Among the first lessons we learn about flying is that lift opposes weight and thrust opposes drag. Within this humble statement there are two profound implications. Few pilots, except perhaps those who exclusively fly gliders, would forget the importance of the second part. Engine failure is a common and not unreasonable fear among students and certificated pilots alike.
But what about the first part of that statement? An airplane’s wing can only produce a certain amount of lift. By applying a bare minimum of logic here, we should conclude that a wing can only oppose so much weight. Yet every year, a few pilots ask more of their wings than those wings can provide by overloading—or incorrectly loading—the airplanes they fly. According to the Air Safety Institute’s Nall Report, eight accidents in 2010 alone, including two fatal accidents, were attributed to aircraft being overweight or the density altitude being too high. Density altitude essentially measures available air (see “Weather: Heavy Air,” page 42). It gets lumped in with weight because without enough air up there, the wing can’t oppose weight as effectively.
This narrative, from the archives of the National Transportation Safety Board, describes an accident attributed to an overloaded aircraft:
“Several witnesses reported observing [a Piper PA-28, N1803T] become airborne about two-thirds of the way down the departure runway. It then pitched up to a nose-high attitude of at least 20 degrees, leveled off about 40 feet above ground level, pitched up again to about the same pitch attitude, and subsequently descended behind a tree line with a high sink rate. Another witness reported seeing the airplane come over the tree line behind his residence and impact the ground in a nose-low attitude. A post-accident examination of the airplane and engine revealed no mechanical malfunctions or failures that would have precluded normal operation. The witness's observations are consistent with the pilot attempting to climb at too steep a pitch angle, which resulted in an aerodynamic stall. According to calculations, the airplane’s weight was about 110 pounds over the maximum allowable takeoff weight, which would have resulted in reduced climb performance.”
Run the numbers. To calculate whether your airplane is overweight, find its basic empty weight (BEW). For nonexperimental, factory-built, and certified airplanes, the BEW is in the weight and balance data specific to that particular airplane.
Add the weight of everything you put in the airplane to the BEW. This includes the weight of passengers, bags, and fuel (avgas weighs an average of six pounds per gallon). Compare that weight to the maximum allowable weight (max gross) found in the aircraft's manual—the operating limitations.
If you have too much weight, it’s decision time. Can you leave behind enough baggage or fuel—or even passengers—to get the airplane within limits? By the way, if you do this quick calculation before fueling or agreeing to take passengers, it makes your decision-making much easier.
Where should the weight go? We can’t treat the subject of weight fairly without also covering balance—in other words, where you put the weight. “Balance” to pilots is the balance of the airplane along its longitudinal axis, its pitch, and your three options are: too far aft, too far forward, and within the envelope. The envelope, based on the airplane’s center of gravity, is defined in the airplane’s handbook.
If you load the airplane with weight that is too far forward, the elevator has
to push down harder on the air to keep the nose up. That “tail down force” is actually lift directed downward for the express purpose of balancing the airplane. The more tail down force you use, the more upward lift the wings must produce to counteract that force. When you load an airplane too far forward, it is as if you have overloaded it. In fact, you have overloaded it—not with mass inside the airplane but rather with air forces outside. The result is the same. Up elevator travel could also be limited, presenting many problems on takeoff and landing.
Another unfortunate example from the NTSB files:
“On February 2, 2005...a Bombardier Challenger CL-600-1A11, N370V, ran off the departure end of Runway 6 at Teterboro Airport (TEB), Teterboro, New Jersey, at a groundspeed of about 110 knots; through an airport perimeter fence; across a six-lane highway (where it struck a vehicle); and into a parking lot before impacting a building. The two pilots were seriously injured, as were two occupants in the vehicle. The cabin aide, eight passengers, and one person in the building received minor injuries. The airplane was destroyed by impact forces and post-impact fire….The National Transportation Safety Board determines that the probable cause of the accident was the pilots’ failure to ensure the airplane was loaded within weight-and-balance limits and their attempt to take off with the center of gravity well forward of the forward takeoff limit, which prevented the airplane from rotating at the intended rotation speed.”
When you load an airplane too far aft of its limits, the airplane no longer acts as if it is too heavy. The problem becomes insufficient stability. With a heavy tail, the nose wants to rise. This may seem like a good thing initially, until you realize that the airplane’s elevator (or stabilator) has stops for both up and down. This means that there is a limit to how much the elevator can counteract that nose-up pitch. If you have more weight in the back than the elevator can overcome, there’s no stopping the upward trend of the nose—in other words, there’s no bringing the nose back down, and stall recovery could be impossible. Again, from the NTSB:
“The pilot and a passenger, a perspective [sic] flight student, completed a preflight inspection of [a Cessna 177 Cardinal, N3431T]. Another passenger subsequently approached the airplane with two heavy bags. The first passenger put the larger bag in the baggage compartment, behind the rear seat. The second passenger then walked around the airplane and sat in the back seat with the smaller bag. During the takeoff, witnesses stated that the airplane appeared slow, with a nose-high pitch attitude, and an immediate dip of the right wing. They recalled the initial climbout was also low and slow. The tower controller asked the pilot if he was experiencing any difficulty, and the pilot responded that he was, and was going to turn back to the airport. A witness noted that the airplane appeared to be having difficulty gaining altitude, that the wings were moving up and down, and that the propeller was spinning. The airplane then made a sudden, sharp left turn and descended to the ground. A post-accident examination of the wreckage revealed no evidence of preimpact mechanical anomalies. Damage was consistent with a left-turning stall/spin at impact. The airplane’s observed nose-high attitude, and pitch trim found in the full nose-down position, indicated the likelihood that the airplane was loaded with an aft center of gravity.”
Calculating balance requires knowing the moment, or torque, that a given weight is putting on the longitudinal axis. To find moment, multiply the weight by the arm, or distance from some fixed reference point, which is found in the airplane manual.
For example, a 170-pound pilot sits in a Cessna 172 with the seat all the way forward (34 inches behind the reference point, or datum). By simply sitting there, that pilot is twisting the longitudinal axis with 5,780 “pound inches” of moment (170 pounds x 34 inches = 5,780 lb-in.) with respect to the reference datum.
Add up the moments of the airplane and all the items (including fuel) that are in the airplane. Once you have the total weight and moment for your proposed flight, use the charts in the manual to determine whether you are within limits.
A few airplanes (and certainly the private pilot knowledge test) may require that you also find the center of gravity mathematically. For that number, divide total moment by total weight. If you were measuring moment in pound-inches, that last equation will give you center of gravity in inches behind the reference datum. Your flight instructor can and should go over these calculations with you a few times. In some airplanes the center of gravity will shift as you burn fuel on a long trip, requiring you to determine the balance at the beginning of the trip—and again for the conclusion of the trip, to make sure you’re still within CG limits when it’s time to land.
For some pilots, all this math can be a drag. Today there are many good programs and apps available for PCs, smartphones, and glass cockpits that make these calculations easier. There’s no good excuse not to know where your weight and balance situation stands.
One day, with the burdens of training and testing behind you, you may even find yourself thinking, I just want to go fly! I’ve already done a million things to prepare, and I’m tired of it. I don’t need to calculate weight and balance every time. We’ll be fine!
The really dangerous part is that you very likely will be correct. The flight will, in fact, be fine. So will the next flight. Maybe even the next. Eventually, however, it won’t. There’s a popular quote attributed to Capt. A.G. Lamplugh in the 1930s: “Aviation in itself is not inherently dangerous. But to an even greater degree than the sea, it is terribly unforgiving of any carelessness, incapacity, or neglect.” Unfortunately, time has shown that aviation can be just forgiving enough to let some lazy habits develop. Try not to confuse good luck with good piloting.
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