  We report magneto-transport measurements in wide GaAs quantum wells with
tunable density to probe the stability of the fractional quantum Hall effect at
filling factor $\nu = $ 5/2 in the vicinity of the crossing between Landau
levels (LLs) belonging to the different (symmetric and antisymmetric) electric
subbands. When the Fermi energy ($E_F$) lies in the excited-state LL of the
symmetric subband, the 5/2 quantum Hall state is surprisingly stable and gets
even stronger near this crossing, and then suddenly disappears and turns into a
metallic state once $E_F$ moves to the ground-state LL of the antisymmetric
subband. The sharpness of this disappearance suggests a first-order transition.
