my girlfriend tells me you must detune the guitar completely.

is this true?

ive never taken any guitars, electric or acoustic, on an airplane.
i'm taking my acoustic with me to san diego next week and i wanna know, will it fuck up my guitar if i dont detune?

it should be fine, depends where you put it. i take my acoustic with me sometimes but i've always managed to take it as hand luggage. if it's put in the baggage hold it might detune easier, either because it's cold in there or because it gets knocked around more. either way, it should help to have a sturdy, well fitting guitar case.

Apparently you do, because of the low temperatures... too much tension could warp the neck when the guitar expands. Detune it a bit and you should be fine.