Why you need to study math in school
Although it does not approach the head-scratching level of advanced algebra, calculus, or trigonometry, weight-and-balance calculations do involve working with some rather esoteric concepts such as arm, moment, center of gravity, stations, ramp weight, taxi fuel, takeoff weight, and landing weight. You must know the empty weight of the airplane and the weight of everything that goes into it, including the heft of each of your passengers, each piece of baggage, the fuel, charts, bulging flight bag, and even refreshments.
The preflight weight and balance calculation determines the answer to two important questions: First, will the airplane be at or below the maximum allowable weight, and second, will the center of gravity be within the allowable range?
Once you've gone through the exercise several times by filling in weight-and-balance forms to arrive at the answer to those two questions, the natural tendency over time is to dispense with the weight-and-balance calculation before every flight. That's because most of the flights we make are similar. Typically there's just one or two people aboard, a very few items--or none--in the baggage hold, and most likely the fuel tanks are full.
In the Cessna 172 I used to own and fly, that loading scenario would put the airplane well below its 2,300-pound maximum gross weight and well within the allowable center-of-gravity range. It didn't take many weight and balance sessions to come to the conclusion that I could safely put one other person of average weight in the airplane, a few bags, plus any amount of fuel up to and including topping off the tanks, and not exceed any weight or balance limitation.
Any significant change in the loading configuration should trigger a call to action--a formal weight-and-balance calculation. If I planned to take two or three passengers plus lots of bags and full fuel, I knew I could be bumping up against the maximum gross weight. That meant I needed to reach for a pencil and a calculator and do the weight-and-balance exercise.
It's pretty easy to recognize when weight may be a problem. Fill the seats and the tanks of almost any light aircraft and you'll probably be near or above the maximum gross weight. That goes for two- or four-seaters. It may seem irresponsible for manufacturers to put four seats in the cabin if the airplane can't handle four adults and full fuel, but remember these are light aircraft. They may be twice the physical size of a car, but they are perhaps one-third the weight of the typical sedan, and typically have a lot less horsepower.
Also, a significant percentage of the maximum gross weight of a light airplane is devoted to fuel and payload compared with large airplanes, and even cars. In fact, the ratio of passenger weight to overall aircraft weight is inversely proportional to the size of the aircraft. For example, the payload of a Boeing 747 represents approximately 9 percent of the total aircraft weight, while the full-fuel payload of my old Cessna 172 accounted for about 25 percent of the total weight of the airplane. If you add fuel weight to payload (useful load, in other words), then the figure is about 35 percent of total aircraft weight.
When viewed in that context, it's understandable why it can be deceptively easy to overload a light airplane simply by adding one more passengers, a few more bags, or a few gallons of fuel. So, if you are planning a flight with a somewhat heavier load than you normally fly with, take a moment to carefully add all of the items that comprise useful load, then add that to the empty weight of the airplane to ensure that you don't exceed the maximum gross weight.
What's the harm of departing on the high side of max gross weight? Don't they say about some airplanes that if you can close the doors (meaning the seats and baggage area are stuffed to overflowing), it will fly? Yes, you do hear that kind of boasting occasionally, but do you really want to play the role of test pilot? If aircraft handling and performance is based on flying at or below max gross weight, how do you know what to expect when you exceed the limits?
At the very least the airplane will perform poorly in all phases of flight. The take-off run will be longer, the airplane will climb and cruise slower, and final approach and stall speeds will be higher--but you won't have any idea how much higher. Every pound of additional weight put into a Normal category airplane (certificated to 3.8 Gs) requires that the wings support an extra 3.8 pounds. Brakes and landing gear will be severely taxed as well. Loading beyond the limits is simply not worth the risk.
That said, center of gravity is the more insidious part of the weight and balance concern because balance is not as obvious as weight. An overweight condition is easy to recognize--you have to put a lot of stuff in the airplane to exceed the weight limit. Not so with balance. It's relatively easy to stay well below max gross weight but still fall outside of the acceptable center of gravity envelope.
Center of gravity--balance--affects the airplane's stability and control, which directly affects your ability to fly the thing. Moving the center of gravity aft reduces pitch forces; the yoke becomes increasingly light and the airplane becomes more difficult to control in pitch. There may be insufficient nose-down elevator authority to recover from a stall. A far-forward center of gravity increases pitch forces. If the center of gravity is forward of the limit, there may be insufficient nose-up elevator authority to rotate on takeoff, or flare on landing. Regardless, takeoff and landing distances will increase.
Fill up the seats, the baggage compartment, and the tanks, and it's hard not to recognize that the airplane most likely is overweight. At the very least that voice in the back of your head will be telling you to check it out by doing some simple addition. If you find you are over the limit, leave something, or someone, behind.
While you're at it, do a little higher math. Convert weights to moments and check the results against the center-of-gravity envelope. If you find it's outside the limits, but you are below the maximum weight, you often can correct the situation by shifting the load around. That's better than leaving someone standing on the ramp.
Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and co-owner of a Piper Aztec.
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