Fighters won't upgrade???

[DaXX]

I built a bunch of fighters right before getting rocketry thinking i could get a quick upgrade the following turn.

now the time has come and i hit Shift-U to upgrade all my fighters and the advistor tells me 'We have not a single Fighter which can be upgraded!'

I check the civlopedia and it says Fighter upgrades to Jet Fighter.... i have the nec. resources, i can build jet fighters, i just can't upgrade my old units

what gives?

[Harovan]

Hmm, I'm unsure if they upgrade at all, because I never build fighters, but always wait for Rocketry...

Have you checked if they are located in a city with an airport? AFAIK this is necessary for upgrading air units (similar to harbor and navy units).

[DaXX]

uh yes this was the problem.