Chapter 10: Weight and Balance
Operating an aircraft within the weight and balance limits is critical to flight safety. Pilots must ensure that the center of gravity (CG) is and remains within approved limits throughout all phases of a flight.
If an aircraft's CG is outside the approved limits, positive control will be difficult or impossible, resulting in a hazardous flight condition. In addition, operating above the aircraft's maximum weight limitation compromises its structural integrity and adversely affects performance.
Weight attracts all bodies to the earth. It is a product of the mass of a body. Weight also is the result of acceleration acting on the body.
Loading an aircraft beyond the manufacturer's recommended weight must be avoided. The force of lift, created by the wings, counteracts weight and sustains an aircraft in flight. However, lift is limited by several factors, including airfoil design, angle of attack (AOA), airspeed, and air density. Pilots must understand and respect these limits.
Manufacturers attempt to make aircraft as light as possible without sacrificing strength or safety. Therefore, any item aboard an aircraft that increases the total weight is undesirable for performance.
If an aircraft is not properly loaded, the initial indication of poor performance usually takes place during takeoff. An overloaded aircraft may not be able to leave the ground. If it does become airborne, it may exhibit unexpected and hazardous flight characteristics.
Preflight planning should include a check of performance charts to determine if the aircraft's weight may contribute to hazardous flight operations.
Pilots who manage to depart in an aircraft loaded beyond its gross weight limit can expect:
The operating weight of an aircraft can be changed by simply altering the fuel load. Gasoline weighs six (6) pounds per gallon, which is considerable. The weight of one passenger can range from 20 to 40 gallons of fuel. When taking passengers and cargo, reducing total fuel on board can keep the aircraft within gross weight limits. Flight planning should factor the total range or flight time available based on the reduced amount of total fuel.
During routine flight, fuel burn is the only weight change that takes place. As fuel is used, an aircraft becomes lighter and performance is improved.
Balance, Stability, and Center of Gravity
Balance refers to the location of the center of gravity (CG) of an aircraft. It is critical to stability and safety in flight.
The CG is a point at which the aircraft would balance if it were suspended at that point.
The CG can be moved along the aircraft's longitudinal axis, based on the distribution of weight in the aircraft. The fore and aft location of the CG is the primary concern when determining an aircraft's balance.
The distance between CG's forward and back limits — called the CG range — is certified for an aircraft by the manufacturer.
if the CG is displaced too far forward on the longitudinal axis, a nose-heavy condition will result. If the CG is displaced too far aft on the longitudinal axis, a tail-heavy condition results. A center of gravity outside of the permissible CG range can result in an unstable or hazardous flight condition.
The position of the lateral CG is not computed in all aircraft. However, pilots must be aware that adverse effects arise as a result of a laterally unbalanced condition. A lateral unbalance occurs if the fuel load is mismanaged by supplying the engine(s) unevenly from tanks on one side of the airplane. This can be offset with rudder trim or constant control pressure. The result will be an out-of-streamline aircraft with more drag and decreased efficiency.
Just as excess weight affects flight characteristics, adverse balance conditions operate in much the same manner. Stability and positive control are impacted by improper balance.
Loading in a nose-heavy condition causes problems in controlling and raising the nose, especially during takeoff and landing.
Loading in a tail-heavy condition impacts longitudinal stability and reduces the pilot's ability to recover from stalls and spins. Tail-heavy loading also produces very light control forces.
The CG's fore and aft limits are published for each aircraft in the Type Certificate Data Sheet (TCDS), or aircraft specification and the AFM or POH. The CG should not exist beyond these limits before flight. If the CG is beyond the CG range, reloading the aircraft or removing items may be necessary before flight.
Manufacturers purposely place the forward CG limit as far rearward as possible to aid pilots in avoiding damage when landing. This also permits sufficient elevator/control deflection at minimum airspeed.
As the CG moves aft, a less stable condition occurs, which decreases the ability of the aircraft to right itself after maneuvering or turbulence.
For some aircraft, both fore and aft CG limits may be specified to vary as gross weight changes.
Pilots can undertake several actions to relocate the CG before flight, including relocating baggage and cargo items, and placing heavier passengers in forward seats.
While fuel burn can affect the CG, this is based on the location of the fuel tanks. Most small aircraft carry fuel in the wings very near the CG. Therefore, while fuel burn will reduce the aircraft's gross weight, it will have little effect on the loaded CG.
Before any flight, the pilot should determine the weight and balance condition of the aircraft. Charts and graphs are provided in the approved AFM/POH to enable pilots to make weight and balance computations.
A typical general-aviation aircraft (two or four seats) cannot remain within the approved weight and balance limits with full passengers, fuel, and baggage compartments. If the maximum passenger load is carried, the pilot typically will need to reduce the fuel load or baggage.
Weight and Balance Computations
Determining the total weight of a loaded aircraft simply requires totalling the aircraft's empty weight, the fuel and oil, and everything loaded on the aircraft.
Determining the distribution of mass around the CG requires additional calculations.
The point at which an aircraft balances can be determined by locating the CG — the imaginary point at which all the weight is concentrated. To provide the necessary balance between longitudinal stability and elevator control, the CG is usually located slightly forward of the center of lift. This loading condition causes a nose-down tendency in flight, which is desirable during flight at a high AOA and slow speeds.
The CG range limits are usually specified in inches, along the longitudinal axis of the airplane, measured from a reference point called a datum. The datum reference is an arbitrary point, established by aircraft designers that may vary in location between different aircraft.
The fulcrum is where the airplane would balance were it placed on a single, raised point. For sample illustrations, the datum can be located at the fulcrum. However, in many aircraft it's at a different location along the longitudinal axis, and it's only used for reference.
The arm is the distance from the datum to any component part or any object loaded on the aircraft. When the object or component is located aft of the datum, it is measured in positive inches; if located forward of the datum, it is measured as negative inches or minus inches
The station is the location of passengers, fuel, oil, baggage, and additional items.
The moment is the weight of a station multiplied by the arm. Each occupied station has a calculated moment.
The moment is is expressed in inch-pounds (in-lb). It is the measurement of the gravitational force that causes a tendency of the weight to rotate about a point or axis.
A classic example of moment weight (or moment effectiveness) is a 50-lb weight and a 100-lb weight on a playground see-saw — a long board balanced on a fulcrum. The large weight is placed closer to the fulcrum, while the smaller weight is more distant from the fulcrum. The lesser weight has a greater arm. It exhibits as much potential energy as the greater weight because of this greater arm. (In physics, this mechanical advantage is called leverage, proven as "the law of the lever" by Archimedes).
If small weight's station is 100 inches from the fulcrum, the moment (downward force of the station) can be determined by multiplying 50 pounds (weight) by 100 inches (station). The specific location on the board produces a moment of 5,000 inch-pounds.
In order to balance the board, the large weight's moment also should be 5,000 inch-pounds. The weight of the object cannot be adjusted, but the station is variable. Thus, in order for the large weight to produce 5,000 in-lb of downward force, the station will need to be 50 inches from the fulcrum.
|Small item||50 pounds||100 inches||5,000 inch-pounds|
|Large item||100 pounds||50 inches||5,000 inch-pounds|
These objects in these locations create equal downward forces.
If additional objects are added to the board, the stations of each object can be adjusted so that the total moments on either side of the fulcrum are equal.
Determining Loaded Weight and CG
There are various methods for determining the loaded weight and CG of an aircraft. There is the computational method as well as methods that utilize graphs and tables provided by the aircraft manufacturer.
A simple computational method, using a table format, can be used to determine total weight and center of gravity.
|Aircraft Empty Weight||2,100||x||78.3||=||164,430|
|Front Seat Occupants||340||x||85.0||=||28,900|
|Rear Seat Occupants||350||x||121.0||=||42,350|
|Baggage Area 1||80||x||150.0||=||12,000|
|Calculate CG:||281,430 ÷ 3,320 = 84.8|
The result is the total arm. This is the location of the CG, as referenced from the datum.
Consult the AFM/POH to determine if the aircraft is at or under the gross weight. In the above example, total aircraft weight is 3,320 pounds.
Also consult the AFM/POH to determine if the CG is within the allowable CG range. In the above example, the CG is 84.8 inches aft of the datum.
If either figure does not permit safe flight operations, the aircraft may need to be loaded differently, and/or some items may need to be removed from the aircraft.
For some aircraft, loaded weight and CG can be determined using graphs provided by the manufacturers. These will be in the AFM/POH. Because moment values can be large numbers, the moment may sometimes be divided by 100, 1,000, or 10,000 to simplify calculations.
To determine the moment using the loading graph, find the weight (along the left-side metric) and draw a line straight across until it intercepts the item for which the moment is to be calculated. These diagonal lines represent stations. Then draw a line straight down to determine the moment.
In the example below, the red line depicts a pilot and passenger weighing a combined 340 pounds, which has a total moment of 127,000 — or 12.7 for purposes of calculation.
Once this has been done for each item, total all weights and all moments. Draw a line for both weight and moment on the CG envelope graph. If the lines intersect within the envelope, the aircraft is loaded within limits.
In the example below, the red line depicts an airplane with a loaded aircraft weight of 2,367 pounds and a total moment of 105.2. The aircraft's CG is within the permissible CG range.
The table method uses information and limitations are contained in tables provided by the manufacturer. It applies the same principles as the computational and graph methods. For each station, a range of moments are provided for a range of weights. Minimum and maximum moments are then provided for the aircraft's gross weight.
Shifting, Adding, and Removing Weight
Please see the Pilot's Handbook of Aeronautical Knowledge for formulas covering weight additional, removal, and shifting. Questions regarding this subject matter are found on the Commercial Pilot, Ground Instructor, and Flight Instructor written tests.
Terms and Definitions
For additional information on weight, balance, CG, and aircraft stability refer to the FAA handbook appropriate to the specific aircraft category.
Commercial Pilot & Flight Instructor Test Questions
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