AIRPLANE WEIGHT AND BALANCE
Airplane Weight and Balance Lesson by wifiCFI
Terminology (W&B Glossary)
Airplane Flight Manual (AFM)
An FAA-approved document, prepared by the holder of a type certificate for an aircraft, that specifies the operating limitations and contains the required markings and placards and other information applicable to the regulations under which the aircraft was certificated.
Arm
The horizontal distance from the reference datum to the CG of an item.
Ballast
A weight installed or carried in an aircraft to move the center of gravity to a location within its allowable limits.
Basic Empty Weight
Standard empty weight plus optional equipment.
Center of Gravity (CG)
The point at which an airplane would balance if suspended. Its distance from the reference datum is determined by dividing the total moment by the total weight of the airplane.
Center of Lift
The location along the chord line of an airfoil at which all the lift forces produced by the airfoil are considered to be concentrated.
CG Arm
The arm obtained by adding the airplane’s individual moments and dividing the sum by the total weight.
CG Limits
The extreme CG locations within which the aircraft must be operated at a given weight.
CG Limit Envelope
An enclosed area on a graph of the airplane loaded weight and the CG location. If lines drawn from the weight and CG cross within this envelope, the airplane is properly loaded.
Datum
An imaginary vertical plane or line from which all measurements of arms are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms and the location of CG range are measured from this point.
Empty Weight
The weight of the airframe, engines, all permanently installed equipment, and unusable fuel.
Load Factor
The ratio of the maximum load an aircraft can sustain to the total weight of the aircraft.
Maximum Zero Fuel Weight
The maximum authorized weight of an aircraft without fuel. This is the total weight for a particular flight minus the fuel. It includes the aircraft and everything that is carried on the flight except the weight of the fuel.
Moment
The product of the weight of an item multiplied by its arm.
Pilot’s Operating Handbook (POH)
An FAA-approved document published by the airframe manufacturer that lists the operating conditions for a particular model of aircraft and its engine(s).
Ramp Weight
The zero fuel weight plus all of the usable fuel on board.
Station
A location along the airplane fuselage usually given in terms of distance from the reference datum.
Unusable Fuel
The fuel remaining after a test has been completed in accordance with governmental regulations.
Usable Fuel
The fuel available for flight planning.
Useful Load
The difference between takeoff weight, or ramp weight if applicable, and basic empty weight.
Zero Fuel Weight
The weight of an aircraft without fuel.
Weight and Balance (W&B C1)
There are many factors in the safe and efficient operation of aircraft, including proper weight and balance control.
The weight and balance system commonly employed among aircraft consists of three equally important elements: the weighing of the aircraft, the maintaining of the weight and balance records, and the proper loading of the aircraft.
An inaccuracy in any one of these elements defeats the purpose of the system.
The final loading calculations are meaningless if either the aircraft has been improperly weighed or the records contain an error.
Effects of Overweight
Most modern aircraft are so designed that, when all seats are occupied, the baggage compartment is full, and all fuel tanks are full, the aircraft is grossly overloaded.
Effects of being overweight include:
Longer takeoff run
Reduced rate and angle of climb
Service ceiling Lowered
Cruise speed reduced
Cruise range is shortened
Maneuverability is decreased
Longer landing roll
Excessive loads on structure and landing gear imposed
Forward CG Effects
Longer Takeoff Roll
Because the weight is concentrated forward in the aircraft, it will need to gain more airspeed before it is able to reach liftoff speed.
Longer Landing Roll
The forward weight will create momentum in pulling the aircraft down the runway.
Also, there won’t be as much weight over the main wheels so braking will be less effective.
Higher Stall Speed
The aircraft will stall at a higher airspeed due to “wing loading.”
The wings will seem to be carrying more weight and will need to fly at a higher airspeed to produce enough lift to carry the additional feel of weight.
Imagine carrying 2 milk gallons from your car to your house.
One is carried down by your side while the other is carried in a straight arm out in front of you.
While both milk jugs weigh the same amount, the milk jug held out in front of your center of gravity will “seem” heavier.
This is what occurs with “wing loading” in an aircraft with a Forward CG.
Easier Stall Recovery
The forward CG will assist the aircraft in recovering from a stall.
Decreased Cruise Speed
Because the CG is forward, the pilot will need to trim the aircraft “nose-up” to maintain altitude at cruise.
The deflected trim tab will cause drag with the relative airflow.
Aft CG Effects
Lower Stall Speed
With the CG Aft, the aircraft will have a lower stall speed due to decreased wing loading.
Reduced Elevator Authority
The Aft CG will cause the Elevator and Rudder to be less effective.
This is because the arm from the CG to the Elevator and Rudder is shorter.
More Difficult Stall Recovery
Because the weight is concentrated aft in the aircraft, it will be more difficult to lower the Angle of Attack in order to recover from a stalled condition.
Faster Cruise Speed
The Aft CG will not cause the Trim Tab on the Elevator to be deflected as far.
Creating less drag at cruise.
Weight and Balance Formula
Weight x Arm = Moment
After making the appropriate computations, you will take your finalized numbers and chart them on the aircraft’s CG Limit Envelope to make sure the aircraft is within balance limitations.
This CG Limit Envelope graph is located in chapter 6 of the POH.
Sample weight and balance calculation.
This is for a Cessna 172XP.
For this example let’s assume:
The front pilot’s weigh 320 lbs.
The rear passengers weigh 150 lbs.
There is 50 lbs in the baggage area.
There are 50 gallons of fuel on board.
Fuel weight = gallons x 6 lbs.
Fuel weight for this example = 300 lbs.
Weight x Arm = Moment
Now, we multiply our weights by the arms in the POH.
Next, we need to find our takeoff weight.
This is done by adding all of our weights together.
Then we do the same for our takeoff moments.
We find the takeoff arm by:
Dividing the Takeoff Moment by the Takeoff Weight.
Lastly, we plot our Takeoff Arm and Weight on the graph.
In this example, we ended up safely in the Normal Category.
FAA Sources Used in this Lesson
Weight and Balance Handbook