How does aircraft weight affect its performance?

Weight and Balance is one of the most important topics taught during the early stages of the aviation career, but why is it so important? Basically, the weight and balance of the airplane will directly affect its performance until at some point, the airplane won’t be able to fly.

Moreover, if the airplane is loaded over its limits it won’t be possible to calculate its performance characteristics (fuel consumption, rate of climb, true airspeed, etc). Airplane manufacturers provide performance charts and aerodynamic characteristics in the Pilot Operating Handbook (POH). Those performance charts are generated based on the airplane’s maximum gross weight (maximum allowable total weight) and center of gravity (CG) location within the specified limits, although sometimes manufacturers also provide performance charts for other than maximum gross weight, allowing pilots to calculate performance characteristics when the airplane is loaded differently (never in excess of maximum gross weight).

According to 14 CFR 91.9 “no person may operate an aircraft without complying with the operating limitations.”. Loading an airplane over its maximum gross weight or its CG limits it’s illegal, but more importantly, dangerous due to the deteriorated performance.

An overloaded condition occurs when an airplane is loaded over its maximum total gross weight, and as mentioned, it has a huge negative effect on aircraft performance. Let’s analyze those negative impacts:

• Increased landing roll
• An airplane’s lift should overcome its weight to be able to fly. The airplane should go faster in order for its wings to be able to produce more lift. In order to speed to a faster speed a greater distance is required.

• Reduced rate of climb
• An airplane will be able to maintain a certain unaccelerated flight attitude (straight and level, climb, descend) as long as all upward forces equal all downward forces, and all forward forces equals all backward forces. Thus, if thrust should equal drag, the aircraft´s total drag defines the amount of thrust or power needed for each specific flight attitude. We can say that the total drag curve is as well the thrust or power required curve.

Note: The drag curve is actually equal to the required thrust curve, not the required power curve. Although they are pretty similar. Remember thrust is the lift produced by the propeller and power is how fast the engine produces energy to move that propeller. For this study, we are going to pretend that the required power curve is equal to the thrust required curve

• Climb performance is directly dependent upon the ability to produce either excess thrust (Angle of Climb — VX) or excess power (Rate of Climb — VY). The excess of thrust or power is determined by the difference between the engine‘s power or propeller thrust output and the one needed for each flight attitude.

Note: Excess thrust is used to counteract the increased drag generated by the weight vector towards the back of the airplane that occurs when an airplane is pitched up. In other words, it’s what makes the airplane to climb at a steep angle of climb.

As mentioned before, power is how fast energy is produced. Excess of power determines how fast an airplane climbs. Energy can be transformed into kinetic energy (airspeed) or potential energy (altitude), thus the rate of climb is how fast we produce potential energy.

• When an airplane is heavier more lift should be produced by increasing its AOA, which in fact is also going to increase the airplane’s total drag and thrust or power required. This will lead to a reduced excess of thrust or power, therefore a reduced rate of climb or angle of climb. VY it’s always at a certain AOA for a specific airfoil, but the airspeed at which this AOA is found will be greater as heavier the airplane. Take a look at the curves

• Greater fuel consumption and reduced range
• The airplane should fly at a higher AOA to produce more lift and overcome the excess weight. As greater the AOA, the greater the induced drag (and total drag). The airplane will slow down unless more power is added to overcome the excess of drag. The extra power needed will make the airplane burn more fuel.
• Reduced cruising speed
• At a specific cruise power setting, let’s say 2300 RPM, an overloaded airplane will fly slower than an airplane loaded within the limits.
• Increased Stall speed

For this example, I like to review the lift equation. It’s fundamental to understand that the Critical angle of Attack, which is the angle of attack at which an airplane will stall, is a design factor and it remains unchanged regardless of the weight, speed, or atmospheric conditions. Pilots can increase the angle of attack or add power to increase the airspeed to generate the desired amount of lift from the wings. By looking at the lift equation, the following statement can be said. When an aircraft’s altitude is wanted to be maintained, whenever the AOA is increased, airspeed should be reduced, and the opposite is true, when airplane airspeed is increased the AOA should be reduced, they are interchangeable. Of course, there are some limits, the AOA can be increased until it reaches its critical angle of attack, after that point the airplane will be in a stalled condition. The airspeed we can produce has also a limitation, which is the engine maximum RPM. For instance, an airplane’s gross weight is 2300 lbs, and its wings are producing 2300 lbs of lift with an angle of attack of 18° (let’s say this is the critical AOA) and an airspeed of 60 knots. Now imagine that this same airplane has a greater gross weight of 2400 lbs. Our lift production has a lack of 100 pounds, the airplane will start losing altitude. How can this problem be solved? Increasing the AOA is not an option because the airplane has already reached its critical angle of attack. The only solution is to increase the engine’s power therefore its airspeed. As you can see, according to this example, when the airplane is heavier its stall speed is increased as well.

• Prone to structural damage
• The airplane’s airworthiness category (normal, utility, aerobatic, etc) determines the airplane’s ability to resist a positive or negative structural load (load factor). This ability to resist is based on the airplane’s maximum gross weight. A normal airworthiness category will guarantee an aircraft a positive structural load of 3.8 G and a negative load of 1.52 G. The Load factor (G) is what the airplane feels it weighs. When the load factor is increased to 2, the airplane will feel it weighs 2 times its current weight. Therefore, manufacturers guarantee that the airplane structure can resist 3.8 times its maximum gross weight. Overloading the airplane may cause structural damage or fatigue even when the load factor is within the airplane’s limit.
• During landings, the nose gear strut and the main landing gear system will suffer from the overweight until at some point it breaks.