Fiona Stevens has nailed it:
I had a very pleasant conversation today with Frau Unger from Airberlin head office who has helped many of us violinists in the past. She allows me to post the following procedure for taking violins on board until the wording of Airberlin terms & conditions is changed.
Anyone wishing to take a violin/viola on board should write to email@example.com stating their flight & booking number, and the dimensions & weight of their instrument case. They should then receive written confirmation that they will be able to take the instrument as handluggage at no extra cost.
For anyone wondering why, whilst top management has decided to change policy in favour of musicians, it is taking so long for the official wording to be changed: this is (much the same as my recent post re Eurowings) because changing the wording of general terms & conditions requires every department (including legal) to ok the new wording, and this simply takes time.