I was on ryanair 605 from budapest to copenhangen on 18 may 2019. noticed object out of plane window expelling a black vapor trail that eventually dissipated. the object was heading towards the plane on the right hand side. at first i assumed an aircraft was having difficulties in flight, but quickly realized it was something else and reached for my cell phone to take picture. during the fraction of a second it took me to lean forward, the object made a 90 degree turn away from the plane and, as it was directly across from me at that point, i noticed it actually was generating two black trails. i thumbed my phone to the picture app and when i looked back, it had again made a 90 degree turn and was pacing the plane. i managed to get three pictures of the object before it just disappeared.