Osprey FarPoint 40. Unlike most convertible travel packs, it has a real hipbelt. It's small enough to fit onto all but the smallest airliners as long as you don't overfill it so that it becomes too thick. And Osprey packs have a reputation for lasting forever. It's a little heavy at 3.1 or 3.2 pounds.

I did a ton of research before settling on the FarPoint. And I wrote up a detailed comment about the results of that research, unfortunately it wouldn't post on reddit and then it blanked out. So here's a shorter re-creation of the disappeared comment.

Tortuga is a new company that makes travel packs and has an article that tries to criticize the FarPoint 40, but its criticisms are so weak that it convinced me to get the FarPoint. http://www.tortugabackpacks.com/pages/osprey-farpoint-vs-tortuga-travel-backpack

I'll leave out the details so here's the summary: the FarPoint 40 is carry-on size, in fact it's a better size than the Tortuga travel pack which is right at the limit of maximum carry-on dimensions. Osprey's background as a backpack company rather than travel pack company make me more confident that its hipbelt will do its job well. The Tortuga's reachable pockets are a nice feature but not very important because I carry my important stuff in my pants pockets and shirt pockets, not in my pack. Tortuga does have an advantage in that its laptop sleeve is on the side of the pack that's next to the body, whereas the FarPoint's laptop sleeve is far away from the body, harming weight distribution and balance. The Tortuga travel pack is almost 3/4 of a pound heavier than the FarPoint.

Rail Riders makes a very innovative travel pack but its hipbelt doesn't look good, and two of its linear dimensions are at the carry-on limit -- I wanted a pack that would be below the limits to guarantee that I could carry it on, as with the FarPoint. https://www.railriders.com/journey-pack-p-1026.html

Eagle Creek makes bombproof travel packs and suitcases that last forever, but their carry-on sized convertible packs either lacked good hipbelts or were a little too small. http://shop.eaglecreek.com/systems-go-duffel-pack-35l/d/1362_cl_2835

As a stopgap until I got the FarPoint, I bought a Mountain Hardwear Agama. Cheap and light and just about the right size. But it lacks a good hipbelt and doesn't convert into a suitcase. Also although Mountain Hardwear has a good reputation, I don't know if this pack will last forever the way that products from Osprey, Eagle Creek, and Rail Riders do, especially given how light it is. http://www.mountainhardwear.com/agama-backpack-OU5605.html

So the FarPoint was and remains my first choice.