As Denti says, not a problem as long as it is not visible in uniform, or at least in the uniform you'll be wearing in front of the public. I asked this question about a year ago. I was told not to worry, so went and got mine done.
It became a conversation topic at interview (A standard "Do you have any piercings or tattoos which are visible and you can't/won't cover up?" question) and I'm fairly certain that it actually went to my benefit as it's an aviation related tattoo!