By Robert N. Rossier, EAA 472091
This piece originally ran in Robert’s Stick and Rudder column in the December 2020 issue of EAA Sport Aviation magazine.
It’s a long way and a heavy load for Santa and his sleigh on his global Christmas Eve excursion. It really is important for him to get the weight and balance right for his sleigh and keep an eye on the elves as they load the bags. As he makes his stops along the way offloading heavy gifts and toys, it might seem that the situation improves. But it’s more than just the weight he needs to be concerned about. Movement of the center of gravity (CG) changes the stability, controllability, and performance of any aircraft. If the CG should wander outside the envelope, all bets are off. Clearly, Santa must have a load on his mind.
Although we might not fly quite as far as Santa, or carry quite as big a load, we too must pay particular attention to our weight and balance when planning a flight. It’s not just a matter of getting off the ground; it’s a matter of performance, stability, and our overall safety.
A Matter of Performance
When it comes to aircraft performance, our weight and balance condition is a good news/bad news kind of deal. We all know that a light aircraft climbs and flies faster, uses less runway for takeoff and landing, and travels farther on a gallon of fuel. However, our gross weight also affects some important speeds.
A higher weight means a higher maneuvering speed, higher stall speed, and higher speeds required for landing, best glide, and best rate of climb. As a general rule of thumb, a 10 percent increase in weight produces a 5 percent increase in maneuvering and stall speed. However, our pilot’s operating handbook (POH) may give us a better handle on such changes.
It’s not just the total weight that’s important. For any given weight, where we put that weight — i.e., the location of our CG — has a marked effect on performance. With the CG forward (nose heavy), we’ll need more speed and a longer runway to get off the ground, as well as for landing. Our cruising speed will be lower, our range will be reduced, and our stall speed will be higher.
When we operate with an aft CG, we need less airspeed and distance for takeoff, will get a higher climb rate, and will need less distance for landing. While it sounds like operating with an aft CG is a winning situation, we have another side of the coin to consider focused on stability and controllability.
An Unstable Situation
In a properly loaded aircraft, several important forces are in balance. We have upward lift from the wings acting aft of the CG and a downward force of gravity acting on the CG. Balancing this is a downward force on the tail (also aft of the CG) that holds the nose up. The total amount of lift needed from the wings is the weight of the loaded aircraft plus the download on the tail.
As the CG moves closer to the center of pressure (or lift) on the wings, the less downward force is needed on the tail. This sounds good because we now need less total lift, and we get those improvements in performance that we talked about. But we lose stability because it takes less change in elevator pressure to move the tail up or down. In fact, with the CG aft, we might encounter a stall as we leave ground effect on our climb-out. We might experience a stall on landing. Worse yet, we might inadvertently stall and be unable to recover (think flat spin).
Having our CG too far forward can also cause controllability problems. Although the controls might feel solid and steady, we might not be able to hold the nose up on landing, resulting in an ignoble end to our flight.
The bottom line here is that it’s all — you guessed it — a balancing act. We need to be squarely within the CG envelope, and we need to be aware of how seemingly subtle changes in our CG will affect performance and stability.
Errors and Omissions
While it might seem that managing our aircraft weight and balance should be an easy task, there are some simple ways to make some rather significant errors.
The first mistake is to use the wrong empty weight and CG location. This can happen when we use the “sample” in the weight and balance section of the POH or use outdated information for our aircraft. While the sample may be a reasonable representation, it seldom corresponds to the aircraft we’re flying. Any time a change is made to the aircraft — such as adding or removing electronics or accessories such as wheel fairings — we change the empty weight and CG. Such changes are recorded in the official weight and balance records for the aircraft, and it’s there that we need to look to ensure we’re starting our calculations off on the right foot.
Another common mistake is to use unrealistic personal and baggage weights in place of actual weights for ourselves, our passengers, and our baggage. The weight recorded when we take a physical exam or weigh ourselves on the bathroom scale might not accurately reflect our weight the way we’re dressed — especially for a winter flight, when we may be wearing a heavy coat, boots, and other accessories. And then there’s our flight bag, possibly loaded with charts and plates, headsets, and other necessities of modern air travel. Oftentimes pilots don’t even consider such weights in their calculations. Some pilots or passengers might carry a pocketbook, handbag, or day pack and not include that as part of their personal or baggage weight. And how about that survival bag and other spare items we stash in the back of the airplane? Eyeballing or hefting baggage to determine its weight is seldom accurate. All told, it is easy to miscalculate our weight by a significant amount.
Making the Calculations
In the old days, we performed our weight and balance calculations by hand or used a “graphical calculator” of sorts to ensure we stayed inside the envelope for our entire flight. These days, we have at our disposal an amazing array of electronic and online tools that make quick work of calculating weight and balance. Here again, we want to make certain that the empty weight and CG used by the calculator or app is the actual empty weight and CG for the aircraft we’re flying.
The Icing on the Cake (or Wings)
One more factor that Santa and the rest of us need to be particularly wary of is the danger of icing. Not only does airframe ice play havoc with our aerodynamic surfaces such as wings, propellers, and control surfaces, it can quickly add a surprising amount to the gross weight of the aircraft, all in a matter of minutes. The NTSB files are rife with examples of how quickly an icing situation can turn fatal, so avoiding such conditions is a critical element in our safety.
This time of year, we might all have Santa on our minds. As we consider his amazing annual journey, we should also keep in mind our own flying, and be sure to keep our balance and weight within the proper limits. Merry Christmas to all, and to all a safe flight!
Robert N. Rossier, EAA 472091, has been flying for more than 30 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.