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Weight and Balance — How Much is Too Much?

by H. Dean Chamberlain
Reprinted with permission from FAA Aviation News

Weight and Balance: A simple concept in aviation, but in an era when the nightly news regularly reports on the increased obesity in the American population, it is a topic of growing concern in the aviation industry. Recently, a few of the general aviation safety inspectors here in the home of FAA Aviation News, Flight Standards Service's General Aviation and Commercial Division, had an interesting discussion on the topic. First, I must confess, none of the inspectors involved in the discussion are as trim as they once were. And when asked, I say I am not gaining weight, I am only losing useful load — more and more each year. Of course, you guessed it, the discussion occurred over lunch in the FAA Headquarters cafeteria. The question asked was: How much could two adults weigh in a Cessna 172 with full fuel and the aircraft still be within weight and balance (W&B) to practice spins? Then the question was asked: How many pilots actually compute weight and balance once they receive their initial pilot certificate?

The consensus is that not many general aviation pilots compute weight and balance for every flight. The reasons offered were many. Some may own their own aircraft, compute their W&B one time, and unless something changes, they use the same numbers thereafter. Some may just use the standard FAA numbers used in the aircraft's flight manual and assume they are good to go. Some may not remember how to compute they're actual W&B. Then there is the final group. That group may be hesitant to ask their passengers for their actual weights because the group may be afraid of offending the passengers. As a result, this last group of pilots may use their best guess for the actual weights. As we said in a previous article, "A Weighty Matter" published in the September/October 2005 issue, this line of questioning becomes even more sensitive when the passenger is your mother-in-law or boss.

It goes without saying, if pilots are not using actual passenger weights, it is doubtful they are using actual baggage weights. After all, how much can a set or two of golf clubs weigh?

And aircraft, like people, tend to add a few extra pounds over their useful life. Little things like an extra pair of chocks, an extra quart or two of oil, an old tow bar, a set of tie-down ropes or chains, a little bit of dirt, and the list goes on. All of which when added together can mean a few extra pounds pilots may not think about. And since most of this type of gear is often thrown in the back of the aircraft, a few extra pounds well aft of the aircraft's datum line can have significant impact on an aircraft's W&B and, possibly, performance.

Generally speaking, in aviation with everything else being equal'more weight results in less performance. Forward center of gravity (CG) beyond the forward limit can result in lack of elevator control necessary to properly flair upon landing and can require a faster airspeed to rotate when taking off. Although aft CG within aft CG limits can increase cruise performance by reducing the amount of down load on the tail, aft CG beyond the limits may prevent a recovery in case of a stall or spin.

TCDS
Weights and Balance Data

So, how much can two pilots in the front seat of a Cessna 172 weigh and still keep the aircraft within its W&B for spin practice?

The answer depends. If you check the Cessna 172 Type Certificate Data Sheet (TCDS) available on the FAA's Internet Web site, you will find the TCDS list all of the C-172 models from the original 1955 model through today's C-172S model. Over those 52 years, the weights listed for the normal and utility categories have changed. As a result, you have to know which make and model of C-172 is being discussed.

And as noted, you have to decide which category you plan on operating in when discussing weights. If you plan on doing spins for example, you will be operating in the reduced gross weight utility category with its respective operating limitations. In the C-172S manual, it states, "In the utility category, the rear seat must not be occupied and the baggage compartment must be empty." It also notes "Abrupt use of controls is prohibited above 98 knots." So your flight purpose also determines your maximum operating weight and limitations. Along with the change in gross aircraft weights by model, there are also changes in CG limits that must be observed as well as other operating limitations.

Using Actual Weights By Make, Model, and Serial Number

The key to calculating an aircraft's W&B is based upon using the latest actual W&B data for the actual aircraft by make, model, and serial number. You need to start with the actual weight of the aircraft you are using. This weight will vary from aircraft to aircraft depending upon the installed equipment. Then using that data, you need to compare the data to the aircraft. Was any equipment removed or added to the aircraft without an appropriate W&B update? If so, a new official W&B needs to be completed and added to the aircraft's records. Then, you must following the manufacturer's guidance to compute the W&B for your flight, paying special attention to any notes or other factors that must be considered in calculating W&B. For example, a note regarding fuel for the C-172S says "Serial Nos. 172S8001 and On, The certificated empty weight and corresponding center of gravity location must include un-usable fuel of 18 pounds at 46.0 inches aft of datum, and full oil of 15.0 pounds at 13.1 inches forward of datum." As you can see, this note applies to a specific range of serial numbers. This is why is it important to not only know which make and model of aircraft is involved, but also its serial number when reviewing manufacturer's W&B and TCDS information.

To emphasize the importance of a correct W&B, Cessna states in the Model 172S NAV III manual, "It is recommended that the airplane be weighed to verify Basic Empty Weight and CG Arm at intervals not to exceed five years."

Maximum Takeoff and Landing Weights

According the Cessna generic flight manual for the C-172S, normal category maximum takeoff and landing weight for a C-172S is 2,550 pounds. Utility maximum takeoff and landing weight for a C-172S is 2,200 pounds. This is a 350-pound difference between the two categories.

Center of Gravity Limits

The center of gravity limits for a C-172S varies from forward CG limit at 35.0 inches aft of datum at 1,950 pounds or less, with straight line variation to 41.0 inches aft of datum at 2,550 pounds to aft CG limit at 47.3 inches aft of datum at all weights in the normal category. The utility limits of forward CG limit at 35.0 inches aft of datum at 1,950 pounds or less, with straight line variation to 37.5 inches aft of datum at 2,200 pounds to the aft CG limitation of 40.5 inches aft of datum at all weights.

As you can see, not only do the weights differ, but also so do the center of gravity limits differ between the normal and utility categories for the C-172S.

Maximum Weights To Spin

Based upon the information presented so far, and the following information, with full fuel, what is the maximum weight that two pilots can weigh in a C-172S in the normal category? What can they weigh in the utility category? Are they within CG limits? Can they do spins with full fuel? Oh, and by the way, do you know the rule dealing with parachutes when doing spins? (Title 14 Code of Federal Aviations sections 91.307, Parachutes and parachuting.) Are parachutes required? If so, how much do two parachutes weigh? Little things do add up.

If you don't know the answers to these questions, we will provide them in the next issue.