Gravity is Undefeated
Why Weight and Balance is Critical to Flight Safety
By James Williams, FAA Safety Briefing Magazine
Is loading an aircraft like loading a car, i.e., if it fits, it goes? Not quite. And while this isn’t entirely true about a car either, it usually does work — you load it until there’s no room left. The idea that you’re only limited by volume doesn’t work for small aircraft. It’s tempting, and frankly understandable, to think that because the airplane has four seats, you can take four people and their bags. But that scenario may be optimistic, to put it mildly. In logistics, running out of space/volume before running out of payload capacity/weight is called cube out. The reverse situation, running out of payload before volume, is called weigh out. In a perfect world, your point of weigh and cube out would be the same, meaning you are using both all available space and payload. But in our imperfect world, passenger cars usually cube out, and small aircraft almost always weigh out.
Where’s the Weight?
An aircraft must generate lift to counter the force of gravity acting on its weight — and that includes everything you loaded on board. This means having at least one pound of lift for every pound the aircraft weighs. So if we have a 2,000-pound airplane loaded with 500 pounds of fuel, people, and baggage, we would need to generate 2,500 pounds of lift to stay in the air. We generate that lift by pulling an airfoil (the wing) through the air with enough velocity to produce the required lift. The heavier the aircraft, the more lift you have to generate, and pilots are very careful not to overload an aircraft.
How weight is distributed in an airplane is critical to flight safety because it determines the aircraft’s center of gravity (CG).
The total amount of weight isn’t the only issue. How the weight is distributed in the airplane can also be critical to flight safety because it determines the aircraft’s center of gravity (CG). You can visualize the CG by thinking of a model airplane hanging from a string. If the string is attached at the CG, the model hangs perfectly level. If you move the weight around within the aircraft, the location of the CG changes. You may have experienced this in a car if you were carrying a heavy load in the trunk and noticed that it’s harder to steer, or the rear feels like it wants to turn faster or slide. Moving the CG in an airplane is even more critical as you don’t have the luxury of a firm connection to the ground.
This is particularly true when you move the CG fore and aft. Think of it like a lever or a see-saw. Archimedes is often quoted as saying, “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” Meaning that the further from the fulcrum, or in this case, the CG, you put a weight, the more effect it will have. This is why you might notice pilots being very particular about putting seemingly insignificant items in a specific place in the aircraft.
As we discussed, the aircraft’s CG moves depending on loading. In aviation, we have developed practical ways of calculating how each load affects an aircraft to ensure we are within safe limits. We do this by computing a “moment” for objects or people placed at several assigned “stations” within the fuselage. We use something called a datum as a reference point in the aircraft to measure from. For example, there are stations for the front seats, the rear seats and the baggage compartment. The moment is computed by multiplying the weight at that station by the distance from a certain location (this is referred to as the arm). The total of all the weights and all the moments has to be within certain limits for the aircraft to operate safely.
The first step is to calculate the moment of each station. You can think of the moment as the total effect of each weighted item (including the aircraft itself) on the CG. So weight times arm equals moment. For many GA airplanes, it would be the total weight in the front seats times the front seat arm, plus the total weight in the back seats times the back seat arm, plus the total weight in the baggage compartment times the baggage compartment arm. That combined total would be the total moment, which is then divided by the total weight to get the CG. However, there’s one quirk with certain GA airplanes that feature a front baggage compartment forward of the firewall. The arm for that station will be negative in most cases as the datum is usually located near the firewall. That moment will be subtracted from the total rather than added in your calculations.
You can then use a chart to determine the acceptable CG limits for that weight. The larger the total moment, the further back the CG will be. In many cases, when traveling in GA airplanes, our primary concern is keeping the CG within the rear limit because we know we will have a certain amount of weight in passengers and cargo that will be aft of the datum. If we are under maximum takeoff weight, we may need to move weight forward to stay in the appropriate CG envelope. This is why airplanes with forward baggage compartments generally load those areas first, as they counterbalance the weight behind the datum, hence the negative arm. But most small GA airplanes don’t have forward baggage compartments, so the next best thing would be to move items forward in the cabin. For example, if we had a 20-pound item in the rear baggage compartment of a generic 1975 Cessna 172 (the airplane I happen to have an owner’s manual for at hand), that item would have a moment of 2,460 (20 x 123). Moving the same item to the rear seat would be 1,460 (20 x 73). Moving it to the front seat would be 740 (20 x 37). You can see what a difference that arm makes.
There’s no pause or respawn in real life, so taking unnecessary risks with weight and balance isn’t a good strategy.
Hard Mode
So what does all of this mean when you go flying? Generally speaking, the lighter the aircraft, the better it will perform and the less fuel it will consume. Heavier aircraft accelerate more slowly, take longer to take off, and have less range. This can be compounded by environmental factors like high temperatures and high airport elevation. Because of that, even if you are within weight limits, you might not have enough performance to get off the ground or clear obstacles on departure.
It gets worse if you exceed the aircraft’s maximum allowable weight. At a certain point, you won’t be able to take off, but far more dangerous is being able to get off the ground and not able to climb or clear obstacles. In the air, the aircraft will be less maneuverable, and stall speed and landing speeds may be higher than normal.
If the CG is outside of the approved envelope, you can have many of the same problems as when the weight is over limits but with the addition of some serious control issues. An extreme forward CG (too much weight too close to the nose of the aircraft) can cause a nose-heavy condition and difficulty raising the nose for takeoff or landing. By contrast, a rear CG can affect longitudinal stability and reduce the capability to recover from stalls and spins.
By failing to adhere to proper weight and balance, you are intentionally increasing risk and cranking up a flight’s difficulty level. In a game, turning up the difficulty can create a fun challenge, but it increases the risk of disaster in real life. There’s no pause or respawn in real life, so taking unnecessary risks with weight and balance isn’t a good strategy.
Understanding some of the complexities of weight and balance will help you help your pilot, especially during the planning stages of a flight. You’ll understand why you need to rearrange things and, if the weight and balance can’t be adjusted, even possibly leave behind some items. Your advance knowledge and understanding can help reduce any self-imposed pressure the pilot may feel to make everything fit.
At the end of the day everyone wants a safe flight, and having a proper weight and balance is an easy way to improve the odds.
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James Williams is FAA Safety Briefing’s associate editor and photo editor. He is also a pilot and ground instructor.
