Guide: Weight & Balance (CG)
An improperly loaded aircraft is a lethal aerodynamic liability. Every aircraft has strict structural and aerodynamic limitations dictated by the manufacturer. If you load an aircraft beyond its Maximum Gross Weight, the structural integrity of the wing spar is compromised, and the aircraft may simply refuse to fly. Even more critically, the distribution of that weight determines the Center of Gravity (CG). The CG is the precise fulcrum point where the aircraft would balance if suspended from a string. If the CG is too far forward, the elevator will lack the aerodynamic authority to lift the nose during landing, resulting in a nose-wheel crash. If the CG is too far aft (behind the limit), the aircraft becomes violently unstable in pitch. An aft-CG stall is often completely unrecoverable, leading to a fatal flat spin. This calculator ensures your total load and its physical distribution adhere to the safe operating envelope.
How to Use This Tool
Open your specific aircraft's Pilot Operating Handbook (POH) to the Weight and Balance section. Enter the aircraft's exact Empty Weight and Empty Arm (the distance from the datum line). This data is unique to your specific tail number, not generic models. Next, input the weights of the Front Passengers, Rear Passengers (or baggage), and the Fuel. You must cross-reference your POH to input the correct "Arm" (station location in inches or meters) for each specific row of seats and the fuel tanks.
The Math Behind It
The underlying physics relies on the lever principle: Weight × Arm = Moment. The engine calculates the Moment for every single item loaded into the aircraft (e.g., a 200lb passenger sitting at an arm of 37 inches generates a moment of 7,400 inch-pounds). It then sums all individual weights to find the Total Gross Weight. Finally, it sums all individual moments and divides that Total Moment by the Total Gross Weight. The resulting number is the exact Center of Gravity (in inches from the datum).
Understanding Your Results
Total Gross Weight is the absolute mass of the aircraft; compare this number directly to your POH MTOW (Maximum Takeoff Weight) to ensure you are legal to fly. Zero Fuel Weight is the weight before fuel is added, a critical structural limitation in larger aircraft. Center of Gravity is your balance point. You must plot this exact decimal on your POH CG Envelope Graph to guarantee you are within the safe boxed limits.
Real-World Example
A pilot wants to fly a Cessna 172 with three adult friends. The aircraft has an empty weight of 1,650 lbs. The pilot and front passenger weigh 400 lbs combined (Arm: 37). The two rear passengers weigh 400 lbs combined (Arm: 73). They load 40 gallons of fuel (240 lbs, Arm: 48). The calculator determines the Total Gross Weight is 2,690 lbs. The POH maximum gross weight is 2,550 lbs. The aircraft is 140 lbs over gross weight, making it illegal and highly dangerous to fly. Furthermore, the calculator reveals the CG is at 45.2 inches, which is drastically aft of the CG limit. The pilot must kick out a passenger and drain fuel before attempting to start the engine.
Frequently Asked Questions
What is the Reference Datum?
The Datum is an imaginary vertical plane from which all horizontal distances (Arms) are measured. Manufacturers arbitrarily set this point. In some aircraft, it is the tip of the propeller spinner; in others, it is the firewall. An arm of 37 inches means the seat is physically 37 inches behind the datum.
Why is an Aft CG so dangerous?
Aircraft are designed to be naturally nose-heavy for stability. The tail provides downward aerodynamic force to keep the nose up. If the CG moves too far backward (Aft), the aircraft loses its natural stall-recovery tendency. If the aircraft stalls, the nose will pitch up uncontrollably, leading to a fatal flat spin.
Does burning fuel change my CG in flight?
Yes. As the engine burns fuel, weight is removed from the fuel tank station. Depending on whether the fuel tanks are located in front of or behind the main CG point, burning fuel will cause the CG to slowly creep forward or backward during the flight. You must calculate your landing CG as well.
What happens if I fly over the maximum gross weight?
Your takeoff roll will be drastically extended, your climb rate will be anemic, your stall speed will increase (meaning you must fly faster on approach), and you risk structural failure if you encounter severe turbulence, as the wings are supporting more mass than they were certified to carry.