I started flying with guitars back in the late 1980's/early 1990's. At the time, I owned a Lowden O25C and brought it along as I moved about from London to New York to L.A., and, on occasion, points beyond. The advice Brynn Hiscox gave to me, no matter that his cases were built to take the rigours of travel and the thousand shocks guitars are heir to, was to first loosen the strings, place a Dampit or similar humidifier in the soundhole, wrap the instrument in a large platic leaf bag to create a stable enviroment, and then put the "package" in the case. If memory serves, Hiscox also recommended placing towels in the empty space between the guitar and the interior case wall to further stabilise it and increase protection from rough handling. Even though I often flew first class and managed to charm the gate agents into allowing the guitar to be brought into the cabin and stored in a closet, there were times they refused. My Lowden then would be taken and placed in the cargo hold, and I would spend the entire flight wondering if it would survive. It always arrived safely and the guitar never suffered any structural or cosmetic damage.
Many UK-based luthiers trust their instruments to Hiscox cases regardless of where it is being shipped, domestically or internationally. I eventually could afford Calton Cases, then built like tanks in Ash (near Aldershot) England, purchasing them for the luthier-built guitars in my collection. Why? As already mentioned, I could afford them and they eliminated the need to wrap the guitar within and any additional padding. I have continued to rely on Hiscox cases for two, recent Martin Custom Shop guitars which are now supplied in crap cases, and the Fender Teles because the manufacturer cases don't travel at all well.
Two new guitars have been commissioned and both luthiers use Hiscox cases. Indeed, because of the drop in the quality of the Calton cases when production was moved from the UK to Canada, one of the luthiers insists on shipping the completed instrument in a Hiscox Pro II case, no matter that Caltons now are manufacturered in Austin, TX.
It is risky to travel by air with cases that may or may not be up to the rigours of airline baggage handling. Security concerns coupled with shrinking storage space in airline cabins make bring instruments on-board increasingly unlikely. One often will be informed (unhelpfully) that if an instrument is so valuable, one should purchase an adjacent seat. If your instrument is valuable and/or irreplacable to you, regardless of its real value, spending the additional money to purchase a proper flight case and making certain your guitar is well and properly packed (one can always ring the builder/manufacturer for advice) is cheaper than trying to replace it after it's been damaged or destroyed. If one travels by air and needs to bring a guitar, purchasing a Hiscox, Calton, Hoffee, Karura, or a Visesnut is a sound, albeit expensive, investment considering the alternatives. As Brynn Hiscox told me many years ago, it is much cheaper to replace an instrument case than it is to replace the instrument within.
'72 Guild Mark II
'76 Gurian S3M
'96 Sobell Model 1 Sicilian
'97 Bown 000
'00 Aram "Hauser"
'09 Martin 000-18 Marquis
'11 Nick Apollonio 12-string Bell
'12 Martin D-41 Custom
'13 Martin M-30 Jorma Kaukonen
'17 Martin CS OMC-21