http://i60.tinypic.com/24zdt2s.jpg
Far left is the farpoint 40,middle is 55L but s/m and the red bag on the right is 55L m/l without the day pack. Day pack is far right.
Can hide the straps and been using my 55L as a duffle.
Don't know about European Airlines but we flew Jetstar from Singapore to Saigon with no problem.
The 40 is fatter when packed than the 55L by almost a double. But you should have no problem with the 40l as its short in height