Probably more than you think
Student pilots are taught the theory of weight and balance soon after their first solo. Operation of an airplane when it's overweight, or when the weight is distributed such that the center of gravity (CG) is beyond the front or rear limits, not only makes an airplane unsafe, but also renders it unairworthy since it's being operated outside the limits stated in the type certificate. A thorough preflight always includes a weight and balance determination.
Nevertheless, everyone seems to know of a time when they, or a friend of theirs, flew in an airplane that was overweight. Often this happens during primary flight instruction when the pilot, his instructor, and a fully fueled trainer launch for a dual cross-country flight.
Because of the nature of aircraft weighing and calculations, there is usually a small amount of wiggle room in weight and balance figures, but the best practice is to know how much your airplane weighs—and always manage the fuel, passenger, and baggage loading to keep the CG and gross weight within limits. It's safer that way. And it's legal.
Aircraft weights
There's a good chance that your current aircraft weight and balance records do not reflect the true weight of your airplane. The variations may be surprisingly large, or they may be so small that they can be considered inconsequential. Reasons for the discrepancy between the true weight and the "paper" weight include the use of fleet, or average, weights during production; the likelihood of miscalculations or guesses as to the weight and arm of components when upgrades or modifications are installed; the wording of the regulations concerning weight and balance procedures; and the fact that there is no regulatory requirement to actually weigh general aviation airplanes.
Federal aviation regulations allow manufacturers to create their own method of establishing an aircraft's weight. Some companies, such as Mooney, weigh every airplane they build. These aircraft have the actual aircraft weight listed in the original weight and balance documents. Cessna, and some other manufacturers, use computed weights for most of the single-engine airplanes they produce. This practice consists of weighing a number of airplanes—in Cessna's case, 20—and averaging the combined weights to get a fleet weight. Similar airplanes are assumed to fall within the tolerances of the fleet weight. To check for deviations during the manufacturing process, every tenth airplane is weighed. If the weight of the check airplane is within 1 percent of the fleet weight, that fleet weight is used for the next 10 airplanes. If the weight of the check airplane is outside the 1-percent tolerance, a new fleet weight is established using the weight-averaging procedure.
An aircraft owner with the original factory weight and balance report in his possession can determine whether his airplane's weight was established by using the actual weight or a computed weight by reading the weight and balance report. Owners who want a copy of the original production weight and balance report for their airplane can usually obtain a certified copy by contacting the manufacturer.
Weight and balance calculations
Since there is no requirement to weigh airplanes that are operated under FAR Part 91 when a new radio stack, STOL kit, or auxiliary fuel tank is installed, the mechanic or shop that does the installation usually computes the weight and balance change, as opposed to reweighing the entire airplane, and creates an updated weight and balance report. The old report then has the word superceded and the date written across its face. This process of determining weights, arms, and moments of both removed and installed equipment is pretty straightforward. Errors can arise when a weight or arm is estimated or when the mathematical computations are in error. Any error, if not found and corrected, will ripple down through subsequent reports.
Can this be dangerous? It depends. Usually small errors of this sort don't compromise safety. And as the student pilot in the trainer learns, airplanes will fly even if they are a few pounds overweight.
The regulations
Advisory Circular 120-27C, titled "Aircraft Weight and Balance Control" (November 7, 1995), provides guidelines for commercial airplane operators who are required to establish a weight and balance program. An area that has the potential to create large discrepancies between an aircraft's true weight and the "paper" weight is the definition of negligible weight in the advisory circular.
According to this advisory circular, an aircraft weight should be reestablished whenever the cumulative change to the operating weight exceeds "plus or minus one-half of one percent of the maximum landing weight or the cumulative change in the CG position exceeds one-half of one percent of the mean aerodynamic chord." This definition means that a cumulative weight change of less than eight pounds for a Cessna 150 or less than 20.5 pounds for a late-model Cessna 210 would not require a recalculation of the airplane's weight and balance paperwork. If two or three of these negligible modifications were performed without being computed and entered into the aircraft records, the potential for weight and balance problems would be substantial.
Fortunately, any maintenance facility or airframe and powerplant mechanic who takes the responsibility seriously wouldn't consider adding eight pounds to an airplane without updating the aircraft's empty weight, moment, and empty weight CG. Still, it shouldn't take an owner long to pull out his aircraft weight and balance paperwork and check to see if the radios and equipment presently in his airplane match the changes noted in the weight and balance records. An airplane with a complete set of records should have every previously superceded weight and balance report extant in the airplane records. For this same reason, weight and balance records should be a part of every aircraft prepurchase inspection.
How about that paint job?
Paint shops contacted during the writing of this article said that post-paint job weighing isn't included in a paint job price quote. The Cessna factory weight and balance record from a 1962 Cessna 182 used the notation "empty, dry & unpainted" before the empty weight totals. The options in the equipment list show that owners could order the airplane bare, with "standard stripes" weighing two and a half pounds, or with "all-over" paint, weighing 19 pounds. A few pounds won't make any appreciable difference in the flight characteristics of the average general aviation airplane, unless the weight is located a long way forward or aft of the CG location, but paint jobs, different brands of tires, and other variables show why it's almost impossible for the true weight and the "paper" weight of an airplane to match perfectly.
Where's the datum?
Mechanics, repair stations, and avionics shops have to determine the center of the weight installed (or removed) and the position of that weight in the airframe when computing weight and balance changes. This position is relative to the datum, an arbitrary point chosen by the manufacturer. Cessna almost always uses the forward face of the firewall as the datum in its single-engine models; the Diamond Katana DA20 uses the leading edge of the wing at the root rib; the Piper Cherokee Six has its datum located 78.4 inches forward of the wing leading edge at the intersection of the straight and tapered sections of the wing.
After a weight, arm, and moment are established, they are entered on a weight and balance change form. There is no official form. The content and scope of the form, following general guidelines, are left to the individual maintenance person or shop. If the position of the weight added is forward of the datum, the arm is a negative value; aft of the datum and it's a positive value. If weight is taken out of the airplane, it's a negative value; added, it's a positive value. Multiplication of the positive or negative weight and arm values will result in a positive or negative moment that is added or subtracted from the existing moment before being divided by the new empty weight to give a new empty weight CG.
What if an owner doesn't like the look of his weight and balance records, or perhaps has invested in a new interior, paint job, and full avionics suite, and won't be satisfied until he knows the true weight of his airplane? It's time to find a set of scales and get the facts.
The Cutlass gets weighed
I flew the 1981 Cessna 172RG Cutlass that I rent from San Luis Obispo, California, to Del Air, a maintenance shop owned by Harry Dellicker in Porterville, California. Dellicker had a set of recently certified low-profile floor scales that he had agreed to let me use. This Cutlass had never been wrecked or repainted; in fact, the only modifications shown in the maintenance and weight and balance records were the installation of King IFR avionics and a loran receiver. All these weights had been computed—the records revealed that this aircraft, being a Cessna, had never been weighed. The Cutlass had gained weight over the years, much like the pilots who fly her, but the calculated weight and the true weight were remarkably close.
How much does fuel weigh?
I weighed the airplane when it was full of fuel and oil. Some airframe manufacturers, and Advisory Circular AC43.13-1B, recommend completely draining the fuel when the airplane is leveled in accordance with the manufacturer's service manual. Since weighing an airplane with full fuel tanks cuts down on time and costs involved, I would wager that the majority of general aviation airplanes are weighed using this method. After recording the scale weights with the airplane leveled, weight of the usable fuel was subtracted to arrive at a computed aircraft empty weight. The calculations were carried out using the commonly accepted weight of a gallon of avgas, which is six lbs. It's only at minus 7 degrees Celsius that a gallon of 100LL actually weighs six pounds—a gallon of 100LL varies from 6.3 pounds at minus 40 degrees Celsius to 5.56 pounds at plus 40 Celsius. The sliding fuel weight introduces yet another variable between an aircraft's true weight and its "paper" weight.
How far off was the Cutlass?
The most recent weight and balance report for the Cutlass calculated that it weighed 1,682.69 pounds with a CG arm of 37.397 inches and a moment of 62,928.35. Certified scale weights totaled 2,069.5 pounds with full fuel and oil. After subtracting 372 pounds (62 gallons at 6 lb. per gallon) at an arm of 46, the result was an empty weight of 1,697.5 lb. The difference between the computed weight and the actual weight is a little more than 15 lb., or less than 1 percent. Not perfect but certainly good enough.
How far off is your airplane? It will depend on how clean it is, since dirt and oily grime do add weight over the years. It will depend on how many times the airframe has been repaired, and how often radios, parts, modifications, and accessories have been removed or installed. It will depend on whether the airplane has been painted—especially if the paint job was a low-budget "scuff and squirt" job, with new color laid over an existing color coat. And it will depend on whether all the weight and balance changes have been entered in the airplane records.
Some pilots like to carry a spare magneto or alternator in the baggage compartment of their airplane to prevent parts problems should a mechanical malfunction occur when away from home. Some Cessna 182 pilots find that a 50-lb. toolbox tied down in the baggage compartment makes the airplane fly better and flare easier as the CG is moved away from the forward limit. Extra stuff must be accounted for during preflight calculations.
Aircraft owners who are truly interested in their airplanes, or owners who think that their airplane doesn't perform as it should because it's heavy, may decide that it's time to determine the true weight. Air taxi operators and well-equipped FBOs should be able to help owners find a calibrated set of scales. After the true weight and balance has been found, the next step in professional aircraft record keeping is to create an up-to-date equipment list.
Links to additional information about aircraft weight may be found on AOPA Online ( www.aopa.org/pilot/links/2001/links0101.shtml). E-mail the author at [email protected].
