y{bbbbbggggggggg}\\
\X{cccccccc} & \Y{dd}
\end{align*}
\end{document}
The left side is specified by \X{...}, the right side by \Y{...}.
Don't use align. Use gather instead.
\documentclass[fleqn]{article}
\usepackage{amsmath}
\begin{document}
\begin{gather}
b := (a \oplus s_1) \oplus s_2 \\
e := 0
\end{gather}
\end{document}
Best Answer
As far as I understand, you want to obtain
C_1in the last line moved to the left. (If no, simply remove\!). However, I would suggest replacing,\,'s by,\quad's.