No, a 50-liter backpack will almost certainly not fit under an airplane seat on any major commercial airline in 2026. The standard dimensions for "personal item" space under the seat are approximately 18 x 14 x 8 inches (45 x 35 x 20 cm), which equates to a volume of roughly 25 to 30 liters. A 50-liter pack is typically designed as a "full-sized" carry-on (which fits in the overhead bin) or a small trekking pack, often measuring 22 to 26 inches in length. Even if the pack is only half-full, its internal frame or back panel is usually too long to slide into the footwell without protruding into your legroom or the aisle, which is a safety violation. If you attempt to use a 50L pack as your only bag, you will be required to store it in the overhead compartment. For travelers aiming for "under-seat only" travel to save on baggage fees, a 28L or 30L "panel-loading" backpack is the absolute maximum size that can reliably squish into the required space. Trying to force a 50L bag under the seat will likely result in a "gate check" fee or significant discomfort as it leaves no room for your feet during the flight.