When I was preparing for my PPL theoretical exam, a fellow student asked our teacher at the end of a lesson about mass and balance: “Do all aircraft have a center of gravity, or is this an optional piece of equipment ?”. I never forget the question, but can’t remind how the teacher did react. All pilots know it’s important to have the center of gravity within acceptable limits, and know how to calculate its position. But do YOU remember why it’s important ?

When a force is applied to an object, it makes it move, and / or rotate. If the object can move freely, like an aircraft in flight, it will rotate around its center of gravity. The farthest the point where the force is applied, the higher the rotation. The so-called “moment” is the force multiplied by the distance to the center of gravity. This is nothing else than the good old lever principle: “Give me a place to stand on, and I will move the Earth.”

Because the center of gravity of an aircraft is never located at the center of gravity, the lift generates a moment. In light aircraft, the center of lift is typically behind the center of gravity, so lift generates a nose-down moment. This moment is compensated by some aerodynamic forces generated by the horizontal stabilizer. To compensate for the nose-down moment, the stabilizer generates some downward forces which create a nose-up moment. Because this force is far away from the center of gravity, a minimal magnitude is sufficient.

So the moment of lift (upwards, behind CoG, nose-down) and the moment from the stabilizer (downwards, behind CoG, nose-up) compensate for each other, making the aircraft stable, easy to control, and reduce the required control forces. But the distance between the center of gravity and the center of lift is not that big…

If the center of gravity moves behind the center of lift, the direction of the moment created by the lift changes direction: lift is still upwards, but it’s now in front of CoG, creating a nose-up moment. The force from the stabilizer is still downwards, and still behind the center of gravity… and still creating a nose-up moment. There is no more compensation and the sole way to compensate for the moment is to use extra control forces, but it’s not granted that the aircraft will remain stable and controllable…

Flying with passengers in the back seats of a PA28 or C172 makes quite a difference in the required control forces. This is why the typical differences training for a new aircraft type includes a flight at maximum take-off mass.

Nobody will check that you’re flying with a properly positioned center of gravity before you go flying. It’s one of the numerous duties of pilots to ensure the plane is loaded correctly, before and during each and every flight. As soon as you operate with a center of gravity outside the approved limits, you become a test pilot.

Insurance companies don’t like test pilots at all.