We spend a considerable amount of time during initial training on the subject of airplane weight and balance computations, making sure the trainee knows how to use the charts, plotters and software applicable to the aircraft. Getting the numbers to come out in an acceptable range assures that the aircraft will perform and handle as expected. Regretfully, we often see pilots, once rated, ignore the W&B calculations. If it fits, it’ll fly, seems to be their motto.
Indeed, it may be difficult to tell the boss he or she can’t bring along one more passenger, or that those steamer-trunk luggage cases have to be left behind. If it’s a hot day in the mountains, the aircraft may be weight/runway-length restricted; will the aircraft owner understand why luggage or people have to be off-loaded? All too often, job security pushes pilots into accepting the risk. And, after the aircraft’s limitations are ignored one time, such procedures will soon become a habit, or expected conduct.
Taking weight-and-balance seriously means more than just doing the math, or having the computer do it. It requires an understanding of the risks and reasons behind proper loading. Pilots overload, or miss-load, because they don’t assign value to the task of load calculation.
How Heavy Is Too Heavy?
I had one pilot tell me that as long as he wasn’t more than 300 pounds over the handbook’s ramp weight, it was okay. The salesman had said so when he bought the airplane. In my mentoring role, I discussed the matter at some length, making the point that no evil magic was going to transpire even if he was 301 pounds over gross weight. The wings weren’t going to break off nor would the engines refuse to accelerate us to liftoff speed. But neither would it be okay to ignore the manufacturer’s limitations, just because he wanted to fill all the seats that came with the airplane.
To heighten our discussion, I picked up his expensive pilot operating handbook and carried it over to the trash can, where I summarily deposited it. I told him he might as well throw it away, because it was meaningless. All the performance numbers in that book were based on operating the aircraft while loaded within its weight-and-balance limits. If he chose to overload it, he was, in effect, becoming a test pilot. No one at the factory could tell him how it going to work, because their testing stopped after assuring that all was well at maximum takeoff weight.
And so, if you don’t want to be a test pilot, you had better stay inside the normal loading envelope. Not because the airplane won’t fly, or because you can’t handle it if nothing unusual comes up, but because you have no tested data to guide you, no normal cushion of safety to make up for nature’s foibles or your ineptitude.
A rather svelte pilot I know was flying a twin Cessna with two porcine individuals ensconced in the rear-most seats. They had flown in this configuration many times, but on one particular trip they encountered thunderstorm-related turbulence and the aircraft pitched up and down so violently it wound up with overstress damage to the tail and aft fuselage. Why? Because the aircraft was loaded with the C.G. well aft of the allowable range, which eliminated the airplane’s normal stability and control response. It became so sensitive it was easy to overcontrol.
Balance Is Vital
Too often, we focus on respecting the airplane’s maximum ramp weight or zero-fuel weight, neglecting the danger of flying with the loaded C.G. outside the stated limits, whether expressed in inches aft of datum or in percentage of mean aerodynamic chord. As seen above, loading too far aft removes the positive stability expected of certificated aircraft and could prove deadly in a stall condition. Loading outside the forward C.G. limit, on the other hand, can make it difficult to rotate the nose up for takeoff or landing and cause stiff handling in pitch. Landing nosewheel first, because you’ve run out of elevator authority, is not a recommended way to conclude a flight.
The tragic loss of a Boeing 747 freighter during takeoff at Bagram, Afghanistan in April, 2013 was caused by heavy cargo of military vehicles shifting as the airplane climbed through 1,200 feet AGL. Given the suddenly-extreme aft C.G. movement, the crew was unable to prevent the stall and the huge widebody went into the ground as a ball of fire.
Therefore, to preserve handling and positive stability, always assure that the center of gravity remains inside the normal loading limits, even if your aircraft weight is under the maximum allowed. What’s positive stability? It’s exhibited by an airplane’s tendency to resume a trimmed speed, after being disturbed, either by control action or turbulence. If the airplane is pitched up and airspeed falls off by 10 knots, it should pitch down on its own, hands-free, accelerating beyond the trimmed speed until it pitches up from the dive, slowing to a lesser speed than before. After a few lessening oscillations, it will come back to the original speed, all on its own. If loaded to the aft-c.g. limit, the airplane may show neutral stability, meaning it’s reluctant to return to a trimmed speed without pilot intervention, and if loaded beyond the aft limit, it can become dangerously unstable, diverging rather than recovering.
Zero Fuel Weight
Many aircraft have been “grown” after their initial certification to allow extra seats or cargo space. In some cases, adding too much weight in the fuselage will exceed a designed wing-bending moment because of the concentration of weight inboard. The answer is to establish a maximum zero-fuel weight; since fuel normally goes into the wings, keeping the span-wise loading distributed more evenly assures airframe integrity. Naturally, it’s the minimum-fuel condition that sets the zero-fuel weight limitation, not a takeoff loading.
A maximum landing weight, on the other hand, usually results from design limits for the landing gear. An overweight landing, if necessary, has to be done with as little vertical descent at touchdown as possible. Bear in mind that reference speeds and stopping distances are usually predicated on max-landing weight, unless a special page in the operating manual is devoted to overweight landing. In small G/A aircraft, you may very well be the test pilot if you land heavier than the maximum landing weight.
Thus, knowledge of your aircraft’s loading is important for survival, not just attaining the rating. The reason most pilots can depart without visibly doing their sums is because they know, or should know, that today’s load falls into an acceptable range. They have confirmed this from prior calculations with similar body counts and luggage, although it pays to check the bags for one with anvils in it.
Wise pilots spend downtime working some sample calculations of passenger, fuel and baggage scenarios. In doing so, it’s possible to find the areas where you will run into trouble, and you won’t have to figure the weight and balance each time if you are familiar with today’s example.
In simple summation, don’t fly heavy and don’t fly out of airplane weight and balance. This is not advice to pass the course for a rating; it’s guidance to stay alive.•T&T