Tape measure in hand. I spent an hour looking at backpacks trying to find the best that would work as a carry on. Its best to go to an outfitting store and tear into them. This one had the most features that I liked. I think as long as the rigid frame fits, the rest will squish. As mentioned by...
I recently measured the Gregory Zulu 45. The frame is 22 inches long which meets the overhead bin size requirements. As long as you don't overpack the top pouch there should be no problem taking this as a carry on. Still possible you may have to gate check it on flights within Europe.
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