An Aviation Safety Staff Report
One hot Saturday in July, I was introduced to aircraft weight and balance, as practiced by some pilots. I was banging on a coffee machine in the FBO, trying either to get something in the cup or my money back, when a FSDO inspector stopped by. I knew he hadnt come to help me, but we were on reasonably friendly terms.
Can you give me a hand, getting a scale out of my car? he asked.
Okay, where does it go? I responded.
He pointed to the ramp, and the row of airplanes parked there. I need to give some advice to these pilots. You will probably be surprised at what we turn up.
We unloaded a large freight scale and rolled it over to the ramp. He looked down the line of airplanes and pointed at a Saratoga. Lets strike up a conversation with this pilot.
We wheeled the scale over to the Saratoga and noticed its tail skid was almost scraping the ground. The pilot, who had been watching the refueling came over immediately. Can I help you? he asked in a concerned voice.
Yes, Im from the FSDO. I need a few minutes of your time to check the weight and balance of your Saratoga.
The inspector showed his credentials and the unloading began. As the pilot removed each item, it went on the scale and its weight was recorded. The collection of cargo was surprising: A large packaged tent was on the bottom, with layers of baggage, tool boxes and other wooden boxes on top. The weight totaled out at a little over 350 pounds.
The airplanes weight and balance data was located and examined. The passengers got into the spirit of things and allowed themselves to be weighed, along with their baggage. The inspector patiently pointed out the difference between the Saratogas maximum gross takeoff weight and the 400 excess pounds planned. The center of gravity was well aft, almost off the chart.
There was no enforcement action. The inspector stayed with the Saratoga until it was properly loaded. One of the passengers volunteered to drive the excess cargo to Lake Tahoe in his pickup truck.
The inspector continued down the line, weighing and advising. Three out of five airplanes were overweight, out of their allowable center of gravity, or both. It was a good object lesson, and may have saved some lives.
Center of Gravity
The center of gravity (CG) of most light aircraft is located between 20% and 25% of the wings chord, near the leading edge. Safe loading limits for an aircraft are always listed relative to the aft (rearward) CG limit or a forward CG limit.
If we establish a reference point-often called a datum-we can standardize how we compute the aircrafts balance by referring to the center of gravity in relation to this datum. The datum can be anyplace-the firewall, the prop or an imaginary point in front of the airplane. Once that point is established, computing the aircrafts center of gravity-expressed as inches aft of the datum-becomes a simple matter of basic math.
For example, lets place two weights-a 12-pound suitcase and a six-pound flightbag-in our imaginary aircraft. The 12-pound weight is 24 inches aft of the datum while the six-pound flightbag is 48 inches aft. The suitcase has a moment-or force-of 12 x 24, or 288 inch-pounds. Meanwhile, the six-pound weights moment is computed by multiplying 6 times 48. This computes out at 288 inch-pounds, the same as our 12-pound suitcase. Combined, the two bags move our center of gravity aft by a total of 576 inch-pounds. To determine if this new loading is within the aircrafts center-of-gravity range, we then need to go to the manufacturers weight and balance data and plot its effect on the overall aircraft. We do that using data in the Pilots Operating Handbook (POH) or the Airplane Flight Manual (AFM), which contain the manufacturers weight and balance data for the specific aircraft. See the sidebar for definitions of terms used in computing an aircrafts weight and balance.
Effects On Performance
Center of lift, center of gravity and the elevators effectiveness in controlling nose-up and nose-down movements are all elements of understanding the effect of weight and balance on an airplanes performance. A range of weight and balance limitations is designed into all aircraft by the manufacturer.
The pilot, before takeoff, is responsible for making sure that the weight and balance limitations are met and must be totally familiar with the related characteristics of each airplane flown. Part of that pre-flight planning also includes determining the aircrafts weight and balance at the end of the planned flight. Because fuel is burned during the flight, the aircraft will not weigh the same at the end of a flight as it did for takeoff and that missing weight may affect its balance and performance.
Similarly, the gross weight of the aircraft affects how it will perform. Performance is computed by the manufacturer based on a standard day-59 degrees Fahrenheit and 29.92 inches of mercury-at sea level. However, high elevations, high temperatures and high humidity (high density altitudes) can seriously affect aircraft performance. On hot days at high elevations, it can be necessary to off-load passengers, fuel or baggage to obtain the required performance.
Few airplanes are designed to allow an FAA-standard 170-pound person in each seat, full fuel tanks and a baggage compartment loaded to its limits while remaining within the weight and balance limitations. If a full load of passengers and baggage is to be carried, fuel often will need to be drained to meet gross-weight limitations. Conversely, if the loading of passengers and their baggage is concentrated behind the wing, fuel often must be added to remain within the allowable CG range. If range or endurance concerns override the need to carry passengers or baggage, someone-or their clothes-must stay home. Juggling these parameters to properly configure the airplane is what puts the balance in a pilots weight and balance computations.
The Envelope, Please
Performing a weight and balance computation results in two important numbers: The aircrafts gross weight and its CG. Both of these values must fall within the aircrafts allowable weight and CG rage-sometimes known as its loading envelope. If either of these two values fall outside of the loading envelope, the aircraft will be out of the allowable CG range, overweight or both. Off-loading fuel, people or bags-or re-positioning these items-is necessary before re-computing the results to determine of the aircraft is properly loaded. Sometimes, this process will have to be repeated several times before the total loaded weight and CG location are within the loading envelope.
To perform these computations, we start with the aircrafts empty weight and CG location. The empty weight comes from the required recorded entries in the aircrafts documents and may result of earlier computations or from actually weighing it. As discussed earlier, the point from which all the balance measurements are made is the datum, that imaginary, arbitrary and established point near the forward part of the aircraft. The CG is measured in inches from-usually aft of-the datum.
The effect of any items weight is determined by calculating its moment. Assume we have a hypothetical airplane, the empty weight of which is 2000 pounds. It has a CG located 100 inches from the datum. From this point, we use simple math to add up the weight of fuel, baggage and passengers as well as their moments. Of course, to compute the moments, we need to know where each item will be located and its weight.
To begin, lets put a 160-pound pilot in his seat, which is located directly over the CG. Weight times the CG equals moment, which would give us a moment of 16,000 pound-inches. This process is repeated for every item loaded. The new center of gravity is determined after dividing the total weight of the loaded airplane by the total moments. In this case, our hypothetical total weight is 2160 lbs. and the CG remains at 100 lb.-in. aft of the datum because the pilots seat is on the CG. Since this loading is within the airplanes envelope (it should be, since we didnt change the CG), were good to go. Of course, in the real world, we would still need to add some fuel, maybe a passenger and some baggage.
Doing this math for every item loaded aboard the airplane can be cumbersome and lead to errors. Realizing this, manufacturers have simplified the process. One method they use is to provide a table in the POH/AFM showing the location of baggage compartments, fuel tanks and seats. By adding the weight and moment numbers in the tables to the airplanes empty weight and CG, determining the airplanes weight and balance is simplified. In another method, the math is performed for you on a loading graph. The fully aware pilot will keep the numbers for maximum loads stored in his or her memory. When it looks as though things are getting critical with the loading, go to the loading charts to be sure.
Outside the Envelope
Out-of-limits forward and aft CG can create serious problems for the pilot, much more than simply being 100 pounds overgross. Moving the CG forward beyond its limit reduces the elevators effectiveness at the same time its forced to perform more work. At low speeds, the pilot runs out of elevator authority and the nose drops uncontrollably. Theres little to do except add power, accelerate or both. In extreme situations, neither power or airspeed is enough to bring up the nose. Nose-heavy airplanes also spin very rapidly, with difficult high-speed recoveries. Trying to land or take off an airplane that is loaded with a CG forward of its limits is an accident close to happening.
Loading the airplane with its CG aft of limits results in the reverse of the nose-heavy situation. On takeoff with normal back pressure, the nose will come up much farther wanted it to. The landing flare would pose another problem, as normal control inputs can pitch the nose much higher than normal, more quickly, resulting in a stall close to the runway. An extreme case, with a critically aft CG location, can result in an airplane that cannot be recovered from a stall. With weight concentrated beyond the aft CG limit, an unrecoverable flat spin can develop from a tail-heavy stall. If this occurs, pilot and passengers are in for a bad day.
The legal limit on aircraft weight is concerned with performance and maintaining structural integrity. In the former instance, there are numerous cases of aircraft too heavy to fly or that couldnt climb out of ground effect. In the latter, the 100-lb. suitcase will suddenly weigh 380 lbs. at the 3.8 G load limit of a Normal category airplane. Is the baggage compartment floor strong enough to support that weight?
In older aircraft, its worth looking into the actual center of gravity. During the lifetime of the airplane things get added that can change the factory original weight. Undocumented or minor alterations can seriously change recorded weights. Meanwhile, the CG quietly moves from where the paperwork says it should be. If there is any serious doubt, the airplane should be weighed by qualified mechanics.
Putting more stuff aboard than the airplane can legally carry happens more than anyone likes to admit. As long as the load is within the CG limits, it can usually be accomplished without a huge risk. Performance will definitely suffer, but the undesirable effects of a CG beyond its forward or aft limits wont be realized.
Is it legal or wise to overload an aircraft? Not a chance. Being even moderately over your gross weight, however, is far preferable to being even slightly out of CG. When overgross, expect and plan for poor runway and climb performance, possible engine overheating and cranky controllers as you slowly slog your way uphill. Plan to burn off enough fuel to be below MGTOW or maximum landing weight when you arrive at your destination. And check to make sure that youre still within CG at the end of your planned flight-the CG on many popular GA airplanes moves aft as fuel is burned.
Adding it Up
Quickly and accurately calculating an airplanes CG should be childs play for the average pilot. Yet, a healthy appreciation for the consequences of loading the airplane outside of its CG envelope is not something every GA pilot has. Now, you do.