One of the more unusual restrictions I found when researching what alcohol you can take on a plane is the limit on proof. For airlines the limit is 140 proof or about 70% ABV. This limit applies to checked luggage only from what I can tell. Bottles in your carry-on don’t seem to get the same treatment. This led to two possible answers. First is the fact that high-proof spirits are actually illegal to sell in at least 15 States and transporting them could lead to significant liability.
Second is the possibility for damage to the aircraft and cargo.
For legality purposes it is illegal to sell alcohol at 190 proof in California, Florida, Hawaii, Iowa, Maine, Maryland, Michigan, Minnesota, Nevada, New Hampshire, North Carolina, Ohio, New York, Virginia, and Pennsylvania. It’s totally legal to own so far as I’ve been able to tell but as with many other things the devil is in the details.
My college chemistry courses taught us about a concept called vapor pressure. Essentially the boiling point of a liquid is decreased depending on the amount of atmospheric pressure applied to it. This is the reason that people who live at high altitude or who go hiking in the mountains have to take care when cooking. Water will boil at a much lower temperature leading to inaccurate cooking times. As you can see below the boiling point drops significantly as the pressure decreases. Standard cruising altitude for most aircraft is 30,000 feet at which point the pressure would be around 226 torr outside. Given the chart below that puts the boiling point somewhere between 40 and 65 C. The closer to 200 proof you get the lower the boiling point.
When alcohol boils it turns into a gas which rapidly increases the pressure inside the bottle and causes either the cap to fail or the bottle itself to shatter, at which point you have a quantity of highly flammable gas loose inside the hold of an aircraft.
Not to mention that the exploding liquor bottle and flying glass could do a bit of damage on their own.
Depending on the aircraft most cargo holds are generally pressurized and heated. Some aren’t heated but regardless the changes in pressure and temperature shouldn’t impact a bottle while in flight.
So in the end this amounts to an overabundance of caution from the airlines. I’ve reached out to some airlines in an effort to better understand this restriction but have not yet heard back from any.