I once queried the CP about an A-310 takeoff from YYC with frost over the tanks. He replied that the FOM allowed for that.
After the Dryden report, attempting takeoff with
any ice or frost was banned, but CD has usefully pointed out CAR 701.25(3):
Notwithstanding subsection (2), a person may conduct a take-off in an aircraft that has frost caused by cold-soaked fuel adhering to the underside of its wings if the take-off is conducted in accordance with the aircraft manufacturer's instructions for take-off under those conditions.
but note that this clause only applies to the
underside.
So the attempted takeoff would have been a violation of CAR 701.25.
Note that pushback and taxi with contaminated wings would not be a violation of the CARs -- only the takeoff attempt.
I do not understand why USAir is not backing the FAs
I would think your average airline management with any concern for maximising bums in seats would be looking for a way to fire the FO. If the union is preventing that, management would want to be seen on the side of the FAs.