Jazzyvee:
I'm hoping the baggage handlers are careful too.
Yea, and I'm hoping I win the lottery! I think you have to prepare for the worst case when it comes to baggage handlers. (Of course one way to prepare for the worst case is with the best case!) Just to be extra careful and provide your axe with extra support, it is always helful to wrap the peghead in a towel or bubble wrap so that it is not just sitting in the case, unsupported. From what I've read, most broken pegheads are the result of the case being thrown around and the peghead breaking off inside the case from the force - kind if like whiplash. The towel/bubble wrap acts kind of like the extended head restraint on a car seat that prevents your head from going back when rear-ended.
I've heard differing opinions on whether to loosen the strings. Some say it makes no difference, others claim it relieves tension on the neck and makes it less likely it will be damaged. I would think that relieving the pressure might also help avoid the situation where the guitar is exposed to very cold conditions causing the strings to contract and put more tension on the neck. I'd be interested in others' thoughts on the loosening strings tactic.
Speaking of strings, when I fly, I always wait to get where I'm going before changing the strings. Have fun in Spain. We'll be on the lookout for a report.
Bill, tgo