There are plenty of empty seats just a few rows ahead. So why won’t the flight attendant let you move up and spread out?

Although changing your seat on an uncrowded flight may seem like a non-issue to passengers, it’s a potential problem that could (quite literally) turn the plane upside down.

It’s a Weighty Issue

Most people realize that aviation is a well-tuned, finely calculated feat of engineering. Although one person’s movement on a commercial jet is unlikely to upend the plane, the issue is so important that the Federal Aviation Administration has an entire handbook regarding weight and balance on aircraft.

The center of gravity on a plane is most critical during takeoff. Pilots need know the distribution of weight on the plane or the “index number” in order to set the trim (used to maintain airspeed). “If the trim is set wrong, then the aircraft may crash at take-off,” pilot Magnar Nordal explained on Quora.

Related: Why You Can’t Get Up to Go to the Bathroom Before Takeoff

“On a large, wide body aeroplane, a single person can move 10 rows of seats and the effect on the balance is negligible,” Darren Patterson, a pilot for a major U.S. airline, told BBC. “Have that same person move just a few rows on a regional plane or turboprop and the effects are far more dramatic; possibly even exceeding the limits of the envelope."

However, passengers need not freak out over throwing off an airplane’s center of gravity for the entire flight. After takeoff, passengers are free to move about the cabin without fear of disrupting critical weight distribution. But that doesn’t necessarily mean it’s OK to take over empty seats without warning. Some seats may be left intentionally empty for “operational reasons,” according to United Airlines, so don’t automatically assume an empty seat is up for grabs.

Or It’s a Question of Money

The best time to switch seats is during check-in, whether that’s online or at the airport counter — especially if you’re trying to get a better seat. Delta writes specifically on their website that even if a passenger spots an empty premium economy seat, they “may not select complimentary Delta Comfort+ seats while on board, regardless of elite status or fare class.” This decision is generally made to placate the customers who have chosen to pay more.

Now that airlines consider seat assignment a “perk” (passengers in basic economy don’t get this luxury), flight attendants may be less lenient about letting passengers sit in whatever’s empty. United, for example, does “not allow complimentary seat selection or changes for Basic Economy tickets.”

While some lucky passengers have been known to nab an empty premium economy seat just by asking nicely, one couple was kicked off a flight for attempting to move into an empty exit row without permission.

This is one instance when asking for permission is much, much better than begging for forgiveness.