Running the Numbers

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Taking time to calculate weight and balance on the ground can save you some nasty surprises in the air. Consider the misfortunes of some pilots who didn't.

Are you looking for trouble?

You know as well as I do that looking for trouble isn't conducive to a long and safe career in aviation. But, we all have probably launched on trips without running the weight-and-balance numbers. And, in doing so, we may have inadvertently punched a few holes in the weight-and-balance envelopes established by the manufacturers of our aircraft.

Weight-and-balance limitations are there for very good reasons, most involving the safety of the occupants while the aircraft is in flight. When we disregard them, knowingly or unknowingly, we are looking for trouble.

Take, for instance, the hapless pilot of a Cessna 402 who attempted to transport nine members of a varsity basketball team in his airplane. The pilot did not get the accurate weights of the individuals and their luggage. As he entered the cabin to close the door, the airplane tilted over onto its tail...not an uncommon occurrence for that type of airplane when too much weight is placed in the rear of the cabin. The pilot and passengers got out of the airplane and raised the tail. Then, the pilot decided to proceed with the flight, still without having "run the numbers."

As he climbed through 300 feet AGL, he turned to those in the cabin and yelled that he had a problem. He instructed his passengers to move as far forward as they could. This apparently solved the pilot's immediate "problem," but the airplane eventually was landed with most of its passengers not seated with restraints fastened.

An inspection revealed that the 402's rear bulkhead, a stringer, the elevator control tube and the housing for the tail navigation light were damaged when the aircraft fell onto its tail. Investigators ran the numbers using actual weights of the passengers and baggage, and found that the center of gravity was well beyond the aft limit. The pilot was very fortunate that he did not lose an engine during the flight; the aft c.g. would have seriously aggravated the asymmetric control problem.

This may be an extreme case of weight and balance mismanagement and total neglect of incurred damage, but the fact remains that many of us don't pay enough attention to how we load our aircraft. While we don't get into accidents every time that happens, there can be consequences that we don't anticipate.

Ask, Don't Guess

There are compromises in the designs of almost every airplane built. We can fill the cabin or the fuel tanks; but, normally, we can't do both. Most of the time, we must juggle fuel and payload to assure ourselves that the airplane is within the weight and balance envelope. In some airplanes, like the Piper Seneca, the c.g. may be forward of its limit with full fuel and only two people aboard (in the front seats). In others, like the Beech Bonanza, the numbers should be run both for takeoff and landing, since the c.g. moves aft in a rather tight envelope as fuel is burned.

If you guess passenger and baggage weights, as the Cessna 402 pilot apparently did, you could find yourself playing test pilot and operating outside the envelope. It's best to ask each individual for their actual weight, check each piece of baggage as it is loaded and run the numbers before takeoff.

Unfortunately, you don't see too many pilots standing around their airplanes with calculators in hand. That's a shame, since it really does not take a lot of time to run the numbers. There are special plotters, calculators and computer programs for many aircraft that produce results within a minute. I have a program that takes the weights of passengers, baggage and fuel, and then shows me where I stand in a graph, similar to that in the AFM.

Still, too many pilots load passengers and baggage with nary a thought about weight and balance. The FARs don't specifically require a record of weight and balance for noncommercial flights, but all pilots are required to operate our aircraft within the limitations prescribed by the manufacturer. We cannot be sure we are doing so unless we run the numbers.

One quick-and-dirty method used by many pilots is what I call the "half-of-the-equation" system: The pilot quickly adds up the weights of the empty aircraft, passengers, baggage and fuel, and gets a warm and fuzzy feeling if the sum is less than the maxmum allowable takeoff weight. The balance portion of the calculation is neglected altogether. This practice reflects an ignorance of what "weight and balance" really is, and of what flying an airplane out of its c.g. envelope could mean to safe operation.

The balance side of the equation could be simply envisioned as the airplane resting on a pinnacle, supported at its center of gravity by one sharp point. As you add or subtract weight to the aircraft, that balance point (e.g., the center of gravity) moves. Add or subtract too much weight forward or aft, and the aircraft will fall off the pinnacle. Before that happens, though, the pilot will experience some handling problems. And, in some cases, those problems can be serious enough to cause a loss of control.

For example, as the c.g. shifts closer to the horizontal tail (toward the aft limit), the elevator becomes more effective the stick forces required to make a pitch change become very light, and the airplane becomes longitudinally unstable.

Inherent Stability

Production airplanes are designed to be longitudinally stable when operated within their c.g. envelopes. You can "feel" this when you push or pull the control wheel to make a pitch change: The airplane, when trimmed for level flight, will try to return to its trimmed condition. But, as the distance between the c.g. and the horizontal tail decreases, the stick force required to make a pitch change also decreases.

At the point of "neutral longitudinal stability," the airplane will tend to maintain its present attitude instead of trying to return to its trimmed condition. The stick force required to make a change will be extremely light, and that could result in overcontrolling. It's possible, in an extreme case, to get into a pitch attitude so high that there won't be enough "down" elevator travel to prevent, or recover from, the impending stall.

You've probably flown your airplane at one time or another with the c.g. at or near the aft limit. You'll probably remember that as you rolled down the runway, the airplane came off the ground before you expected it to. You quickly trimmed the nose down, realizing that although you set the trim forward of where you normally would, it was not enough. Then, as you flew the airplane away from the airport, you noticed that the amount of elevator travel to effect a pitch change was much less than normal.

Some time ago, a Beech 18 captain lifted off from an Alaskan airport, retracted the landing gear and found the aircraft trying to roll to the left. He corrected with right aileron and attempted to decrease the pitch attitude, but the aircraft rolled right and then back to the left. He landed near the departure end of the runway with the gear up.

Investigators discovered that no one had checked the weight of the freight and that the airplane was 1350 pounds over its maximum certified takeoff weight. Also, the c.g. was about three inches aft of the aft limit. Did the pilot run out of forward elevator because of the severe aft loading? We don't know that for sure, but the rolling action of the airplane indicates it was in or near a stall condition from the time the landing gear was retracted.

What may have happened is that the stick forces on the elevator were so light because of the rear loading that, initially, the pilot thought he had the airplane trimmed properly when it was perilously close to a stall. When he realized that, in reality, the nose was pitched way too high and tried to avoid the stall, it was too late to do anything except allow the aircraft to settle back to the ground.

The "behavior" of different airplanes loaded to an aft c.g. varies. But, remember that the manufacturer designed and certified your airplane to be operated within the c.g. limits; and, once you get behind the aft limit, you are on your own. You become a test pilot, and you relegate your passengers to playing the role of potential crash dummies.

Exceeding the forward limit can be trouble, as well. As from the time the c.g. moves forward, the controls become heavier. More effort, and travel, is required to make a pitch change. The level-flight trim position of the elevator is higher than nonmal; and, in some extremes of forward loading, you could run out of elevator before you accomplish the pitch change you want.

For example, you are beginning to flare for a landing and discover that, even with full travel of the elevator, the pitch attitude is not high enough to keep the nose wheel from striking the ground before the mains do. In fact, at the point where you pulled the throttle to idle for the landing, the nose would fall, the airspeed would increase, and you couldn't do anything about it except attempt a power-on landing probably at a higher airspeed than normal.

Adding power would pitch the nose back up, but you'd need a longer runway to help dissipate the extra speed.

On the Spot

Sometimes, it's hard to do an accurate "weight and balance" because someone shows up with more luggage than you expect, or it's obvious that some of your passengers didn't give you their correct weights. But, you should always try to be certain that the aircraft is within its weight and c.g. limits. Programmable calculators can make a weight-and-balance problem easy to do on the ramp with your passengers standing by the airplane.

Sure, doing it on paper would take a while; but if you know you have a large load to go into your airplane and you don't know the exact figures before hand, it would be much safer to delay the flight and do the calculations than to cross your fingers while stuffing the seats and baggage areas with as much as they can physically handle. After becoming airborne and realizing that the airplane is not behaving very well is the wrong time to wonder about all the luggage you loaded in a rear baggage compartment.

In one case, a Piper Lance crashed during a night takeoff from Great Barrington Airport in Massachusetts. There were five passengers and the pilot aboard. They had flown in to go skiing but found that the ski resort was closed. They decided to fly over to nearby Pittsfield, where they knew they could ski. On takeoff, the aircraft hit the top of a 60-foot tree nearly 500 feet from the departure end of the runway.

Two of the occupants died. Investigators determined that the aircraft was 368 pounds over its maximum certificated weight and loaded outside the aft c.g. limit. The runway was just under 2600 feet long, and it was the first time the pilot had flown into the airport.

The NTSB report I reviewed did not say where the flight originated or whether the pilot bought any fuel at Great Barrington; but, if he did not, then the aircraft loading was farther out of tolerance on the leg before the crash occurred. It seems as though this pilot loaded the airplane according to the number of seats and amount of baggage space it contained, rather than considering the full impact of weight and balance.

If you always fly your airplane with the same load, it is probably not necessary to do a weight and balance each time; you should know where you stand in the c.g. envelope. But, when something changes—someone unexpected shows up, or more luggage than you anticipated is sitting on the ramp—take the time to do a thorough weight and balance calculation.

Remember, the manufacturer's limitations are the result of tests of handling characteristics within the envelope (there is little or no testing outside the envelope). By flying outside the envelope, you are not only testing your aircraft and your piloting skills, but, also, the ever-present law of Mr. Murphy.