If you’re lucky enough to get on a flight that’s not full, you may be eyeing up a different seat with more leg room or a nicer view. However, flight attendants have the right to deny you the replacement seat—and there's a reason why. Sitting in the right seat is far more important than you could imagine.

The Federal Aviation Administration recently published a handbook that addresses weight and balance on planes. As Travel + Leisure explains, “the center of gravity on a plane is most critical during takeoff.” Therefore, pilots need to know the distribution of weight on a plane, also known as the “index number.” Pilots use this number to set the trim, something that maintains airspeed. Pilot Magnar Nordal told Quora that “if the trim is set wrong, then the aircraft may crash at take-off.”

So make sure to always ask before seat-swapping on a flight—your life could depend on it.