The trip out is neatly packed. The trip home comes with a damp towel, worn clothes and one pair of shoes you would rather keep away from everything else. The Thule Aion 40L addresses that less photogenic half of travel with a removable TPU-lined rolltop bag, rather than simply offering more pockets.
The feature worth packing around
The removable bag creates a separate place for dirty or damp items inside the main compartment. Lay the backpack open, identify what needs separating, and leave enough room for that bag before filling the remaining space. Separation still uses capacity: it does not turn forty litres into two independent forty-litre compartments. Unpack wet items promptly at your destination.
The photographs show internal compression straps, a zippered mesh area, a small-access pocket and padded device storage. These are different jobs, not a reason to fill every space. Put bulky clothing in the main cavity and keep the few things you need on the move accessible. Check the padded compartment against your actual laptop dimensions and any sleeve you use.
Check the airline before the packing list
Thule lists the exterior at 13 by 9.1 by 20.5 inches and the empty weight at 3.2 pounds. Compare those numbers, plus your loaded weight, with the allowance for your particular airline and fare. A forty-litre description or a carry-on marketing label is not universal permission to board with it. This is the fixed-capacity 40L model, not the expandable 28L Aion.
The waxed canvas is water-resistant, not waterproof. Water beading in a manufacturer photograph does not make this a dry bag. The compatible Aion Sling can act as a hip belt but is sold separately; so are any luggage locks shown in styling photographs.
Who gets the benefit?
Choose it if separating the return-trip mess is a recurring problem and you prefer a backpack that opens for deliberate packing. Skip the extra size if your trips are mostly a laptop, a light layer and lunch. For a weekend away, the useful question is not whether you can squeeze more in—it is whether everything has a sensible place when you come back.
