I'm going to do it soon flying from US to Europe and liitle worry about that. I know how they treat checked in luggage. It will be in SKB hard case but I'm not sure if it's hard enough...
Is there any way to tell airline about fragile content?

In the past, I have flown twice from the UK to Australia, with a Korg synth,(in a soft case) and also an acoustic guitar. In both cases, they were placed in the 'Fragile items' hold, and came to no harm.
The airline informed me that this section of the hold was heated, and used for transporting items as fragile as wedding cakes, etc.

The only problem I had, was that the Korg left Singapore, and ended up in Kuala Lumpur, instead of my destination (Perth, Australia)
However, it was delivered undamaged, to my home the next day.

close enough that's the good one...

BTW did you pay extra for that fragile hold or had to notice them before the flight?
thanks

From memory, it didn't cost any extra, but yes, I checked with the Airline (Singapore Airlines) beforehand. At London Heathrow, there was a large, caged trolley at the check-in, where fragile items were placed, to be loaded onto the plane.

This was about eight years ago, so much could have changed since then.