Datum is a fixed reference point used to measure horizontal distances when calculating an aircraft's weight and balance. Every measurement in a weight and balance calculation starts from this single point.
How It Works#
The manufacturer selects the datum location before an aircraft enters service. It can be any fixed point: the firewall, the nose of the aircraft, or even an imaginary point ahead of the nose. The exact location does not matter as long as every measurement uses the same starting point.
All items on the aircraft, such as seats, fuel tanks, and cargo bays, are assigned a position called an arm. The arm is the horizontal distance from the datum to that item, measured in inches. A positive arm means the item sits behind the datum. A negative arm means it sits ahead of it.
Once you know an item's weight and its arm, you multiply the two together. The result is called a moment. Moments tell you how much rotational force each item applies around the datum point.
Example in Aviation#
A Cessna 172 uses the firewall as its datum. The pilot weighs 180 lb and sits 37 inches behind the datum, giving a moment of 6,660 lb-in. The rear passenger weighs 150 lb and sits 73 inches behind the datum, giving a moment of 10,950 lb-in. The flight instructor adds all weights and all moments separately, then divides total moment by total weight. The result is the center of gravity (CG), expressed as inches from the datum.
Why It Matters#
If you move the datum, every arm value changes. This is why the datum location is fixed in the aircraft's Type Certificate Data Sheet (TCDS) and Pilot's Operating Handbook (POH). Pilots must use the manufacturer's datum, not their own reference point, or every calculation becomes meaningless.
Understanding the datum helps pilots catch weight and balance errors before flight. A CG that sits too far forward or too far aft can make an aircraft uncontrollable. The datum is the anchor that makes every other weight and balance number trustworthy.
Key Takeaways#
- The datum is a fixed reference point for all horizontal weight and balance measurements.
- Its location is set by the manufacturer and published in the POH and TCDS.
- Arms are measured in inches from the datum, positive aft and negative forward.
- Multiplying weight by arm gives a moment, which determines where the CG falls.
- Using the wrong datum invalidates every weight and balance calculation.