Does anyone have any experience on whether a backpack with a frame will work as a carry-on for either the commuter airlines in Europe (like Ryanair) or for the trip from the U.S. to Europe (probably London)? I measured my Osprey Atmos 50 backpack and it looks like it might be close to the requirements I have seen.
I don't know about the commuters but if your Atmos 50 is completely full it could be close on the transatlantic flights. Mine fit OK on American and Air France but it was not quite fully packed. I would say give it a try. If it doesn't fit in the overhead they will check it for you and you haven't lost anything.
Adrian, I suspect that your Osprey Pack will not be suitable for carry-on, especially with the European airlines which have very restrictive baggage limits. My Backpack is supposedly "carry-on size" with the Daypack removed, but I've found in reality that airlines always force me to check it, either on size or weight basis.
You could certainly try and take it as carry-on for the transatlantic flight, but you might want to take a small Daypack and be prepared to check the Osprey if necessary.