AFT Calculator (Aft Center of Gravity)
A professional tool for pilots and loadmasters to calculate the aircraft’s Aft Center of Gravity for safe flight operations.
Aircraft Basic Empty Weight
Enter the Basic Empty Weight (BEW) from the aircraft’s POH/AFM.
Enter the arm for the Basic Empty Weight CG.
Load Stations
Calculation Results
Total Weight: 0.00
Total Moment: 0.00
CG Position Visualizer
What is an AFT Calculator?
An AFT calculator, in the context of aviation, is a tool used to determine the Aft Center of Gravity (CG) of an aircraft. The CG is the single point where the entire weight of the aircraft is considered to be concentrated. Its position along the longitudinal axis (from nose to tail) is critical for stability and control during flight. An Aft CG means the balance point is shifted towards the tail of the aircraft.
This calculator is essential for pilots, flight engineers, and loadmasters. Before every flight, they must perform a weight and balance calculation to ensure the aircraft’s loaded CG is within the permissible forward and aft limits set by the manufacturer. Flying with a CG outside these limits can lead to dangerous flight characteristics and potential loss of control. Our AFT calculator simplifies this complex but vital pre-flight task.
AFT Calculator Formula and Explanation
The core principle of an AFT calculator is based on the concept of moments. A moment is a measure of the tendency of a weight to cause rotation at a distance. It’s calculated by multiplying a weight by its distance (arm) from a reference point called the datum.
The fundamental formula to find the Center of Gravity is:
CG = Total Moment / Total Weight
To use this, you sum the individual moments of every item on board (aircraft itself, crew, passengers, baggage, fuel) and divide by the total weight of the aircraft. For a detailed breakdown, check out our aircraft weight and balance guide.
| Variable | Meaning | Unit (Auto-Inferred) | Typical Range |
|---|---|---|---|
| Weight | The mass of an object (e.g., passenger, cargo). | lbs or kg | 0 – 1,000,000+ |
| Arm (Station) | The distance from the reference datum to the item’s CG. | inches or meters | -50 to 2000+ |
| Moment | The rotational force produced by a weight at its arm. (Weight × Arm). | lb-in or kg-m | Varies widely |
| Datum | An imaginary vertical plane from which all horizontal distances (arms) are measured. | N/A (Reference Point) | Defined by manufacturer |
Practical Examples
Example 1: Light Aircraft Loading
Let’s consider a small aircraft with the following details:
- Aircraft Empty Weight: 1,500 lbs at an arm of 80 in.
- Pilot & Front Passenger: 350 lbs at an arm of 85 in.
- Rear Passengers: 300 lbs at an arm of 120 in.
- Baggage: 50 lbs at an arm of 145 in.
Calculation:
- Aircraft Moment: 1500 * 80 = 120,000 lb-in
- Front Pax Moment: 350 * 85 = 29,750 lb-in
- Rear Pax Moment: 300 * 120 = 36,000 lb-in
- Baggage Moment: 50 * 145 = 7,250 lb-in
- Total Weight: 1500 + 350 + 300 + 50 = 2,200 lbs
- Total Moment: 120000 + 29750 + 36000 + 7250 = 193,000 lb-in
- Final AFT CG: 193,000 / 2200 = 87.73 inches
Example 2: Shifting Cargo
Using the same aircraft, imagine we move the 50 lbs of baggage from the rear compartment (arm 145 in) to an empty front seat (arm 85 in). See how this impacts the Aft CG.
- New Baggage Moment: 50 lbs * 85 in = 4,250 lb-in
- Total Weight: Remains 2,200 lbs
- New Total Moment: 120000 + 29750 + 36000 + 4250 = 190,000 lb-in
- New AFT CG: 190,000 / 2200 = 86.36 inches
As you can see, moving just 50 lbs forward shifted the CG by over an inch, highlighting the importance of precise loading. To better understand these limits, use a CG envelope calculator.
How to Use This AFT Calculator
- Select Units: Start by choosing your preferred units for weight (lbs/kg) and arm (in/m). The calculator will handle all conversions.
- Enter Basic Empty Weight: Input the aircraft’s empty weight and its corresponding CG arm from the official flight manual.
- Add Load Items: Click “+ Add Load Item” for each group of passengers, cargo bay, or fuel tank. For each item, enter its total weight and its arm (distance from datum).
- Analyze Results: The calculator instantly updates the “Total Weight,” “Total Moment,” and the final “Aft Center of Gravity” in real-time.
- Visualize CG: The CG Position Visualizer chart shows the calculated CG relative to the safe limits (defaults are for a typical light aircraft, not specific). A green dot indicates the CG is within limits.
Key Factors That Affect AFT CG
- Passenger Seating: Where passengers sit has a significant impact. A passenger moving from a front to a back seat will shift the CG aft.
- Cargo Loading: Heavy cargo placed in rear baggage compartments will have a strong aft-shifting effect on the CG.
- Fuel Burn: As fuel is consumed during flight, its weight decreases. The location of the fuel tanks determines how the CG shifts. In many aircraft, fuel burn causes the CG to move forward. A proper fuel burn calculator is essential for long flights.
- Aircraft Modifications: Installing new equipment, like avionics in the nose or an antenna on the tail, permanently alters the empty weight CG.
- Use of Ballast: Sometimes, fixed or removable ballast is used to bring the CG into the acceptable range when loading is not ideal.
- Gear Retraction/Extension: On retractable-gear aircraft, the movement of the landing gear can cause a slight shift in the CG.
Understanding these factors is as crucial as the calculation itself. For pilots, this knowledge informs decisions related to both pre-flight planning and in-flight performance, such as when planning with a takeoff performance calculator.
Frequently Asked Questions (FAQ)
An excessively aft CG makes the aircraft less stable and can reduce the elevator’s effectiveness, making it difficult or impossible to recover from a stall or a spin. The aircraft becomes “tail-heavy” and wants to pitch up.
The datum is an imaginary vertical plane chosen by the aircraft manufacturer. It’s a fixed reference point from which all horizontal distances (arms) are measured for weight and balance purposes. Its location is specified in the aircraft’s flight manual.
Our AFT calculator automatically converts all inputs to a consistent internal unit system for calculation. When you select ‘kg’ or ‘m’, the values are converted before being used in the formula, and the final result is converted back to your chosen unit for display.
% MAC (Percent Mean Aerodynamic Chord) is another way to express the CG location, common in larger aircraft. While related, the direct arm measurement is more fundamental and universally applicable. We may add a percent MAC calculator as a separate tool in the future.
Yes, the principle (Weight × Arm = Moment) is universal. However, you MUST use the correct Empty Weight, Arm, and CG limits specified in the Pilot’s Operating Handbook (POH) or Aircraft Flight Manual (AFM) for your specific aircraft.
The calculator is designed to ignore non-numeric input and will treat those fields as zero to prevent calculation errors.
No. Shifting an existing weight (like a passenger or baggage) inside the aircraft only changes the total moment and thus the CG position. The total weight remains the same.
The safe forward and aft CG limits are critical for flight and are found in the “Weight and Balance” section of your aircraft’s POH/AFM. The visualizer uses typical values, but you must always refer to your official documentation.