Occasionally one blade may produce slightly more thrust than another, even when flying at an identical pitch angle. The consequent imbalance of aerodynamic forces causes vibration and will cause the rotational path taken by each blade to be different. When a blade suffers from this condition it is said to be “out-of-track”.

The main rotor blade tracking involves adjusting individual blade angle, so that they all fly in the same TPP and ensures that they are equally spaced within the rotor disc. This in turn ensures that the centrifugal force on each blade is equal and in this case it is said that the blades are "in-track".

Vertical vibrations on the rotor head arise from different amounts of lift produced by an out-of-track blade, particularly when it becomes the advancing blade.

An out-of-track blade may be a result of:

• A faulty blade tracking process
• Blades of unequal weight or balance
• Faulty Drag Dampers

The Tip Path Plane (TPP) is the one taken by the tips of the rotor blades as they rotate. This plane is also known as the “no flapping plane”.

The same shall apply when lift is generated, thrust is created and the blades are risen forming a coning angle.

The Coning angle is the angle between the blades and the TPP.