Library Contents

Aircraft performance and handling characteristics are affected by the gross weight and center of gravity limits. If every pilot were to understand and respect this fact, general aviation accidents could be reduced dramatically. An overloaded or improperly balanced aircraft will require more power and greater fuel consumption to maintain flight, and the stability and controllability will be seriously affected. Lack of appreciation for the effects of weight and balance on the performance of aircraft, particularly in combination with such performance reducing factors as high density altitude, frost or ice on the wings, low engine power, severe or uncoordinated maneuvers, and emergency situations, is a prime factor in many accidents.

This review of the fundamentals of aircraft weight and balance is provided to remind pilots of the effects that adverse loading can have on aircraft performance.

The lifting capability of an aircraft depends upon the airfoil design of the wing, the speed at which the wing moves through the air, and the density of the air. It is the design of the aircraft wing that limits the amount of available lift, and it is the available power from the engine(s) that likewise limit(s) the speed at which the wing can be made to move through the air. The efficiency of the engine/wing combination is reduced when air is less dense than the established standard day (barometric sea level pressure of 29.92 inches of mercury at a temperature of 59 degrees F). Therefore, every pilot should ensure during preflight preparation that the aircraft gross weight is within safe limits for the intended flight, considering the aircraft performance capabilities. The total weight of baggage, cargo, and fuel load should be adjusted accordingly to provide an adequate margin of safety.

Pilots must understand that in many general aviation aircraft, it is not possible to fill all seats, load the baggage compartment to capacity, carry full fuel, and remain within approved weight and balance center of gravity (CG) limits. In many four-place and six-place airplanes, the fuel tanks may not be filled to capacity when a full complement of passengers and their baggage is carried. It will be necessary to reduce the number of passengers or baggage weight if the proposed flight distance requires a full fuel load.

The aircraft performance characteristics adversely affected by overweight are:

• Increased takeoff speed.
• Increased takeoff runway length.
• Rate of climb.
• Maximum altitude capability.
• Operational range.
• Maneuverability.
• Controllability.
• Stall Speed.
• Approach speed.
• Landing distance.

Every pilot must consider how these characteristics would affect the aircraft in an emergency situation. Another consideration is high elevations, and/or hot and humid weather (density altitude), which is the subject of another publication in the Accident Prevention Program series.

Balance refers to the location of the CG along the longitudinal axis of the aircraft. This factor is of primary importance to safety of flight. There are forward and aft limit beyond which the CG should not be located for flight. These limits are established by the aircraft design engineers to ensure proper predictable aircraft control about the horizontal, vertical, and lateral axes. The operational weight and balance limits for each aircraft are contained in the Pilot's Operating Handbook or flight manual. This information may also be obtained from the FAA Aircraft Specification or Type Certificate Data Sheets, which are available at most aircraft maintenance facilities. The weight and balance information for each aircraft must be amended when repairs or alterations have been made that effect a change in the aircraft empty weight or CG location (reference 14 CFR 43.5(a)(4) and 91.31 (b)). To ensure aircraft controllability during flight, the aircraft must be loaded within the design weight and CG limits.

A forward CG limit is specified to ensure that sufficient elevator deflection is available at minimum speed as for landing. The aft CG limit is the most critical during flight maneuvers or operation of the aircraft. Aircraft stability decreases as the CG moves aft, and the ability of the aircraft to right itself after maneuvering will be correspondingly decreased. The aircraft will be highly unstable in gusting or turbulent air, making attitude and directional control extremely difficult.

If, after the aircraft is loaded, the CG does not fall within the allowable limits, it is essential to shift loads before flight is attempted. The actual location of the CG is determined by a number of factors under control of the pilot:

• Placement of baggage and cargo.
• Assignment of seats to passengers according to each individual's weight.
• Fuel load. Selective use of fuel from various tank locations during flight may aid in maintaining safe balance conditions.

All aircraft are delivered with the empty weight and CG data, which remain with the aircraft records. The aircraft owner is responsible for ensuring that maintenance personnel make appropriate entries in the aircraft records when repairs or alterations are made to the aircraft. Unless the aircraft flight manual is amended when the aircraft empty weight or CG changes, the pilot has no baseline for loading calculations and decisions.

All pilots need to be familiar with the terms related to aircraft weight and balance. Some of the more common terms are:

Arm (moment arm) - the horizontal distance in inches from the reference datum to the item. The algebraic sign is plus ( + ) If measured aft of the datum and minus ( - ) if measured forward of the datum;

Center of gravity (CG) - the point about which an aircraft would balance if it were possible to suspend it at that point. It is the mass center of the aircraft or the theoretical point at which the entire weight of the aircraft is assumed to be concentrated;

Center of gravity limits - the specified forward and aft points beyond which the CG must not be located during flight. The CG moment envelope is contained in the aircraft flight manual and FAA Aircraft Specifications or Type Certificate Data Sheets;

Center of gravity range - the distance between the forward and aft CG limits;

Datum line - an imaginary vertical plane or line from which all measurements of arm are taken. The datum is established by the manufacturer. After the datum is selected, all moment arms and the CG range must be computed with reference to that point;

Fuel load - the expendable part of the aircraft load. Fuel load includes only usable fuel and not the fuel required to fill the lines or that which remains trapped in the tank sumps;

Moment - the product of the weight of an item multiplied by its arm. Moments are expressed in inch pound (in.-lb.);

Total moment - the weight of the aircraft multiplied by the distance between the datum and the CG;

Moment index - the moment divided by a constant such as 100, 1,000, or 10,000. The purpose of using a moment index is to simplify computations of weight and balance on large aircraft where heavy items and long arms result in large unmanageable numbers. It is simply a matter of reduction to the least common denominator;

Mean aerodynamic chord (MAC) - the average distance from the leading edge to the trailing edge of the wing. The MAC is specified for the aircraft by determining the average chord of an imaginary wing which has tile same aerodynamic characteristics of the actual wing. Center of gravity is usually located at or near the forward 25 percent of the chord;

Station - a location in the aircraft which is identified by a number designating its distance in inches from the datum. The datum is therefore identified as zero and the station and arm are usually identical;

Useful load - the weight of the pilot, copilot, passengers, baggage, usable fuel, and drainable oil;

Empty weight - the airframe, engines, and all items of operating equipment that have fixed locations and are permanently installed in the aircraft. It includes optional and special equipment, fixed ballast, hydraulic fluid, unusable (residual) fuel, and undrainable (residual) oil.

Reminders:

A simple and fundamental weight check should always be made before flight to assure that the aircraft useful load is not exceeded. If there is the slightest doubt about the loading, it will be advisable to calculate it by using actual weights and moment arms to determine that the aircraft is loaded within safe limits.

• Aircraft Empty WT x CG(ARM) = Moment
• Oil WT x ARM = Moment
• Pilot and Passenger WT x SEAT (ARM) = Moment
• Passengers WT x Seat (ARM) = Moment
• Baggage WT x Compartment(ARM)=Moment
• Fuel WT x Tank (ARM) = Moment
• Aux. Fuel WT x Tank (ARM) = Moment

Add total weight and check against maximum takeoff weight. If within limits, add the total moment and divide by total weight to determine loaded CG The loaded CG should be within the forward and aft CG limits shown in the aircraft flight manual weight and balance information. If not, a few minor load adjustments may correct the problem.

For your safety and the safety of your passengers, check the weight and balance of your aircraft before each flight. Keep the aircraft gross weight and center of gravity within prescribed limits.

Aircraft Weight and Balance Handbook -- FAA-S-8083-1 (http://www.faa.gov/library/manuals/aircraft/media/FAA-S-8083-1.pdf)

Pilot's Handbook of Aeronautical Knowledge -- FAA-H-8083-25 (http://www.faa.gov/library/manuals/aviation/pilot_handbook/)

AC 43.13-1A Acceptable Methods Techniques and Practices - Aircraft Inspection and Repair (http://www.airweb.faa.gov/Regulatory_and. . .9BAAC81B86256B4500596C4E?OpenDocument)

The purpose of this series of Federal Aviation Administration (FAA) Aviation Safety Program publications is to provide the aviation community with safety information that is informative, handy, and easy to review. Many of the publications in this series summarize material published in various FAA advisory circulars, handbooks, other publications, and various audiovisual products developed by the FAA and used in its Aviation Safety Program.

Some of the ideas an materials in this series were developed by the aviation industry. The FAA acknowledges the support of the aviation industry and its various trade and membership groups in the production of this series.

Comments regarding these publications should be directed to the National Aviation Safety Program Manager, Federal Aviation Administration, Flight Standards Service.