a (hopefully) simple quaternion question

weg22@drexel.edu

Say my aircraft is in a current orientation of
phi (roll, x-axis) = 0 degrees
theta (pitch, y-axis) = 30 degrees
psi (yaw, z-axis) = 20 degrees

which corresponds to a quaternion
Q = 0.952 + 0.045i + 0.255j + 0.168k

But my desired orientation is
phi = 0 degrees
theta = 0 degrees
psi = 0 degrees
-OR-
Q = 1 + 0i + 0j + 0k

If I am thinking in terms of Euler notation, then I know there is a
pitch error of 30 degrees and need to deflect the elevator to correct
this. Likewise, the 20 degree yaw error must be countered by rudder
deflection. However, if I'm thinking in terms of quaternions, then how
can I go about this?

Thanks in advance,
weg