The proposal makes the assumption that the tax is valid in the first place and, secondly, that the aviation business contributes significantly to global emissions.
In addition, it also tacitly infers that the tax revenue is used to offset the carbon emissions; we all know that isn't the case.
If this, or any other Govt, is genuinely concerned with carbon emissions they wold have embraced the Kyoto accord and would put their own houses in order by reducing ground based fossil fuel consumption by industry etc first.
They would also level the playing field by levying the tax on trains, buses, ferries, taxis and other passenger or cargo carrying systems.