  We give a conjectural construction of Bridgeland stability conditions on the
derived category of fibred threefolds. The construction depends on a
conjectural Bogomolov-Gieseker type inequality for certain stable complexes. It
can be considered as a relative version of the construction of Bayer, Macr\`i
and Toda. We prove the conjectural Bogomolov-Gieseker type inequality in the
case of relative projective planes over curves. This gives the the existence of
Bridgeland stability conditions on such threefolds.
