Running the Numbers

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.

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Are you looking for trouble?

You know as well as I do that looking for trouble isn’t conduciveto a long and safe career in aviation. But, we all have probablylaunched on trips without running the weight-and-balance numbers.And, in doing so, we may have inadvertently punched a few holesin the weight-and-balance envelopes established by the manufacturersof our aircraft.

Weight-and-balance limitations are there for very good reasons,most involving the safety of the occupants while the aircraftis 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 attemptedto transport nine members of a varsity basketball team in hisairplane. The pilot did not get the accurate weights of the individualsand their luggage. As he entered the cabin to close the door,the airplane tilted over onto its tail…not an uncommon occurrencefor that type of airplane when too much weight is placed in therear of the cabin. The pilot and passengers got out of the airplaneand raised the tail. Then, the pilot decided to proceed with theflight, still without having "run the numbers."

As he climbed through 300 feet AGL, he turned to those in thecabin and yelled that he had a problem. He instructed his passengersto move as far forward as they could. This apparently solved thepilot’s immediate "problem," but the airplane eventuallywas landed with most of its passengers not seated with restraintsfastened.

An inspection revealed that the 402’s rear bulkhead, a stringer,the elevator control tube and the housing for the tail navigationlight were damaged when the aircraft fell onto its tail. Investigatorsran 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 duringthe flight; the aft c.g. would have seriously aggravated the asymmetriccontrol problem.

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

Ask, Don’t Guess

There are compromises in the designs of almost every airplanebuilt. 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 payloadto assure ourselves that the airplane is within the weight andbalance envelope. In some airplanes, like the Piper Seneca, thec.g. may be forward of its limit with full fuel and only two peopleaboard (in the front seats). In others, like the Beech Bonanza,the numbers should be run both for takeoff and landing, sincethe c.g. moves aft in a rather tight envelope as fuel is burned.

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

Unfortunately, you don’t see too many pilots standing around theirairplanes with calculators in hand. That’s a shame, since it reallydoes not take a lot of time to run the numbers. There are specialplotters, calculators and computer programs for many aircraftthat produce results within a minute. I have a program that takesthe weights of passengers, baggage and fuel, and then shows mewhere I stand in a graph, similar to that in the AFM.

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

One quick-and-dirty method used by many pilots is what I callthe "half-of-the-equation" system: The pilot quicklyadds up the weights of the empty aircraft, passengers, baggageand fuel, and gets a warm and fuzzy feeling if the sum is lessthan the maxmum allowable takeoff weight. The balance portionof the calculation is neglected altogether. This practice reflectsan ignorance of what "weight and balance" really is,and of what flying an airplane out of its c.g. envelope couldmean to safe operation.

The balance side of the equation could be simply envisioned asthe airplane resting on a pinnacle, supported at its center ofgravity by one sharp point. As you add or subtract weight to theaircraft, that balance point (e.g., the center of gravity) moves.Add or subtract too much weight forward or aft, and the aircraftwill fall off the pinnacle. Before that happens, though, the pilotwill experience some handling problems. And, in some cases, thoseproblems 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 thestick 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 stablewhen 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 returnto its trimmed condition. But, as the distance between the c.g.and the horizontal tail decreases, the stick force required tomake a pitch change also decreases.

At the point of "neutral longitudinal stability," theairplane will tend to maintain its present attitude instead oftrying to return to its trimmed condition. The stick force requiredto make a change will be extremely light, and that could resultin overcontrolling. It’s possible, in an extreme case, to getinto 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 withthe c.g. at or near the aft limit. You’ll probably remember thatas you rolled down the runway, the airplane came off the groundbefore you expected it to. You quickly trimmed the nose down,realizing that although you set the trim forward of where younormally would, it was not enough. Then, as you flew the airplaneaway from the airport, you noticed that the amount of elevatortravel 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 rollto the left. He corrected with right aileron and attempted todecrease the pitch attitude, but the aircraft rolled right andthen back to the left. He landed near the departure end of therunway with the gear up.

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

What may have happened is that the stick forces on the elevatorwere so light because of the rear loading that, initially, thepilot thought he had the airplane trimmed properly when it wasperilously 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 settleback to the ground.

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

Exceeding the forward limit can be trouble, as well. As from thetime the c.g. moves forward, the controls become heavier. Moreeffort, and travel, is required to make a pitch change. The level-flighttrim position of the elevator is higher than nonmal; and, in someextremes of forward loading, you could run out of elevator beforeyou accomplish the pitch change you want.

For example, you are beginning to flare for a landing and discoverthat, even with full travel of the elevator, the pitch attitudeis not high enough to keep the nose wheel from striking the groundbefore the mains do. In fact, at the point where you pulled thethrottle to idle for the landing, the nose would fall, the airspeedwould increase, and you couldn’t do anything about it exceptattempt a power-on landing probably at a higher airspeed thannormal.

Adding power would pitch the nose back up, but you’d need a longerrunway 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 theircorrect weights. But, you should always try to be certain thatthe aircraft is within its weight and c.g. limits. Programmablecalculators can make a weight-and-balance problem easy to doon the ramp with your passengers standing by the airplane.

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

In one case, a Piper Lance crashed during a night takeoff fromGreat Barrington Airport in Massachusetts. There were five passengersand the pilot aboard. They had flown in to go skiing but foundthat the ski resort was closed. They decided to fly over to nearbyPittsfield, where they knew they could ski. On takeoff, the aircrafthit the top of a 60-foot tree nearly 500 feet from the departureend of the runway.

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

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

If you always fly your airplane with the same load, it is probablynot necessary to do a weight and balance each time; you shouldknow where you stand in the c.g. envelope. But, when somethingchanges—someone unexpected shows up, or more luggage than youanticipated is sitting on the ramp—take the time to do a thoroughweight and balance calculation.

Remember, the manufacturer’s limitations are the result of testsof handling characteristics within the envelope (there is littleor 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.

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