Oh - I should've noticed there's a problem using the $4B profit - $7B (earnings before interest, tax, depreciation and amortisation) should be used instead. You shouldn't double count depreciation charged on existing runways, interest paid on debt used to fund the existing runways, or tax on existing equity return.

Using half of current EBITDA (two exisiting runways) to pay for the runway's current cost ($86.2B), we get 24.6 years.

If we take into account that HKAA may be possibly underpricing its landing fees like cross-harbour tunnels (in effect subsidising airlines using HK *and* air travellers) then payback is reduced to 13 years.