I am having the same problem with another airline. Under present rules, the permitted single piece of hand baggage must not exceed 56x45x25 cms. A typical violin case is less wide, less deep than this, but a bit longer, and in total has less than half the volume of the allowed dimension. It is also very light and fits easily into the cabin lockers and a security check can verify that a violin is not a bomb. But will it be permitted on board? Answers on a postcard. On seconds thoughts, the answer is too obvious...