This is probably is not a question that you would wake up in the middle of the night and think, “How do commercial airlines calculate weight and balance?”
Unless you are a flight crew member you probably have not walked onto an airplane and thought, Gee I wonder if the weight and balance is correct on this airplane?
You may have stepped onto a smaller commuter-type airplane (Regional Airline) and been told what seat you could occupy on the flight.
Possibly you knew the reason for that or if you did not then you may have wondered why you needed to sit in that seat.
Allow me to give you some insight as to why it is so critical for the weight and balance to be correct on an airplane.
How Do Commercial Airlines Calculate Weight and Balance?
Airlines compute the weight, (determined by the manufacturer) and the balance (location of the center of gravity of the aircraft) by using the average weights of passengers plus the amount of cargo and fuel loaded onto the aircraft. Many airlines will use computer programs to determine the weight and balance, or they are computed by the crew.
1. What is Weight and Balance of an Airplane?
The weight of an airplane determined by the manufacturer remains a constant number. Unless the configuration of the aircraft is changed then this number is always the same.
The weight of an aircraft changes with the weight of passengers and cargo loaded onto the airplane plus the fuel weight.
The weight and balance of an aircraft are important because you must have the stability of the airplane in flight.
Just like a seesaw if you load too much weight to either end it becomes too heavy. In order to have an equilibrium of the airplane, it must be loaded correctly so that all loads are balanced.
2. Center of Gravity
If you take a chain and hook it to a hypothetical skyhook and suspend an airplane from its center of gravity it will be suspended perfectly horizontally. This is the perfect configuration for the safest flight.
The airplane manufacturer determines a point in the aircraft called the reference datum. It is a vertical line through the airplane from which all measurements fore and aft are taken to determine the CG, (center of gravity) envelope.
The aircraft dispatcher and the pilots will then know how to load your airplane safely to properly achieve a CG that is within the CG limits. It is how you know how much weight to put in each zone or area in the airplane. Each zone will have a weight limit.
This is the reason that you may be asked to move to a different location on the airplane once the aircraft has been boarded so that the airplane remains within its CG limit.
3. Gross Weight
The gross weight of the airplane is all of the weights of the passengers, crew, cargo, and fuel loaded onto the airplane. Each airplane has a maximum gross weight which simply means if you exceed it you must remove some weight.
The maximum take-off weight of an airplane is very important and includes among other things the numbers required for takeoff roll. How much runway do we have? And is it enough. It also determines the landing weight. You can read about some common problems that arise from improper loading here.
You’ve likely heard about airplanes having to dump fuel and this is the reason; the airplane weighs too much to safely land. Fuel must be dumped ’cause you just can’t throw out people. There’s a rule somewhere about that too.
4. Big Airplane, Small Airplane
When it comes to weight and balance computations it doesn’t make any difference whether you’re in a bigger plane or a small airplane.
The weight and balance of any aircraft must be completed to assure a safe journey.
Another consideration of loading procedures is the securing of loads and the necessity to assure that the proper restraints are placed on cargo. A sudden shift in cargo weight can have catastrophic effects.
There is a video that is very graphic of a 747 crash that you can view here if you choose. It shows the result of a weight shift on takeoff. Every aviation accident disturbs me greatly, especially when someone dies and more so, if the accident could have easily been prevented.
As boring as the study of aircraft weight and balance may appear to be, I cannot emphasize it’s importance enough.
If you ever find yourself in an aviation ground school you will be subjected to the gruesome task of learning weight and balance for every airplane that you ever will fly. Most especially in the airlines.
It isn’t really all that hard but I can guarantee you that the FAA exam will make you think that it is. They’ll do anything to trip you up on a question.
Lastly, if you are ever involved in an aircraft accident or incident you had better have your weight and balance paperwork in order because it will be scrutinized heavily as a possible or probable cause.