@Saluki, GAAP is mostly concessions which are expiring soon if I remember correctly. In general airports are great businesses, however:
a) Watch out for concessions versus owned outright
b) In Europe, Aena is a great business, however may be hurt by environmental movement of switching trips under a thousand miles to rail
c) Regulatory framework can be a blessing or a curse. While you want dual till structure (Aena has it), if I am not mistaken, Spanish government forced Aena to cut contract prices during Covid, that's why I no longer own Aena, I do not trust the Spanish government.
d) Capital allocation is very important - the Mexican airports (concessions) historically paid very high dividends. Aena is empire building.