I went abroad to France on some business in fall of 2017 and brought along my McSpadden in a soft case. I'm not sure about what the luggage allowance difference is between international and domestic flights but I was able to find under "musical instruments" the size limitations and discovered I could bring my McSpadden with soft case as a carry-on. It fit into the overhead bin on top of other people's luggage. I was nervous about it, I must say, but it traveled fine. I would prefer doing what Aaron does next time. Anyway, it was possible in 2017 to bring it onboard an international flight at least.
Thank you Garret. From what I am reading it appears that you can bring your dulcimer on the airplane as carry-on, as long as it fits in the overhead bin. The problem is I am booked on a CRJ200 going from Huntsville, AL to Washington DC and bins measure 18" wide! Ridiculous. So unless, the crew is willing to put it in one of their personal lockers which would be bigger and longer, I don't think they're going to let me take it. Such a shame...