In our Ask a Pilot series, pilot Spencer Marker answers one of your aviation-related questions each week. See past installments here and submit your own to Whitney@johnnyjet.com.
Hey Spencer. Recently on a flight from Detroit to Ft. Wayne, IN, the pilot requested two passengers to move from the front of the airplane to the back for “weight and balance.” Why did they need to move and what does this mean?
Hey William. Thanks for writing in this week to be featured in our Ask a Pilot column. Weight and balance distribution are things that all pilots, from small two-seat Cessnas to mammoth Airbus A380s, have to consider every time they take to the skies. The reason is simple: to be able to properly control an aircraft, the weight has to be within a predetermined limit. Additionally, and more confusingly, the center of that weight has to reside within a certain range. Let’s look at why both numbers are so important.
Why is weight so important?
All airplanes, from the smallest single-seat planes to the largest jet-transports, have a maximum weight at which a takeoff is allowed. Unsurprisingly, this is called an aircraft’s maximum takeoff weight. It includes the weight of the plane, its fuel, baggage, its passengers, and its crew.
The maximum takeoff weight of most aircraft is not enough to carry a full load of passengers and baggage as well as a full load of fuel. Therefore, a balance must be struck. Generally speaking, airlines will load an airplane with only enough fuel to make it to its destination and comply with all other federally regulated fuel requirements. Fueling an airplane exactingly will ensure a flight complies with federal regulations and is able to carry as many passengers and bags as possible.
Runway length will also affect an aircraft’s ability to carry a full load. In airports with relatively short runways, like Chicago Midway Airport or John Wayne Airport in Orange County, CA, a flight’s passenger load can be restricted due to short runways. A shorter runway means that a jet has less opportunity to attain a high enough speed to take off before reaching a critical point down the runway (this is known as balanced field length, which is an article unto itself). Therefore, flights will carry fewer passengers or bags in order to meet the runway length restriction imposed by smaller airports.
Balancing the load
William, in the scenario you mentioned, your flight crew made the determination that, in order to take off safely, passengers would have to be moved from the front of the aircraft to the back. While it is annoying to passengers being forced to move, this relocation satisfies a basic aerodynamic principle: for a flight to takeoff, the longitudinal load onboard, whether passengers or cargo, must be balanced properly.
So, why do we need to balance an airplane? Well, when an airplane is loaded, the location of each passenger, piece of baggage or drop of fuel is calculated as to its effect on an airplane’s longitudinal center of gravity. This is an axis that runs from an aircraft’s nose to its tail. An airplane with a center of gravity too far forward or aft (toward the rear) is unstable and difficult to control.
The center of gravity is like a see-saw that must be balanced before a flight is allowed to leave. Weight on one end of the balance must be compensated with weight on the other. Now, aircraft are able to compensate for variances in the balance of the aircraft by utilizing a movable stabilizer (the horizontal tail) so aircraft can be flown within a center of gravity range. However, when the airplane is too nose- or tail-heavy, the crew must relocate baggage or passengers in the cabin compensate.
While it’s possible to load every airplane outside of its center of gravity range, most often, smaller aircraft are more susceptible to variances in how the airplane is loaded. And when the airplane is loaded out of the center of gravity range, it cannot legally leave the ground until it is back within parameters.
To sum up
Thanks for the excellent question, William. While the arrangement of passengers in a jet’s cabin may seem unimportant, it’s actually something that is well-documented by the gate agents and carefully watched by your flight crew. While moving people around the cabin is no fun, especially for those that were expecting a seat near the front, managing the weight and balance of the jet is of utmost importance for safety of flight. The controllability of the aircraft depends on it.
Have you been on a flight with weight and balance issues? If so, post in the comments below! If anyone has a burning aviation question or something you would like cleared up, drop us a line at Whitney@johnnyjet.com to get your question featured in an upcoming Ask a Pilot column.
Spencer didn’t explain how the aircraft’s center of gravity is determined. In the late 1930’s, Lewis Imm invented a small portable instrument that was used prior to take off for determining the aircrafts center of gravity. This was done by entering the weight and location of each item loaded. See the following website for a little more history: librascopememories dot com.