We investigate the thermal QCD phase transition and its scaling properties on the lattice.
The simulations are performed with $N_f=2+1+1$ Wilson twisted mass fermions at
pion masses from physical up to heavy quark regime. We introduce a novel chiral order parameter,
which is free from linear mass contributions and turns out to be very useful for
the study of scaling behaviour. Our results are compatible with $O(4)$ universal scaling for the physical pion mass
and the temperature range $[120:300]$ MeV. Violations to scaling at larger masses and other possible scenarios,
including mean field behaviour and $Z(2)$ scaling scenario are also discussed.
We provide an estimation for the critical temperature in the chiral limit $T_0$.