Barring the occasional free-spirited passenger who may try to take an empty seat, everyone usually goes to their allocated seats, self-selected or distributed by the airline, and gets comfortable for the ride. Sometimes the flight crew will inform everyone to sit in their assigned seat, but why is that?
Some travelers may believe that this is because the airline has a chart showing who is sitting where, which would be helpful information in case of an emergency. This is not the case, however. Especially on lighter aircraft, the distribution of human weight is factored in before the plane leaves the ground, and your seat assignment plays a role.
A critical balancing act
According to Air New Zealand’s Chief Flight Operations and Safety Officer, Captain David Morgan, ensuring everyone is in their correct seat is related to an aircraft’s “center of gravity envelope” (COG). As all aircraft come with a manufacturer-calculated COG envelope, operators must ensure the total weight added to the plane before a flight takes off doesn’t exceed these parameters. Recently, Simple Flying covered weight and balance, a factor that has impacted some aviation accidents, and passengers constitute one of the major “weights” of an aircraft.
Of course, if one single passenger is moved up or down a few rows on a massive widebody filled with fuel and luggage, it’s unlikely to cause an issue for takeoff. But an aircraft with a COG too far forward or aft can indeed be comparatively difficult to control or require more fuel. Morgan went on to mention,
“We want the center of gravity, if we can, towards the back of the aircraft because the airplanes produce less drag if it’s there.”
Once all the fuel, luggage, and passengers have been accounted for, calculations begin to determine where the COG is, based on the exact distribution of all those weights. Pilots then receive what is known as a “load sheet,” which includes this essential information, which can impact takeoff thrust and speed.
“It’s important that the aircraft’s center of gravity is actually where we expected it to be from the calculations.”
Just as important for landing
Interestingly, the distribution of passengers is equally crucial for landing. Most will understand that an aircraft landing is significantly lighter than when it took off since it burned off lots of fuel during flight. So, again, before the flight has left the ground, an estimation of where the center of gravity will be upon landing, based on the allocation of human weight, is calculated.
Especially for landing, with much of the weight gone, the impact of cargo and passengers now affects the characteristics of flight much greater. If passengers were free to move around as they want during the last segment of a given flight, and ten travelers of an average weight went from one end to another with their potential carry-on luggage, that would mean several hundred kilograms of weight shifting. For some aircraft types, this could be too significant.
Therefore, airlines ensure passengers are in their allotted seats before takeoff and landing. And it’s because of this reason that sometimes the carrier will change your seat last minute; perhaps one party of travelers has canceled their plans, and this caused a weight and balance issue.
Source: simpleflying.com